Meeting Abstract

76.1  Monday, Jan. 6 08:00  Molgulid Ascidians have a Radical Heterochronic Shift in their Metamorphic Gene Network MALISKA, M.; LOWE , E.; WEBER, C.; STOLFI, A.; PEYRIERAS, N.; CHRISTIAEN, L.; BROWN, C.T.; SWALLA, B.J.*; Univ. of Washington, Seattle; Michigan State University, East Lansing; Univ. of Washington, Seattle; New York University, New York; Institut de Neurobiologie Alfred Fessard, Gif sur Yvette; New York University, New York; Michigan State University, East Lansing; Univ. of Washington, Seattle bjswalla@uw.edu

Transcriptome and genomic data offer an exciting new approach to examine the origin and evolution of the chordate body plan. One system for studying chordate body plan evolution are two tunicate species with radically different larval body plans—the tailed ascidian Molgula oculata and the tailless M. occulta. Tailed M. oculata embryos, like most solitary ascidians, have 40 notochord cells that are converged and extended in the tadpole larvae. The larvae also have tail muscle cells flanking the notochord in the tail, and, in the head, an otolith, a gravity sensory organ. 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, M. tectiformis, and is now reported for both the tailed, M. oculata and tailless M. occulta. Furthermore, both species have already formed siphons at the time of hatching, so morphologically metamorphosis has began as well. 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.