Meeting Abstract

51.4  Saturday, Jan. 5  Molgulid Ascidians have a Radical Heterochronic Shift in the Metamorphic Gene Network MALISKA, ME*; LOWE, E; WEBER, C; PIERCE, T; BROWN, CT; SWALLA, BJ; University of Washington; Michigan State University; University of Washington; Michigan State University; Michigan State University; University of Washington

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—the tailed ascidian Molgula oculata and the tailless M. occulta. Tailed M. oculata embryo have 40 notochord cells that are converged and extended, tail muscle cells flanking the notochord, and the otolith, a gravity sensory organ located in the head. The tailless M. occulta does not form a tail in their larval stage, and have only 20 notochord cells that do not converge and extend during larval development. We show by transcriptome analyses that the ascidian metamorphosis program begins earlier in molgulid ascidians. 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 is necessary to determine if this pronounced heterochrony is the necessary preadaptation for tailless tadpole to evolve in molgulid ascidians. However, we forecast that these studies will facilitate the elucidation of the metamorphic signal in ascidian tadpole larvae, which is still currently unknown.