Developmental Mechanisms of Body Plan Evolution

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Although there are several million species of animals, they fall into groups representing only 35 basic body plans (phyla). This proposal is to study development of the unusual body plan of adult echinoderms, which, because of the radical reorganization it has undergone, offers a unique model for body plan evolution. Echinoderms are related to chordates. Thier common ancestor was bilateral, but the echinoderm ancestor produced a new body plan by generation of its major body axis from the original ancestral left side. In addition, echinoderms evolved a pentameral, radial symmetry. The new body plan had to have involved a reorganization of development, and this is evident in the formation of the adult rudiment as an outgrowth of left-side tissues of the echinoderm larva. Unlike the model systems most used in studies of development, echinoderms transform their body plan in development, making a transformation from the bilateral symmetry of the larva to the pentameral radial symmetry of the adult. This developmental transformation parallels their evolutionary history. As the crucial steps in echinoderm evolution as well as in echinoderm development are the processes that produce left-right asymmetry and pentamery, the project will focus on these. Development of the echinoderm adult has been little studied. Heliocidaris erythrogramma, a sea urchin with a very large egg develops directly into a juvenile adult in only four days, and is highly amenable to study of adult development. The study will use the following approaches: (1) The mechanisms of development of the asymmetries will be studied by microsurgical techniques to investigate autonomous and local inductive interactions in this embryo. Tissue-specific gene markers will be used to analyze these processes in operated embryos. (2) The roles of regulatory genes involved in asymmetry generation in vertebrates will be investigated, and genes involved in development of asymmetry in H. erythrogramma identified. (3) The function of genes implicated in symmetry determination will be tested by over/ectopic expression experiments into appropriate cells of the H. erythrogramma embryo. The results of this project will provide an insight into the mechanisms that underlie evolutionary modifications of developmental processes, and will yield understanding of the development of asymmetries in embryos.