Developmental Mechanisms of Body Plan Evolution

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Raff 9724003 Although there are several million species of animals, they fall into groups representing 35 basic body plans (phyla). The phyla appeared in the Cambrian radiation, and have existed for over a half billion years. The study of the evolution of development has revealed two important and unexpected observations. The first is that radically different developmental modes can evolve rapidly among closely related taxa. The second is that master regulatory genes are highly conserved, even among morphologically widely disparate phyla. These results suggest that developmental reorganization has occurred largely by co-option of existing genes and pathways, and that we can exploit the expression of conserved regulatory genes to determine how body plan transformations took place. This study investigates the evolution of the pentameral body plan of echinoderms with the bilaterally symmetric body plan of their sister group, the chordates. This proposal focuses on the transformations that have converted a bilaterally symmetric body plan into a pentameral one, and on finding shared processes that govern axial determination in disparate phyla. The proposed work will seek to establish a potential echinoderm homolog to the notochord in the induction of axial structures. The hypothesis that such a homolog exists can be confirmed by defining patterns of expression of echinoderm homologs of vertebrate axial determination genes. The project will exploit a sea urchin species that develops directly into a juvenile adult in only four days. Genes involved in specification of vertebrate anterior-posterior and dorsal-ventral axis induction pathways will be isolated from that sea urchin. The project will characterize spatial expression patterns of the genes to infer echinoderm homologs to vertebrate axial features. The function of axial determination genes will be tested by over/ectopic expression experiments. Finally, the project will determine the role of induction processes in axial determination in echin oderms by experimental embryology and use of gene markers. This testing of inductive processes will further test the potential homologies of developmental processes in two morphologically distinct but related phyla. The results of this project will provide an insight into the evolutionary modifications of developmental processes, and into the role of developmental processes in constraining the course of morphological evolution.