A tale of death and life: natural apoptosis in the colonial ascidian Botryllus schlosseri (Urochordata, Ascidiacea).

The colonial ascidian Botryllus schlosseri forms new zooids by blastogenesis, through the formation of palleal buds which progressively grow and mature until adults are formed. At a temperature of 19°C, adult zooids remain active for about one week; then they contract, close their siphons and are gradually resorbed, being replaced by buds which reach functional maturity, open their siphons and begin their filtering activity as adult zooids. This recurrent generation change, known as take-over, is characterised by the occurrence of diffuse programmed cell death by apoptosis. Immediately before the take-over, an increase in the expression of molecules recognised by anti-Bax antibodies and a parallel decrease in the expression of molecules immunopositive to anti-Bcl-2 antibodies were observed in zooid tissues, suggesting a mitochondrion-dependent apoptotic pathway. During the take-over, circulating phagocytes infiltrate the zooid tissues and engulf apoptotic cells; in addition, the frequency of haemocytes showing nuclear condensation and annexin-V labelling significantly increases. Previous experiments showed the involvement of phosphatidylserine and CD36 in the recognition of effete cell. The resorption of old zooids is closely related to the rejuvenation of the colony occurring at the take-over. The death of adult zooids puts a quantity of material at the colony disposal. This material is represented by senescent cells, which, once ingested and digested by phagocytes, can be recycled and used to sustain the burden of blastogenesis: this involves a cross-talk between old tissues, phagocytes and developing buds. Therefore, B. schlosseri can be considered a new and promising model organism for the study of natural apoptosis.