Chapter 7 - Evolution of Bilaterian Animals: Processes of Change and Stasis in Hierarchical Developmental Gene Regulatory Networks

Abstract

Evolution of the animal body plan is the outcome of change in the encoded genomic regulatory program for development. Major features of Phanerozoic animal evolution relate directly to developmental gene regulatory network (GRN) hierarchy. In this chapter, we consider rapid and continuously adaptive changes occurring at the species level, in terms of regulation of effector gene expression at the periphery of developmental GRNs. Developmental processes responsible for the generation of definitive characters of the body plan shared among all members of given phyla or classes occur at upper levels of GRN hierarchy. We discuss regulatory mechanisms accounting for the evolutionary stasis of such developmental characters. Evolutionary mechanisms operating at different levels of GRN hierarchy are fundamentally distinct, ranging from cis-regulatory adaptation at individual genes to co-optive redeployment of whole regulatory circuits. Conservation of regulatory circuitry within GRN hierarchy leads to an explanation for the nested organization of shared character sets underlying animal phylogeny.