How organisms adapt to new environments is of fundamental biological interest, but poorly understood at the genetic level. Chemosensory systems provide attractive models to address this problem, because they lie between external environmental signals and internal physiological responses. To investigate how selection has shaped the well-characterized chemosensory system of Drosophila melanogaster, this study analysed genome-wide data from five diverse populations. By couching population genomic analyses of chemosensory protein families, including odorant receptors, gustatory receptors, and odorant-binding proteins, within parallel analyses of other large families, it was demonstrated that chemosensory proteins are not outliers for adaptive divergence between species. However, chemosensory families often display the strongest genome-wide signals of recent selection within D. melanogaster. Recent adaptation has operated almost exclusively on standing variation, and patterns of adaptive mutations predict diverse effects on protein function. Finally, evidence is provided that chemosensory proteins have experienced relaxed constraint, and it is argued that this has been important for their rapid adaptation over short timescales (Arguello, 2016).
Gene expression profiling is one of the most reliable high-throughput phenotyping methods, allowing researchers to quantify the transcript abundance of expressed genes. Because many biotic and abiotic factors influence gene expression, it is recommended to control them as tightly as possible. This study shows that a 24 h age difference of Drosophila simulans females that were subjected to RNA sequencing (RNA-Seq) five and six days after eclosure resulted in more than 2000 differentially expressed genes. This is twice the number of genes that changed expression during 100 generations of evolution in a novel hot laboratory environment. Importantly, most of the genes differing in expression due to age introduce false positives or negatives if an adaptive gene expression analysis is not controlled for age. These results indicate that tightly controlled experimental conditions, including precise developmental staging, are needed for reliable gene expression analyses, in particular in an evolutionary framework (Hsu, 2019).
Divergent populations across different environments are exposed to critical sensory information related to locating a host or mate, as well as avoiding predators and pathogens. These sensory signals generate evolutionary changes in neuroanatomy and behavior; however, few studies have investigated patterns of neural architecture that occur between sensory systems, or that occur within large groups of closely-related organisms. This study examine 62 species within the genus Drosophila and describes an inverse resource allocation between vision and olfaction, which was consistently observe at the periphery, within the brain, as well as during larval development. This sensory variation was noted across the entire genus and appears to represent repeated, independent evolutionary events, where one sensory modality is consistently selected for at the expense of the other. Moreover, evidence is provided of a developmental genetic constraint through the sharing of a single larval structure, the eye-antennal imaginal disc. In addition, the ecological implications of visual or olfactory bias were examined, including the potential impact on host-navigation and courtship (Keesey, 2019).
Thermal adaptation is typically detected by examining the tolerance of a few populations to extreme temperatures within a single life stage. However, the extent to which adaptation occurs among many different populations might depend on the tolerance of multiple life stages and the average temperature range that the population experiences. This study examined local adaptation to native temperature conditions in eleven populations of the well-known cosmopolitan fruit fly, Drosophila melanogaster. These populations were sampled from across the global range of D. melanogaster. Traits related to fitness were measured during each life stage to determine if certain stages are more sensitive to changes in temperature than others. D. melanogaster appeared to show local adaptation to native temperatures during the egg, larval, and adult life stages, but not the pupal stage. This suggests that across the entire distribution of D. melanogaster, certain life stages might be locally adapted to native temperatures, while other stages might use phenotypic plasticity or tolerance to a wide range of temperatures experienced in the native environment of this species (Austin, 2019).
During evolution, organisms have acquired variable feeding habits. Some species are nutritional generalists that adapt to various food resources, while others are specialists, feeding on specific resources. However, much remains to be discovered about how generalists adapt to diversified diets. Larvae of the generalists Drosophila melanogaster and D. simulans develop on three diets with different nutrient balances, whereas specialists D. sechellia and D. elegans cannot develop on carbohydrate-rich diets. The generalist D. melanogaster downregulates the expression of diverse metabolic genes systemically by transforming growth factor beta (TGF-beta)/Activin signaling, maintains metabolic homeostasis, and successfully adapts to the diets. In contrast, the specialist D. sechellia expresses those metabolic genes at higher levels and accumulates various metabolites on the carbohydrate-rich diet, culminating in reduced adaptation. Phenotypic similarities and differences strongly suggest that the robust carbohydrate-responsive regulatory systems are evolutionarily retained through genome-environment interactions in the generalists and contribute to their nutritional adaptabilities (Watanabe, 2019).
Climate warming is threatening biodiversity worldwide. Climate specialists such as alpine species are especially likely to be vulnerable. Adaptation by rapid evolution is the only long-term option for survival of many species, but the adaptive evolutionary potential of heat resistance has not been assessed in an alpine invertebrate. This study show that the alpine fly Drosophila nigrosparsa cannot readily adapt to heat stress. Heat-exposed flies from a regime with increased ambient temperature and a regime with increased temperature plus artificial selection for heat tolerance were less heat tolerant than the control group. Increased ambient temperature affected negatively both fitness and competitiveness. Ecological niche models predicted the loss of three quarters of the climatically habitable areas of this fly by the end of this century. These findings suggest that, alongside with other climate specialists, species from mountainous regions are highly vulnerable to climate warming and unlikely to adapt through evolutionary genetic changes (Kinzner, 2019).
Relationships between an organism and its environment can be fundamental in the understanding how populations change over time and species arise. Local ecological conditions can shape variation at multiple levels, among these are the evolutionary history and trajectories of coding genes. This study examines the rate of molecular evolution at protein-coding genes throughout the genome in response to host adaptation in the cactophilic Drosophila mojavensis. These insects are intimately associated with cactus necroses, developing as larvae and feeding as adults in these necrotic tissues. Drosophila mojavensis is composed of four isolated populations across the deserts of western North America and each population has adapted to utilize different cacti that are chemically, nutritionally, and structurally distinct. High coverage Illumina sequencing was performed on three previously unsequenced populations of D. mojavensis. Genomes were assembled using the previously sequenced genome of D. mojavensis from Santa Catalina Island (USA) as a template. Protein coding genes were aligned across all four populations and rates of protein evolution were determined for all loci using a several approaches. Loci that exhibited elevated rates of molecular evolution tend to be shorter, have fewer exons, low expression, be transcriptionally responsive to cactus host use and have fixed expression differences across the four cactus host populations. Fast evolving genes were involved with metabolism, detoxification, chemosensory reception, reproduction and behavior. Results of this study give insight into the process and the genomic consequences of local ecological adaptation (Allan, 2019).
Population genomic data has revealed patterns of genetic variation associated with adaptation in many taxa. Yet understanding the adaptive process that drives such patterns is challenging; it requires disentangling the ecological agents of selection, determining the relevant timescales over which evolution occurs, and elucidating the genetic architecture of adaptation. Doing so for the adaptation of hosts to their microbiome is of particular interest with growing recognition of the importance and complexity of host-microbe interactions. This study tracked the pace and genomic architecture of adaptation to an experimental microbiome manipulation in replicate populations of Drosophila melanogaster in field mesocosms. Shifts in microbiome composition altered population dynamics and led to divergence between treatments in allele frequencies, with regions showing strong divergence found on all chromosomes. Moreover, at divergent loci previously associated with adaptation across natural populations, the more common allele was found in fly populations experimentally enriched for a certain microbial group was also more common in natural populations with high relative abundance of that microbial group. These results suggest that microbiomes may be an agent of selection that shapes the pattern and process of adaptation and, more broadly, that variation in a single ecological factor within a complex environment can drive rapid, polygenic adaptation over short timescales (Rudman, 2019).
Drosophila melanogaster recently spread from its tropical origin in Africa and became a cosmopolitan species that has adapted to a wide range of different thermal environments, including temperate climates. An important limiting factor of temperate climates has probably been their low and varying temperatures. The transcriptional output of genes can vary across temperatures, which might have been detrimental while settling in temperate environments. The reduction of temperature-sensitive expression of functionally important genes to ensure consistent levels of gene expression might have been relevant while adapting to such environments. This study focuses on the gene vestigial (vg) whose product is a key factor in wing development. Evidence is provided that temperature-sensitivity of vg has been buffered in populations from temperate climates. Temperature-sensitivity of vg gene expression was investigated in six natural populations, including four temperate populations (three from Europe and one from high-altitude Africa), and two tropical populations from the ancestral species range. All temperate populations exhibited a lower degree of temperature-induced expression plasticity than the tropical populations (Voigt, 2019).
Drosophila melanogaster is thought to originate from sub-Saharan Africa. This study documents the collection of 288 D. melanogaster individuals from multiple African wilderness areas in Zambia, Zimbabwe, and Namibia. The presence of D. melanogaster in these remote woodland environments is consistent with an ancestral range in southern-central Africa, as opposed to equatorial regions. After sequencing the genomes of 17 wilderness-collected flies collected from Kafue National Park in Zambia, reduced genetic diversity relative to town populations, elevated chromosomal inversion frequencies, and strong differences at specific genes including known insecticide targets were found. Combining these genomes with existing data, this study probed the history of this species' geographic expansion. Demographic estimates indicated that expansion from southern-central Africa began approximately 10,000 years ago, with a Saharan crossing soon after, but expansion from the Middle East into Europe did not begin until roughly 1,400 years ago. This improved model of demographic history will provide an important resource for future evolutionary and genomic studies of this key model organism. These findings add context to the history of D. melanogaster, while opening the door for future studies on the biological basis of adaptation to human environments (Sprengelmeyer, 2019).
In animals, the most common type of RNA editing is the deamination of adenosines (A) into inosines (I). Because inosines base-pair with cytosines (C), they are interpreted as guanosines (G) by the cellular machinery and genomically encoded G alleles at edited sites mimic the function of edited RNAs. The contribution of this hardwiring effect on genome evolution remains obscure. This study looked for population genomics signatures of adaptive evolution associated with A-to-I RNA edited sites in humans and Drosophila melanogaster. Single nucleotide polymorphisms at edited sites occur 3 (humans) to 15 times (Drosophila) more often than at unedited sites, the nucleotide G is virtually the unique alternative allele at edited sites and G alleles segregate at higher frequency at edited sites than at unedited sites. This study study reveals that a significant fraction of coding synonymous and nonsynonymous as well as silent and intergenic A-to-I RNA editing sites are likely adaptive in the distantly related human and Drosophila lineages (Popitsch, 2020).
The pervasive occurrence of sexual dimorphism demonstrates different adaptive strategies of males and females. While different reproductive strategies of the two sexes are well-characterized, very little is known about differential functional requirements of males and females in their natural habitats. The impact environmental change on the selection response was studied in both sexes. Exposing replicated Drosophila populations to a novel temperature regime, sex-specific changes were demonstrated in gene expression, metabolic and behavioral phenotypes in less than 100 generations. This indicates not only different functional requirements of both sexes in the new environment but also rapid sex-specific adaptation. Supported by computer simulations it is proposed that altered sex-biased gene regulation from standing genetic variation, rather than new mutations, is the driver of rapid sex-specific adaptation. This discovery of environmentally driven divergent functional requirements of males and females has important implications-possibly even for gender aware medical treatments (Hsu, 2020).
Cold stress is a critical environmental challenge that affects an organism's fitness-related traits. In Drosophila, increased resistance to specific environmental stress may lead to increased resistance to other kinds of stress. This study aimed to understand whether increased cold stress resistance in Drosophila melanogaster can facilitate their ability to tolerate other environmental stresses. This study used successfully selected replicate populations of D. melanogaster against cold shock and their control population. The present work investigated egg viability and mating frequency with and without heat and cold shock conditions in the selected and their control populations. Resistance to cold shock, heat shock, desiccation, starvation, and survival post-challenge with Staphylococcus succinus subsp. succinus PK-1 were also examined in the selected and their control populations. After cold-shock treatment, it was found a 1.25 times increase in egg viability and a 1.57 times increase in mating frequency in the selected populations compared to control populations. Moreover, more males (0.87 times) and females (1.66 times) of the selected populations survived under cold shock conditions relative to their controls. After being subjected to heat shock, the selected population's egg viability and mating frequency increased by 0.30 times and 0.57 times, respectively, compared to control populations. Additionally, more selected males (0.31 times) and females (0.98 times) survived under heat shock conditions compared to the control populations. Desiccation resistance slightly increased in the females of the selected populations relative to their control, but no change was observed in the case of males. Starvation resistance decreased in males and females of the selected populations compared to their controls. These findings suggest that the increased resistance to cold shock correlates with increased tolerance to heat stress, but this evolved resistance comes at a cost, with decreased tolerance to starvation (Singh, 2022).
The influence of pleiotropy on adaptive responses is a highly controversial topic, with limited empirical evidence available. Recognizing the pivotal role of the correlation of fitness effects, an experiment was designed to compare the adaptive gene expression evolution of natural and experimental populations. To test this, we studied the evolution of gene expression in response to temperature in two Drosophila species on a natural temperature cline in North America and replicated populations evolving in hot and cold temperature regimes. If fitness effects of affected traits are independent, pleiotropy is expected to constrain the adaptive response in both settings. However, when fitness effects are more correlated in natural populations, adaptation in the wild will be facilitated by pleiotropy. Remarkably, evidence was found for both predicted effects. In both settings, genes with strong pleiotropic effects contribute less to adaptation, indicating that the majority of fitness effects are not correlated. In addition, this study also discovered that genes involved in adaptation exhibited more pleiotropic effects in natural populations. It is proposed that this pattern can be explained by a stronger correlation of fitness effects in nature. More insights into the dual role of pleiotropy will be crucial for the understanding of polygenic adaptation (Thorholludottir, 2023).
The evolution of endosymbionts and their hosts can lead to highly dynamic interactions with varying fitness effects for both the endosymbiont and host species. Wolbachia, a ubiquitous endosymbiont of arthropods and nematodes, can have both beneficial and detrimental effects on host fitness. This study documented the occurrence and patterns of transmission of Wolbachia within the Hawaiian Drosophilidae and examined the potential contributions of Wolbachia to the rapid diversification of their hosts. Screens for Wolbachia infections across a minimum of 140 species of Hawaiian Drosophila and Scaptomyza revealed species-level infections of 20.0%, and across all 399 samples, a general infection rate of 10.3%. Among the 44 Wolbachia strains identified using a modified Wolbachia multi-locus strain typing scheme, 30 (68.18%) belonged to supergroup B, five (11.36%) belonged to supergroup A, and nine (20.45%) had alleles with conflicting supergroup assignments. Co-phylogenetic reconciliation analysis indicated that Wolbachia strain diversity within their endemic Hawaiian Drosophilidae hosts can be explained by vertical (e.g., co-speciation) and horizontal (e.g., host switch) modes of transmission. Results from stochastic character trait mapping suggest that horizontal transmission is associated with the preferred oviposition substrate of the host, but not the host's plant family or island of occurrence. For Hawaiian Drosophilid species of conservation concern, with 13 species listed as endangered and one listed as threatened, knowledge of Wolbachia strain types, infection status, and potential for superinfection could assist with conservation breeding programs designed to bolster population sizes, especially when wild populations are supplemented with laboratory-reared, translocated individuals. Future research aimed at improving the understanding of the mechanisms of Wolbachia transmission in nature, their impact on the host, and their role in host species formation may shed light on the influence of Wolbachia as an evolutionary driver, especially in Hawaiian ecosystems (Corpuz, 2023).
