Meeting Abstract
Transcriptome and genome data offer new approaches to examine the origin and evolution of the chordate body plan. Chordate body plan evolution has been studied by comparing two closely related ascidian species with radically different larval body plans—the tailed Molgula oculata and the tailless M. occulta. Embryos of tailed M. oculata have 40 notochord cells that converge and extend to form the notochord in the center of the tadpole larva’s tail. Muscle cells flank the notochord in the tail of M. oculata and are critical for larval swimming. In the head is the otolith, a gravity-sensing organ that is important for larval settlement at metamorphosis. In contrast, the larva of M. occulta does not have a tail or an otolith. The embryo has only 20 notochord cells, and these cells do not converge and extend during larval development, but they do form a “notoball”. We show by transcriptome analyses that the ascidian metamorphosis program begins earlier in molgulid ascidians than in other ascidian species. This radical heterochronic shift has been documented in another tailless ascidian, Molgula tectiformis, and is now reported for both the tailed, Molgula oculata and tailless Molgula occulta. Further functional data will be needed to test the hypothesis that this pronounced heterochrony is a preadaptation for evolution of tailless development in molgulid ascidians. These studies will also facilitate the identification of the genes involved in initiating metamorphosis in ascidian tadpole larvae.
