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Zygotically transcribed genes

Gap Genes and Pair Rule Genes

  • Pair rule genes: the origin of the segment-polarity pattern
  • Dynamic maternal gradients control timing and shift-rates for Drosophila gap gene expression
  • An enhancer's length and composition are shaped by its regulatory task
  • Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation
    Gap genes

    Pair rule genes

    Unclassified genes involved in regulation of gap and pair rule genes

    Pair rule genes: the origin of the segment-polarity pattern
    The pair-rule class of segmentation genes was identified in a Nobel award winning genetic screen, performed in the late 1970s by Christiane Nusslein-Volhard, Eric Wieschaus and co-workers. Pair-rule mutants affect homologous parts of the cuticle pattern in alternate segments, resulting in the so-called "double-segment" periodicity of defects. As noted by Nusslein-Volhard and Wieschaus (1980) in their original report, the pair-rule class of segmentation genes was unexpected; the presence of this class suggests that during development, the embryo initially has a repeat unit corresponding to two segments, which are later subdivided into individual segments. Since their discovery, all eight 'classical' pair-rule genes (even-skipped, hairy, runt, fushi-tarazu, odd-skipped, paired, odd-paired, and sloppy-paired) have been studied at the genetic and molecular levels. Each gene encodes a transcription factor (Pankratz, 1993) that is expressed in regions corresponding to each gene's domains of function, as deduced from the gene's mutant phenotype.

    A hallmark of the pair-rule genes is their striped pattern of expression at the end of the cellular blastoderm stage. Interestingly, analysis of pair-rule gene expression during early cellularization indicates that, as a class, the pair-rule genes are initially expressed in domains larger than their 'final' pair-rule pattern. In fact, the products of most pair-rule genes initially accumulate throughout the entire metameric region. During cellularization, pair-rule stripes are formed by a combination of events: accumulation of products within the stripes and loss of pair-rule products in the intervening interstripes (Carroll, 1990). Formation of the interstripes must reflect a combination of transcriptional repression coupled with the turnover of gene products, whereas, establishment of stripes must reflect continued (and possibly increased) expression. Since the equivalent sequence of stripe patterns is seen for pair-rule mRNA and protein, neither translational regulation nor differential rates of protein turnover (relative to mRNA) appear likely to have a s ignificant influence on pair-rule pattern. This suggests that as a class, the pair-rule genes share a common mechanism for the rapid turnover of their gene products. Since the pair-rule genes are posited to function in discrete domains of the embryo, determining how the pair-rule gene products are specifically synthesized and degraded (or turned-over) will be important for a thorough understanding of how this class functions during development.

    Interestingly, although the pair-rule genes were identified genetically by their pattern defects in alternate segments, this class of segmentation genes is expressed in a wide variety of tissues during embryogenesis. One of the most fascinating examples of unexpected expression subsequent to cellularization is the 'segment-polarity' pattern of expression observed with gene products from the even-skipped, runt, odd-skipped, paired and odd-paired loci. In addition, select pair-rule genes are expressed in the mesoderm, gut and most notably the central nervous system.

    During segmentation, the pair-rule genes function as intermediates between the nonperiodic expression of gap genes and the repeated expression patterns of the segment-polarity genes. Understanding how each pair-rule gene functions within the segmentation hierarchy involves addressing at least three questions:

    1. What genes are required for the correct expression of a particular pair-rule gene?
    2. For a given pair-rule gene, what are its downstream targets?
    3. What other pair-rule genes, if any, interact with an individual pair-rule gene in regulating these targets?

    Initially, it was proposed that the gap genes were the sole regulators of pair-rule expression, and in turn, the segment-polarity genes were thought to be the sole targets of pair-rule function. However, studies have shown that some pair-rule genes are regulated by other members of the pair-rule class, suggesting that the pair-rule class contains both regulators and targets of pair-rule genes (Pankratz, 1993). According to this view, only a subset of pair-rule genes (even-skipped, hairy, and runt) respond directly to gap information. These so-called "primary" pair-rule genes regulate the expression of the remaining so-called "secondary" pair-rule genes (fushi-tarazu, odd-skipped, paired, odd-paired, and sloppy-paired), which in turn play a more direct role in the establishment of segment-polarity gene expression. While this probably over-simplifies the nature of these interactions, it is clear that understanding the role of a particular pair-rule gene in the segmentation hierarchy requires not only an analysis of the individual gene's role as a mediator between the gap and segment-polarity classes, but also requires identifying its function within the pair-rule class.

