For my dissertation research, I've studied the mechanistic evolution of coloniality in ascidians by comparing embryonic, larval and adult development of solitary and colonial ascidians. I propose that flexibility in the specification of progenitor stem cell lineages and cell numbers in late embryonic and pre-metamorphic larval stages are necessary for life history transitions between solitary and colonial species. First, I studied expression of vasa, a DEAD box RNA helicase gene found in germ cells across the metazoans by in situ hybridization. I found that vasa mRNAs in the colonial ascidian Botryllus schlosseri are localized and segregated early in development to a posterior lineage of cells, resembling that of the germline of strictly sexual solitary ascidians. In mature colonies of botryllid ascidians, bot-vasa mRNA was expressed in putative spermatogonia, in oocytes of zooids, and in unexpectedly in some circulating cells in the zooids and differentiating buds. Contrary to our predictions, vasa knockdown assays by siRNA in sexually mature colonies of Botryllus schlosseri, did not have obvious effects on germ cells, but resulted in heterochronic shifts in the development of buds during adult asexual cycles. These results suggest a novel and previously undescribed role of vasa in the regulation of budding cycles for Botryllus schlosseri. I hypothesize that circulating vasa-positive cells may be important for communication between the developing buds and the regressing mature zooid at the end of every blastogenic generation. I described the formation of early buds in the vasculature after inducing whole body regeneration in Botrylloides violaceus to characterize and identify circulating progenitor germline and somatic stem cells. I reported the expression of Piwi protein, involved in microRNA processing and stem cell maintenance, in hemocytes surrounding the regenerates, some of which were proliferating, as examined by proliferating cell nuclear antigen (PCNA) expression. In conclusion, I propose that multiple stem cell types are found in hemocytes of Botrylloides violaceus and that they undergo proliferation in the peripheral vasculature before differentiating into epithelial tissues for all three germ layers during whole body regeneration.
|Advisor:||Swalla, Billie J.|
|School:||University of Washington|
|School Location:||United States -- Washington|
|Source:||DAI-B 69/06, Dissertation Abstracts International|
|Subjects:||Molecular biology, Cellular biology, Microbiology|
|Keywords:||Ascidians, Germ cells, Germline, Hemocytes, Regeneration, Stem cells|
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