Neuronal activity is temperature-sensitive and affects behavioral traits important for individual fitness, such as locomotion and courtship. Yet not enough is known about the evolutionary response of neuronal phenotypes in new temperature environments. This study used long-term experimental evolution of Drosophila simulans populations exposed to novel temperature regimes. A direct relationship was demonstrated between thermal selective pressure and the evolution of neuronally expressed molecular and behavioral phenotypes. Several essential neuronal genes evolve lower expression at high temperatures and higher expression at low temperatures, with dopaminergic neurons standing out by displaying the most consistent expression change across independent replicates. The link between evolved gene expression and behavioral changes was functionally validated by pharmacological intervention in the experimentally evolved D. simulans populations as well as by genetically triggered expression changes of key genes in D. melanogaster. Since natural temperature clines confirm these results for Drosophila and Anopheles populations, it is concluded that neuronal dopamine evolution is a key factor for temperature adaptation (Jaksic, 2020).
Many organisms enter a dormant state in their life cycle to deal with predictable changes in environments over the course of a year. The timing of dormancy is therefore a key seasonal adaptation, and it evolves rapidly with changing environments. The hypothesis that differences in the timing of seasonal activity are driven by differences in the rate of development during diapause was tested in Rhagoletis pomonella, a fly specialized to feed on fruits of seasonally limited host plants. Transcriptomes from the central nervous system across a time series during diapause show consistent and progressive changes in transcripts participating in diverse developmental processes, despite a lack of gross morphological change. Moreover, population genomic analyses suggested that many genes of small effect enriched in developmental functional categories underlie variation in dormancy timing and overlap with gene sets associated with development rate in Drosophila melanogaster. These transcriptional data also suggested that a recent evolutionary shift from a seasonally late to a seasonally early host plant drove more rapid development during diapause in the early fly population. Moreover, genetic variants that diverged during the evolutionary shift were also enriched in putative cis regulatory regions of genes differentially expressed during diapause development. Overall, these data suggest polygenic variation in the rate of developmental progression during diapause contributes to the evolution of seasonality in R. pomonella. Patterns that suggest hourglass-like developmental divergence early and late in diapause development are discussed along with an important role for hub genes in the evolution of transcriptional divergence (Dowle, 2020).
One hypothesis for the function of sleep is that it serves as a mechanism to conserve energy. Recent studies have suggested that increased sleep can be an adaptive mechanism to improve survival under food deprivation in Drosophila melanogaster. To test the generality of this hypothesis, Sleep and its plastic response to starvation was compared in a temperate and tropical population of Drosophila melanogaster. Flies from the temperate population were found to be more starvation resistant, and it was hypothesized that they would engage in behaviors that are considered to conserve energy, including increased sleep and reduced movement. Surprisingly, temperate flies slept less and moved more when they were awake compared to tropical flies, both under fed and starved conditions, therefore sleep did not correlate with population-level differences in starvation resistance. In contrast, total sleep and percent change in sleep when starved were strongly positively correlated with starvation resistance within the tropical population, but not within the temperate population. Thus, unexpectedly complex relationships between starvation and sleep were observed that vary both within and across populations. These observations falsify the simple hypothesis of a straightforward relationship between sleep and energy conservation. The hypothesis that starvation is correlated with metabolic phenotypes was tested by investigating stored lipid and carbohydrate levels; stored metabolites were found to partially contributed towards variation starvation resistance. These findings demonstrate that the function of sleep under starvation can rapidly evolve on short timescales and raise new questions about the physiological correlates of sleep and the extent to which variation in sleep is shaped by natural selection (Sarikaya, 2020).
Ecological adaptation is frequently inferred by the comparison of natural populations from different environments. Nevertheless, the inference of the selective forces suffers the challenge that many environmental factors covary. With well-controlled environmental conditions, experimental evolution provides a powerful approach to complement the analysis of natural populations. On the other hand, it is apparent that laboratory conditions differ in many ways from natural environments, which raises the question to what extent selection responses in experimental evolution studies can inform about adaptation processes in the wild. This study compared the expression profiles of replicated Drosophila melanogaster populations which have been exposed to two distinct temperature regimes (18/28 °C and 10/20 °C) in the laboratory for more than 80 generations. Using gene-wise differential expression analysis and co-expression network analysis, 541 genes and three co-regulated gene modules were identified that evolved in the same direction in both temperature regimes, and most of these changes probably reflect an adaptation to the space constrain or diurnal temperature fluctuation that is common in both selection regimes. 203 genes and seven modules evolved temperature-specific expression changes. Remarkably, a significant overlap was detected of these temperature-adaptive genes/modules from experimental evolution with temperature-adaptive genes inferred from natural Drosophila populations covering two different temperature clines. It is concluded that well-designed experimental evolution studies are a powerful tool to dissect evolutionary responses (Hsu, 2020).
Phenotypic plasticity is the ability of a single genotype to produce different phenotypes in response to environmental variation. The importance of phenotypic plasticity in natural populations and its contribution to phenotypic evolution during rapid environmental change is widely debated. This study shows that thermal plasticity of gene expression in natural populations is a key component of its adaptation: evolution to novel thermal environments increases ancestral plasticity rather than mean genetic expression. The evolution of plasticity in gene expression was determined by conducting laboratory natural selection on a Drosophila simulans population in hot and cold environments. After more than 60 generations in the hot environment, 325 genes evolved a change in plasticity relative to the natural ancestral population. Plasticity increased in 75% of these genes, which were strongly enriched for several well-defined functional categories (e.g. chitin metabolism, glycolysis and oxidative phosphorylation). Furthermore, this study showed that plasticity in gene expression of populations exposed to different temperatures is rather similar across species. It is concluded that most of the ancestral plasticity can evolve further in more extreme environments (Mallard, 2020).
A fundamental aim of adaptation genomics is to identify polymorphisms that underpin variation in fitness traits. In D. melanogaster latitudinal life-history clines exist on multiple continents and make an excellent system for dissecting the genetics of adaptation. Previous work has identified numerous clinal SNPs in insulin/insulin-like growth factor signaling (IIS), a pathway known from mutant studies to affect life history. However, the effects of natural variants in this pathway remain poorly understood. This study investigated how two clinal alternative alleles at foxo, a transcriptional effector of IIS, affect fitness components (viability, size, starvation resistance, fat content). This polymorphism from the North American cline was assessed by reconstituting outbred populations, fixed for either the low- or high-latitude allele, from inbred DGRP lines. Since diet and temperature modulate IIS, alleles were phenotyped across two temperatures (18 ° C, 25 ° C) and two diets differing in sugar source and content. Consistent with clinal expectations, the high-latitude allele conferred larger body size and reduced wing loading. Alleles also differed in starvation resistance and expression of InR, a transcriptional target of FOXO. Allelic reaction norms were mostly parallel, with few GxE interactions. Together, these results suggest that variation in IIS makes a major contribution to clinal life-history adaptation ( , ).
While several studies in a diverse set of species have shed light on the genes underlying adaptation, knowledge on the selective pressures that explain the observed patterns lags behind. Drosophila melanogaster is a valuable organism to study environmental adaptation because this species originated in Southern Africa and has recently expanded worldwide, and also because it has a functionally well-annotated genome. This work aimed to decipher which environmental variables are relevant for adaptation of D. melanogaster natural populations in Europe and North America. 36 whole-genome pool-seq samples of D. melanogaster natural populations were ezamined, collected in 20 European and 11 North American locations. The BayPass software was used to identify SNPs and transposable elements showing signature of adaptive differentiation across populations, as well as significant associations with 59 environmental variables related to temperature, rainfall, evaporation, solar radiation, wind, daylight hours, and soil type. Besides temperature and rainfall, wind related variables are also relevant for D. melanogaster environmental adaptation. Interestingly, 23% to 51% of the genes that showed significant associations with environmental variables were not found overly differentiated across populations. Besides SNPs, ten reference transposable element insertions associated with environmental variables were identified. These results showed that genome-environment association analysis can identify adaptive genetic variants that are undetected by population differentiation analysis while also allowing the identification of candidate environmental drivers of adaptation (Bogaerts-Marquez, 2020).
To detect the genomic mechanisms underlying evolutionary dynamics of adaptation in sexually reproducing organisms, this study analyze multigenerational whole genome sequences of Drosophila melanogaster adapting to extreme O(2) conditions over an experiment conducted for nearly two decades. Methods to analyze time-series genomics data and predict adaptive mechanisms were developed. This study report a remarkable level of synchronicity in both hard and soft selective sweeps in replicate populations as well as the arrival of favorable de novo mutations that constitute a few asynchronized sweeps. Additionally direct experimental observations were made of rare recombination events that combine multiple alleles on to a single, better-adapted haplotype. Based on the analyses of the genes in genomic intervals, this study provides a deeper insight into the mechanisms of genome adaptation that allow complex organisms to survive harsh environments (Iranmehr, 2021).
Periods of nutrient shortage impose strong selection on animal populations. Experimental studies of genetic adaptation to nutrient shortage largely focus on resistance to acute starvation at adult stage; it is not clear how conclusions drawn from these studies extrapolate to other forms of nutritional stress. The genomic signature of adaptation to chronic juvenile malnutrition was studied in six populations of Drosophila melanogaster evolved for 150 generations on an extremely nutrient-poor larval diet. Comparison with control populations evolved on standard food revealed repeatable genomic differentiation between the two set of population, involving >3,000 candidate SNPs forming >100 independently evolving clusters. The candidate genomic regions were enriched in genes implicated in hormone, carbohydrate, and lipid metabolism, including some with known effects on fitness-related life-history traits. Rather than being close to fixation, a substantial fraction of candidate SNPs segregated at intermediate allele frequencies in all malnutrition-adapted populations. This, together with patterns of among-population variation in allele frequencies and estimates of Tajima's D, suggests that the poor diet results in balancing selection on some genomic regions. Candidate genes for tolerance to larval malnutrition showed a high overlap with genes previously implicated in acute starvation resistance. However, adaptation to larval malnutrition in this study was associated with reduced tolerance to acute adult starvation. Thus, rather than reflecting synergy, the shared genomic architecture appears to mediate an evolutionary trade-off between tolerances to these two forms of nutritional stress (Kawecki, 2021).
Forecasting which species/ecosystems are most vulnerable to climate warming is essential to guide conservation strategies to minimize extinction. Tropical/mid-latitude species are predicted to be most at risk as they live close to their upper critical thermal limits (CTLs). However, these assessments assume that upper CTL estimates, such as CTmax, are accurate predictors of vulnerability and ignore the potential for evolution to ameliorate temperature increases. This study used experimental evolution to assess extinction risk and adaptation in tropical and widespread Drosophila species. Tropical species were found to succumb to extinction before widespread species. Male fertility thermal limits, which are much lower than CTmax, are better predictors of species' current distributions and extinction in the laboratory. Little evidence was found of adaptive responses to warming in any species. These results suggest that species are living closer to their upper thermal limits than currently presumed and evolution/plasticity are unlikely to rescue populations from extinction (van Heerwaarden, 2021).
Changes in gene regulation at multiple levels may comprise an important share of the molecular changes underlying adaptive evolution in nature. However, few studies have assayed within- and between-population variation in gene regulatory traits at a transcriptomic scale, and therefore inferences about the characteristics of adaptive regulatory changes have been elusive. This study assessed quantitative trait differentiation in gene expression levels and alternative splicing (intron usage) between three closely-related pairs of natural populations of Drosophila melanogaster from contrasting thermal environments that reflect three separate instances of cold tolerance evolution. The cold-adapted populations were known to show population genetic evidence for parallel evolution at the SNP level, and this study found evidence for parallel expression evolution between them, with stronger parallelism at larval and adult stages than for pupae. A flexible method was implemented to estimate cis- versus trans-encoded contributions to expression or splicing differences at the adult stage. The apparent contributions of cis- versus trans-regulation to adaptive evolution vary substantially among population pairs. While two of three population pairs show a greater enrichment of cis-regulatory differences among adaptation candidates, trans-regulatory differences are more likely to be implicated in parallel expression changes between population pairs. Genes with significant cis-effects are enriched for signals of elevated genetic differentiation between cold- and warm-adapted populations, suggesting that they are potential targets of local adaptation. These findings expand knowledge of adaptive gene regulatory evolution and the ability to make inferences about this important and widespread process (Huang, 2021).
A collection of forty populations were used to study the phenotypic adaptation of Drosophila melanogaster larvae to urea-laced food. Fifteen populations subjected to direct selection for urea tolerance and five controls were studied. In addition, another twenty populations were studied that had not been exposed to urea but were subjected to stress or demographic selection. This study describes the differentiation in these population for six phenotypes: (1) larval feeding rates, (2) larval viability in urea-laced food, (3) larval development time in urea-laced food, (4) adult starvation times, (5) adult desiccation times, and (6) larval growth rates. No significant differences were observed for desiccation resistance. The demographically/stress-selected populations had longer times to starvation than urea-selected populations. The urea-adapted populations showed elevated survival and reduced development time in urea-laced food relative to the control and nonadapted populations. The urea-adapted populations also showed reduced larval feeding rates relative to controls. This study showed that there is a strong linear relationship between feeding rates and growth rates at the same larval ages feeding rates were measured. This suggests that feeding rates are correlated with food intake and growth. This relationship between larval feeding rates, food consumption, and efficiency has been postulated to involve important trade-offs that govern larval evolution in stressful environments. These results support the idea that energy allocation is a central organizing theme in adaptive evolution (Bitner, 2021).
Animals may vary in their utilization of plants depending on plant availability, and also on the sex of the animal.
Evolutionary adaptations may arise, particularly in specialist animals to the chemistry of the host plants, and these adaptations may differ between the sexes due to differences in their interactions with the plants. Drosophila mojavensis uses different host cacti across its range, and volatile chemicals emitted by the host are the primary cue for host plant identification. This study measured responses of individual olfactory sensory neurons to a large suite of odorants across males and females of the two southern D. mojavensis populations. A switch in host plant was shown to be accompanied by changes in the olfactory system, but the effect of this switch is minor compared to that of sex. That is, differences were observed in olfactory receptor neuron specificity and sensitivity to odorants between sexes, and to a lesser extent between populations. The majority of sensory differences are restricted to only three of the 17 sensory neurons measured. Further, numerous differences between sexes were found that only occur within one population, i.e., sex-by-population interactions (Ammagarahalli, 2021).
