bicoid


DEVELOPMENTAL BIOLOGY

Oogenesis and Embryonic expression

Localization of maternally provided RNAs during oogenesis is required for formation of the antero-posterior axis of the Drosophila embryo. This paper describes a subcellular structure in nurse cells and oocytes that may function as an intracellular compartment for assembly and transport of maternal products involved in RNA localization. This structure, termed a "sponge body," consists of ER-like cisternae, embedded in an amorphous electron-dense mass. It lacks a surrounding membrane and is frequently associated with mitochondria. Sponge bodies are not identical to the Golgi complexes. It is suggested that the sponge bodies are homologous to the mitochondrial cloud in Xenopus oocytes, a granulo-fibrillar structure that contains RNAs involved in patterning of the embryo. Exuperantia protein, the earliest factor known to be required for the localization of Bicoid mRNA to the anterior pole of the Drosophila oocyte, is highly enriched in the sponge bodies but not an essential structural component of these. RNA staining indicates that sponge bodies contain RNA. However, neither the intensity of this staining nor the accumulation of Exuperantia in the sponge bodies is dependent on the amount of Bicoid mRNA present in the ovaries. Sponge bodies surround nuage, a possible polar granule precursor. Microtubules and microfilaments are not present in sponge bodies, although transport of the sponge bodies through the cells is implied by their presence in cytoplasmic bridges. It is proposed that the sponge bodies are structures that are involved in localization of mRNAs in Drosophila oocytes by means of the assembly and transport of included molecules or associated structures (Wilsch-Bräuninger, 1997).

Earliest zygotic translation of the Bicoid mRNA is detected immediately after fertilization and egg deposition (stage 1). The anterior-posterior gradient of Bicoid is immediately apparent. The level increases slightly until onset of cellularization and decreases more rapidly during gastrulation [Images]. Traces are still apparent at the end of germ band elongation (Driever, 1988). In embryos from females with a mutation in bicoid, head and thorax are lacking and replaced by a posterior telson (Fröhnhofer, 1986).

Different combinations of gap repressors for common stripes in Anopheles and Drosophila embryos

Embryos lacking both maternal and zygotic hb display a reduction and an anterior shift of ems and btd expression at the blastoderm stage. Thus, it has been proposed that head-specific ems expression at the blastoderm stage requires synergistic activation by bcd and hb. However, no hb consensus site could be detected within the 304 bp enhancer element. It cannot be excluded that hb binding sites exist in the ems enhancer outside this element. However, the results suggest that hb plays a relatively minor role in ems expression control in the head and brain (Hartmann, 2002).

Drosophila segmentation is governed by a well-defined gene regulation network. The evolution of this network was investigated by examining the expression profiles of a complete set of segmentation genes in the early embryos of the mosquito, Anopheles gambiae. There are numerous differences in the expression profiles as compared with Drosophila. The germline determinant Oskar is expressed in both the anterior and posterior poles of Anopheles embryos but is strictly localized within the posterior plasm of Drosophila. The gap genes hunchback and giant display inverted patterns of expression in posterior regions of Anopheles embryos, while tailless exhibits an expanded pattern as compared with Drosophila. These observations suggest that the segmentation network has undergone considerable evolutionary change in the dipterans and that similar patterns of pair-rule gene expression can be obtained with different combinations of gap repressors. The evolution of separate stripe enhancers in the eve loci of different dipterans is discussed (Goltsev, 2004).

Anopheles lacks bicoid and contains a lone Hox3 gene that is more closely related to zen and specifically expressed in the serosa. How is hunchback activated in the presumptive head and thorax in Anopheles? The homeobox gene orthodenticle can substitute for bicoid in Tribolium. However, orthodenticle does not appear to be maternally expressed in Anopheles, but instead, staining is strictly zygotic and restricted to anterior regions, similar to the pattern seen in Drosophila. Sequential patterns of orthodenticle, giant, and hunchback expression are established by differential threshold readouts of the Bicoid gradient in Drosophila. It is possible that an unknown maternal regulatory gradient emanating from the anterior pole is responsible for producing similar patterns of expression in Anopheles. It is proposed that this unknown regulatory factor may be localized to the anterior pole by Oskar. Oskar coordinates the assembly of polar granules and is essential for the localization of Nanos in the posterior plasm. It might also localize one or more unknown determinants in anterior regions of Anopheles embryos (Goltsev, 2004).

The eve stripe 2 enhancer is the most thoroughly characterized enhancer in the segmentation gene network. It can be activated throughout the anterior half of the embryo by Bicoid and Hunchback, but the Giant and Kruppel repressors delimit the pattern and establish the anterior and posterior stripe borders, respectively. Removal of the Giant repressor sites within the stripe 2 enhancer in cis or removal of the repressor in trans causes an anterior expansion of the stripe 2 pattern. However, ectopic expression does not extend to the anterior pole, suggesting that an additional anterior repressor regulates the stripe 2 enhancer. Recent studies identified Sloppy-paired as the likely anterior repressor. The limits of the giant and Kruppel expression patterns seen in Anopheles suggest that they might define the eve stripe 2 borders, just as in Drosophila. However, at the critical time when eve stripe 2 is formed in Anopheles, the giant staining pattern extends to the anterior pole, while the corresponding Drosophila gene is repressed in these regions. It is therefore possible that Giant is sufficient to form the anterior border in Anopheles and that repression by Sloppy-paired represents an innovation in Drosophila (Goltsev, 2004).

Why do some enhancers generate two stripes, while others direct just one? Consider the eve stripe 2 and stripe 3/7 enhancers in Drosophila. The stripe 3/7 enhancer is activated by ubiquitous activators, including dSTAT, and the two stripes are 'carved out' by the localized Hunchback and Knirps repressors. Knirps establishes the posterior border of stripe 3 and anterior border of stripe 7, while Hunchback establishes the anterior border of stripe 3 and posterior border of stripe 7. The stripe 2 enhancer directs just a single stripe due to the localized distribution of the stripe 2 activators, particularly Bicoid. In principle, a ubiquitous activator would cause the stripe 2 enhancer to direct two stripes, stripes 2 and 5. Opposing Giant and Kruppel repressor gradients would carve out the borders of the two stripes, similar to the way in which Hunchback and Knirps regulate the stripe 3/7 and stripe 4/6 enhancers. Presumably, the eve stripe 5 enhancer directs a single stripe of expression because it is regulated by a localized activator, possibly Caudal (Goltsev, 2004).

Stability and nuclear dynamics of the Bicoid morphogen gradient

Patterning in multicellular organisms results from spatial gradients in morphogen concentration, but the dynamics of these gradients remain largely unexplored. This study characterized, through in vivo optical imaging, the development and stability of the Bicoid morphogen gradient in Drosophila embryos that express a Bicoid-eGFP fusion protein. The gradient is established rapidly (1 hr after fertilization), with nuclear Bicoid concentration rising and falling during mitosis. Interphase levels result from a rapid equilibrium between Bicoid uptake and removal. Initial interphase concentration in nuclei in successive cycles is constant (+-10%), demonstrating a form of gradient stability, but it subsequently decays by approximately 30%. Both direct photobleaching measurements and indirect estimates of Bicoid-eGFP diffusion constants (D <=1 microm2/s) provide a consistent picture of Bicoid transport on short (min) time scales but challenge traditional models of long-range gradient formation. A new model is presented emphasizing the possible role of nuclear dynamics in shaping and scaling the gradient (Gregor, 2007a).

