USING FOSSILS AND PHYLOGENIES TO DATE THE TIMING OF KEY GENE REGULATORY NETWORK INNOVATIONS: AN EXAMPLE USING ECHINOIDS

Interdisciplinary studies provide one of the best opportunities to understand the mechanisms and timing of key evolutionary innovations. Molecular developmental biology lends the ability to understand the genetic underpinning behind the origin of evolutionary novelties through the study of Gene Regulatory Networks (GRN’s). GRN’s are the system of genetic interactions responsible for the development and formation of disparate body plans and morphologies. The resultant products of the interactions of GRN’s are thus visible in the rock record in the form of fossil morphologies. Fossil data has wide applicability for interdisciplinary studies, as fossils show us direct evidence of past morphologies, and bring with them the element of evolutionary time. Because fossils show us direct evidence for the outcome of genetic interactions taking place in GRN’s, they allow us to infer the activity and existence of portions of GRN topologies in the fossil record, and thus date the timing of these genetic evolutionary innovations. Using paired fossil data and phylogenetic analyses, we have been able to date the appearance of a novel piece of GRN circuitry essential for skeletogenesis in larval echinoids, the Double-Negative Gate. The Double-Negative Gate has been demonstrated to be present in a number of phylogenetically diverged euechinoid echinoids, however, is absent in their sister clade, the cidaroids. Using well-dated fossil taxa and a robust phylogeny for echinoids, we are able to date the appearance of this innovation to at least the Permian period, well before the end-Permian mass extinction. This paleogenomic approach has wide utility for integration of fossil and molecular developmental data in a number of taxa, and we hope to promote the utility of fossils in molecular developmental research.