JEFFERY, J.E.*; BININDA-EMONDS, O.R.P.; COATES, M.I.; RICHARDSON, M.K.: Heterochrony in Amniote Evolution

Heterochrony is thought to be a major link between developmental mechanisms and the emergence of phenotypic difference above the species level (macroevolution). Heterochronies affecting the timing of discrete events are of particular importance during organogenesis. Within vertebrates, this is the mid-embryonic period when the body plan is specified under the control of regulatory genes, and when major organ primordia develop. Previous studies have identified putative heterochronic changes (e.g. in somite formation) during this key period, but these have not been quantified or tracked systematically across evolutionary history. In part, this is due to the difficulties inherent in analysing comparative developmental data. We have developed a new protocol, based on ‘event-pairing’, for quantifying heterochrony and mapping it across the evolution of different animal groups. It improves upon earlier event-pairing methods by adding an en bloc comparison of the shifts along each branch of the tree. This can determine the shifts of individual events relative to all other developmental events surveyed, and can identify the stronger trends. Using data from vertebrates, we track timing shifts in embryonic organ development. These include delayed eye development in mammals and advanced heart development within the amniotes. We explore possible functional and adaptive explanations for these shifts (e.g. the evolution of endothermy). We also subject the data to a parsimony analysis to identify instances of convergence (homoplasy). In future, our approach may be adapted to analyse other forms of comparative dynamic data.