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Gene regulatory network architecture in different developmental contexts influences the genetic basis of morphological evolution

ABSTRACT: Convergent phenotypic evolution is often caused by recurrent changes at particular nodes in the underlying gene regulatory networks (GRNs). The genes at such evolutionary 'hotspots' are thought to maximally affect the phenotype with minimal pleiotropic consequences. This has led to the suggestion that if a GRN is understood in sufficient detail, the path of evolution may be predictable. The repeated evolutionary loss of larval trichomes among Drosophila species is caused by the loss of shavenbaby (svb) expression. svb is also required for development of leg trichomes, but the evolutionary gain of trichomes in the 'naked valley' on T2 femurs in Drosophila melanogaster is caused by the loss of microRNA-92a (miR-92a) expression rather than changes in svb. We used RNAseq of second (T2) pupal legs (20-28 h after puparium formation) to compare the expression of trichome GRN genes between strains with a large and a small naked valley. We also compared the function of components between the larval and leg trichome GRNs to investigate why the genetic basis of trichome pattern evolution differs in these developmental contexts. Moreover, we used ATAC-seq of second (T2) pupal legs (20-28 h after puparium formation) to identify the pupal leg enhancers of svb. We found key differences between the two networks in both the genes employed, and in the regulation and function of common genes. These differences in the GRNs reveal why mutations in svb are unlikely to contribute to leg trichome evolution and how instead miR-92a represents the key evolutionary switch in this context. Our work shows that variability in GRNs across different developmental contexts, as well as whether a morphological feature is lost versus gained, influence the nodes at which a GRN evolves to cause morphological change. Therefore, our findings have important implications for understanding the pathways and predictability of evolution.

ORGANISM(S): Drosophila melanogaster

PROVIDER: GSE113240 | GEO | 2018/04/18

REPOSITORIES: GEO

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Project description:Developmental programs are implemented by regulatory interactions between Transcription Factors (TFs) and their target genes, which remain yet poorly understood. While recent studies have focused on regulatory cascades of TFs that govern early development, little is known on how these are selected and controlled the ultimate cellular effectors of terminal differentiation. We addressed this question during late Drosophila embryogenesis when the finely tuned expression of a TF, Ovo/Shavenbaby (Svb), triggers the morphological differentiation of epidermal trichomes. We used chromatin immunoprecipitation and microarray profiling to identify Svb downstream target genes and show that Svb directly regulates a large set of terminal effectors of trichome formation. Functional assays delineated 18 Svb bound sequences driving specific expression of trichome effectors, with highly similar pattern and dynamics. Coupling computational modeling to functional dissection, we further investigated the regulatory logic of these enhancers. We find that these “terminal” enhancers harbor remarkable features with respect to their functional architectures. Trichome enhancers display weak if any clustering of Svb binding sites. Moreover, the in vivo function of each site relies on its intimate context, with a critical importance of the nucleotides flanking Svb binding sites. Finally, we identify additional cis-regulatory motifs, showing a broad diversity of positioning among trichome enhancers, and that critically contribute to their activity. Taken together, these results show that trichome formation is underpinned by unexpectedly diverse modes of regulation, and shed light on the functional architecture of enhancers governing a terminal differentiation program. Chromatin from 12-14 hour old embryos expressing svb tagged with GFP (Kondo T, Plaza S, Zanet J, Benrabah E, Valenti P, Hashimoto Y, Kobayashi S, Payre F, Kageyama Y: Small peptides switch the transcriptional activity of Shavenbaby during Drosophila embryogenesis. Science 2010, 329:336-339) was collected in duplicate. Chromatin was immunoprecipitated using an antiGFP antibody and input chromatin was used as a control.

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Gene regulatory network architecture in different developmental contexts influences the genetic basis of morphological evolution

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