Adaptation to rapid environmental changes must occur within a short-time scale. In this context, studies of invasive species may provide insights into the underlying mechanisms of rapid adaptation as these species have repeatedly encountered and adapted to novel environmental conditions. This study investigated how invasive and noninvasive genotypes of Drosophila suzukii deal with oxidative stress at the phenotypic and molecular levels. Also studied was the impact of transposable element (TE) insertions on the gene expression in response to stress. Results show that flies from invasive areas (France and the United States) live longer in natural conditions than the ones from native Japanese areas. As expected, lifespan for all genotypes was significantly reduced following exposure to paraquat, but this reduction varied among genotypes (genotype-by-environment interaction) with invasive genotypes appearing more affected by exposure than noninvasive ones. A transcriptomic analysis of genotypes upon paraquat treatment detected many genes differentially expressed (DE). Although a small core set of genes were DE in all genotypes following paraquat exposure, much of the response of each genotype was unique. Moreover, it was shown that TEs were not activated after oxidative stress and DE genes were significantly depleted of TEs. In conclusion, it is likely that transcriptomic changes are involved in the rapid adaptation to local environments. This study provides new evidence that in the decade since the invasion from Asia, the sampled genotypes in Europe and the United States of D. suzukii diverged from the ones from the native area regarding their phenotypic and genomic response to oxidative stress (Marin, 2021).
Adaptive evolution is key in mediating responses to global warming and may sometimes be the only solution for species to survive. Such evolution will expectedly lead to changes in the populations' thermal reaction norm and improve their ability to cope with stressful conditions. Conversely, evolutionary constraints might limit the adaptive response. This study tests these expectations by performing a real-time evolution experiment in historically differentiated Drosophila subobscura populations. The phenotypic change was addressed after nine generations of evolution in a daily fluctuating environment with average constant temperature, or in a warming environment with increasing average and amplitude temperature across generations. The results showed that (1) evolution under a global warming scenario does not lead to a noticeable change in the thermal response; (2) historical background appears to be affecting responses under the warming environment, particularly at higher temperatures; and (3) thermal reaction norms are trait dependent: although lifelong exposure to low temperature decreases fecundity and productivity but not viability, high temperature causes negative transgenerational effects on productivity and viability, even with high fecundity. These findings in such an emblematic organism for thermal adaptation studies raise concerns about the short-term efficiency of adaptive responses to the current rising temperatures (Santos, 2021).
Establishing causal links between adaptive mutations and ecologically relevant phenotypes is key to understanding the process of
adaptation, which is a central goal in evolutionary biology with applications for conservation, medicine, and agriculture. Yet despite recent progress, the number of identified causal adaptive mutations remains limited. Linking genetic variation to fitness-related effects is complicated by gene-by-gene and gene-by-environment interactions, among other processes. Transposable elements, which are often ignored in the quest for the genetic basis of adaptive evolution, are a genome-wide source of regulatory elements across organisms that can potentially result in adaptive phenotypes. This work combined gene expression, in vivo reporter assays, CRISPR/Cas9 genome editing, and survival experiments to characterize in detail the molecular and phenotypic consequences of a natural Drosophila melanogaster transposable element insertion: the roo solo-LTR FBti0019985. This transposable element provides an alternative promoter to the transcription factor Lime, involved in cold- and immune-stress responses. The effect of FBti0019985 on Lime expression were found to depend on the interplay between the developmental stage and environmental condition. A causal link between the presence of FBti0019985 and increased survival to cold- and immune-stress was established. The results exemplify how several developmental stages and environmental conditions need to be considered to characterize the molecular and functional effects of a genetic variant, and add to the growing body of evidence that transposable elements can induce complex mutations with ecologically relevant effects (Merenciano, 2023).
Insects have adapted to a multitude of environmental conditions, including the presence of xenobiotic noxious substances. Environmental microorganisms, particularly rich on ephemeral resources, employ these noxious chemicals in a chemical warfare against predators and competitors, driving co-evolutionary adaptations. In order to analyse how environmental microbes may be driving such evolutionary adaptations, this study experimentally evolved Drosophila melanogaster populations by exposing larvae to the toxin-producing mould Aspergillus nidulans that infests the flies' breeding substrate. To disentangle the effects of the mycotoxin Sterigmatocystin from other substrate modifications inflicted by the mould, the following four selection regimes were used: (i) control without fungus, (ii) A. nidulans wild type, (iii) a mutant of A. nidulans ΔlaeA with impaired toxin production, (iv) synthetic Sterigmatocystin. Experimental evolution was carried out in five independent D. melanogaster populations each, for a total of 11 generations. This evolution experiment was further combined with transcriptome analysis to identify evolutionary shifts in gene expression due to the selection regimes and mould confrontation. Populations that evolved in presence of the toxin-producing mould or the pure mycotoxin rapidly adapted to the respective conditions and showed higher viability in subsequent confrontations. Yet, mycotoxin-selected populations had no advantage in A. nidulans wild type confrontation. Moreover, distinctive changes in gene expression related to the selection-regime contrast were only associated with the toxin-producing-fungus regime and comprised a narrow set of genes. Thus, it needs the specific conditions of the selection agent to enable adaptation to the fungus (Trienens, 2023).
Genetic correlations concentrate genetic variation in certain directions of the multivariate phenotype. Adaptation and, under some models, plasticity is expected to occur in the direction of the phenotype containing the greatest amount of genetic variation (g(max)). However, this may hinge on environmental heterogeneity, which can affect patterns of genetic variation. This study used experimental evolution to test whether plasticity and phenotypic evolution follow g(max) during adaptation to environments that varied in environmental heterogeneity. For >25 generations, Drosophila melanogaster populations were exposed to six homogeneous or spatially and temporally heterogeneous treatments involving hot (25°C) and cold (16°C) temperatures. Five wing traits were assayed in both temperatures. Wing morphology diverged between populations evolving in homogeneous hot and cold temperatures in a direction of the phenotype containing a large proportion of genetic variance and that aligned closely with g(max) at 16°C but not at 2°C. Spatial heterogeneity produced an intermediate phenotype, which was associated with similar genetic variance across assay temperatures compared with all other treatments. Surprisingly, plasticity across assay temperatures was in a different direction to phenotypic evolution and aligned better with maternal variance than g(max). Together, these results provide experimental evidence for evolution along genetic lines of least resistance in homogeneous environments but no support for predicting plastic responses from the orientation of genetic variation. These results also suggest that spatial heterogeneity could maintain genetic variation that increases the stability of genetic variance across environments (Walter, 2023).
C-to-U RNA editing in plants is believed to confer its evolutionary adaptiveness by reversing unfavorable DNA mutations. This "restorative hypothesis" has not yet been tested genome-wide. In contrast, A-to-I RNA editing in insects like Drosophila and honeybee is already known to benefit the host by increasing proteomic diversity in a spatial-temporal manner (namely "diversifying hypothesis"). This study profiled the RNA editomes of multiple tissues of Arabidopsis thaliana, Drosophila melanogaster, and Apis melifera. The haplotype diversity (HD) of RNA molecules was unprecedentedly defined based on nonsynonymous editing events (recoding sites). Signals of adaptation is confirmed in Arabidopsis by observing higher frequencies and levels at nonsynonymous editing sites over synonymous sites. Compared to A-to-I recoding sites in Drosophila, the C-to-U recoding sites in Arabidopsis show significantly lower HD, presumably due to the stronger linkage between C-to-U events. It is concluded that C-to-U RNA editing in Arabidopsis is adaptive but it is not designed for diversifying the proteome like A-to-I editing in Drosophila. Instead, C-to-U recoding sites resemble DNA mutations. These observation supports the restorative hypothesis of plant C-to-U editing which claims that editing is used for fixing unfavorable genomic sequences (Duan, 2023).
Determining the mechanisms by which a species adapts to its environment is a key endeavor in the study of evolution. In particular, relatively little is known about how transcriptional processes are fine-tuned to adjust to different environmental conditions. This study examined Drosophila melanogaster from 'Evolution Canyon' in Israel, which consists of two opposing slopes with divergent microclimates. Several hundred differentially expressed genes and dozens of differentially edited sites were identified between flies from each slope; these changes were correlate with genetic differences, and CRISPR mutagenesis was used to validate that an intronic SNP in prominin regulates its editing levels. It was also demonstrated that while temperature affects editing levels at more sites than genetic differences, genetically regulated sites tend to be less affected by temperature. This work shows the extent to which gene expression and RNA editing differ between flies from different microclimates, and provides insights into the regulation responsible for these differences (Yablonovitch, 2017).
This study analyzed the genomes, transcriptomes, and editomes of individual fly lines from the two slopes of Evolution Canyon, building upon previous studies examining Drosophila from this canyon1. Evolution Canyon is uniquely suited for studying the biodiversity, evolution and adaptation of organisms that live in relatively close proximity to each other, and is a potential model for incipient sympatric speciation. Although genetic and gene expression differences have been discovered previously in flies from different environments, most studies have examined fly populations at different latitudinal clines, and many signatures of adaptation found so far may be related to migration out of Africa. This is the first study to systematically examine gene expression and RNA editing differences in flies from different microclimates (Yablonovitch, 2017).
Several candidate genes were identified whose expression may play an adaptive role in the Evolution Canyon flies. The most striking of these are the Glutathione S-transferase genes, which show global under-expression in the NFS1 (north facing slope 1) flies compared to the SFS (south facing slope), and tend to be under-expressed in the NFS2 flies compared to the SFS as well. These detoxification enzymes metabolize antioxidants, and their decreased expression in the NFS1 and NFS2 relative to the SFS flies may be related to the decreased sun exposure, as there is 200–800% more solar radiation on the SFS18. It is interesting to note that a previous study examining flies from Evolution Canyon showed enrichment of glutathione metabolism and transferase activity in genomic regions with evidence of inter-slope differentiating selection. In addition, some Glutathione S-transferase genes have been shown to have significantly decreased expression in European flies compared to African flies in brain tissue. Other genes involved in pigmentation, stress response, digestion, and chitin-related processes also showed significant gene expression differences between flies from the two slopes. Future studies will be needed to address the potential adaptive role of the expression of these genes, and any regulatory mutations that are responsible for these gene expression changes (Yablonovitch, 2017).
A strong connection was shown between the genetic differences and the gene expression and RNA editing differences of the flies from the two slopes. This implies that, despite the flies being collected from Evolution Canyon years prior to these experiments, genetic regulation of gene expression and RNA editing still persists in the isofemale lines. In particular, it was confirmed that an intronic SNP regulates the editing levels of two sites in the prominin transcript, although the exact amount of editing level regulation contributed by the SNP could not be determined, since a CRISPR-associated PAM mutation was made in the same mutant. Since both the editing sites and the intronic SNP are conserved in many Drosophila species, and since most of the NFS1 lines contain the mutant allele that causes a decrease in editing, it is possible that the less stressful NFS environment decreased the strength of selection against this mutation. Although an editing-related prominin phenotype has not been identified, previous studies examining RNA editing evolution in different Drosophila species have demonstrated evidence of selection, especially for conserved, non-synonymous site. As the sites in prominin dealt with in this study are likewise conserved and code for non-synonymous amino acid changes, it's still possible that they play an adaptive role (Yablonovitch, 2017).
For RNA editing, both population genetic differences and environmental differences were capable of regulating editing between flies from the two slopes, and that the regulation seems to act through changing the structural stability of the RNA editing substrate. A change in environment regulates editing to a large extent for dozens of sites, most likely by affecting the stability of many RNA structures simultaneously. In contrast, genetic regulation is more site-specific, likely due to particular SNPs nearby editing sites which change the stability of the RNA structure containing those sites. This result is also supported by previous studies that examined editing level differences between and within Drosophila species. Population genetic differences in editing tend to be maintained regardless of the environment in which they were measure, suggesting that genetic regulation may be more influential than environmental regulation of these sites. One 3' UTR site in the falafel transcript was found that exhibits a genotype-environment interaction between the NFS1 and SFS fly populations, as well as several non-synonymous sites in the cacophony transcript between the NFS2 and SFS fly populations. Future studies will be needed in different populations and environments to determine whether these trends in editing happen universally (Yablonovitch, 2017).
To conclude, this study found surprising connections between genetics, gene expression, and RNA editing in flies from the distinct microclimates of Evolution Canyon. By sequencing individual lines, it was possuble to show a clear correspondence between genotype and gene expression differences between flies from the two opposing slopes, some of which may be important for adaptation. In addition, both genetic and environmental regulation of RNA editing was observed in these flies, though the two modes of regulation seem to operate mostly independently of each other. This study sets the stage for future examinations of the regulation of adaptive gene expression and RNA editing differences, not only in other fly populations, but in other species as well (Yablonovitch, 2017).
An animal's vision depends on terrain features that limit the amount and distribution of available light. Approximately 10,000 years ago, vinegar flies (Drosophila melanogaster) transitioned from a single plant specialist into a cosmopolitan generalist. Much earlier, desert flies (D. mojavensis) colonized the New World, specializing on rotting cactuses in southwest North America. Their desert habitats are characteristically flat, bright, and barren, implying environmental differences in light availability. This study demonstrated differences in eye morphology and visual motion perception under three ambient light levels. Reducing ambient light from 35 to 18 cd/m(2) causes sensitivity loss in desert but not vinegar flies. However, at 3 cd/m(2), desert flies sacrifice spatial and temporal acuity more severely than vinegar flies to maintain contrast sensitivity. These visual differences help vinegar flies navigate under variably lit habitats around the world and desert flies brave the harsh desert while accommodating their crepuscular lifestyle (Currea, 2022).
Proteins are the building blocks for almost all the functions in cells. Understanding the molecular evolution of proteins and the forces that shape protein evolution is essential in understanding the basis of function and evolution. Previous studies have shown that adaptation frequently occurs at the protein surface, such as in genes involved in host-pathogen interactions. However, it remains unclear whether adaptive sites are distributed randomly or at regions associated with particular structural or functional characteristics across the genome, since many proteins lack structural or functional annotations. This study sought to tackle this question by combining large-scale bioinformatic prediction, structural analysis, phylogenetic inference, and population genomic analysis of Drosophila protein-coding genes. Protein sequence adaptation was found to be more relevant to function-related rather than structure-related properties. Interestingly, intermolecular interactions contribute significantly to protein adaptation. Intermolecular interactions, such as physical interactions, may play a role in the coadaptation of fast-adaptive proteins. It was found that strongly differentiated amino acids across geographic regions in protein-coding genes are mostly adaptive, which may contribute to the long-term adaptive evolution. This strongly indicates that a number of adaptive sites tend to be repeatedly mutated and selected throughout evolution in the past, present, and maybe future. These results highlight the important roles of intermolecular interactions and coadaptation in the adaptive evolution of proteins both at the species and population levels (Peng, 2022).
