Evidence for an instructive role of apoptosis during the metamorphosis of Hydractinia echinata (Hydrozoa).

Apoptosis is a highly conserved mechanism of cell deletion that destroys redundant, dysfunctional, damaged, and diseased cells. Furthermore, apoptotic cell death is essential during the development of multicellular organisms. However, there are only a few examples where the occurrence of apoptosis has been shown to be a direct prerequisite for developmental processes. As described previously by our group, the degradation of larval tissue during the first half of the metamorphosis of Hydractinia echinata involves extensive cell death. A large number of cells are removed, and we observed several cellular features of apoptotic cell death in the dying tissue, e.g., nucleosomal DNA fragmentation and nuclear condensation. Furthermore, we showed that metamorphosis in the basal cnidarian H. echinata depends on the activity of caspases, the central enzymes of apoptosis. In the present study, we build on these previous investigations of apoptosis in H. echinata by characterising a caspase-3 sequence in this species and placing it in an evolutionary context by performing phylogenetic analyses. Furthermore, we report the successful knockdown of a caspase by RNAi and show that apoptosis plays a role as an instructive mechanism in the metamorphosis of H. echinata.