Meeting Abstract

P2-80   -   Tail Loss in Molgulid Ascidians: Investigating Key Notochord Genes Powers, MM*; Stolfi, A; Di Gregorio, A; Swalla, BJ; University of Washington, Friday Harbor Laboratories; Georgia Institute of Technology; New York University; University of Washington, Friday Harbor Laboratories mpowers4@uw.edu

Ascidians are invertebrate chordates and the closest extant relatives to vertebrates based on phylogenomic analyses. Prior to metamorphosizing into sessile adults, most ascidians begin their lives as tadpole-like larvae that have a tail with a notochord, dorsal neural tube, and muscle cells, and a head containing a light-sensing organ (ocellus) and a gravity-sensing organ (otolith). However, in one clade of ascidians, the Molgulidae, both the larval tail and pigmented otolith cells have been lost multiple times independently. One of these tailless species, Molgula occulta, and a closely related tailed species, Molgula oculata, both live near Roscoff, France and can be hybridized to produce a half-tailed larva with an otolith. Larval tails are formed through convergent extension and then swelling of 40 notochord cells within the tail. This process fails to occur in tailless species leaving a 20-cell aggregate known as the “notoball”. In the hybrid, larval notochords undergo normal convergent extension forming a smaller, 20-cell notochord. We are using this system to study changes in the gene regulatory network that may be responsible for the breakdown of the notochord developmental pathway and lead to the lack of convergent extension in tailless ascidians. We identified 10 genes that are upregulated as the larval tail forms (tailbud stage) in tailed M. oculata, but not in tailless M. occulta based on transcriptome data. We examined the genomes of M. occulta and M. oculata to determine whether these candidate genes were intact, without premature stop codons or other disruptions to translation. For intact genes, we used in situ hybridization to visualize whether these candidate genes were expressed in the “notoball” of tailless M. occulta and verify expression in notochord cells of tailed M. oculata.