Direct observation of evolution in response to natural environmental change can resolve fundamental questions about adaptation, including its pace, temporal dynamics, and underlying phenotypic and genomic architecture. This study tracked the evolution of fitness-associated phenotypes and allele frequencies genome-wide in 10 replicate field populations of Drosophila melanogaster over 10 generations from summer to late fall. Adaptation was evident over each sampling interval (one to four generations), with exceptionally rapid phenotypic adaptation and large allele frequency shifts at many independent loci. The direction and basis of the adaptive response shifted repeatedly over time, consistent with the action of strong and rapidly fluctuating selection. Overall, clear phenotypic and genomic evidence were found of adaptive tracking occurring contemporaneously with environmental change, thus demonstrating the temporally dynamic nature of adaptation (Rudman, 2022).
Understanding the dynamics of species adaptation to their environments has long been a central focus of the study of evolution. Theories of adaptation propose that populations evolve by "walking" in a fitness landscape. This "adaptive walk" is characterised by a pattern of diminishing returns, where populations further away from their fitness optimum take larger steps than those closer to their optimal conditions. Hence, it is expected that young genes evolve faster and experience mutations with stronger fitness effects than older genes because they are further away from their fitness optimum. Testing this hypothesis, however, constitutes an arduous task. Young genes are small, encode proteins with a higher degree of intrinsic disorder, are expressed at lower levels, and are involved in species-specific adaptations. Since all these factors lead to increased protein evolutionary rates, they could be masking the effect of gene age. While controlling for these factors, this study used population genomic data sets of Arabidopsis and Drosophila and estimated the rate of adaptive substitutions across genes from different phylostrata. A gene's evolutionary age was found to significantly impact the molecular rate of adaptation. Moreover, it was observed that substitutions in young genes tend to have larger physicochemical effects. This study, therefore, provides strong evidence that molecular evolution follows an adaptive walk model across a large evolutionary timescale (Moutinho, 2022).
Populations of short-lived organisms can respond to spatial and temporal environmental heterogeneity through local adaptation. Local adaptation can be reflected on both phenotypic and genetic levels, and it has been documented in many organisms. Although complex fitness-related phenotypes have been shown to vary across latitudinal clines and seasons in similar ways in Drosophila melanogaster populations, the comparative signals of local adaptation across space and time remain poorly understood. This study examined patterns of allele frequency change across a latitudinal cline and between seasons at previously reported expression quantitative trait loci (eQTLs). eQTLs were divided into groups by using differential expression profiles of fly populations collected across latitudinal clines or exposed to different environmental conditions. In general, eQTLs were found to be enriched for clinally varying polymorphisms, and these eQTLs changed in frequency in concordant ways across the cline and in response to starvation and chill-coma. The enrichment of eQTLs among seasonally varying polymorphisms is more subtle, and the direction of allele frequency change at eQTLs appears to be somewhat idiosyncratic. Taken together, it is suggested that clinal adaptation at eQTLs is at least partially distinct from seasonal adaptation (Yu, 2022).
Local adaptation can lead to elevated genetic differentiation at the targeted genetic variant and nearby sites. Selective sweeps come in different forms, and depending on the initial and final frequencies of a favored variant, very different patterns of genetic variation may be produced. If local selection favors an existing variant that had already recombined onto multiple genetic backgrounds, then the width of elevated genetic differentiation (high FST) may be too narrow to detect using a typical windowed genome scan, even if the targeted variant becomes highly differentiated. Therefore a simulation approach was used to investigate the power of SNP-level FST (specifically, the maximum SNP FST value within a window, or FST_MaxSNP) to detect diverse scenarios of local adaptation, and compared it against whole-window FST and the Comparative Haplotype Identity statistic. It was found that FST_MaxSNP had superior power to detect complete or mostly complete soft sweeps, but lesser power than full-window statistics to detect partial hard sweeps. Nonetheless, the power of FST_MaxSNP depended highly on sample size, and confident outliers depend on robust precautions and quality control. To investigate the relative enrichment of FST_MaxSNP outliers from real data, the two FST statistics were applied to a panel of Drosophila melanogaster populations. FST_MaxSNP had a genome-wide enrichment of outliers compared to demographic expectations, and though it yielded a lesser enrichment than window FST, it detected mostly unique outlier genes and functional categories. These results suggest that FST_MaxSNP is highly complementary to typical window-based approaches for detecting local adaptation, and merits inclusion in future genome scans and methodologies (da Silva Ribeiro, 2022).
In spatially structured populations, local adaptation improves organisms' fitness in their native environment. Hosts and pathogens can rapidly adapt to their local antagonist. Since males and females can differ in their immunocompetence, the patterns of local adaptation can be different between the sexes. However, there is little information about sex differences in local adaptation in host-pathogen systems. This study experimentally coevolved four different replicate populations of Drosophila melanogaster (host) and Pseudomonas entomophila (pathogen) along with appropriate controls. The four host-pathogen coevolution populations were used to investigate the occurrence of local adaptation separately in males and females of the coevolving hosts. Local adaptation was also assessed in pathogens. A reciprocal infection experiment was set up where each of the four coevolving hosts were infected with their local pathogen or non-local pathogens from the other three replicate populations. Overall, male and female hosts had better survivorship when infected with local pathogens, indicating that they were locally adapted. Interestingly, males were more susceptible to non-local pathogens compared to females. In addition, no fecundity cost was found in females infected with either local or non-local pathogens. No evidence was found of local adaptation among the pathogens. This study showed sex-specific adaptation in the coevolving hosts where female hosts had a broader response against allopatric coevolving pathogens with no cost in fecundity. Thus, these results might suggest a novel mechanism that can maintain variation in susceptibility in spatially structured populations (Ahlawat, 2022).
Previous work demonstrated that one Antimicrobial peptide, Metchikowin (Mtk), has a single residue that segregates as either proline (P) or arginine (R) in populations of four different Drosophila species, some of which diverged more than 10 million years ago. The recurrent finding of this polymorphism regardless of geography or host species, coupled with evidence of balancing selection in Drosophila AMPs, suggest there is a distinct functional importance to each allele. To assess their functional differences, D. melanogaster lines were created with the P allele, R allele, or Mtk null mutation using CRISPR/Cas9 genome editing. This study report results from experiments assessing the two hypotheses using these lines. In males, testing of systemic immune responses to a repertoire of bacteria and fungi demonstrated that the R allele performs as well or better than the P and null alleles with most infections. With some pathogens, however, females show results in contrast with males where Mtk alleles either do not contribute to survival or where the P allele outperforms the R allele. In addition, measurements of life history traits demonstrate that the R allele is more costly in the absence of infection for both sexes. These results provide strong in vivo evidence that differential fitness with or without infection and sex-based functional differences in alleles may be adaptive mechanisms of maintaining immune gene polymorphisms in contrast with expectations of rapid evolution. Therefore, a complex interplay of forces including pathogen species and host sex may lead to balancing selection for immune genotypes. Strikingly, this selection may act on even a single amino acid polymorphism in an AMP (Perlmutter, 2023).
Antimicrobial peptides are host-encoded immune effectors that combat pathogens and shape the microbiome in plants and animals. However, little is known about how the host antimicrobial peptide repertoire is adapted to its microbiome. This study characterized the function and evolution of the Diptericin antimicrobial peptide family of Diptera. Using mutations affecting the two Diptericins (Dpt) of Drosophila melanogaster, the specific role of DptA for the pathogen Providencia rettgeri and DptB for the gut mutualist Acetobacter. The presence of DptA- or DptB-like genes across Diptera correlates with the presence of Providencia and Acetobacter in their environment. Moreover, DptA- and DptB-like sequences predict host resistance against infection by these bacteria across the genus Drosophila. This study explains the evolutionary logic behind the bursts of rapid evolution of an antimicrobial peptide family and reveals how the host immune repertoire adapts to changing microbial environments (Hanson, 2023).
Periodic food shortage is a common ecological stressor for animals, likely to drive physiological and metabolic adaptations to alleviate its consequences, particularly for juveniles that have no option but to continue to grow and develop despite undernutrition. This study examined changes in metabolism associated with adaptation to nutrient shortage, evolved by replicate Drosophila melanogaster populations maintained on a nutrient-poor larval diet for over 240 generations. In a factorial metabolomics experiment it was shown that both phenotypic plasticity and genetically-based adaptation to the poor diet involved wide-ranging changes in metabolite abundance; however, the plastic response did not predict the evolutionary change. Compared to nonadapted larvae exposed to the poor diet for the first time, the adapted larvae showed lower levels of multiple free amino acids in their tissues-and yet they grew faster. By quantifying accumulation of the nitrogen stable isotope (15)N it was shown that adaptation to the poor diet led to an increased use of amino acids for energy generation. This apparent "waste" of scarce amino acids likely results from the trade-off between acquisition of dietary amino acids and carbohydrates observed in these populations. The three branched-chain amino acids (leucine, isoleucine, and valine) showed a unique pattern of depletion in adapted larvae raised on the poor diet. A diet supplementation experiment demonstrated that these amino acids are limiting for growth on the poor diet, suggesting that their low levels resulted from their expeditious use for protein synthesis. These results demonstrate that
selection driven by nutrient shortage not only promotes improved acquisition of limiting nutrients, but also has wide-ranging effects on how the nutrients are used. They also show that the abundance of free amino acids in the tissues does not, in general, reflect the nutritional condition and growth potential of an animal (Cavigliasso, 2023).
The processes underlying mate choice profoundly influence the dynamics of sexual selection and the evolution of male sexual traits. Consistent preference for certain phenotypes may erode genetic variation in populations through directional selection, whereas divergent preferences (e.g., genetically compatible mates) provide one mechanism to maintain such variation. However, the relative contributions of these processes across episodes of selection remain unknown. Using Drosophila melanogaster, this study followed the fate of male genotypes, previously scored for their overall reproductive value and their compatibility with different female genotypes, across pre- and postmating episodes of selection. When pairs of competitor males differed in their intrinsic quality and their compatibility with the female, both factors influenced outcomes from mating success to paternity but to a varying degree between stages. These results add further dimensions to understanding of how the interactions between genotypes and forms of selection shape reproductive outcomes and ultimately reproductive trait evolution (Mahdjoub, 2023).
Seasonally polyphenic types have been documented in many Drosophilids, which differ significantly during thermal stress. Although Drosophila simulans is a sibling species to Drosophila melanogaster, both thrive in the temperate and tropical climates, but various climatic factors are expected to impact their distribution and abundance. As a result, D. simulans may use phenotypic plasticity to adapt to colder and drier circumstances in temperate zones, although such studies are less known. The main aim of this study was to find a link between adaptive plasticity and thermal tolerance in D. simulans. Two morphs in D. simulans flies were characterized based on the abdominal melanization collected from the same locality and season, as this trait is highly associated with the larval developmental conditions. The results suggested that flies reared from dark and light morph showed significant differences in the basal level of proline, carbohydrates (trehalose, glycogen), and lipids (cuticular lipids and total body lipids) within simulated seasons and morph lineages in D. simulans flies. It was further shown that D. simulans reared from dark morph are better adapted to cold conditions, whereas light flies are more adapted to warm conditions. The flies, both from light and dark morph lineages, when reared at 15 °C, showed an increase in the level of total body lipids after acclimation at 0 °C but a decrease in the level of proline and carbohydrates (trehalose, glycogen). Heat acclimation increases glycogen levels in the flies from light morph lineage while decreases trehalose and proline (Tamang, 2022).
Mitochondria are organelles that produce cellular energy in the form of ATP through oxidative phosphorylation, and this primary function is conserved among many taxa. Locomotion is a trait that is highly reliant on metabolic function and expected to be greatly affected by disruptions to mitochondrial performance. To this end, this study aimed to examine how activity and sleep vary between Drosophila melanogaster strains with different geographic origins, how these patterns are affected by mitochondrial DNA (mtDNA) variation, and how breaking up co-evolved mito-nuclear gene combinations affect the studied activity traits. The results demonstrate that Drosophila strains from different locations differ in sleep and activity, and that females are generally more active than males. By comparing activity and sleep of mtDNA variants introgressed onto a common nuclear background in cytoplasmic hybrid (cybrid) strains, it was possible to quantify the among-line variance attributable to mitochondrial DNA, and it was established that mtDNA variation affects both activity and sleep, in a sex-specific manner. Altogether this study highlights the important role that mitochondrial genome variation plays on organismal physiology and behaviour (Anderson, 2022).
Cactophilic species of the Drosophila buzzatii cluster (repleta group) comprise an excellent model group to investigate genomic changes underlying adaptation to extreme climate conditions and host plants. In particular, these species form a tractable system to study the transition from chemically simpler breeding sites (like prickly pears of the genus Opuntia) to chemically more complex hosts (columnar cacti). This study reports four highly contiguous genome assemblies of three species of the buzzatii cluster. Based on this genomic data and inferred phylogenetic relationships, candidate taxonomically restricted genes (TRGs) likely involved in the evolution of cactophily and cactus host specialization were identified. Functional enrichment analyses of TRGs within the buzzatii cluster identified genes involved in detoxification, water preservation, immune system response, anatomical structure development, and morphogenesis. In contrast, processes that regulate responses to stress, as well as the metabolism of nitrogen compounds, transport, and secretion were found in the set of species that are columnar cacti dwellers. These findings are in line with the hypothesis that those genomic changes brought about key mechanisms underlying the adaptation of the buzzatii cluster species to arid regions in South America (Moreyra, 2023).