The principal results provide the following foundation for any mechanistic model for the formation or read out of the Bcd gradient:

Earlier work established that molecular motion in the embryo is described well by the diffusion equation on the time (1 hr) and space scales of relevance for morphogenesis. The present work shows that diffusion is an equally good description of Bicoid transport on the scale of minutes and microns. The difficulty is that the relevant diffusion constants differ by more than an order of magnitude. To explain the observation that the Bcd gradient reaches a nuclear steady state very quickly the larger diffusion constants are needed, but the dynamics of transport into and out of the nuclei are consistent with the smaller diffusion constant, which were also measured directly. New experiments will be required to decide which of these is correct (Gregor, 2007a).

The most dramatic qualitative feature that is see in watching the development of the embryos expressing Bcd-GFP is the filling and emptying of the nuclei. Quantitatively, this results in a startling juxtaposition of dynamics and stability. Thus, although Bcd concentrations vary in time over a factor of four during the course of a mitotic cycle, the nuclear concentration near the start of interphase is reproducible with 10% accuracy from cycle to cycle. Although the number of nuclei is changing by a factor of 16 from cycle 10 to cycle 14, the total number of Bcd molecules that are localized in nuclei changes hardly at all. At the present level of understanding, both these examples of stability in the presence of change seem to be the result of cancellation among several independent processes, which is implausible. One way of summarizing the problem is that the simplest model looks like it works, but this is only because many parameters have been adjusted to make it work, leaving the simplest model as an effective description of the dynamics after the mechanisms responsible for this adjustment have done their job. This layer of mechanisms remains to be discovered (Gregor, 2007a).

The Bicoid gradient is shaped independently of nuclei

Morphogen gradients provide embryos with positional information, yet how they form is not understood. Binding of the morphogen to receptors could affect the formation of the morphogen gradient, in particular if the number of morphogen binding sites changes with time. For morphogens that function as transcription factors, the final distribution can be heavily influenced by the number of nuclear binding sites. This study has addressed the role of the increasing number of nuclei during the formation of the Bicoid gradient in embryos of Drosophila melanogaster. Deletion of a short stretch of sequence in Bicoid impairs its nuclear accumulation. This effect is due to a approximately 4-fold decrease in nuclear import rate and a approximately 2-fold reduction in nuclear residence time compared with the wild-type protein. Surprisingly, the shape of the resulting anterior-posterior gradient as well as the centre-surface distribution are indistinguishable from those of the normal gradient. This suggests that nuclei do not shape the Bicoid gradient but instead function solely during its interpretation (Grimm, 2010).

This paper has addressed the role of nuclear import and retention on the formation of the Bicoid gradient by generating a mutant form of Bicoid that does not accumulate within nuclei. Dual-colour imaging of mutant and wt Bicoid within the same embryo were used to reduce the variability due to timing in cross-embryo observations. The principal finding is that nuclei do not affect the anterior-posterior or the centre-to-surface distribution of the protein. A failure to detect an effect makes it unlikely that nuclei represent a major site of Bicoid degradation, or that they play a significant role in shaping or scaling the morphogen gradient. Instead, nuclei appear to be passive sensors of the Bicoid gradient. This is a surprising finding given that the transcription factor Bicoid accumulates in nuclei, the number of which increases by three orders of magnitude in less than ~2 hours from fertilisation to cycle 14. Unfertilised eggs provide an alternative means of testing a role of nuclei in the formation of the Bicoid gradient. Previous observations were, however, inconclusive, possibly reflecting the difficulties associated with staging and imaging the Bicoid gradient in unfertilised eggs (Grimm, 2010).

One interpretation of these results is that the gradient reaches its steady state before cycle 10, at a stage when the number of nuclei is still low (~750 nuclei) and therefore trapping of Bicoid by nuclei would not affect the shape of the gradient. However, the subsequent increase in nuclear density that occurs between cycles 10 and 14 might still have been predicted to alter the distribution of Bicoid if trapping or degradation became more efficient as the number of nuclei increased with each cycle. Such an effect has been observed for the Torso-dependent gradient of activated MAPK, which is activated at the two poles of the egg and sharpens with each round of mitosis in the embryo (Coppey et al., 2008). Whether such a sharpening would also be expected for the Bicoid gradient depends on its mobility and its lifetime and whether its distribution in the final cleavage stages continues to depend on movement from an anterior source. Using previously measured parameters and an assumed lifetime significantly longer than that of activated MAPK, it has been calculated that the shape of the Bicoid gradient would be refractory to the number of nuclei and it is argued that the late distribution of Bicoid protein at cycle 14 reflects local synthesis from a graded mRNA source. Distinguishing these and other models will require more accurate measurements of the relevant parameters, but the exclusion of nuclear import, retention and degradation in shaping the gradient greatly simplifies such an analysis (Grimm, 2010).

Fundamental origins and limits for scaling a maternal morphogen gradient

Tissue expansion and patterning are integral to development; however, it is unknown quantitatively how a mother accumulates molecular resources to invest in the future of instructing robust embryonic patterning. A model, Tissue Expansion-Modulated Maternal Morphogen Scaling [TEM(3)S], was developed to study scaled anterior-posterior patterning in Drosophila embryos. Using both ovaries and embryos, a core quantity was measred of the model, the scaling power of the Bicoid (Bcd) morphogen gradient's amplitude nA. Model-derived predictions about Bcd gradient and patterning properties were also evaluate directly. The results show that scaling of the Bcd gradient in the embryo originates from, and is constrained fundamentally by, a dynamic relationship between maternal tissue expansion and bcd gene copy number expansion in the ovary. This delicate connection between the two transitioning stages of a life cycle, stemming from a finite value of nA~3, underscores a key feature of developmental systems depicted by TEM(3)S (He, 2015).

Mechanisms of gap gene expression canalization in the Drosophila blastoderm

Extensive variation in early gap gene expression in the Drosophila blastoderm is reduced over time because of gap gene cross regulation. This phenomenon is a manifestation of canalization, the ability of an organism to produce a consistent phenotype despite variations in genotype or environment. The canalization of gap gene expression can be understood as arising from the actions of attractors in the gap gene dynamical system. In order to better understand the processes of developmental robustness and canalization in the early Drosophila embryo, this study investigated the dynamical effects of varying spatial profiles of Bicoid protein concentration on the formation of the expression border of the gap gene hunchback. At several positions on the anterior-posterior axis of the embryo, attractors and their basins of attraction were analyzed in a dynamical model describing expression of four gap genes with the Bicoid concentration profile accounted as a given input in the model equations. This model was tested against a family of Bicoid gradients obtained from individual embryos. These gradients were normalized by two independent methods, which are based on distinct biological hypotheses and provide different magnitudes for Bicoid spatial variability. It was shown how the border formation is dictated by the biological initial conditions (the concentration gradient of maternal Hunchback protein) being attracted to specific attracting sets in a local vicinity of the border. Different types of these attracting sets (point attractors or one dimensional attracting manifolds) define several possible mechanisms of border formation. The hunchback border formation is associated with intersection of the spatial gradient of the maternal Hunchback protein and a boundary between the attraction basins of two different point attractors. How the positional variability for hunchback is related to the corresponding variability of the basin boundaries was demonstrated. The observed reduction in variability of the hunchback gene expression can be accounted for by specific geometrical properties of the basin boundaries. The mechanisms of gap gene expression canalization in early Drosophila embryos were clarified. These mechanisms were specified in the case of hunchback in well defined terms of the dynamical system theory (Gursky, 2011).

This study presents the dynamical analysis of the simplified model of the gap gene network on the ensemble of early Drosophila embryos. The main goal was to decode the mechanistic basis of the gap gene border formation and stability under the Bcd morphogen variance. The hb border formation mechanisms were described in terms of attracting sets and their attraction basins calculated in the nuclei surrounding the border position (Gursky, 2011).