    A well-characterized target of pair-rule function is the segment-polarity gene engrailed (en), which is expressed in 14 stripes during gastrulation and at all subsequent stages of embryogenesis. Each engrailed stripe marks the posterior margin of a segment, or the anterior margin of a parasegment. Correct establishment of the 14 engrailed stripes requires the activities of all the pair-rule genes (DiNardo, 1987). However, in general, mutations in individual pair-rule genes affect either odd- or even-numbered engrailed stripes. Thus, the correct establishment of alternate engrailed stripes requires the combined activities of a subset of pair-rule genes.

    How do these combinations of pair-rule activities regulate engrailed? At the time when en is initially expressed, the various pair-rule genes are present in distinct stripes that frequently overlap. It is widely held that the resulting overlapping stripes can be translated into vertical rows of cells containing unique combinations of pair-rule gene products capable of interacting with one another to establish the correct pattern of en and other segmentation genes (DiNardo, 1987). According to this model, overlapping stripes of activators (e.g. Fushi-tarazu) and repressors (such as Odd-skipped) define the narrow engrailed stripes.

    While this 'combinatorial model' provides a basis for understanding how narrow engrailed stripes are specified by broad pair-rule stripes, it does not define the precise interactions between the individual pair-rule genes. For example, Odd-skipped (Odd), a repressor of en, and Fushi-tarazu (Ftz), an activator of en, are proposed to interact with one another to specify the even-numbered en stripes. Two hypotheses have been proposed to account for the correct positioning of a narrow en stripe within a broad Ftz stripe. In the Hierarchy model, en responds to a critical threshold level of Ftz activity within the Ftz stripe (Lawrence, 1989). In this model, Odd and Ftz are proposed to interact in a hierarchy, with Odd protein functioning to repress ftz, presumably at the transcriptional level (Mullen, 1995). A key feature of this model is that Odd represses en indirectly by reducing Ftz levels. In the alternate view, the Combinatorial model, the presence of a negative regulator of en prevents en expression within portions of the Ftz stripe. According to this model, Odd is proposed to interact in a combinatorial (parallel) manner with Ftz. The distinguishing feature of this model is that Odd does not affect en by altering Ftz levels; rather, Odd acts to prevent en activation within the Ftz stripe. Recent experimental data (see Odd) strongly supports the Combinatorial model of interactions between Odd and Ftz (Ward, 1997 and Ward and Coulter, manuscript in prep.). Thus, in this case, two "secondary" pair-rule genes appear to act independently of one another in order to define precisely the even-numbered en stripes. Interestingly, the primary pair-rule gene even-skipped (eve) appears to directly regulate ftz and odd to define the anterior margins of these two secondary pair-rule genes, thereby allowing en to be activated in a narrow stripe (Ward, 1997, Ward and Coulter, manuscript in prep., Fujioka, 1995 and Manoukian, 1992).

    This essay courtesy of and copyright © 1997, Ellen J. Ward

    Dynamic maternal gradients control timing and shift-rates for Drosophila gap gene expression

    This study simulated dynamic morphogen interpretation by the gap gene network in Drosophila. Gap genes are activated by maternal morphogen gradients encoded by bicoid (bcd) and caudal (cad). These gradients decay at the same time-scale as the establishment of the antero-posterior gap gene pattern. This study used a reverse-engineering approach, based on data-driven regulatory models called gene circuits, to isolate and characterise the explicitly time-dependent effects of changing morphogen concentrations on gap gene regulation. To achieve this, the system was simulate in the presence and absence of dynamic gradient decay. Comparison between these simulations reveals that maternal morphogen decay controls the timing and limits the rate of gap gene expression. In the anterior of the embyro, it affects peak expression and leads to the establishment of smooth spatial boundaries between gap domains. In the posterior of the embryo, it causes a progressive slow-down in the rate of gap domain shifts, which is necessary to correctly position domain boundaries and to stabilise the spatial gap gene expression pattern. A newly developed method was used for the analysis of transient dynamics in non-autonomous (time-variable) systems to understand the regulatory causes of these effects. By providing a rigorous mechanistic explanation for the role of maternal gradient decay in gap gene regulation, this study demonstrates that such analyses are feasible and reveal important aspects of dynamic gene regulation which would have been missed by a traditional steady-state approach. More generally, it highlights the importance of transient dynamics for understanding complex regulatory processes in development (Verd, 2017).

