Meeting Abstract
Transcriptome and genome data offer an exciting new approach to examine the origin and evolution of the chordate body plan. Chordate body plan evolution can be studied with two tunicate species with radically different larval body plans that are found sympatrically off the coast of Roscoff, France – the tailed ascidian Molgula oculata and the tailless M. occulta . Tailed M. oculata embryos have forty notochord cells in the center of the tail, muscle cells flanking the notochord in the tail, and in the head is the otolith, a gravity sensory organ. The tailless M. occulta does not form a tail in their larval stage, and have only twenty notochord cells that do not converge and extend during larval development. We have sequenced the genomes of these two species and a third species, M. occidentalis in collaboration with the Lionel Christiaen lab at NYU, and they are available on Aniseed. We show by transcriptome and in situ hybridization analysis that the notochord gene network is expressed at the right time and place in the tailless M. occulta embryos and larvae, although the notochord collapses into a “notoball” near the posterior. We show by transcriptome analyses that the ascidian metamorphosis program begins much earlier in molgulid ascidians, during early development. This radical heterochronic shift has been documented in another tailless ascidian, M. tectiformis , and is now reported for three additional species: the tailed molgulid species, M. oculata , M. occidentalis , and the tailless M. occulta. Further functional data is necessary to determine if this pronounced heterochrony is the necessary preadaptation for tailless tadpole to evolve in molgulid ascidians. We’ve shown that downstream genes become pseudogenes in muscle and the sensory otolith, suggesting that there is an initial upstream hit. This is an excellent model system to study the evolution of gene networks underlying morphology.