Chemosensory-driven hostplant specialization is a major force mediating insect ecological adaptation and speciation. Drosophila sechellia, a species endemic to the Seychelles islands, feeds and oviposits on Morinda citrifolia almost exclusively. This fruit is harmless to D. sechellia but toxic to other Drosophilidae, including the closely related generalists D. simulans and D. melanogaster, due to its high content of fatty acids. While several olfactory adaptations mediating D. sechellia's preference for its host have been uncovered, the role of taste has been much less examined. This study found that D. sechellia has reduced taste and feeding aversion to bitter compounds and host fatty acids that are aversive to D. melanogaster and D. simulans. The loss of aversion to canavanine, coumarin, and fatty acids arose in the D. sechellia lineage, as its sister species D. simulans showed responses akin to those of D. melanogaster. D. sechellia has increased taste and feeding responses towards M. citrifolia. These results are in line with D. sechellia's loss of genes encoding bitter gustatory receptors (GRs) in D. melanogaster. It was found that two gustatory receptor (GR) genes which are lost in D. sechellia, GR39a.a and GR28b.a, influence the reduction of aversive responses to some bitter compounds. Also, D. sechellia has increased appetite for a prominent host fatty acid compound that is toxic to its relatives. These results support the hypothesis that changes in the taste system, specifically a reduction of sensitivity to bitter compounds that deter generalist ancestors, contribute to the specialization of D. sechellia for its host (Reisenman, 2023).
Adaptation to increasingly warmer environments may be critical to avoid extinction. Whether and how these adaptive responses can arise is under debate. Though several studies have tackled evolutionary responses under different thermal selective regimes, very few have specifically addressed the underlying patterns of thermal adaptation under scenarios of progressive warming conditions. Also, considering how much past history affects such evolutionary response is critical. This study reports a long-term experimental evolution study addressing the adaptive response of Drosophila subobscura populations with distinct biogeographical history to two thermal regimes. The results showed clear differences between the historically differentiated populations, with adaptation to the warming conditions only evident in the low latitude populations. Furthermore, this adaptation was only detected after more than 30 generations of thermal evolution. These findings show some evolutionary potential of Drosophila populations to respond to a warming environment, but the response was slow and population specific, emphasizing limitations to the ability of ectotherms to adapt to rapid thermal shifts (Santos, 2023).
Understanding the genetic properties of adaptive trait evolution is a fundamental crux of biological inquiry that links molecular processes to biological diversity. Important uncertainties persist regarding the genetic predictability of adaptive trait change, the role of standing variation, and whether adaptation tends to result in the fixation of favored variants. This study used the recurrent evolution of enhanced ethanol resistance in Drosophila melanogaster during this species' worldwide expansion as a promising system to add to understanding of the genetics of
adaptation. Elevated ethanol resistance was found to have evolved at least three times in different cooler regions of the species' modern range-not only at high latitude but also in two African high-altitude regions. Applying a bulk segregant mapping framework, this study found that the genetic architecture of ethanol resistance evolution differs substantially not only between the three resistant populations, but also between two crosses involving the same European population. Population genetic scans were applied for local adaptation within the quantitative trait locus regions, and potential contributions were found of genes with annotated roles in spindle localization, membrane composition, sterol and alcohol metabolism, and other processes. Simulation-based analyses were appleid that confirm the variable genetic basis of ethanol resistance and hint at a moderately polygenic architecture. However, these simulations indicate that larger-scale studies will be needed to more clearly quantify the genetic architecture of adaptive evolution and to firmly connect trait evolution to specific causative loci (Sprengelmeyer, 2021).
Local adapation can result in variation in seasonal responses, but the genetic basis and evolutionary history of this variation remains elusive. Many insects, including Drosophila melanogaster, are able to undergo an arrest of reproductive development (diapause) in response to unfavorable conditions. In D. melanogaster, the ability to diapause is more common in high latitude populations, where flies endure harsher winters, and in the spring, reflecting differential survivorship of overwintering populations. Using a novel hybrid swarm-based genome wide association study, this study examined the genetic basis and evolutionary history of ovarian diapause. Outbred females were exposed to different temperatures and day lengths, ovarian development was characterized for over 2800 flies, and their complete, phased genomes were reconstructed. Diapause, scored at two different developmental cutoffs, was found to be modest heritability, and hundreds of SNPs associated with each of the two phenotypes were identified. Alleles associated with one of the diapause phenotypes tend to be more common at higher latitudes, but these alleles do not show predictable seasonal variation. The collective signal of many small-effect, clinally varying SNPs can plausibly explain latitudinal variation in diapause seen in North America. Alleles associated with diapause are segregating in Zambia, suggesting that variation in diapause relies on ancestral polymorphisms, and both pro- and anti-diapause alleles have experienced selection in North America. Finally, outdoor mesocosms were used to track diapause under natural conditions. Hybrid swarms reared outdoors were found to evolve increased propensity for diapause in late fall, whereas indoor control populations experienced no such change. These results indicate that diapause is a complex, quantitative trait with different evolutionary patterns across time and space (Erickson, 2020).
The repeatability or predictability of evolution is a central question in evolutionary biology and most often addressed in experimental evolution studies. This study inferred how genetically heterogeneous natural systems acquire the same molecular changes to address how genomic background affects adaptation in natural populations. In particular, advantage was taken of independently formed neo-sex chromosomes in Drosophila species that have evolved dosage compensation by co-opting the dosage-compensation male-specific lethal (MSL) complex to study the mutational paths that have led to the acquisition of hundreds of novel binding sites for the MSL complex in different species. This complex recognizes a conserved 21-bp GA-rich sequence motif that is enriched on the X chromosome, and newly formed X chromosomes recruit the MSL complex by de novo acquisition of this binding motif. Recently formed sex chromosomes were identified in the D. melanica and D. robusta species groups by genome sequencing and generate genomic occupancy maps of the MSL complex to infer the location of novel binding sites. Diverse mutational paths were utilized in each species to evolve hundreds of de novo binding motifs along the neo-X, including expansions of microsatellites and transposable element (TE) insertions. However, the propensity to utilize a particular mutational path differs between independently formed X chromosomes and appears to be contingent on genomic properties of that species, such as simple repeat or TE density. This establishes the 'genomic environment' as an important determinant in predicting the outcome of evolutionary adaptations (Ellison, 2019).
This study took advantage of naturally occurring variation in sex chromosome karyotype in Drosophila species to study independent replicates of solving the same evolutionary challenge: to dosage compensate newly formed neo-X chromosomes by acquiring hundreds of MSL-binding sites in response to Y degeneration (Ellison, 2019).
The independent acquisition of dosage compensation in Drosophila allows several important questions in evolutionary biology and gene regulation to be addressed: first, how repeatable is evolution? Evolutionary biologists have long debated the predictability of the evolutionary process. At one extreme, evolution could be highly idiosyncratic and unpredictable, since the survival of the fittest could occur along a great number of forking paths. Alternatively, constraints on evolution may force independent lineages to frequently converge on the same genetic solutions for the same evolutionary challenge. Second, how do regulatory networks evolve? And what is the contribution of TEs to regulatory evolution? Evolutionary innovations and adaptations often require rapid and concerted changes in regulation of gene expression at many loci. TEs constitute the most dynamic part of eukaryotic genomes, and the dispersal of TEs that contain a regulatory element may allow for the same regulatory motif to be recruited at many genomic locations, thereby drawing multiple genes into the same regulatory network. Third, what makes a binding motif functional? The genomes of complex organisms encompass megabases of DNA, and regulatory molecules must distinguish specific targets within this vast landscape. Regulatory factors typically identify their targets through sequence-specific interactions with the underlying DNA, but they typically bind only a fraction of the candidate genomic regions containing their specific target sequence motif. An unresolved mystery in regulatory evolution is what drives the specificity of binding to a subset of genomic regions that all appear to have a sequence that matches the consensus binding motif (Ellison, 2019).
Several features make dosage compensation in Drosophila a promising system to tackle these questions. The genetic architecture for most adaptations -- especially those involving regulatory changes -- as well as the timing and exact selective forces driving them is generally little understood. In contrast, detailed knowledge is available of the molecular mechanism of dosage compensation in Drosophila. The cis- and trans-acting components of this regulatory network and the regulatory motif for targeting the MSL complex to the X are known. Clear expectations are available of which genomic regions should acquire dosage compensation and about the timing and the evolutionary forces that drive wiring of hundreds of genes into the dosage-compensation network on newly evolved X chromosomes. Specifically, Y degeneration is a general facet of sex chromosome evolution, creating selective pressures to up-regulate X-linked genes in males. Dosage compensation should thus only evolve on neo-X chromosomes whose neo-Y homologs have started to degenerate and should evolve simultaneously or shortly after substantial gene loss has occurred on the neo-Y. Indeed, comparative data in Drosophila support this model of dosage-compensation evolution. Drosophila species with partially eroded neo-Y chromosomes exist that have not yet evolved MSL-mediated dosage compensation, including D. busckii and D. albomicans, lending empirical support to the notion that dosage compensation evolves in response to Y degeneration and not the other way round. Thus, a refined understanding of how, when, why, and where dosage compensation in Drosophila evolves makes this an ideal model system to study the repeatability of evolution and the evolution of regulatory networks (Ellison, 2019).
Results from evolution experiments indicate that although evolution is not identical in replicate populations, there is an important degree of predictability. Experimentally evolved populations under controlled, identical conditions consistently show parallelism in which mutations in certain genes are repeatedly selected. However, organisms adapting to similar environments are not genetically identical, but their genome instead carries the legacy of their unique evolutionary trajectory, raising the question of how genomic differences affect genetic parallelism (Ellison, 2019).
Sex chromosome-autosome fusions have independently created neo-sex chromosomes in different Drosophila lineages. This provides everal independent replicates to study how, on the molecular level, evolution has solved the same adaptive challenge: acquiring hundreds of binding sites to recruit the MSL complex to newly formed X chromosomes. This allows quantification of how much variation there is, both within and between species, in the underlying mutational paths to acquire hundreds of MSL-binding sites on neo-X chromosomes and identify genomic contingencies that will influence the repeatability of evolutionary trajectories. Importantly, neo-sex chromosomes of Drosophila are evolutionarily young (between 0.1-15 MY old), which allows, in many cases, inferring of the causative mutations that have resulted in the gain of a regulatory element and decipher the evolutionary processes at work to draw hundreds of genes into a new regulatory network (Ellison, 2019).
The results suggest that the evolution of MSL-binding sites is highly opportunistic but contingent on genomic background. In particular, each independently evolved neo-X chromosome was found to use a diverse set of mutational pathways to acquire MSL-binding sites on a new neo-X chromosome, ranging from microsatellite expansions to the utilization of presites to TE insertions. However, different lineages differ with regards to the frequency of which mutational paths are most often followed to acquire novel binding sites, and this propensity may depend on the genomic background. In particular, species found with the higher density of simple repeats are more prone to utilize expansions in GA microsatellites to gain a novel MSL-binding site. In contrast, D. robusta has an elevated TE density compared to its sibling species, and it was found that the dispersal of a TE has played an important role in the acquisition of MSL-binding sites on its neo-X chromosome. Thus, this suggests that the genomic background of a species predisposes it to evolve along a particular path, yet the evolutionary process is random and resourceful with regards to utilizing a variety of mutations to create novel MSL-binding sites. However, different phenotypes show drastic differences in their underlying genetic architecture, and the importance of genomic background likely differs among traits and (Ellison, 2019).
Evolutionary innovations and adaptations often require rapid and concerted changes in regulation of gene expression at many loci. It has been suggested that TEs play a key role in rewiring regulatory networks, since the dispersal of TEs that contain a regulatory element may allow for the same regulatory motif to be recruited at many genomic locations. A handful of recent studies have implicated TEs as drivers of key evolutionary innovations, including placentation in mammals or rewiring the core regulatory network of human embryonic stem cells. While these studies demonstrate that TEs can, in principle, contribute to the creation or rewiring of regulatory networks, they do not address the question of how often regulatory elements evolve by TE insertions versus by other mutations. That is, the importance of TEs in contributing to regulatory evolution is not known. Quantification of the role of TEs would require a priori knowledge of how and when regulatory networks evolve and a detailed molecular understanding of which genes are being drawn into a regulatory network and how. As discussed above, these parameters are well understood for dosage compensation in flies (Ellison, 2019).
Previous work in D. miranda has shown that a helitron TE was recruited into the dosage-compensation network at two independent time points. The younger 1.5-MY-old neo-X chromosome of D. miranda is in the process of evolving dosage compensation, and dozens of new CESs on this chromosome were created by insertions of the ISX element. The domesticated ISX TE gained a novel MRE motif by a 10-bp deletion in the ISY element, which is a highly abundant TE in the D. miranda genome. The remnants of a related (but different) TE at CES was found on the older neo-X of this species (which formed roughly 13-15 MY ago), but the TE was too eroded to reconstruct its evolutionary history. This study, identified another domesticated TE that was utilized to deliver MSL-binding sites to a newly formed neo-X chromosome, but no significant TE contribution was found for MSL-binding site evolution in two independent neo-X chromosomes (Ellison, 2019).
The data shed light on the question of when TEs are expected to be important in regulatory evolution. For TEs to contribute to regulatory rewiring, two conditions have to be met: a regulatory element (or a progenitor sequence that can easily mutate into the required binding motif) needs to be present in the TE, and TE needs to be active in the genome (and not yet silenced by the host machinery). TEs undergo a characteristic life cycle in which they invade a new species (or escape the genome defense by mutation) and transpose until they are silenced by the host genome. Once a TE is robustly repressed, it no longer can serve as a vehicle to disperse regulatory elements, so the time window when a particular TE family can be domesticated is probably short and needs to coincide with a necessity to disperse regulatory motifs. A high TE burden does increase that chance, but at a cost: maintaining active TEs in the genome allows a rapid response to evolutionary challenges but also creates a major source of genomic mutation, illegitimate recombination, genomic rearrangements, and genome size inflation (Ellison, 2019).
The current findings support this view of a TE tradeoff. The ISY element in D. miranda is the most highly abundant transposon in the D. miranda genome and is massively contributing to the degeneration of the neo-Y in this specie. Indeed, the genomic analysis has revealed >20,000 novel insertions of the ISY element on the neo-Y, often within genes. Yet, it contained a sequence that was only one mutational step away from a functional MSL-binding site (that is, a single 10-bp deletion), and domestication of this element allowed for the rapid dispersal of functional binding sites for the MSL complex along the neo-X. The domestication of the TE in D. robusta occurred too long ago for to reconstruct its exact evolutionary history and the potential damage its mobilization may have caused while it was active. However, consistent with a tradeoff that the host genome faces, it was found that D. robusta has a higher TE density than its sister species and also a considerably larger genome size, yet a TE contributed to wiring hundreds of genes into the dosage-compensation network on its neo-X (Ellison, 2019).
