Origin of Metazoan Developmental Toolkits and Their Expression in the Fossil Record

Abstract

Developmental regulatory genes (largely transcription factors and signaling pathways) were once viewed as tightly connected to the origin of the morphological features with which they are associated in bilaterians. With the increased study of basal metazoans (sponges and cnidarians) as well as other eukaryotic clades, it is now clear that many of these highly conserved genes arose much earlier in evolution, and served different biological purposes. This provides a new view of the nature of developmental toolkits associated with the early origin of Metazoa: ancient regulatory genes were only later co-opted for the various developmental roles associated with bilaterian morphology. Here we review the nature of the toolkits at the origin of Metazoa, the Placozoan-Eumetazoan last common ancestor (LCA), the Cnidarian-Bilaterian LCA, and the Protostome-Deuterostome LCA. Integrating this data with recent molecular clock results and data on the fossil record reveals long macroevolutionary lags between the origin of the molecular toolkits and their developmental potential, and the origin of crown group morphologies as documented in the fossil record.