JACOBS, David/K; HUGHES, Nigel/C; WINCHELL, Christopher/J; UCLA; UC Riverside; UCLA: Terminal Addition and the Evolution of Bilaterian Form

Diverse bilaterian taxa, including representative lophotorchozoa, ecdysozoa and deuterosomes, share aspects of a developmental process where repeated pattern elements are added posteriorly during differentiation. Developmental genetic data suggest that this process of �terminal addition� derives from a shared ancestral mode of development. However, terminal addition is not an obvious feature of development in many bilaterian lineages, leading to a prediction of multiple modifications of terminal addition in the early Paleozoic radiation of Bilateria. This then leads to a framework for interpretation of the evolution of bilaterian development. We integrate information relating to the serial organization, terminal addition process and developmental type in modern taxa. These data, traced as characters on phylogenetic trees, permit assessment of the polarity of evolutionary change and demonstrate multiple avenues of evolutionary modification of terminal addition. Eight separate modifications of terminal addition are identified, for example, series of terminally added elements can be truncated, the discreteness of serial elements can be reduced or lost, or the number of axes can be multiplied. Incorporation of fossil data permits assignment of timing to these changes. In addition detailed study of lineages in the fossil record where ontogenetic series are preserved demonstrate trends in change in the terminal addition process- often towards greater stereotypy in number of serial elements. A similar examination of variation in developmental genetic mechanisms associated with the terminal addition should illuminate how evolution has modified these developmental systems.