    An enhancer's length and composition are shaped by its regulatory task

    Enhancers drive the gene expression patterns required for virtually every process in metazoans. It is proposed that enhancer length and transcription factor (TF) binding site composition-the number and identity of TF binding sites-reflect the complexity of the enhancer's regulatory task. In development, regulatory task complexity is defined as the number of fates specified in a set of cells at once. It is hypothesized that enhancers with more complex regulatory tasks will be longer, with more, but less specific, TF binding sites. Larger numbers of binding sites can be arranged in more ways, allowing enhancers to drive many distinct expression patterns, and therefore cell fates, using a finite number of TF inputs. This study compared ~100 enhancers patterning the more complex anterior-posterior (AP) axis and the simpler dorsal-ventral (DV) axis in Drosophila and found that the AP enhancers are longer with more, but less specific binding sites than the (DV) enhancers. Using a set of ~3,500 enhancers, enhancer length and TF binding site number were found to increase with increasing regulatory task complexity. Therefore, to be broadly applicable, computational tools to study enhancers must account for differences in regulatory task (Li, 2017).

    Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation

    Drosophila segmentation is a well-established paradigm for developmental pattern formation. However, the later stages of segment patterning, regulated by the "pair-rule" genes, are still not well understood at the system level. Building on established genetic interactions, a logical model of the Drosophila pair-rule system was constructed that takes into account the demonstrated stage-specific architecture of the pair-rule gene network. Simulation of this model can accurately recapitulate the observed spatiotemporal expression of the pair-rule genes, but only when the system is provided with dynamic "gap" inputs. This result suggests that dynamic shifts of pair-rule stripes are essential for segment patterning in the trunk and provides a functional role for observed posterior-to-anterior gap domain shifts that occur during cellularisation. The model also suggests revised patterning mechanisms for the parasegment boundaries and explains the aetiology of the even-skipped null mutant phenotype. Strikingly, a slightly modified version of the model is able to pattern segments in either simultaneous or sequential modes, depending only on initial conditions. This suggests that fundamentally similar mechanisms may underlie segmentation in short-germ and long-germ arthropods (Clark, 2017).



    References

    Carroll, S. B. (1990). Zebra patterns in fly embryos: activation of stripes or repression of interstripes? Cell 60: 9-16. PubMed Citation: 2403844

    Clark, E. (2017). Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation. PLoS Biol 15(9): e2002439. PubMed ID: 28953896

    DiNardo, S. and O'Farrell, P. (1987). Establishment and refinement of segmental pattern in the Drosophila embryo: spatial control of engrailed expression by pair-rule genes. Genes & Development 1: 1212-25. PubMed Citation: 3123316

    Fujioka, M., et al. (1995). Early even-skipped stripes act as morphogenetic gradients at the single cell level to establish engrailed expression. Development 121 (12): 4371-82. PubMed Citation: 8575337

    Lawrence, P. A. and Johnston, P. (1989). Pattern formation in the Drosophila embryo: allocation of cells to parasegments by even-skipped and fushi tarazu. Development. 105: 761-7. PubMed Citation: 2598812

    Li, L. and Wunderlich, Z. (2017). An enhancer's length and composition are shaped by its regulatory task. Front Genet 8: 63. PubMed ID: 28588608

    Manoukian, A. and Krause, H. (1992). Concentration-dependent activities of the even-skipped protein in Drosophila embryos. Genes & Development. 6: 1740-51. PubMed Citation: 1355458

    Mullen, J. R. and DiNardo, S. (1995). Establishing parasegments in Drosophila embryos: roles of odd-skipped and naked genes. Dev. Biol. 169: 295-308. PubMed Citation: 7750646

    Nusslein-Volhard, C. and Wieschaus, E. (1980). Mutations affecting segment number and polarity in Drosophila. Nature 287: 795-801. PubMed Citation: 6776413

    Pankratz, M. J. and Jackle, J. (1993). Blastoderm segmentation. In The Development of Drosophila melanogaster. M. Bate and A. M. Arias, eds. Cold Spring Harbor Lab Press, Vol. 1. pp. 467-516

    Verd, B., Crombach, A. and Jaeger, J. (2017). Dynamic maternal gradients control timing and shift-rates for Drosophila gap gene expression. PLoS Comput Biol 13(2): e1005285. PubMed ID: 28158178

    Ward, E.J. Dissertation. Characterization of Odd-skipped Protein Pattern of Accumulation During Embryogenesis in D. Melanogaster. 1997. St. Louis University, St. Louis, Missouri.

    Ward, E.J. and Coulter, D.E. Interactions among odd-skipped, fushi-tarazu and even-skipped define the even-numbered engrailed stripes in the Drosophila embryo (Personal communication to The Interactive Fly)




    Zygotically transcribed genes

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