Apoptosis and phosphatidylserine-mediated recognition during the take-over phase of the colonial life-cycle in the ascidian Botryllus schlosseri

Colonies of the ascidian Botryllus schlosseri undergo recurrent generation changes in which adult zooids are gradually resorbed and replaced by new blastogenic generations. During these periods, known as take-over phases, programmed cell death, which, on the basis of morphological analysis is ascribed to apoptosis, occurs widely in zooid tissues. In the present report, we re-investigate cell death during the take-over process. Results confirm the occurrence of diffuse apoptosis, as evidenced by chromatin condensation, positivity to the TUNEL reaction and expression of phosphatidylserine on the outer leaflet of the plasma membrane. Apoptosis also occurs among haemocytes, and senescent blood cells are actively recognised and ingested by circulating professional phagocytes. Both phosphatidylserine and CD36, a component of the thrombospondin receptor, are involved in the recognition of apoptotic haemocytes, which fosters the idea that fundamental recognition mechanisms are well conserved throughout chordate evolution.