Life and death of haemocytes of the colonial ascidian Botryllus schlosseri during the blastogenetic cycle

A recurrent blastogenetic cycle characterizes the colonies of the ascidian Botryllus schlosseri. It starts when a new zooid generation opens its siphon and ends with take-over, when adult zooids cease filtering, are progressively resorbed and replaced by a new generation of buds, reaching functional maturity. During the generation change, massive apoptosis occurs in the colony, mainly in the tissues of old zooids. In the present study, we investigated the behaviour of haemocytes during the colonial blastogenetic cycle, in terms of occurrence of cell death and expression of molecules involved in the induction of apoptosis. Results indicate that, during take-over: i) caspase-3 activity in haemocyte lysates increases; ii) about 20-30% of haemocytes express phosphatidylserine on the outer leaflet of their plasma membrane, show DNA fragmentation, are immunopositive for caspase-3 and –8, and express the death receptor Fas on their surface; iii) FasL is massively expressed by immunocytes; iv) there is an increase in the fraction of cells expressing molecules recognized by the anti-Bax antibody, and a parallel decrease in the number of cell immunopositive to anti-Bcl-2. On the whole, these data are consistent with the involvement of both the extrinsic, death-receptor-mediated, and the intrinsic, stress-related pathway in the induction of haemocyte apoptosis. Effete cells and corpses are quickly ingested by phagocytes and replaced by a new generation of cells which appears in the circulation during the generation change.