Both GRNs serve to highlight the ways in which redeployment of trans-acting factors can lead to GRN rewiring and network co-option. The pigmentation GRN for butterflies of the Heliconius species group is less fully detailed but it is emerging as a useful model for exploring important questions about redundancy and modularity in cis-regulatory systems. The pigmentation GRN in Drosophila melanogaster and other drosophilids is a well-defined network for which studies from closely related species illuminate the different ways co-option of regulators can occur. Here, we review two insect pigmentation GRNs in order to examine common themes in GRN evolution and to reveal some of the challenges associated with investigating changes in GRNs across different evolutionary distances at the molecular level. Comprehending evolution therefore requires an understanding of the nature of changes in GRN structure and the responsible mechanisms. cis-regulatory module (CRM) level control of gene expression, restricts splice variants, interaction partners, and modifying proteins to distinct spatiotemporal contexts.Ī major driving force behind the evolution of species-specific traits and novel structures is alterations in gene regulatory networks (GRNs). Likewise, post-translational modifications are important for altering transcription factor modularity, and are context specific owing to the requirement of co-expression with a modifying enzyme. However, both interaction partners must be present to exert function, which means that these interactions can be controlled by limiting expression domain (C). Protein-protein interactions are particularly important to transcription factor function, since this ability determines whether the protein can successfully alter chromatin or recruit RNA polymerase. Here, the version with the purple exon may have different functional abilities than the all blue version. Alternate splicing can lead to tissues that differ in the version of a transcription factor. Alternative splicing, protein-protein interactions, and post-translational modifications also increase transcription factor diversity, but these mechanisms also offer context specificity. Specificity for the blue site could change without altering functions governed by the red site. Here, the red homolog recognizes the red binding site, but the purple homolog can bind both red and blue binding sites. DNA binding can evolve in modular ways too. Exon shuffling allows transcription factors to evolve new function through acquisition of domains, shown here as a red exon swapped for blue exon. While gene duplicates are frequently lost, retention of both copies relaxes constraint and allows the paralogs to diverge through acquisition of mutations (indicated by purple ancestral copy splitting into red and blue versions). Gene duplication, exon shuffling, and modular DNA binding allow transcription factors to increase and change their functionality. Many of these mechanisms are modular and may be mixed and matched to offer even greater evolutionary flexibility. Mechanisms for generating transcription factor diversity and limiting novel function to specific contexts. Here, we review the recent works that have led to this unexpected change in the field of Evolution and Development (Evo-Devo) and consider the implications these studies have had on our understanding of the evolution of developmental processes. Just as cis-regulatory changes make use of modular binding site composition and tissue-specific modules to avoid pleiotropy, transcription factor coding regions also predominantly evolve in ways that limit the context of functional effects. A growing body of evidence suggests that changes to the coding regions of transcription factors play a much larger role in the evolution of developmental gene regulatory networks than originally imagined. Most studies of GRN evolution focus on changes to cis-regulatory DNA, and it was historically theorized that changes to the transcription factors that bind to these cis-regulatory modules (CRMs) contribute to this process only rarely. Genetically encoded modifications to these networks have generated the wide range of metazoan diversity that exists today. Developmental GRNs interpret maternally deposited molecules and externally supplied signals to direct cell-fate decisions, which ultimately leads to the arrangements of organs and tissues in the organism. The form that an animal takes during development is directed by gene regulatory networks (GRNs).
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