The results reveal that the border formation can be associated with the event of intersection between a boundary separating the attraction basins of two di®erent point attractors and the initial Hb profile presenting the input from the maternally expressed hb gene. Attracting sets of another type, the unstable manifolds of saddle equilibria, actively participate in the adjustment of the border position. They do so by attracting the solution trajectories in the nuclei surrounding this position. The model predicts that these attracting manifolds can be involved in the border formation for some Bcd profiles (Gursky, 2011).

The hb border correctly forms in the model by the onset of gastrulation for all individual Bcd profiles. For about a half of these profiles, however, the Kr and Gt patterns in the solutions exhibit defects in the anterior part of the spatial domain (solution classes II and III). It turns out that the hb border formation mechanism involving the attracting manifolds is mostly associated with these cases. This may lead to the conclusion about restricted applicability of this mechanism in the case of hb expression. However, this mechanism exists and plays an important role for the gap domain borders in a posterior part of the embryo, where the domains form and vary in time under the control of an unstable manifold. To analyze canalization for the posterior borders, the variation for external inputs from Cad and Tll should be taken into account, where these transcription factors are among the key regulators, and a modified model should be considered including an input from the terminal gene huckebein (Gursky, 2011).

As previously reported, the model exhibits a significant filtration (canalization) of the Bcd positional variability at the level of hb border formation. The results show how this filtration stems from the stable behavior of the attraction basin boundaries. Has been shown that the mutual regulatory repression between the gap genes accounts for the observed variance reduction, thus presenting a buffering mechanism for canalization. This buffering mechanism was translated to the level of attractors and their attraction basins. As the hb border position is well encoded by the intersection between the initial Hb profile and corresponding attraction basin boundaries, the stability of hb border predicted by the model can be explained by inspecting the geometrical properties of these attraction basins (Gursky, 2011).

From this inspection, the following two mechanisms responsible for the observed robustness can be elucidated. First, the initial Hb profile is a monotonously decreasing function of A/P position, while the basin boundary to be crossed is a monotonously increasing one, i.e., these curves have opposite dependencies on the A/P position. This purely geometrical fact evidently prescribes a smaller variation of the intersection point when the basin boundary is changing due to the variance of Bcd concentration, as opposed to the case if the curves would jointly rise or jointly fall along the A/P axis (Gursky, 2011).

The second mechanism is associated with the specific nonlinear form of the response curve. The gap gene cross regulation of hb bends the response line exhibited in absence of this regulation. This bending e®ectively reduces the Hb positional variance by about half. In terms of attractors, this bending is controlled by the fact that a basin boundary responsible for the hb border formation does not change monotonously, but oscillate in the state space with the changing Bcd profile (Gursky, 2011).

The results show that the full range of the hb positional variance is broken down into two almost equal parts, the anterior and posterior ones. These parts are associated with two families of the Bcd individual profiles (Family I and Family II, respectively) and two di®erent mechanisms of hb border formation. The Bcd profiles from Family I lead to the hb border formation as a switch from a hb/ON attractor in a hb-expressing nucleus to a hb/OFF attractor in a hb-nonexpressing nucleus, while for Family II the border forms with the help of an attracting invariant manifold in a hb-nonexpressing nucleus. Since the difference between the two families is in the amplitude of the Bcd profiles, it is concluded that Bcd profiles of high amplitude canalize by a dynamical mechanism different from those of lower amplitude. Each dynamical mechanism provides only half of the full variance for the hb border, but in two adjacent spatial domains. Therefore, the change of the dynamical mechanism that happens with rising Bcd amplitude e®ectively doubles the variance (Gursky, 2011).

The hb border positions from the more posterior range are placed posterior to the spatial position of a bifurcation annihilating attractor A3. This bifurcation position delimits the anterior and posterior dynamical regimes in the model. Therefore, the Bcd profiles from the second family shift the hb border to the posterior dynamical regime, which is characterized by an active role of an attracting invariant manifold in the pattern formation (Gursky, 2011).

The results indicate that the posterior range of hb positional variation is almost equal to the anterior one only due to smaller variation of the Bcd profiles in Family II compared to Family I. This suggests that the solutions in the anterior and posterior dynamical regimes have quite different sensitivity rates to variation of the Bcd concentration. For Family I, the standard deviation for the hb border position is 2.6 times less than for the Bcd threshold position, while it is only 1.4 times less in the case of Family II. This difference can be explained by an observation that Bcd profiles of higher amplitude correspond to the linear part of the response curve, and this is a consequence of specific regulatory interactions in the gap gene circuit as explained further (Gursky, 2011).

The model was used (1) to study the canalization mechanisms based on the assessment that the model provides one of the best spatio-temporal precision for the description of gap gene expression. This model is an approximation to a more general model of gene regulation, which should be grounded on the statistical-mechanical formalism. One possible limitation is the linear approximation for the argument of the nonlinear regulation function g. The canalization mechanisms described in terms of attractors and attraction basins generally depend on the structure of the model that predicts these attracting states. Therefore, an important direction for future investigations should be verification of the proposed mechanisms in a phase space of a more general model (Gursky, 2011).

The nonlinear nature of the Bcd readout by the gap gene circuit is clearly represented in a specific nonlinear form of the response curve showing the Bcd dependence of the hb border position in the model. The nonlinear part of the curve can be explained by the regulatory actions on hb from the other gap genes. In particular, a regulatory analysis in the full model revealed that the regulatory interactions between hb, gt, and Kr underlie the folding part of the response curve. The gap gene cross-regulation also participate in the linear parts of the response curve by tuning the incline of these parts (Gursky, 2011).

It was previously pointed out that the gt and Kr expression borders in the anterior part of the A/P axis show large variation in the model in response to Bcd variation because the model is missing some regulators in this part. For example, these gt and Kr borders are absent in the solutions from class III. This fact raises doubts on the specific folding part that the response curve exhibits in the middle range of the Bcd concentration values. On the other hand, the folding part exists only for the Bcd profiles associated with the solutions from class I, with all expression borders formed correctly, which means that an essential portion of the artificial variation of the gt and Kr borders can be excluded from the consideration without affecting the folding form of the curve (Gursky, 2011).

The model was investigated on the ensemble of Bcd profiles normalized by the alternative method, which provided lower Bcd variance. One used this method as an artificial limit case, in which the ensemble possessing minimal Bcd variance was dealt with, and it was applied for the crosschecking purposes (Gursky, 2011).

No essential discrepancy was found in the mechanisms of hb border formation and canalization for the two normalization methods. A distinct bifurcation structure in the model with the new parameter values does not lead to changes in the solutions during the biologically important time. The model preserves an attracting invariant manifold related to the posterior dynamical regime. The same border formation mechanisms appear except the one associated with the attractor/manifold transition. It is important that, even though the second family of Bcd profiles does not appear in the alternative normalization case, the invariant manifolds still play their role in adjusting the border position. The model also demonstrates an essentially nonlinear response curve for the hb border. Therefore, the conclusions formulated above are robust with respect to the choice of the normalization method, and, in more general terms, they should be valid for different estimates of the actual Bcd variance (Gursky, 2011).

This correspondence can be explained by the fact that the parameters A and l obtained for the alternatively normalized Bcd profiles form a subset in similar parameters obtained in the case of the basic normalization method. Roughly speaking, the alternatively normalized Bcd profiles can be associated with Family I. In particular, this means that the Bcd data rescaled according to the alternative algorithm support the conclusion formulated above about different dynamical mechanisms of canalization for Bcd profiles of different amplitude (Gursky, 2011).

