ZENG, L.*; SWALLA, B. J.; University of Washington; University of Washington: Evolution of Coloniality in the Tunicates: How many times has it evolved?

The phylogenetic relationships of ascidians are very important for studying Chordate and Deuterostome evolution. We are studying phylogenetic relationships in order to understand the evolution of coloniality in the tunicates. Coloniality and the ability to reproduce asexually may confer strong selective advantage under certain environmental circumstances. However, only a few phyla within the invertebrates contain colonial species. Shifting between colonial and solitary lifestyles involves alteration of a whole suite of life history characteristics, and it is possible that developmental or morphological constraints influence these transitions. Tunicates are divided into three classes: the ascidians, larvaceans and thaliaceans. MP, ML, NJ and ME analyses of the 18S rDNA of the Tunicata shows that it contains four distinct clades: Styelidae+Pyuridae, Molgulidae, Phlebobranchia+Thaliacean and Larvacean. We constructed phylogenies with 37 ascidian species� full-length 18S rDNA and mitochondrial cytochrome oxidase b gene sequences. Within the Stolidobranchia, there is a single clade in the family Styelidae that has colonial, social and solitary ascidians. Therefore, coloniality has evolved only once in Stolidobranch ascidians. Our analyses also suggest that the family Botryllidae is best classified as a genus within the Styelidae. Furthermore, our analyses show that the Phlebobranch ascidians and planktonic, colonial Thaliaceans are sister groups. We are working to place Aplousobranchs in this analysis, because they are all colonial ascidians. In summary, coloniality has evolved at least 3 times within the tunicates (but maybe 4, depending on where the Aplousobranchs come out in the trees). Research is ongoing in the Swalla Lab to investigate this interesting and important evolutionary question.