Phagocytosis-induced apoptosis in the compound ascidian Botryllus schlosseri

Colonies of the ascidian Botryllus schlosseri contain three blastogenetic generations: functional zooids, their buds and budlets on buds. Generation change or take-over (TO) occur cyclically and assure the recurrent renewal of the colony. During these events, lasting 24-36 h, diffuse apoptosis occurs in zooid tissues, as indicated by morphological, biochemical and molecular investigations. Tissues are rapidly infiltrated by circulating phagocytes, selectively recruited by dying cells, which recognize and greedily ingest them. Using hemocytes as selected cell population for investigation, we studied the transcription rate of three recently characterized genes involved in apoptosis, bsbax, bsaif1 and bsparp1, during the TO as compared to colony developmental phases far from it. In addition, we observed that the massive ingestion leads phagocytes themselves to undergo apoptosis, probably as a consequence of the oxidative stress related to the sustained respiratory burst, as suggested by biochemical analysis. Therefore, a large fraction of circulating phagocytes needs to be replaced by new, young hemocytes, entering the circulation at the end of the generation change.