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Diverse gene regulatory mechanisms alter rattlesnake venom gene expression at fine evolutionary scales

ABSTRACT: Understanding and predicting the relationships between genotype and phenotype is often challenging, largely due to the complex nature of eukaryotic gene regulation. A step towards this goal is to map how phenotypic variation evolves through genomic changes that modify gene regulatory interactions. Using the Prairie Rattlesnake (Crotalus viridis) and related species, we integrate mRNA-seq, proteomic, ATAC-seq and whole genome resequencing data to understand how specific evolutionary modifications to gene regulatory network components produce variation in venom gene expression. Through comparisons within and between species, we find a remarkably high degree of gene expression and regulatory network variation across even a shallow level of evolutionary divergence. We use these data to test hypotheses about the roles of specific trans-factors and cis-regulatory elements, how these roles may vary across venom genes and gene families, and how variation in regulatory systems drive variation in venom phenotypes. Our results illustrate that variation in chromatin and genotype at regulatory elements plays major roles in modulating expression. However, we also find that enhancer deletions, variation in transcription-factor expression, and variation in activity of the insulator protein CTCF also impact downstream venom phenotypes. Our findings provide insight into the diversity and gene-specificity of gene regulatory features and highlight the value of comparative studies to link gene regulatory network variation to phenotypic variation.

ORGANISM(S): Crotalus concolor Crotalus viridis Crotalus lutosus

PROVIDER: GSE254420 | GEO | 2024/07/17

REPOSITORIES: GEO

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Project description:Robustness to perturbation, or canalization, is a fundamental and required feature of complex organisms.1 Mutations are the raw material for evolution, but robustness to their effects is required for organisms to tolerate mutational loads.1 Similarly, robustness to environmental perturbations, or limiting environmental responses to an adaptive range, is necessary for organisms to tolerate environmental variation. Robustness is heritable2-4, but the mechanisms that produce it are poorly understood. Explanations range from the role of specific processes such as heat shock proteins to more general embedded features of developmental systems. Nonlinearities are an embedded, ubiquitous feature of development that may modulate how variation in development relates to phenotypic robustness.5,6 Here, we show that variation in Fibroblast Growth Factor 8 (Fgf8) expression across an allelic series has a nonlinear relationship to phenotypic variation, predicting the magnitude of variation within genotypes. These differences in phenotypic variance are independent of genetic variance and so reflect differential robustness to minor environmental variation. Differences in robustness are not related to differences in the variance of gene expression within genotypes. However, mean gene expression levels do vary across genotypes, particularly in genes downstream to FGF8 signaling. Gene expression changes thus explain the genotype-phenotype curve but not the changes in robustness along the curve. This suggests that mechanisms above the gene-regulatory network are important determinants of robustness. Our results show that nonlinearities in developmental mechanisms can persist from genotype to phenotype to produce variation in robustness between genotypes. This suggests that embedded features of development rather than specific canalizing mechanisms explain robustness. How such features vary among individuals in natural populations and relate to genetic variation more generally are key questions for unravelling the origin and evolvability of this fundamental feature of organismal development.

Project description:Elucidating the molecular mechanisms underlying snake venom variability provides important clues for understanding how the biological functions of this powerful toxic arsenal evolve. We analyzed in detail individual transcripts and venom protein isoforms produced by five specimens of a venomous snake (Bothrops atrox) from two nearby but genetically distinct populations from the Brazilian Amazon rainforest which show functional similarities in venom properties. Individual variation was observed among the venoms of these specimens, but the overall abundance of each general toxin family was conserved both in transcript and in venom protein levels. However, when expression of independent paralogues was analyzed, remarkable differences were observed within and among each toxin group, both between individuals and between populations. Transcripts for functionally essential venom proteins (“core function” proteins) were highly expressed in all specimens and showed similar transcription/translation rates. In contrast, other paralogues (“adaptive” proteins) showed lower expression levels and the toxins they coded for varied among different individuals. These results provide support for the inferences that (a) expression and translational differences play a greater role in defining adaptive variation in venom phenotypes than does sequence variation in protein coding genes and (b) convergent adaptive venom phenotypes can be generated through different molecular mechanisms. Significance: Analysis of individual transcripts and venom protein isoforms produced by specimens of a venomous snake (Bothrops atrox), from the Brazilian Amazon rainforest, revealed that transcriptional and translational mechanisms contribute to venom phenotypic variation. Our finding of evidence for high expression of toxin proteins with conserved function supports the hypothesis that the venom phenotype consists of two kinds of proteins: conserved “core function” proteins that provide essential functional activities with broader relevance and less conserved “adaptive” proteins that vary in expression and may permit customization of protein function. These observations allowed us to suggest that genetic mechanisms controlling venom variability are not restricted to selection of gene copies or mutations in structural genes but also to selection of the mechanisms controlling gene expression, contributing to the plasticity of this important phenotype for venomous snakes.

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Diverse gene regulatory mechanisms alter rattlesnake venom gene expression at fine evolutionary scales

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