Meeting Abstract
Ascidians share several features with the vertebrates including pharyngeal slits, an endostyle, a notochord and a muscular tail. The molgulids are a monophyletic clade of ascidians in which a tailless phenotype has independently evolved multiple times. We are searching for the molecular basis of this tail loss by investigating the differential gene expression of Molgula oculata and Molgula occulta. M. oculata has the tailed larval phenotype, but M. occulta has lost their tail and sensory organs. The two species can be hybridized: if the eggs of the tailless species are used, then some of the resulting hybrids extend a half-tail and the sensory pigment cell is re-expressed. We have sequenced the genomes and developmental transcriptomes of these two species and the hybrid, and are searching for the mechanisms responsible for the loss of the ascidian tailed phenotype. We are investigating cymric, a SHARK tyrosine kinase located in the myoplasm in M. oculata. The tailed M. oculata makes the full cymric transcript, but the tailless M. occulta cymric transcript is missing the tyrosine kinase domain, suggesting that it is nonfunctional. Furthermore, the M. occulta cymric gene lacks the exons for the tyrosine kinase domain; making it one of the M. occulta genes on its way towards becoming a pseudogene, as has been shown for the tail muscle genes and the sensory pigment gene tyrosinase. We are investigating whether other tailless molgulid species also have an altered cymric gene, but our preliminary results suggest that the altered cymric is not the original mutation causing taillessness in molgulid ascidians, but rather a secondary effect observed in Molgula occulta.
