New data on phagocytosis-induced apoptosis in the colonial ascidian Botryllus schlosseri

Colonies of the ascidian Botryllus schlosseri undergo cyclical generation change or take-over (TO) during which 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. Previous observations led us to suggest that phagocytes, once ingested apoptotic cells or corpses, undergo phagocytosis-induced apoptosis (PIA). We already demonstrated, by Western blot and immunocytochemical analyses, the release of cytochrome c by hemocytes and the expression of apoptosis-related molecules, such as BAX and caspases, in phagocytes during the TO. In order to corroborate the above assumption, we carried out new morphological analyses at the transmission electron microscope (TEM) and started a molecular analysis of apoptosis in Botryllus phagocytes looking for transcripts differentially expressed in phagocytes at TO. We identified and characterized transcript sequences for BAX, AIF and PARP (poly ADP ribose polymerase), and studied their expression and the location of the corresponding mRNA in hemocytes. The collected data clearly indicate the diffuse occurrence of PIA among phagocytes which guarantee the disposal of apoptotic cells or corpses. In addition, they extend the classical view of PIA, intended as a mechanism for the prevention of microbial diffusion within the organism, and reveal an undescribed role of the process in the control of asexual development and colonial homeostasis.