There is an important issue concerning the comparison of the Bcd variance filtration rates. The calculations reveal that, for the basic normalization method, the Hb positional variation of 1.3%EL in the model output follows from the Bcd positional variation of 4.5%EL, thus implying that more than 70% of the positional variance has been filtrated. The same calculations for the alternative normalization method give the filtration rate of approximately 60%. Therefore, the filtration still happens in the model even if Bcd profiles are normalize according to the precisionist hypothesis. This result is quite expected since the reported dynamical mechanisms underlying the processing of the Bcd variation in the model are valid irrespective of the absolute variation range. Whatever actual variation the Bcd morphogen exhibits, the nonlinear model response translates it to a smaller variation of the target gene patterns (Gursky, 2011).

It is concluded that the formation of hb border is coded by the intersection between the maternal Hb gradient and a boundary between attraction basins in the gap gene dynamical system. Small positional variance for hb border can be explained by the geometrical properties of this basin boundary and its nonmonotonic dependence on the Bcd concentration. Main features of the phase portraits underlying the canalization mechanisms do not depend on the normalization method for Bcd (Gursky, 2011).

Adaptation of the length scale and amplitude of the Bicoid gradient profile to achieve robust patterning in abnormally large Drosophila melanogaster embryos

The formation of patterns that are proportional to the size of the embryo is an intriguing but poorly understood feature of development. Molecular mechanisms controlling such proportionality, or scaling, can be probed through quantitative interrogations of the properties of morphogen gradients that instruct patterning. Recent studies of the Drosophila morphogen gradient Bicoid (Bcd), which is required for anterior-posterior (AP) patterning in the early embryo, have uncovered two distinct ways of scaling. Whereas between-species scaling is achieved by adjusting the exponential shape characteristic of the Bcd gradient profile, namely, its length scale or length constant (lambda), within-species scaling is achieved through adjusting the profile's amplitude, namely, the Bcd concentration at the anterior (B0). This study reports a case in which Drosophila melanogaster embryos exhibit Bcd gradient properties uncharacteristic of their size. The embryos under investigation were from a pair of inbred lines that had been artificially selected for egg size extremes. B0 in the large embryos is uncharacteristically low but lambda is abnormally extended. Although the large embryos have more total bcd mRNA than their smaller counterparts, as expected, its distribution is unusually broad. The large and small embryos develop gene expression patterns exhibiting boundaries that are proportional to their respective lengths. These results suggest that the large-egg inbred line has acquired compensating properties that counteract the extreme length of the embryos to maintain Bcd gradient properties necessary for robust patterning. This study documents a case of within-species Bcd scaling achieved through adjusting the gradient profile's exponential shape characteristic, illustrating at a molecular level how a developmental system can follow distinct operational paths towards the goal of robust and scaled patterning (Cheung, 2014).

Effects of Mutation or Deletion

Embryos from mothers with either fewer or greater than the normal two copies of bicoid show initial alterations in the expression of the gap, segmentation and segment polarity genes, as well as changes in early morphological markers. In the absence of a fate map repair system, one would predict that these initial changes would result in drastic changes in the shape and size of larval and adult structures. However, these embryos develop into relatively normal larvae and adults. This indicates a certain plasticity in Drosophila embryonic development along the anterior-posterior axis. Embryos laid by mothers with six copies of bcd have reduced viability, indicating a threshold for repairing anterior-posterior mispatterning. The cephalic furrow (CF) is displaced posteriorly in embryos generated for 6bcd females. Embryos that form the CF more posterior to the mean die at almost twice the frequence of the population of embryos that form the CF anterior to the mean position. Embryos of 4bcd animals either have one missing or a pair of fused denticle belts. Cell death plays a major role in correcting expanded regions of the fate map. There is a concomitant decrease of cell death in compressed regions of the fate map. Compression of the fate map does not appear to be repaired by the induction of new cell divisions. In addition, some tissues are more sensitive to fate map compression than others. 6bcd embryos display increased levels of cell death in the expanded presumptive head region, while 1bcd embryos have increased cell death in the abdominal segments (Namba, 1997).

I>bicoid-independent formation of thoracic segments in Drosophila

The maternal determinant Bicoid (Bcd) represents the paradigm of a morphogen that provides positional information for pattern formation. However, since bicoid seems to be a recently acquired gene in flies, the question has been raised as to how embryonic patterning is achieved in organisms with more ancestral modes of development. Because the phylogenetically conserved Hunchback (Hb) protein acts as a morphogen in abdominal patterning, it was asked which functions of Bcd could be performed by Hb. By reestablishing a proposed ancient regulatory circuitry in which maternal Hb controls zygotic hunchback expression, it has been shown that Hb is able to form thoracic segments in the absence of Bcd (Wimmer, 2000).

A functional hb transgene has been generated that is missing all P2 promoter sequences and relies solely on the P1 promoter (hbP1only). hbP1only constructs do not respond to bcd and do not mediate gene expression in the anterior cap domain. Therefore, hbP1only uncouples the direct link between the Bcd and Hb morphogen systems. Zygotic hb mutants derived from heterozygous parents do not develop labial or thoracic structures, and they also show a fusion of abdominal segments A7 and A8. When one copy of the hbP1only transgene is provided zygotically (by the father) to a hb mutant embryo, it rescues the posterior phenotype, and A7 and A8 developed normally. The labial/thoracic phenotype is not rescued. However, when hbP1only is provided as one copy by the mother to a hb mutant embryo, the posterior and part of the anterior phenotype are rescued. These embryos exhibit normal labial and prothoracic (T1) segments, and only lack meso- and meta-thoracic segments (T2 and T3). The anterior rescue is due to the maternal contribution of hbP1only because sibling embryos that do not inherit the hbP1only construct zygotically also exhibit the partial anterior (but not the posterior) rescue. This indicates that restoring high levels of maternal hb expression (i.e., two copies: one wild type plus one copy of hbP1only) is sufficient to rescue the labial and prothoracic segments in the zygotic hb mutant progeny. Therefore, the lack of zygotic hb leads only to the loss of T2 and T3 and to the fusion of A7 and A8, whereas the previously reported zygotic hb phenotype represents a combination of a haploinsufficient maternal plus a zygotic phenotype (Wimmer, 2000).

The loss of zygotic hb activity affects regions of the embryo that correspond to the two late stripes of zygotic hb expression: The A7-A8 fusion corresponds to the posterior stripe, whereas the loss of T2 and T3 corresponds to the PS4 stripe, which starts as a fairly wide domain covering the anlagen of T2 and T3. This correlation between the zygotic hb phenotype and the late stripe expression pattern led to a reconsideration of the importance of the early bcd-dependent anterior cap domain. Under some conditions, hbP1only (maternal hb contribution plus stripe expression) might suffice for normal segmentation of head and thorax, making superfluous the bcd-dependent anterior cap domain. Hence, the hb PS4 stripe is activated without bcd-dependent hb expression. This stripe is repressed by the knirps abdominal gap-gene product and is activated by high levels of Hb itself, either directly or indirectly (through repression of kni). Embryos that lack the bcd-dependent hb cap domain have been generated that contain an increased maternal hb contribution (to four copies) and kni reduced to one copy. These embryos display a range of partially rescued hb phenotypes, including some embryos with a full set of head and thoracic segments. Thus, bcd-dependent hb expression can in principle be dispensable for embryonic segmentation, and the only critical anterior domain of zygotic hb expression appears to be the PS4 stripe, with the bcd-dependent cap domain serving to activate this stripe. This role is likely achieved by the maternal hb contribution in species where zygotic hb is not under the control of bcd or where a bcd homolog might not exist (Wimmer, 2000).

