The degenerate tale of ascidian tails

January 3 – Febuary 28, 2021

Meeting Abstract

S5-13  Tue Jan 5 19:00 – 19:30  The degenerate tale of ascidian tails Swalla, BJ; University of Washington

Ascidians are chordates, with a swimming chordate tadpole larva that has a distinct head and a tail. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve. There is one group of ascidians, the Molgulidae, where tailless larvae have evolved multiple times independently. My lab has been studying the evolution of tailless ascidian larvae in this clade for over thirty years and have shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of these two species reveals much synteny, but there have been insertions and deletions that disrupt genes in the tailless species, Molgula occulta. Genomics and transcriptomics show that there are a number of expressed pseudogenes in the tailless embryos, and hybrid features are due to the intact genes from the tailed species being expressed in the tailless species. Yet we find that the notochord gene network is essentially intact, although the notochord does not converge and extend and remains as notoball in the tailless embryos. We expect that eventually many of the larval gene networks will be lost in tailless ascidians and the larval body plan abandoned, so eggs will develop directly into an adult.

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