Most invertebrate phyla contain species which include a larval phase in their life cycle. In certain instances, however, there has been a tendancy to modify or eliminate the larval phase. Little is known about how modifications in development arise during evolutoin. In this proposal, a model system is developed and exploited to study the mechanisms underlying evolutionary changes in development. This model system consists of two closely related species of ascidians that exhibit different larval forms and are capable of interspecific hybridizatrion. Molgula oculata (the urodele species) develops a conventional tailed (or urodele) larva, which contains brain pigment cells, a notochord, and tail muscle cells. In contrast, Molgula occulta (the anural species) develops a tailless (or anural) larva, which lacks brain pigment cells, a notochord, and differentiated muscle cells. Hybrids produced by fertilizing eggs of the anural speies with sperm of the urodele species develop a brain pigment cell and notochrod but do not form differentiated muscle cells, suggesting that both maternal and zygotic changes have occured during the evolution of anural development. This proposal combines embryological and molecular approaches 1) to characterize the maternal and zygotic changes involved in the switch from urodele to anural larval development, 2) to examine the molecular mechanisms responsible for lack of muscle cell differentiation in the anural species and 3) to determine the phylogenetic realtionship of anural and urodele ascidians. The results are expected to provide a more comprehensive understanding of the mechanisms underying evolutionary changes in development.