The rescue of T2 and T3 structures by bcd-independent hb expression raises the question of whether these structures could develop in a completely bcd-independent manner. Embryos derived from bcd mutant mothers develop ectopic tail structures that replace head and thorax and exhibit a disruption of some abdominal segments. Although previous work has shown that, in the absence of bcd, high levels of maternal hb can rescue a normal abdomen and some thoracic structures, no complete thoracic segments can be induced. A bcd-independent source for high levels of zygotic hb expression was introduced into a bcd mutant background. By establishing this artificial zygotic Hb gradient, two notable results were obtained, with variable expressivity: (1) about 20% of the embryos exhibit rescued T2 and T3 segments. The maintenance of high Hb levels that lead to the rescue of thoracic segments is likely due to the activation of the hb stripe element because the hbP1AB reporter is activated as a stripe where T2 and T3 form. (2) Most of the ectopic tail structures that are anteriorly duplicated in bcd mutants are suppressed, suggesting further redundancy between Hb and Bcd. However, Hb and Bcd must act at different levels in suppressing these tail structures, which depend on the activity of the caudal (cad) gene: Bcd acts by repressing cad mRNA translation, whereas Hb does not but might instead interfere with Cad protein function. This bcd-independent suppression of cad function might be important in organisms where the Cad gradient only forms late and represents another variation as to how cad activity is suppressed at the anterior of the embryo (Wimmer, 2000).

Different species use various strategies for repression of Cad function: In Drosophila, translational repression of CAD mRNA involves the Bcd homeoprotein, whereas in Caenorhabditis elegans repression involves the KH-domain protein MEX-3. In vertebrates, a mutually antagonistic relation between otd-like and cad-like genes has been proposed to reflect an ancestral system to pattern the anteroposterior axis of the embryo. In arthropods, ancestral head determinants are probably encoded by otd-like genes as well. Thus, in the beetle Tribolium, where no bcd homologs but Bcd-like activities have been found, these activities are probably also covered by Otd or KH-domain proteins. This is consistent with the Otd-like DNA binding specificity of Bcd, which is atypical for a factor encoded by a gene duplication in the Hox cluster. This change in specificity was probably crucial for Bcd to acquire its key role in anterior patterning, because it allowed Bcd to function both as an RNA binding protein and as an Otd-like transcription factor. In this respect, it is not surprising that the zinc-finger protein Hb cannot completely replace Bcd in the head region. Even the highest levels of Hb obtained in these experiments were not able to induce head formation in the absence of Bcd. However, Hb is required for the posterior head region (maxillary and labial segment) and supports anterior head development by synergizing with Bcd. It will be interesting to see whether such a synergism can also take place between Hb and other more ancestral head determinants (Wimmer, 2000).

These results indicate that the two morphogenetic systems, Bcd and Hb, do not need to be directly linked. Hence, the direct regulation of hb by Bcd might represent a recent evolutionary addition to the insect body plan. In Drosophila, the abundance of bcd-dependent hb expression eventually renders superfluous the maternal hb contribution, which is widespread within arthropods. Consistent with the idea that the bcd-dependent hb expression represents a recent evolutionary acquisition, the P2 promoter contains only activator sites that allow the direct response to a specific threshold level of a morphogen. This might be a unique situation, given that most other developmentally regulated promoters contain, in addition to activator sites, repressor elements for setting the exact borders of gene expression. By tinkering with the rather plastic mechanisms of early development, the ontogeny of Drosophila could be changed toward an inferred ancestral state where maternal Hb controls zygotic hb. This change could be brought about by altering patterns and levels of gene expression; this presents the most likely variation on which evolutionary processes are based (Wimmer, 2000).

Two new roles for the Drosophila AP patterning system in early morphogenesis

Although gastrulation is regarded as the stage during Drosophila development when the AP patterning system first influences morphological processes, transcription is already regulated in complex patterns by cycle 10. How soon this transcriptional complexity produces spatial differences in morphology has been unclear. Two new processes are described that establish visible morphological inhomogeneities before the onset of gastrulation. The first of these is the regulation of syncytial nuclear densities in the anterior end of the egg and represents the first zygotically driven AP asymmetry in the embryo. The second process is the generation of a fine-scale pattern in the actin/myosin array during cellularization. Three domains of different yolk stalk diameters as well as depths of cellularization along the AP axis have been found. These domains are established under the control of the AP patterning system and require bicoid activity. The anterior-most domain is a region of large yolk stalk diameters and corresponds to the region of decreased nuclear densities observed during syncytial stages. The middle domain shows smaller yolk stalk diameters and more rapid cellularization. Its establishment requires wild-type paired activity and thus indirectly requires bicoid. It occurs in a region of the embryo that ultimately gives rise to the cephalic furrow and may account for the effect of paired on that structure during gastrulation. These results therefore suggest a link between cytoskeletal organization during cellularization and subsequent morphogenetic processes of gastrulation (Blankenship, 2001).

To assay the degree of uniformity along the AP axis during cellularization, the furrow canals of wild-type embryos were visualized by staining for Myosin. By mid-cellularization, three domains of differing diameters of yolk stalks could be observed. The first domain is centered around the anterior pole of the embryo, and possesses the largest diameters. The second domain, immediately posterior to the first anterior domain, has the smallest diameters. This domain is centered around the location where the cephalic furrow will eventually form. The third domain lies posterior to this pre-CF domain and has furrow canals of intermediate diameter (Blankenship, 2001).

These three domains also differ in the depth of the cellularization front. The anterior domain, which has large yolk stalk diameters, is the shallowest part of the embryo in its depth of cellularization. By the end of cellularization, the most anterior part of this domain can possess a depth of cellularization (15-20 µm) half that of the rest of the embryo (~35 µm on average). The region posterior to the pre-CF domain has an intermediate depth, whereas the pre-CF domain, which has the smallest yolk stalk diameters, has the greatest depth of cellularization. The regions of differing yolk stalk diameters correspond to the regions of varying depth during cellularization (Blankenship, 2001).

To test whether these cellularization phenomena are due simply to the position or geometry of the embryo (e.g. the curvature of the anterior part of the egg), or if these phenomena are being specified and positioned by the AP patterning system, the three domains were sought in embryos that lacked all positional information along this axis. In embryos derived from females triply mutant for bicoid nanos torso-like, yolk stalk diameters are uniform, even though other aspects of cellularization occur normally. The shallow cellularization front of the anterior domain, as well as the greater depth of the pre-CF, is also lost. Thus, these phenomena require the activity of the AP patterning system for their morphogenesis (Blankenship, 2001).

These cellularization phenomena respond to specific levels of the bicoid gradient. Embryos that carry six copies of bicoid produce more Bicoid protein, shifting any given concentration of Bicoid to a position posterior to where it would be in a wild-type embryo. In embryos carrying six copies of bicoid, the pre-CF domain of small yolk stalk diameters and greater depth is shifted posteriorly, while the anterior domain of large yolk stalk diameters expands to cover close to half of the embryo. bicoid appears to be the important factor specifying these phenomena, since embryos derived from females mutant for bicoid lack the anterior and pre-CF domains (Blankenship, 2001).

Since the pre-CF domain is a relatively narrow domain, it seemed unlikely that the broad Bicoid gradient would be directly specifying the pre-CF domain. Several pair-rule genes are expressed in approximately the right time and place to be mediating the formation of the pre-CF domain. While Even-skipped (Eve) is expressed in the posterior half of the pre-CF domain, Prd expression is directly centered on the pre-CF domain. Prd expression also has an unusual feature. When Prd expression is first detected during cycle 13, it is observed in a single, gap gene-like domain. Thus, Prd is expressed before the pre-CF domain is formed, and in the right location in the embryo. Moreover, the early expression of Prd in a single stripe, rather than in the stereotypical seven-stripe pattern, suggests that it could specify the pre-CF domain independent of other factors (Blankenship, 2001).

