The Comparative Experimental Embryology of Brachiopods

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9513068 Freeman One goal of the field of developmental biology is to understand how and why the utilization of the mechanisms responsible for the spatial organization of multicellular organisms changes during the course of evolution. This proposal will study this problem in the Phoronida and the related phylum, Brachiopoda. The Phoronida are unknown as fossils; however, because of their shell, brachiopods have an excellent fossil record which has allowed paleontologists to do a rather convincing job of reconstructing the history of this phylum. A number of extant brachiopod species are derived from, and closely resemble, ancestors that represent important nodes in this phylogeny. Experimental studies will be performed on the embryos of these species in order to establish when, during development, different parts of the embryo become specified and to identify the mechanisms that are responsible for regional specification. These experiments begin with the construction of a fate map for each embryo indicating the regions of hte larva that develop from specific parts of the egg and early embryo. Then diffenrent regions of the embryo will be isolated at various stages of development, and their ability to differentiate will be compared to that of the same region in a whole embryo as indicated by the fate map. By isolating parts of eggs and embryos in combination with other parts and at different developmental stages, one can infer whether the development of a given region is autonomous or conditional. By comparing the timing and mode of regional specification during embryogenesis for a series of species at different nodes in the phyylogeny, one can document the changes in the utilization of mechanisms for regional specification that accompany evolutionary change. Some of these species have larvae that are morphologically similar and have similar life histories although these species are not closely related, while other species are closely related but have very different larvae with different life history strategies. Comparisons beteen these species will allow one to dissect out the role of larval evolution versus the evolution of adult structures in bringing about developmental changes involving early embryogenesis. ***