BROWN, FD*; SWALLA, BJ; University of Washington; University of Washington: Germ Cells and Vasa Expression in Ascidians

Several evolutionary transitions between solitary and colonial lifestyles have been identified in the tunicates. These transitions are accompanied by dramatic morphological, developmental, and reproductive changes. We are studying whether stem cells involved in blastogenesis (asexual reproduction) of a colonial ascidian are able to generate both germline and soma. We compare the expression of vasa, a DEAD box RNA helicase found in germ cells across the metazoans, in the solitary ascidian Boltenia villosa and the closely related colonial ascidian Botrylloides violaceus. In B. villosa, in situ hybridization studies show expression of vasa in the posterior lineage of cells during embryogenesis and in germ cells of the adult gonad. Boltenia vasa mRNA is strongly expressed in the cytoplasm and nuclei of small oocytes, and expression decreases qualitatively in larger oocytes, where it is observed perinuclearly. In contrast, in colonies of the adult ascidian B. violaceus, vasa mRNA is expressed in the presumptive germ cells and oocytes in the gonads, and in certain isolated cells in the differentiating zooid. The timing of specification of the germline in colonial ascidians and the functionality of vasa positive cells in adult colonies is being investigated. Additionally we have conducted a phylogenetic analysis of the conserved DEAD box of Vasa in 22 highly divergent species of Metazoans, including Boltenia and Botrylloides Vasa. Our reconstruction shows that the vasa gene tree corroborates with known phylogenetic trees.