Jacobs, M.W.*; Davis, J.M.; Swalla, B.J.: The Evolution of Coloniality in Stolidobranch Ascidians: a Phylogenetic Analysis

Coloniality and the ability to reproduce asexually may confer strong advantages 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 make that transition difficult to accomplish. The Styelidae is a family of ascidians in the suborder Stolidobranchiata that contains both compound and solitary species, as well as several species with intermediate morphologies. In the ascidians, coloniality is strongly associated with other reproductive traits including brooding, large egg size, and production of large, short-lived, well-differentiated larvae. Solitary ascidians cannot reproduce asexually and are generally free-spawners with small eggs and simple larvae. To examine how many times coloniality and its associated traits have evolved within the Styelidae, a region of about 1000 base pairs in the central part of the 18S rDNA of 27 ascidian species was used to construct phylogenetic trees. In addition, complete 18S sequence was used in selected species from the original dataset to obtain better resolution on some branches with low bootstrap support. Preliminary results suggest that coloniality has arisen only once within the Stolidobranchs and that ‘social’ species with intermediate morphologies are sister taxa to the rest of the colonial species. This implies that ancestry, rather than adaptation, may be the key to understanding the evolution of coloniality in the Ascidians. A detailed comparative analysis of other life history and developmental traits may begin to elucidate the critical evolutionary steps that can lead to a switch between solitary and colonial lifestyles.