Various patterning mutants were examined for a disruption of the pre-CF domain. prd homozygotes show a normal anterior domain of larger yolks, but there is very little difference in the yolk stalk diameters between the pre-CF domain and the posterior domain. The relatively subtle appearance of the pre-CF domain in different genetic backgrounds required the scoring of embryos in a blind test. On a classification scale of 0-5, with 0 indicating a complete absence of the pre-CF domain, prd homozygotes scored a 1.2, while their non-homozygous siblings scored a 3.3. These classifications are significant (P <0.001, n=61). Thus, prd is already active in the regulation of morphogenesis during the process of cellularization (Blankenship, 2001).

The pre-CF domain is first visible when the cellularization front has reached about 25% of its depth. However, the larger yolk stalks of the anterior domain are visible throughout cellularization. The density of nuclei in the anterior is reduced by about 30% from the nuclear density that is found in the rest of the embryo. This anterior domain of lower nuclear density is centered around the anterior pole and extends to approximately two to three nuclei in front of the first stripe of Eve, or ~70% EL. Judging from the staining patterns of Eve and Prd, this is the same area of the embryo in which the anterior domain of large yolk stalk diameters meets the pre-CF domain. During cellularization the anterior nuclear domain is consistently observed in OreR embryos, with an average decrease in nuclear density of 27.6%. In this sample, the values for individual embryos ranged from 34% to 20%. This domain of lower nuclear densities is maintained and stays constant throughout the process of cellularization, and can still be observed in gastrulating embryos. This greater spacing of nuclei in the anterior would necessarily lead to a cellularization network with larger diameters. Additionally, by mid-cellularization, when a pre-CF domain is already visible, a slight clustering of nuclei in this region occurs (Blankenship, 2001).

Since the anterior domain of lower nuclear densities is present at the start of cycle 14, it was asked when this AP asymmetry arises. While both the sphere of nuclei that migrate to the surface of the embryo during cycle 9, and the cortical nuclei of cycle 10 are uniform along the AP axis, the first sign of increased nuclear spacing in the anterior can be observed in cycle 11 embryos. By cycle 12, a pronounced AP asymmetry is observed, which is maintained through the subsequent mitosis to the start of cellularization. The anterior domain of lower nuclear densities is also observable in living embryos. Nuclear counts on GFP-histone embryos demonstrate a similar ~30% reduction in nuclear density in vivo. These observations on living embryos also reveal that, except for when the nuclei are undergoing mitosis and the associated yolk contractions, the anterior nuclear domain is present throughout the rest of the cell cycle (Blankenship, 2001).

Before cycle 14, cortical nuclei are associated with large actin caps. In embryos without actin caps, the regular spacing of nuclei is disrupted. One possible mechanism for the formation of the anterior domain of lower nuclear densities would use the regulation of the size of these actin caps. To see if asymmetries in actin cap size occur along the AP axis, wild-type embryos were stained with fluorescently labeled phalloidin to visualize F-actin. Measurements show that anterior actin caps are larger than caps in the rest of the embryo in cycle 11 and cycle 12, but not during cycle 10. This difference, although small, is highly reproducible from embryo to embryo (Blankenship, 2001).

The exceptionally early appearance of the AP asymmetries in nuclear distributions led to an examination of whether the anterior domain of lower nuclear densities is specified by the AP patterning system. In embryos derived from bicoid nanos torso-like mutant females, nuclear spacing is uniform along the AP axis. In addition, the anterior nuclear domain can be expanded posteriorly in embryos carrying six copies of bicoid. Additionally, embryos from bicoid females are uniform in their nuclear densities, arguing that the anterior domain of lower nuclear densities is set up by the Bicoid gradient. Finally, the larger diameters of the actin caps in the anterior do not occur in embryos from bicoid nanos torso-like females (Blankenship, 2001).

Since zygotic transcription only starts around cycle 10, the early appearance of the anterior domain by cycle 11 raised the question of whether zygotic transcription is required for this domain's formation. It is possible that bicoid might be regulating nuclear spacing through a post-transcriptional mechanism. To assay the effect of transcription on nuclear spacing, embryos were injected with alpha-amanitin to block RNA polymerase II. Consistent with earlier studies, such embryos develop to cycle 14 with normal gross morphology. However, when these embryos were examined with nuclear stains, they showed a total absence of asymmetric nuclear distributions. Embryos injected with alpha-amanitin fail to form the anterior domain of lower nuclear densities. It is concluded that bicoid regulates the zygotic transcription to bring about the formation of the anterior domain of lower nuclear densities (Blankenship, 2001).

Two genes necessary for cephalic furrow formation have been identified. Embryos mutant for either eve or buttonhead (btd) lack cephalic furrows. Both eve and btd have a defect in the earliest phase of CF formation. At no point during development are the stereotypical cell shape changes of shortening along the apical-basal axis and widening of the cell observed in these mutants. Mutations in prd also cause a disruption in CF formation. At the onset of gastrulation, the cephalic furrow does not form, nor do initiator cells undergo their characteristic cell shape change. Because there is an absolute correlation between initiator cell behavior and the middle nucleus of the first stripe of Eve in wild-type embryos, the location where initiator cells should form in prd embryos can still be identified. In prd embryos these Eve-marked cells are indistinguishable from their neighbors at stages during gastrulation when the ventral furrow has formed and the CF would normally be visible in wild-type embryos. However, by mid-germband extension (GBE), prd embryos have formed a regular, CF-like invagination. This late fold is thought to arises through the same mechanisms that govern normal CF formation, but these processes are delayed relative to the development of wild-type embryos (in the stage 7 prd embryo there are no cell shape changes in the region where the CF would form, while the stage 6 wild-type embryo has an obvious CF). Initiator cells form in wild-type embryos at stage 6, at the onset of gastrulation. Imaging of prd embryos reveals initiator cell activities beginning at stage 7. At this stage, wild-type embryos have already begun to deform the yolk sack with a basal bulge of the epithelium. At the beginning of stage 8, or germband extension, wild-type embryos have a furrow that is many cells deep, while prd embryos have just begun to deform the yolk sack. It is only by mid-GBE that most, but not all, prd embryos have a regular CF stretching around the entire circumference of the embryo (Blankenship, 2001).

The abnormalities in CF formation observed at the beginning of gastrulation in prd embryos are superficially similar to those observed in embryos mutant for eve or btd. In the absence of the activity of eve, btd or prd, the cell shape changes that occur in the row of cells that initiate CF formation at the beginning of gastrulation do not occur. In contrast to eve or btd, however, early activities of prd during cellularization have been identified that may account for the later differences observed in CF formation. Consistent with this view the analysis indicates that prd embryos often recover and form a fairly regular CF by mid-germband extension, unlike the severe disruption of the CF in eve and btd embryos. While the start of gastrulation occurs normally in prd embryos, CF formation and initiator cell behavior is delayed. It is proposed that this delay is due not to a function of prd during gastrulation in the specification of initiator cells, as has been proposed proposed for eve and btd function. These latter mutants completely block initiator cell and CF formation. Instead, prd function is necessary for the formation of the pre-CF domain, and it is suggested that prd functions in CF formation only through this disruption of the pre-CF domain. The recovery of the CF observed in prd embryos may be a reflection that cellularization throughout the embryo has finally reached the point that the pre-CF domain reaches at the very beginning of gastrulation, and so CF formation, although delayed, may be correctly initiated. The advanced rate of cellularization in the pre-CF domain may reflect a required premature closing of the base of the initiator cells so that cell volumes may be maintained during the severe cell shape changes of gastrulation, or that cytoskeletal components involved in cellularization must be freed for initiator cell movements. Because little deformation, or loss of volume, occurs in the presumptive initiator cells of early prd embryos, the latter of these two possibilities is favored. Thus, the subtle regulation of one stage of development can have profound effects upon a later, seemingly discrete, process of development (Blankenship, 2001).