Perhaps surprisingly, in many instances, it was not possible to detect specific mutations that would generate a novel MSL binding motif. Instead, it was found that functional MSL-binding sites are derived from presites containing the GA-rich motif that was already present in an ancestor in which the neo-X is autosomal and in which these sequences do not recruit the MSL complex. The MSL binding motif is only modestly enriched on the X chromosome compared to the autosomes (only approximately 2-fold), and only a small fraction of putative binding sites are actually bound by the MSL complex. The dosage-compensation machinery shares this characteristic with many other sequence-specific binding factors whose predicted target motifs are often in vast excess to the sites actually utilized. It has been speculated that other genomic aspects, such as chromatin context or the 3D organization of the genome, could help to distinguish between utilized and nonutilized copies of a motif. The finding that a large number of sites can acquire the ability to recruit the MSL complex, without any apparent associated changes at the DNA level, supports the view that epigenetic modifications or changes to the 3D architecture of the genome help to ultimately determine which putative binding sites in the genome are actually utilized. In D. melanogaster, the X chromosome has a unique satellite DNA composition, and it was suggested that these repeats play a primary role in determining X identity during dosage compensation. Furthermore, localization of the MSL complex to MREs is dependent on an additional cofactor, the CLAMP protein. CLAMP binds directly to GA-rich MRE sequences and targets MSL to the X chromosome but also binds to GA-rich sequence elements throughout the genome. Recent work has shown that variability in sequence composition within similar GA-rich motifs drive specificity for CLAMP binding, and variation within seemingly similar cis elements may also drive context-specific targeting of the MSL complex. Future investigations of changes in the chromatin level, the repeat content, and the genomic architecture of these newly formed sex chromosomes will help to resolve this outstanding question (Ellison, 2019).
The insulin/insulin-like growth factor signaling pathway has been hypothesized as a major determinant of life-history profiles that vary adaptively in natural populations. In Drosophila melanogaster, multiple components of this pathway vary predictably with latitude; this includes foxo, a conserved gene that regulates insulin signaling and has pleiotropic effects on a variety of fitness-associated traits. It was hypothesized that allelic variation at foxo contributes to genetic variance for size-related traits that vary adaptively with latitude. Patterns of variation were examined among natural populations along a latitudinal transect in the eastern United States; thorax length, wing area, wing loading, and starvation tolerance were found to exhibit significant latitudinal clines for both males and females but that development time does not vary predictably with latitude. Recombinant outbred populations were generated, naturally occurring allelic variation at foxo, which exhibits stronger clinality than expected, were shown to be associated with the same traits that vary with latitude in the natural populations. These results suggest that allelic variation at foxo contributes to adaptive patterns of life-history variation in natural populations of this genetic model (Betancourt, 2021).
The relationships between adaptive evolution, phenotypic plasticity, and canalization remain incompletely understood. Theoretical and empirical studies have made conflicting arguments on whether adaptive evolution may enhance or oppose the plastic response. Gene regulatory traits offer excellent potential to study the relationship between plasticity and adaptation, and they can now be studied at the transcriptomic level. This study took advantage of three closely-related pairs of natural populations of Drosophila melanogaster from contrasting thermal environments that reflect three separate instances of cold tolerance evolution. The transcriptome-wide plasticity in gene expression levels and alternative splicing (intron usage) were measured between warm and cold laboratory environments. Suspected adaptive changes in both gene expression and alternative splicing tended to neutralize the ancestral plastic response. Further, the hypothesis was tested that adaptive evolution can lead to decanalization of selected gene regulatory traits. Strong evidence was found that suspected adaptive gene expression (but not splicing) changes in cold-adapted populations are more vulnerable to the genetic perturbation of inbreeding than putatively neutral changes. Some evidence was found that these patterns may reflect a loss of genetic canalization accompanying adaptation, although other processes including hitchhiking recessive deleterious variants may contribute as well. These findings augment our understanding of genetic and environmental effects on gene regulation in the context of adaptive evolution (Huang, 2022).
To respond to changing environmental conditions, a population may either shift toward better-adapted genotypes or adapt on an individual level. The present work aimed to quantify the relevance of these two processes by comparing the responses of defined Drosophila melanogaster populations to different stressors. To do this, two homogeneous populations (isofemale lines), which differ significantly in fitness, and a synthetic heterogeneous population were infected with a specific pathogen and/or exposed to food restriction. Pectobacterium carotovorum was used to infect Drosophila larvae either fed standard or protein-restricted diet. In particular, the two homogeneous groups, which diverged in their fitness, showed considerable differences in all parameters assessed (survivorship, protein and lipid contents, Phenol oxidase (PO) activity, and antibacterial rate). Under fully nutritious conditions, larvae of the homogeneous population with low fitness exhibited lower survivorship and protein levels, as well as higher PO activity and antibacterial rate compared with the fitter population. A protein-restricted diet and bacterial infection provoked a decrease in survivorship, and antibacterial rate in most populations. Bacterial infection elicited an opposite response in protein and lipid content in both isofemale lines tested. Interestingly, the heterogeneous population showed a complex response pattern. The response of the heterogeneous population followed the fit genotype in terms of survival and antibacterial activity but followed the unfit genotype in terms of PO activity. In conclusion, these results show that defined genotypes exhibit highly divergent responses to varying stressors that are difficult to predict. Furthermore, the responses of heterogeneous populations do not follow a fixed pattern showing a very high degree of plasticity and differences between different genotypes (Meshrif, 2022).
Traits that allow species to survive in extreme environments such as hot-arid deserts have independently evolved in multiple taxa. However, the genetic and evolutionary mechanisms underlying these traits have thus far not been elucidated. This study shows that Drosophila mojavensis, a desert-adapted fruit fly species, has evolved high desiccation resistance by producing long-chain methyl-branched cuticular hydrocarbons (mbCHCs) that contribute to a cuticular lipid layer reducing water loss. The ability to synthesize these longer mbCHCs is due to evolutionary changes in a fatty acyl-CoA elongase (mElo). mElo knockout in D. mojavensis led to loss of longer mbCHCs and reduction of desiccation resistance at high temperatures but did not affect mortality at either high temperatures or desiccating conditions individually. Phylogenetic analysis showed that mElo is a Drosophila-specific gene, suggesting that while the physiological mechanisms underlying desert adaptation may be similar between species, the genes involved in these mechanisms may be species or lineage specific (Wang, 2023).
Local adaptation, where fitness in one environment comes at a cost in another, should lead to spatial variation in trade-offs between life history traits and may be critical for population persistence. Recent studies have sought genomic signals of local adaptation, but often have been limited to laboratory populations representing two environmentally different locations of a species' distribution. This study measured gene expression, as a proxy for fitness, in males of Drosophila subobscura, occupying a 20 degrees latitudinal and 11 ° C thermal range. Uniquely, six populations were sampled, and both common garden and semi-natural responses to identify signals of local adaptation were identified. Contrasting patterns of investment were found: transcripts with expression positively correlated to latitude were enriched for metabolic processes, expressed across all tissues whereas negatively correlated transcripts were enriched for reproductive processes, expressed primarily in testes. When using only the end populations, to compare the results to previous studies, it was found that locally adaptive patterns were obscured. While phenotypic trade-offs between metabolic and reproductive functions across widespread species are well-known, the results identify underlying genetic and tissue responses at a continental scale that may be responsible for this. This may contribute to understanding population persistence under environmental change (Porcelli, 2016).
Little is known of how gene expression and its plasticity evolves as populations adapt to different environmental regimes. Expression is expected to evolve adaptively in all populations but only those populations experiencing environmental heterogeneity are expected to show adaptive evolution of plasticity. This study measured the transcriptome in a cadmium-enriched diet and a salt-enriched diet for experimental populations of Drosophila melanogaster that evolved for ~130 generations in one of four selective regimes: two constant regimes maintained in either cadmium or salt diets and two heterogeneous regimes that varied either temporally or spatially between the two diets. For populations evolving in constant regimes, a strong signature of counter-gradient evolution was found; the evolved expression differences between populations adapted to alternative diets is opposite to the plastic response of the ancestral population that is naive to both diets. Based on expression patterns in the ancestral populations, a set of genes was identified for which selection in heterogeneous regimes was predicted to result in increases in plasticity, and the expected pattern was found. In contrast, a set of genes where reduced plasticity was predicted did not follow expectation. Nonetheless, both gene sets showed a pattern consistent with adaptive expression evolution in heterogeneous regimes, highlighting the difference between observing 'optimal' plasticity and improvements in environment-specific expression. Looking across all genes, there is evidence in all regimes of differences in biased allele expression across environments ('allelic plasticity') and this is more common among genes with plasticity in total expression (Huang, 2016).
Species that exhibit broad ranges of distribution may successfully navigate environmental changes by modifying some of their life history traits. Environmental humidity imposes a critical stress that organisms may overcome by increasing their resistance to desiccation. This study used experimental evolution to investigate adaptation to desiccation in the tephritid Anastrepha ludens, a species with high fecundity, late maturation and long lifespan. This study measured morphological, physiological, developmental as well as demographic changes involved in the adaptation to desiccation. Notwithstanding a low heritability (h2 = 0.237), desiccation resistance evolved extremely rapidly and few negative trade-offs were detected. Selected flies exhibited correlated increases in longevity, body size, the amount of body lipids and bulk water content, and in the duration of the pupal stage. Females further delayed sexual maturation, decreased daily fecundity but retained high lifetime reproductive potential. No differences in male mating competitiveness were found. Selected and control lines differed in longevity but not in total female fecundity, demonstrating that A. ludens flies have the capability for fast adaptation to desiccation without loosing their reproductive capability. Thus, it seems that a rapid evolutionary response to desiccation in this polyphagous insect works as a buffer for environmental variation and reduces the strength of selection on reproductive traits (Tejeda, 2016).
Metabolic flexibility is an important component of adaptation to stressful environments, including thermal stress and latitudinal adaptation. The direct relationship between selection on thermal stress hardiness and metabolic flux has not previously been tested. This study investigated flexibility of nutrient catabolism during cold stress in Drosophila artificially selected for fast or slow recovery from chill coma (i.e. cold-hardy or -susceptible), specifically testing the hypothesis that stress adaptation increases metabolic turnover. Using 13C-labelled glucose, this study first showed that cold-hardy flies more rapidly incorporate ingested carbon into amino acids and newly synthesized glucose, permitting rapid synthesis of proline, a compound shown elsewhere to improve survival of cold stress. Second, using glucose and leucine tracers cold-hardy flies were shown to have higher oxidation rates than cold-susceptible flies before cold exposure, similar oxidation rates during cold exposure, and returned to higher oxidation rates during recovery. Additionally, cold-hardy flies transferred compounds among body pools more rapidly during cold exposure and recovery. Increased metabolic turnover may allow cold-adapted flies to better prepare for, resist and repair/tolerate cold damage. This work illustrates for the first time differences in nutrient fluxes associated with cold adaptation, suggesting that metabolic costs associated with cold hardiness could invoke resource-based trade-offs that shape life histories (Williams, 2016).
The effect of temperature on the evolution of metabolism has been the subject of debate for a century; however, no consistent patterns have emerged from comparisons of metabolic rate within and among species living at different temperatures. This study used experimental evolution to determine how metabolism evolves in populations of Drosophila melanogaster exposed to one of three selective treatments: a constant 16 ° C, a constant 25 ° C, or temporal fluctuations between 16 and 25 ° C. August Krogh's controversial hypothesis was tested that colder environments select for a faster metabolism. Given that colder environments also experience greater seasonality, the hypothesis was also tested that temporal variation in temperature may be the factor that selects for a faster metabolism. The metabolic rate of flies from each selective treatment was measured at 16, 20.5, and 25 ° C. Although metabolism was faster at higher temperatures, flies from the selective treatments had similar metabolic rates at each measurement temperature. Based on variation among genotypes within populations, heritable variation in metabolism was likely sufficient for adaptation to occur. It is concluded that colder or seasonal environments do not necessarily select for a faster metabolism. Rather, other factors besides temperature likely contribute to patterns of metabolic rate over thermal clines in nature (Alton, 2016).
Identifying mechanisms of adaptation to variable environments is essential in developing a comprehensive understanding of evolutionary
dynamics in natural populations. Phenotypic plasticity allows for
phenotypic change in response to changes in the environment, and as such
may play a major role in adaptation to environmental heterogeneity. This
study examined the plasticity of stress
response in D. melanogaster originating from two distinct
geographic regions and ecological habitats. Adults were given a
short-term, 5-day exposure to combinations of temperature and photoperiod
to elicit a plastic response for three fundamental aspects of stress
tolerance that vary adaptively with geography. This was replicated in both
the laboratory and in outdoor enclosures in the field. In the laboratory,
geographic origin was found to be the primary determinant of the stress
response. Temperature and the interaction between temperature and
photoperiod were also found to significantly affect stress resistance. In
the outdoor enclosures, plasticity was distinct among traits and between
geographic regions. These results demonstrate that short-term exposure of
adults to ecologically relevant environmental cues results in predictable
effects on multiple aspects of fitness. These patterns of plasticity vary
among traits and are highly distinct between the two examined geographic
regions, consistent with patterns of local adaptation to climate and
associated environmental parameters (Mathur, 2016).
Adaptation to environmental stress is critical for long-term species persistence. With climate change and other anthropogenic stressors compounding natural selective pressures, understanding the nature of adaptation is as important as ever in evolutionary biology. This study investigated this issue in a set of replicated Drosophila lines selected for increased desiccation resistance, a classical physiological trait that has been closely linked to Drosophila species distributions. Pooled whole-genome sequencing was used to compare the genetic basis of their selection responses. While selected SNPs in replicates of the same treatment (desiccation-selection or lab adaptation) tended to change frequency in the same direction, suggesting some commonality in the selection response, candidate SNP and gene lists often differed among replicates. Three of the five desiccation-selection replicates showed significant overlap at the gene and network level. All five replicates showed enrichment for ovary-expressed genes, suggesting maternal effects on the selected trait. Divergence between pairs of replicate lines for desiccation-candidate SNPs was greater than between pairs of control lines. This difference also far exceeded the divergence between pairs of replicate lines for neutral SNPs. Overall, while there was overlap in the direction of allele frequency changes and the network and functional categories affected by desiccation selection, replicates showed unique responses at all levels likely reflecting hitchhiking effects, and highlighting the challenges in identifying candidate genes from these types of experiments when traits are likely to be polygenic (Griffin, 2016).
A painful event establishes two opponent memories: cues that are associated with pain onset are remembered negatively, whereas cues that coincide with the relief at pain offset acquire positive valence. Such punishment- versus relief-memories are conserved across species, including humans, and the balance between them is critical for adaptive behaviour with respect to pain and trauma. In the fruit fly, Drosophila melanogaster as a study case, this study found that both punishment- and relief-memories display natural variation across wild-derived inbred strains, but they do not covary, suggesting a considerable level of dissociation in their genetic effectors. This provokes the question whether there may be heritable inter-individual differences in the balance between these opponent memories in man, with potential psycho-clinical implications (Appel, 2016).
