I have my notochord cells, but where did I leave my tail: A notochord gene expression analysis of tailed, tailless and hybrid Molgulids.

In the molgulid clade of ascidians several species have individually evolved tail loss during their larval stage of development. The notochord, one of the key features of chordates, is found in the center of the tail in most ascidian larvae. In almost all solitary ascidians, including M. oculata, 40 notochord cells have been found extended in the tail. However, in a closely related species, M. occulta (tailless), only 20 notochord cells were found, and these express brachyury. These two species are only 10% divergent at a transcriptomic level and are able to cross hybridize. Some of the hybrid also have 20 notochord cells, however the notochord cells extend and converge in a shortened version of M. oculata tail. Through the use of high throughput sequencing technologies and experimental techniques, we are able to investigate the express of genes associated with notochord development in the parent species and in the hybrid. Several genes—FGF9/16/20, prickle (pk), noto6, leprecan, merlin, and noto17—were analyzed for presence, temporal and spatial expression. FGF9/16/20, pk, noto6, leprecan, merlin, and noto1 were found to be present in both species. Pk expression pattern was found to be similar to Ci-pk2