Natural apoptosis during the blastogenetic cycle of the colonial ascidian Botryllus schlosseri.

Colonies of the compound ascidian Botryllus schlosseri undergo regular generation changes during which adult zooids are progressively resorbed and replaced by growing buds. The generation change, or take-over, is characterized by massive cell death by apoptosis, changes in the expression of surface molecules by senescent cells of zooid tissues and recruitment of circulating phagocytes in zooid tissues which assure the complete clearing of the dying cells. The entire process lasts 24-36 h at 20 °C and has been subdivided, on the basis of the degree of contraction of old zooids, in four substages. It has an antero-posterior progression, at least in the digestive tube, as trace of apoptosis can be found in the epidermis, peribranchial epithelium, and heart in the late take-over, whereas they are easily found in the branchial basket after 2-4 h from the beginning of the generation change. During the take-over, massive recruitment of phagocytes, which ingest senescent cells, occurs: this suggests the release of diffusible chemoattracting factors by effete cells, able to attract phagocytes toward them. We are going to investigate the expression of genes related to apoptosis, such as IAP (inhibitor of apoptosis) and HSP70 during the various phases of the colonial blastogenetic cycle.