Meeting Abstract
Ascidians are invertebrate chordates that share a number of developmental features in common with the vertebrates including a branchial basket, an endostyle, and a notochord inside a functional swimming larval tail. During typical ascidian development 10 notochord precursor cells divide twice to make 40 cells and then converge and swell to extend out a functional swimming tail. The Molgulids are a monophyletic clade of ascidians in which a tailless phenotype has evolved multiple times independently. Here we investigate the differential gene expression of two species of molgulids, Molgula oculata and Molgula occulta. Molgula oculata, has the tailed phenotype, but its sister species, Molgula occulta, has lost the tail and notochord, developing 20 notochord cells that do not converge and extend. The two species can be hybridized: if the egg of the tailless species is used, then some of the resulting hybrids have 20 notochord cells that do converge and extend into a short, non-functional half tail. We have sequenced the genomes and embryonic developmental transcriptomes of these two sister species as well as hybrid embryos, and are searching for differential gene expression of known notochord related genes to attempt to identify the developmental changes responsible for the loss of the tailed phenotype. We are using the transcriptomes to identify the potential genes, then using qPCR to verify expression and in situ hybridization to visually confirm spatio-temporal expression. Thus far VASA in situ hybridization has revealed variation in cellular positions as early as gastrulation and premature cell divisions of the germ line progenitors after neurulation in the tailless M. occulta embryos.