The cellularization front that arises during early cycle 14 is rich in actin and myosin, and is thought to provide a contractile force that orients and drives the process of cellularization. This network, as well as cellularization itself, proceeds through a two-phase process. The first phase is a basal movement of the cellularization front towards the interior of the embryo. During this phase, the furrow canals stay constant in their small size, and the actin/myosin array has a hexagonal shape. The second phase is a lateral movement of the cellularization front, which creates a pinching off at the bases of cells in the newly forming cell sheet. The morphology of the cellularization front is not uniform along the AP axis. The pre-CF domain is distinguished from other regions of the embryo by its small yolk stalk diameters and greater depth of cellularization (Blankenship, 2001).

These results suggest the following model for the generation of AP asymmetries during cellularization. In the pre-CF domain, the Bicoid gradient directs the correct localization of the early gap gene-like expression domain of prd, which, in turn, directs a greater local contraction of the cellularization network. When cellularization is in its first phase of inward directed movement, the greater contraction of the pre-CF domain leads to a greater advance inwards of the cellularization front, thus creating the greater depth of the pre-CF domain. Then, when cellularization shifts to the second phase of a lateral, pinching-off movement, the greater contractility of the pre-CF domain leads to a greater widening of the furrow canals, which creates the smaller yolk stalk diameters observed in the pre-CF domain. The creation of this domain of advanced cellularization may be necessary for the initiator cell shape change required for cephalic furrow formation (Blankenship, 2001).

In certain respects, the anterior domain of large yolk stalk diameters and shallow cellularization appears to be the opposite of the pre-CF domain, and thus might be produced by a downregulation of the same contractile mechanisms that are proposed to operate in the pre-CF domain. However, the formation of the larger yolk stalk diameters in the anterior domain clearly involves a different mechanism. The anterior cellularization domain is pre-figured by an anterior domain of lower nuclear densities, while the pre-CF domain initiates in a region where nuclear densities are uniform. The greater spacing of nuclei in the anterior necessarily causes the formation of actin/myosin arrays of greater diameter during cellularization. A model for the formation of the anterior domain is favored in which the bicoid gradient, by cycle 11, has regulated the transcription of a set of zygotic genes, which in turn regulate the size of the actin caps overlaying the nuclei. This regulation of the actin caps results in larger caps in the anterior that necessitates a greater spacing of nuclei. By cycle 14, when cellularization is initiated, the greater spacing of nuclei dictates the generation of a cellularization network in the anterior in which the hexagonal components are larger in diameter. The initiation of large hexagons would thus lead to larger yolk stalks. It may be that the larger hexagons of the cellularization network contract less efficiently, thus generating the shallower depth of cellularization that is observed for the anterior domain (Blankenship, 2001).

The concept of a mid-blastula transition (MBT) has generally referred to a time when the genome of an embryo begins to exert an influence on development, presumably through zygotic transcription. The best-defined MBT is for Xenopus, where the MBT was characterized by a cessation of synchronous mitotic cycles, the start of zygotic transcription, and a change in the morphology of blastula cells (i.e., the acquisition of cell motility). Various aspects of this characterization have since been called into question. There is low level zygotic transcription before the MBT, and it appears as though cell motility may not be a function of zygotic transcription, but of slower mitosis. Although these discrepancies call into question the usefulness of the MBT as a concept, the idea of an MBT remains an attractive model for explaining the changing morphology of the Drosophila embryo (Blankenship, 2001).

In flies, the MBT has traditionally been discussed in terms of cycle 14 development. It is at this point that the fly embryo ceases its synchronous syncytial divisions and several morphogenetic processes require zygotic transcription for their genesis. At a superficial level, the first observable defects in embryos deficient for zygotic transcription is at cycle 14, when alpha-amanitin-injected embryos show defects in the processes of lipid droplet clearing and cellularization. However, as in Xenopus, defining a precise MBT presents some difficulties. Although cycle 14 marks a major increase in transcriptional efficiency, the start of zygotic transcription occurs in different nuclear cycles for different genes. In general, for the early acting genes involved in patterning, transcription begins around cycle 10, although some genes are transcribed as early as cycle 8, and mutations in specific segmentation genes show subtle disruption as early as cycle 10. What is striking in the results reported for Bicoid and Prd is the correspondence between spatial patterns of expression and regional alterations in morphology. These results show that the genome of the embryo is active in the spatial regulation of morphogenesis at a much earlier time than previously described. The genome directs reproducible asymmetries in morphology by cycle 11. The generation of an anterior domain of lower nuclear densities argues against a single discrete MBT at cycle 14. These results further suggest a more refined view of the Drosophila MBT in which there is a gradual shifting in the guidance of development from maternal to zygotic gene products, one that stretches from as early as cycle 8 until the cessation of mitosis and formation of a cellularized embryo at cycle 14 (Blankenship, 2001).

Maternal expression relaxes constraint on innovation of the anterior determinant

The origin of evolutionary novelty is believed to involve both positive selection and relaxed developmental constraint. In flies, the redesign of anterior patterning during embryogenesis is a major developmental innovation and the rapidly evolving Hox gene bicoid plays a critical role. This study reports evidence for relaxation of selective constraint acting on bicoid as a result of its maternal pattern of gene expression. Evolutionary theory predicts 2-fold greater sequence diversity for maternal effect genes than for zygotically expressed genes, because natural selection is only half as effective acting on autosomal genes expressed in one sex as it is on genes expressed in both sexes. Single individuals have been sampled from ten populations of Drosophila melanogaster and nine populations of D. simulans for polymorphism in the tandem gene duplicates bcd, which is maternally expressed, and zerknüllt, which is zygotically expressed. In both species, the ratio of bcd to zen nucleotide diversity was found to be two or more in the coding regions but one in the noncoding regions, providing the first quantitative support for the theoretical prediction of relaxed selective constraint on maternal-effect genes resulting from sex-limited expression. These results suggest that the accelerated rate of evolution observed for bcd is owing, at least partly, to variation generated by relaxed selective constraint (Barker, 2005).

Mis-specified cells die by an active gene-directed process, and inhibition of this death results in cell fate transformation in Drosophila

Incorrectly specified or mis-specified cells often undergo cell death or are transformed to adopt a different cell fate during development. The underlying cause for this distinction is largely unknown. In many developmental mutants in Drosophila, large numbers of mis-specified cells die synchronously, providing a convenient model for analysis of this phenomenon. The maternal mutant bicoid is a particularly useful model with which to address this issue because its mutant phenotype is a combination of both transformation of tissue (acron to telson) and cell death in the presumptive head and thorax regions. A subset of these mis-specified cells die through an active gene-directed process involving transcriptional upregulation of the cell death inducer hid. Upregulation of hid also occurs in oskar mutants and other segmentation mutants. In hid bicoid double mutants, mis-specified cells in the presumptive head and thorax survive and continue to develop, but they are transformed to adopt a different cell fate. Evidence is provided that the terminal torso signaling pathway protects the mis-specified telson tissue in bicoid mutants from hid-induced cell death, whereas mis-specified cells in the head and thorax die, presumably because equivalent survival signals are lacking. These data support a model whereby mis-specification can be tolerated if a survival pathway is provided, resulting in cellular transformation (Werz, 2005).

