Abstract
A major driving force behind the evolution of species-specific traits and novel structures is alterations in gene regulatory networks (GRNs). Comprehending evolution therefore requires an understanding of the nature of changes in GRN structure and the responsible mechanisms. 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. 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. 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. Both GRNs serve to highlight the ways in which redeployment of trans-acting factors can lead to GRN rewiring and network co-option. To gain insight into GRN evolution, we discuss the importance of defining GRN architecture at multiple levels both within and between species and of utilizing a range of complementary approaches.
Highlights
The nodes of a gene regulatory networks (GRNs) consist of genes and their cis-regulatory modules (CRMs), which control the spatio-temporal patterns of gene expression, while trans-acting transcription factors (TFs) and signaling pathways serve as the network “edges”
GRN evolution has been well studied in the genus Drosophila, and much of our understanding of modularity in regulatory evolution comes from studies in Drosophila species
A comprehensive approach in which an identical set of genetic and sequence-level assays was applied to a carefully selected range of species would be a benefit in this regard, to allow a clearer view of the cis- and trans-changes responsible for evolution of yellow regulation and the pigmentation GRN
Summary
Gene regulatory networks (GRNs) provide a potent framework for conceptualizing the interactions of the genes, regulatory proteins, and signaling pathways that comprise the coordinated gene expression programs at the root of both embryonic and postembryonic development [1,2,3,4]. 1. Gene regulatory network showing conserved kernels and both shared and species-specific. 1. Gene regulatory network showing conserved kernels and both shared and species-specific subcircuits. Purple genes and linkages are unique to sea urchin, while green is specific to sea stars, and black are those in common. Each species would have distinctthese morphological andtraits otherinphenotypic their homology is an additional important step in defining the mechanisms responsible for a given GRN’s evolutionary changes [8], as elegantly shown in the extensive comparison of the GRNs responsible for sea urchin and sea star endomesoderm specification [7,9]. While there are entire subcircuits that are specific to sea urchins, there are common subcircuits and highly conserved kernels (Figure 1). We discuss how studies of GRN evolution can be enhanced by having a broad toolbox of both traditional and contemporary methods and a perspective from multiple GRN levels for GRNs of similar function from a range of closely and more distantly related species
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