Drosophila melanogaster is able to thrive in harsh northern climates through adaptations in life-history traits and physiological mechanisms that allow for survival through the winter. This examined the genetic basis of natural variation in one such trait, female virgin egg retention, which was previously shown to vary clinally and seasonally. To further understanding of the genetic basis and evolution of virgin egg retention, a genome-wide association study (GWAS) was performed using the previously sequenced Drosophila Genetic Reference Panel (DGRP) mapping population. Twenty-nine single nucleotide polymorphisms (SNPs) associated with virgin egg retention were found, and six available mutant lines, each harboring a mutation in a candidate gene, were examined for effects on egg retention time. Four out of the six mutant lines had defects in egg retention time as compared with the respective controls: mun, T48, Mes-4, and Klp67A Surprisingly, none of these genes has a recognized role in ovulation control, but three of the four genes have known effects on fertility or have high expression in the ovaries. The SNP set associated with egg retention time was enriched for clinal SNPs. The majority of clinal SNPs had alleles associated with longer egg retention present at higher frequencies in higher latitudes. These results support previous studies that show higher frequency of long retention times at higher latitude, providing evidence for the adaptive value of virgin egg-retention (Akhund-Zade, 2016).
Predicting how species will respond to the rapid climatic changes predicted this century is an urgent task. Species distribution models (SDMs) use the current relationship between environmental variation and species' abundances to predict the effect of future environmental change on their distributions. However, two common assumptions of SDMs are likely to be violated in many cases: (i) that the relationship of environment with abundance or fitness is constant throughout a species' range and will remain so in future and (ii) that abiotic factors (e.g. temperature, humidity) determine species' distributions. These assumptions were tested by relating field abundance of the rainforest fruit fly Drosophila birchii to ecological change across gradients that include its low and high altitudinal limits. Then, how such ecological variation affects the fitness of 35 D. birchii families transplanted in 591 cages to sites along two altitudinal gradients, was tested to determine whether genetic variation in fitness responses could facilitate future adaptation to environmental change. Overall, field abundance was highest at cooler, high-altitude sites, and declined towards warmer, low-altitude sites. By contrast, cage fitness (productivity) increased towards warmer, lower-altitude sites, suggesting that biotic interactions (absent from cages) drive ecological limits at warmer margins. In addition, the relationship between environmental variation and abundance varied significantly among gradients, indicating divergence in ecological niche across the species' range. However, there was no evidence for local adaptation within gradients, despite greater productivity of high-altitude than low-altitude populations when families were reared under laboratory conditions. Families also responded similarly to transplantation along gradients, providing no evidence for fitness trade-offs that would favour local adaptation. These findings highlight the importance of (i) measuring genetic variation in key traits under ecologically relevant conditions, and (ii) considering the effect of biotic interactions when predicting species' responses to environmental change (O'Brien, 2017)
What are the genomic foundations of adaptation in sexual populations? This question was addressed using fitness-character and whole-genome sequence data from 30 Drosophila laboratory populations. These 30 populations are part of a nearly forty-year laboratory radiation featuring three selection regimes, each shared by ten populations for up to 837 generations, with moderately large effective population sizes. Each of three sets of ten populations that shared a selection regime consist of five populations that have long been maintained under that selection regime, paired with five populations that had only recently been subjected to that selection regime. A high degree of evolutionary parallelism in fitness phenotypes was found when most-recent selection regimes are shared, as in previous studies from this laboratory. Genomic parallelism was also found with respect to the frequencies of single-nucleotide polymorphisms, transposable elements, insertions, and structural variants, which was expected. Entirely unexpected was a high degree of parallelism for linkage disequilibrium. The evolutionary genetic changes among these sexual populations are rapid and genomically extensive. This pattern may be due to segregating functional genetic variation that is abundantly maintained genome-wide by selection, variation that responds immediately to changes of selection regime (Graves, 2017).
Seasonal overwintering in insects represents an
adaptation to stressful environments and in European Drosophila melanogaster females, low temperatures and short photoperiods can induce an ovarian diapause. Diapause may represent a recent (<15Ky) adaptation to the colonisation of temperate Europe by D. melanogaster from tropical sub-Saharan Africa, because African D. melanogaster and the sibling species D. simulans, have been reported to fail to undergo diapause. Over the past few centuries, D. melanogaster have also invaded North America and Australia, and eastern populations on both continents show a predictable latitudinal cline in diapause induction. In Europe however, a new diapause-enhancing timeless allele, ls-tim, is observed at high levels in southern Italy ( approximately 80%), where it appears to have arisen and has spread throughout the continent with a frequency of approximately 20% in Scandinavia. Given the phenotype of ls-tim and its geographical distribution, it was predicted that it would work against any latitudinal cline in diapause induction within Europe. Indeed this study revealed that any latitudinal cline for diapause in Europe is very weak, as predicted by ls-tim frequencies. In contrast, ls-tim frequencies were determined in North America and it was observed that they would be expected to strengthen the latitudinal pattern of diapause. The results reveal how a newly arisen mutation, can, via the stochastic nature of where it initially arose, blur an otherwise adaptive geographical pattern (Pegoraro, 2017)
Organisms are believed to have evolved circadian clocks as adaptations to deal with cyclic environmental changes, and therefore it has been hypothesized that evolution in constant environments would lead to regression of such clocks. This study examined whether circadian clocks and the associated properties evolve differently under constant light and constant darkness. Activity-rest, adult emergence and oviposition rhythms were measured of D. melanogaster populations that have been maintained for over 19 years (~330 generations) under three different light regimes - constant light (LL), light-dark cycles of 12:12 h (LD) and constant darkness (DD). While circadian rhythms in all the three behaviors persist in both LL and DD stocks with no differences in circadian period, they differed in certain aspects of the entrained rhythms when compared to controls reared in rhythmic environment (LD). Interestingly, it was also observed that DD stocks have evolved significantly higher robustness or power of free-running activity-rest and adult emergence rhythms compared to LL stocks. Thus, this study, in addition to corroborating previous results of circadian clock evolution in constant light, also highlights that, contrary to the expected regression of circadian clocks, rearing in constant darkness leads to the evolution of more robust circadian clocks which may be attributed to an intrinsic adaptive advantage of circadian clocks and/or pleiotropic functions of clock genes in other traits (Shindey, 2017).
During colonization of new areas, natural populations have to deal with changing environments, and transposable elements (TEs) can be useful "tools" in the
adaptation process since they are major contributor to the structural and functional evolution of genomes. In this general context, the activity (copy number, transcriptional and excision rate) of the mariner mos1 element was estimated in 19 natural populations of D. simulans. It is shown (1) that mos1 expression is always higher and more variable in testes than in ovaries; (2) that mos1 activity is higher in colonizing populations compared to the sub-Saharan African ones (ancestral populations); (3) that mos1 variations in transcript levels and copy number are negatively correlated to transcriptional variations of piRNA genes, aubergine and argonaute3. Furthermore, mos1 levels of expression in testes highly contrast with the low expression patterns of ago3. These results strongly suggest that the expression polymorphism of piRNA genes could be responsible for the mos1 variations, first between male and female germlines and second, according to the status of natural populations (colonizing or not). These results provide new perspectives about TEs and piRNA genes co-evolution in Drosophila germlines (Saint-Leandre, 2017).
The Hawaiian Drosophila are one of the most species-rich endemic groups in Hawaii and a spectacular example of adaptive radiation. Drosophila silvestris and D. heteroneura are two closely related picture-winged Drosophila species that occur sympatrically on Hawaii Island and are known to hybridize in nature, yet exhibit highly divergent behavioral and morphological traits driven largely through sexual selection. Their closest-related allopatric species, D. planitibia from Maui, exhibits hybrid male sterility and reduced behavioral reproductive isolation when crossed experimentally with D. silvestris or D. heteroneura. A modified four-taxon test for gene flow was applied to recently obtained genomes of the three Hawaiian Drosophila species. The analysis indicates recent gene flow in sympatry, but also, although less extensive, between allopatric species. This study underscores the prevalence of gene flow, even in taxonomic groups considered classic examples of allopatric speciation on islands. The potential confounding effects of gene flow in phylogenetic and population genetics inference are discussed, as well as the implications for conservation (Kang, 2017).
Identifying the genetic basis for adaptive differences between species requires explicit tests of historical hypotheses concerning the effects of past changes in gene sequence on molecular function, organismal phenotype and fitness. This challenge was addressed by combining ancestral protein reconstruction with biochemical experiments and physiological analysis of transgenic animals that carry ancestral genes. A widely held hypothesis of molecular adaptation was tested in this study-that changes in the alcohol dehydrogenase protein (ADH) along the lineage leading to Drosophila melanogaster increased the catalytic activity of the enzyme and thereby contributed to the ethanol tolerance and adaptation of the species to its ethanol-rich ecological niche. These experiments strongly refute the predictions of the adaptive ADH hypothesis and caution against accepting intuitively appealing accounts of historical molecular adaptation that are based on correlative evidence. The experimental strategy employed can be used to decisively test other adaptive hypotheses and the claims they entail about past biological causality (Siddiq, 2017).
Gene expression levels are important quantitative traits that link genotypes to molecular functions and fitness. In Drosophila, population-genetic studies have revealed substantial adaptive evolution at the genomic level, but the evolutionary modes of gene expression remain controversial. This study presents evidence that adaptation dominates the evolution of gene expression levels in flies. 64% of the observed expression divergence across seven Drosophila species are adaptive changes driven by directional selection. The results are derived from time-resolved data of gene expression divergence across a family of related species, using a probabilistic inference method for gene-specific selection. Adaptive gene expression is stronger in specific functional classes, including regulation, sensory perception, sexual behavior, and morphology. Moreover, a large group of genes was identifed with sex-specific adaptation of expression, which predominantly occurs in males. This analysis opens an avenue to map system-wide selection on molecular quantitative traits independently of their genetic basis (Nourmohammad, 2017).
Gene expression levels are important quantitative traits that link genotypes to molecular functions and fitness. In Drosophila, population-genetic studies have revealed substantial adaptive evolution at the genomic level, but the evolutionary modes of gene expression remain controversial. This study presents evidence that adaptation dominates the evolution of gene expression levels in flies. 64% of the observed expression divergence across seven Drosophila species are adaptive changes driven by directional selection. The results are derived from time-resolved data of gene expression divergence across a family of related species, using a probabilistic inference method for gene-specific selection. Adaptive gene expression is stronger in specific functional classes, including regulation, sensory perception, sexual behavior, and morphology. Moreover, a large group of genes was identified with sex-specific adaptation of expression, which predominantly occurs in males. This analysis opens an avenue to map system-wide selection on molecular quantitative traits independently of their genetic basis (Nourmohammad, 2017).
The inference of adaptation exploits the complex dependence of the expression divergence on the evolutionary distance between species. It reflects two fundamental evolutionary features of quantitative traits. First, such traits generate a divergence pattern with two distinct molecular clocks: at a short evolutionary distance, the divergence is always near the expected value under neutrality; at a longer distance, it depends jointly on stabilizing and directional selection. This feature reconciles seemingly contradictory results of previous studies: analysis of closely related species produces a signal of neutral evolution, whereas evolutionary constraint becomes apparent for more distant species. Second, the phenotypic evolution of gene expression decouples from details of its genetic basis. This explains why overall strong selection on gene expression levels was found even though selection on individual QTLs is often weak. The probabilistic extension of the curren inference scheme, which is based on gene-specific expression divergence, identifies functional gene classes associated with adaptive evolution of regulation (Nourmohammad, 2017).
The selection model underlying this analysis is a single-peak fitness seascape, which contains components of stabilizing and directional selection on a quantitative trait. These components are well-established notions of quantitative genetics on micro-evolutionary timescales. Each of them can provide a snapshot of the predominant selection pressure in a population. However, the description of selection remains incomplete as a description of selection over macro-evolutionary periods. If selection on a trait is directional at a given evolutionary time, will that selection relax after the trait value has significantly adapted in the direction of selection? If selection is stabilizing, can it be assumed that the optimal trait value will remain invariant in the context of a different species? To address these questions, a conceptual and quantitative synthesis of stabilizing and directional selection is need. The single-peak seascape model arguably provides the simplest such synthesis. It also provides a simple picture of continual adaptation over macro-evolutionary periods: a species follows a moving fitness peak, and this process generates positive fitness flux but no net increase in fitness (Nourmohammad, 2017).
This method of selection inference can be applied to a spectrum of molecular quantitative traits with a complex genetic basis, provided that comparative data from multiple, sufficiently diverged species are available. Such traits include genome-wide protein levels, protein-DNA binding interactions, and enzymatic activities. For most of these traits, only partial knowledge is available of the underlying genetic loci and their effects on trait and fitness. This method complements QTL studies and opens a way to infer quantitative phenotype-fitness maps at the systems level (Nourmohammad, 2017).
The genomes of species that are ecological specialists will likely contain signatures of genomic adaptation to their niche. This study describes the genome of Drosophila montana, which is the most extremely cold-adapted Drosophila species. Branch tests were used to identify genes showing accelerated divergence in contrasts between cold- and warm adapted species, and about 250 genes were identified that show differences, possibly driven by a lower synonymous substitution rate in cold-adapted species. Evidence was sought of accelerated divergence between D. montana and D. virilis, a previously sequenced relative, and no strong evidence was found for divergent selection on coding sequence variation. Divergent genes are involved in a variety of functions, including cuticular and olfactory processes. Three populations of D. montana were resequenced from its ecological and geographic range. Outlier loci were more likely to be found on the X chromosome and there was a greater than expected overlap between population outliers and those genes implicated in cold adaptation between Drosophila species, implying some continuity of selective process at these different evolutionary scales (Parker, 2018).
Studies combining experimental evolution and next-generation sequencing have found that adaptation in sexually reproducing populations is primarily fueled by standing genetic variation. Consequently, the response to selection is rapid and highly repeatable across replicate populations. Some studies suggest that the response to selection is highly repeatable at both the phenotypic and genomic levels, and that evolutionary history has little impact. Other studies suggest that even when the response to selection is repeatable phenotypically, evolutionary history can have significant impacts at the genomic level. This study tests two hypotheses that may explain this discrepancy. Hypothesis 1: Past intense selection reduces evolutionary repeatability at the genomic and phenotypic levels when conditions change. Hypothesis 2: Previous intense selection does not reduce evolutionary repeatability, but other evolutionary mechanisms may. These hypotheses were tested using D. melanogaster populations that were subjected to 260 generations of intense selection for desiccation resistance and have since been under relaxed selection for the past 230 generations. It was found that, with the exception of longevity and to a lesser extent fecundity, 230 generations of relaxed selection has erased the extreme phenotypic differentiation previously found. No signs were found of genetic fixation, and only limited evidence of genetic differentiation between previously desiccation resistance selected populations and their controls. These findings suggest that evolution in this system is highly repeatable even when populations have been previously subjected to bouts of extreme selection. It is therefore concluded that evolutionary repeatability can overcome past bouts of extreme selection in Drosophila experimental evolution, provided experiments are sufficiently long and populations are not inbred (Phillips, 2018).