Although this study largely focus on the maternal effect mutants bicoid and oskar, it is likely that the principles uncovered are of broader significance. Segmentation mutants acting downstream of bicoid and oskar, including mutants of gap genes (Krüppel, knirps), pair-rule genes (odd, fushi-tarazu) and segment polarity genes (wg, hedgehog, engrailed) induce expression of hid. These mutants are characterized by loss of larval tissue. As in the case of bicoid and oskar, hid expression is upregulated during stage 9 of embryogenesis in the regions of the mutant embryos that are later deleted in the larvae. In addition, hid mutants rescue the cuticle phenotype of armadillo mutants. Finally, hid expression accompanied by TUNEL-positive cell death was found in dorsal and Toll10b mutants, which cause dorsalizing and ventralizing phenotypes, respectively, along the dorsoventral axis of Drosophila embryos. Thus, these data support the notion that upregulation of hid appears to be a common trigger for a caspase-dependent cell death program in mis-specified cells of patterning mutants (Werz, 2005).

Furthermore, mutations affecting imaginal disc development result in loss of the adult appendage due to inappropriate cell death. It is currently being determined whether these mutants also require hid expression to develop the final phenotypes. Moreover, many gene disruptions in mice result in inappropriate cell death in the tissue that requires the function of the disrupted gene, suggesting that similar mechanisms might exist in mammalian development. Finally, cell death may be an important contributing factor to human congenital birth defects. Thus, an understanding of the underlying mechanisms is of general interest (Werz, 2005).

Interestingly, not all segment polarity mutants analyzed induce hid expression and cell death. Embryos mutant for patched, which encodes the hedgehog receptor, were not found to express hid and do not exhibit increased cell death, although hedgehog mutants both upregulate hid and contain increased amounts of cell death. The reasons for these differences are not known, but partial redundancy might account for lack of hid expression in patched mutants. The Drosophila genome encodes another patched homolog, patched-related, which might provide the survival requirement for mis-specified cells in patched mutants (Werz, 2005).

Mis-specified cells in bicoid and oskar mutants induce expression of hid. No increased reaper or grim expression was observed in these mutants. However, expression of reaper has been reported in crumbs mutants, which affect epithelial integrity. X-ray-treated embryos also preferentially respond by upregulation of reaper, rather than hid. Although crumbs mutants were not analyzed for hid expression, it appears that cells contain several developmental checkpoints, which activate different cell death-inducing regulators depending on the type of abnormal cellular development (Werz, 2005 and references therein).

Mis-specified cells can survive if an alternative survival pathway is provided. The example presented here is the acron into telson transformation in bicoid mutants, which is mediated by the torso signaling pathway. Although the cells giving rise to telson structures at the anterior tip are mis-specified based on Abd-B-labeling experiments, they survive because they receive a survival signal from the torso signaling system. In this case, transformation rather than cell death is favored. It has been shown that activation of the Ras/Mapk pathway protects cells from hid-induced apoptosis, both by transcriptional repression of hid and by phosphorylation of Hid protein by Mapk. Because Torso, which encodes a receptor tyrosine kinase (RTK), is known to activate Ras and Mapk, tests were performed to see whether manipulation of active Mapk levels using a gain-of-function allele, MapkSem, can suppress hid expression and cell death in bicoid mutants. However, this was found not to be the case. Thus, torso appears to protect mis-specified cells independently of Mapk activation (Werz, 2005).

The hid bicoid double mutant analysis reveals that the transformation of anterior into posterior identity expands beyond the telson, and that this expansion undergoes hid-induced cell death in bicoid single mutants. The rescued cells secrete larval cuticle elements, suggesting that mis-specified cells have the developmental capacity to terminally differentiate. However, in hid+ background, they instead die, presumably because equivalent survival signals are lacking. It is proposed that mis-specified cells undergo cell death if no alternative survival pathway is provided to protect them (Werz, 2005).

An alternative survival mechanism might also operate in other developmental mutants where transformation rather than cell death occurs. Mutations in the sev RTK and its ligand boss result in transformation of the R7 photoreceptor cell into a non-neuronal cone cell. Survival of this cell could be mediated by the Drosophila Egf receptor (Egfr), another RTK, which is required to maintain cell survival in the developing eye disc. Accordingly, activation of the Ras/Mapk pathway by Egfr would inhibit hid expression and support survival of the presumptive R7 photoreceptor cell. This interpretation is also consistent with observations that egfr- clones are small and undergo cell death, and that this death can be suppressed in hid mutants. Thus, transformation of the R7 photoreceptor to a cone cell rather than R7 cell death in sev and boss mutants could occur because of survival signaling by the Egfr (Werz, 2005).

The hid bicoid double mutant analysis suggests that mis-specified cells can continue to develop and differentiate. Yet, they die. Presumably, this cell death protects the organism from potentially dangerous cells. For example, it is conceivable that in mammals, surviving mis-specified cells might lie dormant in the host organism for years. During this time, they might acquire additional genetic alterations that could drive the progressive transformation of these cells into malignant cancer. In wild-type embryos, mis-specification probably occurs in cells in isolation, and elimination of these cells does not interfere with development and survival of the organism. Only in extreme situations, such as the patterning mutants analyzed in this study, is the mis-specification caused by aberrant development so severe that the affected organism dies (Werz, 2005).

The cause of mis-specification in each segmentation mutant is different. Usually, the expression of other segmentation genes is shifted and expanded, resulting in flattened gradients. Yet, irrespective of the cause of mis-specification, most of these mutants have in common that they induce hid expression. It is currently unknown how the mis-specified fate of cells is recognized, and how hid expression is induced. One possibility might be that the protein gradients established by bicoid+ and oskar+, as well as other segmentation genes are used as readout for proper cellular specification. The steepness of protein gradients as a means to determine life or death decisions has recently been proposed. Such a model would imply that cells are able to determine their position in a graded field and compare this readout with their neighbors. Because in bicoid and oskar mutants these gradients do not form, the concentration difference between neighboring cells would be zero. If the concentration difference between two neighboring cells is below a crucial threshold, they induce the expression of hid and undergo cell death. This model could also explain embryonic pattern repair, which was described in embryos that express six copies of the bicoid gene. In these embryos, the head and thorax primordia are expanded because of the presence of six copies of bicoid. However, this expansion is corrected for by induction of cell death, and relatively normal larvae develop. In this case, the Bicoid protein gradient does form, but would be flatter compared with wild type. Thus, the concentration difference between neighboring cells would be below a critical threshold, sufficient to induce hid-dependent cell death. However, it is largely unknown how cells compare their position in a graded field with those of their neighbors. It has been proposed that short-range cell interactions mediated via the cell-surface proteins Capricious and Tartan provide cues that support cell survival during wing development. Cells unable to participate in these interactions are eliminated by cell death. It is unclear, however, whether short-range interactions are sufficient to explain the cell death phenotype in bicoid and oskar mutants (Werz, 2005).

Irrespective of the underlying mechanism for sensing mis-specification, the current results highlight the role of an active gene-directed process that removes mis-specified cells during development. However, if a survival mechanism is provided, mis-specified cells can survive and adopt a different fate. In wild-type embryos, mis-specification probably occurs in cells in isolation, and hence is difficult to study. However, in bicoid and oskar mutants, large regions of neighboring cells are mis-specified and undergo cell death simultaneously, providing a unique opportunity to clarify the signals that initiate cell death in situations where cells are developmentally mis-specified (Werz, 2005).


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bicoid: Biological Overview | Evolutionary Homologs | Regulation | Targets of Activity | Protein Interactions | Miscellaneous Interactions | Developmental Biology | Effects of Mutation

date revised: 30 April 2015

 

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