Molgulid ascidians share a unique gene complex

Abstract

Typical chordate features found in ascidian tadpole larvae have been evolutionarily lost several times independently within the Molgulidae family. While tailed molgulids retain a tail with muscle cells, a notochord, and a dorsal neural tube, the notochord and muscle cells have been lost within the tailless species. A locus containing an unusual gene arrangement of the Bobcat gene within the first intron of the Manx gene has been shown to be essential for the development of chordate features in molgulid tadpole larvae. Sequencing and closer examination of ascidian genomes show that there is a unique gene arrangement of SSNA1 upstream and adjacent to Manx and Bobcat within the Molgulidae which is not found in cionid ascidians; however, a similar arrangement of SSNA1 directly upstream of Bobcat was found in Oikopleura dioica supporting Appendicularia as a sister group to the Molgulidae. SSNA1 is expressed in tailed Molgula oculata gonads and not in tailless M. occulta gonads, suggesting SSNA1 could have a role in the development of tailed larvae. Expression of these key genes could be affected by one another’s close proximity, disturbing normal gene expression and thereby larval development of chordate features. We propose that the rearrangement that took place in the molgulid ancestor may be contributing to the numerous instances of the evolution of taillessness found in the Molgulidae.