Adaptation to new hosts in phytophagous insects often involves mechanisms of host recognition by genes of sensory pathways. Most often the molecular evolution of sensory genes has been explained in the context of the birth-and-death model. The role of positive selection is less understood, especially associated with host adaptation and specialization. This study aimed to contribute evidence for this latter hypothesis by considering the case of Drosophila mojavensis, a species with an evolutionary history shaped by multiple host shifts in a relatively short time scale, and its generalist sister species, D. arizonae. A phylogenetic and population genetic analysis framework was used to test for positive selection in a subset of four chemoreceptor genes, one gustatory receptor (Gr) and three odorant receptors (Or), for which their expression has been previously associated with host shifts. Strong evidence was found of positive selection at several amino acid sites in all genes investigated, most of which exhibited changes predicted to cause functional effects in these transmembrane proteins. A significant portion of the sites identified as evolving positively were largely found in the cytoplasmic region, although a few were also present in the extracellular domains. The pattern of substitution observed suggests that some of these changes likely had an effect on signal transduction as well as odorant recognition and protein-protein interactions. These findings support the role of positive selection in shaping the pattern of variation at chemosensory receptors, both during the specialization onto one or a few related hosts, but as well as during the evolution and adaptation of generalist species into utilizing several hosts (Dizz, 2018).
Ecological diversification of the endemic Hawaiian Drosophilidae has been accompanied by striking divergence in egg morphology, and ovarian structure and function. To determine how these flies successfully oviposit in a variety of breeding substrates, Scanning Electron Microscopy was used to examine the ultrastructure of the ovipositor of a sample of 65 Drosophila species and five Scaptomyza species of this hyperdiverse monophyletic group. The Drosophila species analyzed included representatives of the fungus-breeding haleakalae group, the leaf-breeding antopocerus and modified tarsus groups, the modified mouthparts species group, the nudidrosophila, and the picture wing clade; the latter sample of 41 species from four species groups included stem- and bark-breeders, as well as tree sap flux-breeders. Ovipositor length was found to vary more than 12-fold among Hawaiian drosophilids, with the longest ovipositors observed in the bark-breeding species and the shortest among the Scaptomyza and fungus-breeders. More noteworthy is the striking variation in overall shape and proportions of the ovipositor, in the shape of the apical region, and in the pattern of sensory structures or ovisensilla. Ultrastructural observations of the pair of long subapical sensilla on the ventral side identify these as taste bristles. Ovipositor form correlates strongly with the oviposition substrate used by the species, being of a distinctive shape and size in each case. It is inferred that the observed morphological divergence in the ovipositor is adaptive and the product of natural selection for successful reproduction in alternate microhabitats (Craddock, 2018).
Insights into the genetic capacities of species to adapt to future climate change can be gained by using comparative genomic and transcriptomic data to reconstruct the genetic changes associated with such adaptations in the past. This study investigated the genetic changes associated with
adaptation to arid environments, specifically climatic extremes and new cactus hosts, through such an analysis of five repleta group Drosophila species. Disproportionately high rates of gene gains were found in internal branches in the species' phylogeny where cactus use and subsequently cactus specialisation and high heat and desiccation tolerance evolved. The terminal branch leading to the most heat and desiccation resistant species, Drosophila aldrichi, also shows disproportionately high rates of both gene gains and positive selection. Several Gene Ontology terms related to metabolism were enriched in gene gain events in lineages where cactus use was evolving, while some regulatory and developmental genes were strongly selected in the Drosophila aldrichi branch. Transcriptomic analysis of flies subjected to sublethal heat shocks showed many more downregulation responses to the stress in a heat sensitive versus heat resistant species, confirming the existence of widespread regulatory as well as structural changes in the species' differing adaptations. Gene Ontology terms related to metabolism were enriched in the differentially expressed genes in the resistant species while terms related to stress response were over-represented in the sensitive one. It is concluded that daptations to new cactus hosts and hot desiccating environments were associated with periods of accelerated evolutionary change in diverse biochemistries. The hundreds of genes involved suggest adaptations of this sort would be difficult to achieve in the timeframes projected for anthropogenic climate change (Rane, 2019).
Phenotypic plasticity can allow organisms to respond to environmental changes by producing better matching phenotypes without any genetic change. Because of this, plasticity is predicted to be a major mechanism by which a population can survive the initial stage of colonizing a novel environment. This prediction was tested by challenging wild Drosophila melanogaster with increasingly extreme larval environments and then examining expression of alcohol dehydrogenase (ADH) and its relationship to larval survival in the first generation of encountering a novel environment. Most families responded in the adaptive direction of increased ADH activity in higher alcohol environments and families with higher plasticity were also more likely to survive in the highest alcohol environment. Thus, plasticity of ADH activity was positively selected in the most extreme environment and was a key trait influencing fitness. Furthermore, there was significant heritability of ADH plasticity that can allow plasticity to evolve in subsequent generations after initial colonization. The adaptive value of plasticity, however, was only evident in the most extreme environment and had little impact on fitness in less extreme environments. The results provide one of the first direct tests of the adaptive role of phenotypic plasticity in colonizing a novel environment (Wang, 2019).
Karyotype refers to the configuration of the genome into a set of chromosomes. The karyotype difference between species is expected to impede various biological processes, such as chromosome segregation and meiotic chromosome pairing, potentially contributing to incompatibility. Karyotypes can rapidly change between closely related species and even among populations of the same species. However, the forces driving karyotype evolution are poorly understood. This study describes a unique karyotype of a Drosophila melanogaster strain isolated from the Seychelles archipelago. This strain has lost the ribosomal DNA (rDNA) locus on the X chromosome. Because the Y chromosome is the only other rDNA-bearing chromosome, all females carry at least one Y chromosome as the source of rDNA. Interestingly, it was found that the strain also carries a truncated Y chromosome (YS) that is stably maintained in the population despite its inability to support male fertility. Modeling and cytological analysis suggest that the Y chromosome has a larger negative impact on female fitness than the YS chromosome. Moreover, an independent strain was generated that lacks X rDNA and has a karyotype of XXY females and XY males. This strain quickly evolved multiple karyotypes: two new truncated Y chromosomes (similar to YS), as well as two independent X chromosome fusions that contain the Y-derived rDNA fragment, eliminating females' dependence on the Y chromosome. Considering that Robertsonian fusions frequently occur at rDNA loci in humans, it is proposed that rDNA loci instability may be one of driving forces of karyotype evolution (Li, 2022).
The karyotype, or number and arrangement of chromosomes, has varying levels of stability across both evolution and disease. Karyotype changes often originate from DNA breaks near the centromeres of chromosomes, which generally contain long arrays of tandem repeats or satellite DNA. Drosophila virilis possesses among the highest relative satellite abundances of studied species, with almost half its genome composed of three related 7 bp satellites. This study discovered a strain of D. virilis that is inferred to have recently undergone three independent chromosome fusion events involving the X and Y chromosomes, in addition to one subsequent fission event. This study isolated and characterized the four different karyotypes discovered in this strain which is believed to demonstrate remarkable genome instability. One of the substrains with an X-autosome fusion has a X-to-Y chromosome nondisjunction rate 20x higher than the D. virilis reference strain (21% vs. 1%). Finally, an overall higher rate of DNA breakage was found in the substrain with higher satellite DNA compared to a genetically similar substrain with less satellite DNA. This suggests satellite DNA abundance may play a role in the risk of genome instability. Overall, this study introduces a novel system consisting of a single strain with four different karyotypes, which will be useful for future studies of genome instability, centromere function, and sex chromosome evolution (Flynn, 2023).
Secondary contact between formerly isolated populations may result in hybrid breakdown, in which untested allelic combinations in hybrids are maladaptive and limit genetic exchange. Studying early-stage reproductive isolation may yield key insights into the genetic architectures and evolutionary forces underlying the first steps toward speciation. This study leveraged the recent worldwide expansion of Drosophila melanogaster to test for hybrid breakdown between populations that diverged within the last 13,000 years. Clear evidence was found for hybrid breakdown in male reproduction, but not female reproduction or viability, supporting the prediction that hybrid breakdown affects the heterogametic sex first. The frequency of non-reproducing F2 males varied among different crosses involving the same southern African and European populations, as did the qualitative effect of cross direction, implying a genetically variable basis of hybrid breakdown and a role for uniparentally inherited factors. The levels of breakdown observed in F2 males were not recapitulated in backcrossed individuals, consistent with the existence of incompatibilities with at least three partners. Thus, some of the very first steps toward reproductive isolation could involve incompatibilities with complex and variable genetic architectures. Collectively, these findings emphasize this system's potential for future studies on the genetic and organismal basis of early-stage reproductive isolation (Lollar, 2023).
The strong reduction in the frequency of recombination in heterozygotes for an inversion and a standard gene arrangement causes the arrangements to become partially isolated genetically, resulting in sequence divergence between them and changes in the levels of neutral variability at nucleotide sites within each arrangement class. Previous theoretical studies on the effects of karyotype on neutral variability have assumed either that the population is panmictic or that it is divided into 2 populations subject to divergent selection. In this study, the theory is extended to a model of an arbitrary number of demes connected by migration, using a finite island model with the inversion present at the same frequency in all demes. Recursion relations for mean pairwise coalescent times are used to obtain simple approximate expressions for diversity and divergence statistics for an inversion polymorphism at equilibrium under recombination and drift, and for the approach to equilibrium following the sweep of an inversion to a stable intermediate frequency. The effects of an inversion polymorphism on patterns of linkage disequilibrium are also examined. The reduction in effective recombination rate caused by population subdivision can have significant effects on these statistics. The theoretical results are discussed in relation to population genomic data on inversion polymorphisms, with an emphasis on Drosophila melanogaster. Methods are proposed for testing whether or not inversions are close to recombination-drift equilibrium, and for estimating the rate of recombinational exchange in heterozygotes for inversions; difficulties involved in estimating the ages of inversions are also discussed (Charlesworth, 2023).
Since the pioneering work of Dobzhansky in the 1930s and 1940s, many chromosomal inversions have been identified, but how they contribute to adaptation remains poorly understood. In Drosophila melanogaster, the widespread inversion polymorphism In(3R)Payne underpins latitudinal clines in fitness traits on multiple continents. This study used single-individual whole-genome sequencing, transcriptomics, and published sequencing data to study the population genomics of this inversion on four continents: in its ancestral African range and in derived populations in Europe, North America, and Australia. The results confirm that this inversion originated in sub-Saharan Africa and subsequently became cosmopolitan; marked monophyletic divergence of inverted and noninverted karyotypes was observed, with some substructure among inverted chromosomes between continents. Despite divergent evolution of this inversion since its out-of-Africa migration, derived non-African populations exhibit similar patterns of long-range linkage disequilibrium between the inversion breakpoints and major peaks of divergence in its center, consistent with balancing selection and suggesting that the inversion harbors alleles that are maintained by selection on several continents. Using RNA-sequencing, this study identified overlap between inversion-linked single-nucleotide polymorphisms and loci that are differentially expressed between inverted and noninverted chromosomes. Expression levels are higher for inverted chromosomes at low temperature, suggesting loss of buffering or compensatory plasticity and consistent with higher inversion frequency in warm climates. These results suggest that this ancestrally tropical balanced polymorphism spread around the world and became latitudinally assorted along similar but independent climatic gradients, always being frequent in subtropical/tropical areas but rare or absent in temperate climates (Kapun, 2023).
In all species, new chromosomal inversions are constantly being formed by spontaneous rearrangement and then stochastically eliminated from natural populations. In Drosophila, when new chromosomal inversions overlap with a preexisting inversion in the population, their rate of elimination becomes a function of the relative size, position, and linkage phase of the gene rearrangements. These altered dynamics result from complex meiotic behavior wherein overlapping inversions generate asymmetric dyads that cause both meiotic drive/drag and segmental aneuploidy. In this context, patterns in rare inversion polymorphisms of a natural population can be modeled from the fundamental genetic processes of forming asymmetric dyads via crossing-over in meiosis I and preferential segregation from asymmetric dyads in meiosis II. Here, a mathematical model of crossover-dependent female meiotic drive is developed and parameterized with published experimental data from Drosophila melanogaster laboratory constructs. This mechanism is demonstrated to favor smaller, distal inversions and accelerate the elimination of larger, proximal inversions. Simulated sampling experiments indicate that the paracentric inversions directly observed in natural population surveys of D. melanogaster are a biased subset that both maximizes meiotic drive and minimizes the frequency of lethal zygotes caused by this cytogenetic mechanism. Incorporating this form of selection into a population genetic model accurately predicts the shift in relative size, position, and linkage phase for rare inversions found in this species. The model and analysis presented in this study suggest that this weak form of female meiotic drive is an important process influencing the genomic distribution of rare inversion polymorphisms (Koury, 2023).
The adaptive value of the Drosophila subobscura chromosomal inversion polymorphism with regard to environmental effects is well-known. However, the specific details of the inversion adaptations to the global warming scenario deserve to be analyzed. Toward this aim, polymorphism and karyotypes were studied in 574 individuals from Petnica (Serbia) in annual samples taken in June for the period 2019-2022. Comparing the results of Petnica (Cfa: humid subtropical climate) with those from Avala (Serbia: Cfb, temperate oceanic climate) and Font Groga (Barcelona, Spain; Csa: hot-summer Mediterranean climate), significant differences were observed for their chromosomal polymorphism. In Petnica, inversions from U and E chromosomes mainly reacted significantly with regard to temperature, humidity, and rainfall. Moreover, the inversion polymorphism from Petnica (2019-2022) was compared with that from 1995. In this period, a significant increase in mean and maximum temperature was observed. However, to properly explain the observed variations of inversions over time, it was necessary to carefully analyze annual seasonal changes and particular heat wave episodes. Interestingly, yearly fluctuations of U chromosome 'warm'-adapted inversions corresponded with opposite changes in 'non-thermal' inversions. Perhaps these types of inversions were not correctly defined with regard to thermal adaptation, or these fluctuations were also due to adaptations to other physical and/or biological variables. Finally, a joint study of chromosomal inversion polymorphism from many Balkan populations of D. subobscura indicated that different climatic regions presented distinct composition, including thermal-adapted inversions (Zivanovic, 2023).