Meeting Abstract
Transcriptomic and genomic data offer exciting new approaches to examine the genetic networks underlying the origin and evolution of the chordate body plan. We study two closely related tunicate species with very divergent 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 center of the tail of 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 with a single pigmented cell. The tailless M. occulta do not form a tail in their larval stage and lack the otolith, and the pigment cell associated with it. We have sequenced the genomes and analyzed developmental transcriptomes for both species and the hybrid embryos. Hybrid embryos made from the sperm of the tailed, Molgula oculata and the egg of the tailless Molgula occulta undergo convergence and extension of the notochord to form a short tail and, in some cases, also a pigmented otolith. We have shown that the muscle genes and tyrosinase pigment genes are pseudogenes in the tailless M. occulta species, but in some cases, the mutated transcripts are found in the transcriptome. We have also found examples where the tailless genes are intact, but show different expression levels in the hybrids, suggesting that there are changes in the cis regulation of the genes. We are continuing analyses of gene expression in the parental species and also in hybrid embryos in an effort to understand the evolution of the genetic networks necessary for tadpole larval development in ascidian embryos.
