Before fertilisation, animal eggs are maintained in cell cycle arrest, to prevent parthenogenetic activation. In vertebrates, this is achieved by MAPK- and Emi2-mediated inhibition of the anaphase promoting complex/cyclosome (APC/C). Sperm induce egg activation by calcium-dependent activation of CaMKII, which triggers the destruction of Emi2, activating APC/C. However, invertebrates do not possess an Emi2 homologue, raising the question of how egg activation is achieved in these species. On , Alex McDougall and colleagues address this problem in ascidians, the closest relatives to the vertebrates. They find no role for CaMKII, but show that the phosphatase calcineurin (CN) is required, acting to promote APC/C activity. Moreover, basal activity of the phosphatase PP2A is also essential for full APC/C function and egg activation. As CN is involved in egg activation in Drosophila, and plays an auxiliary role in Xenopus, the authors suggest that this may represent the ancestral mechanism of egg activation, which has been lost in mammals and replaced by a CaMKII-dependent pathway.
