Sperm extract injection into ascidian eggs signals Ca2+ release by the same pathway as fertilization

Injection of eggs of various species with an extract of sperm cytoplasm stimulates intracellular Ca2+ release that is spatially and temporally like that occurring at fertilization, suggesting that Ca2+ release at fertilization may be initiated by a soluble factor from the sperm. Here we investigate whether the signalling pathway that leads to Ca2+ release in response to sperm extract injection requires the same signal transduction molecules as are required at fertilization. Eggs of the ascidian Ciona intestinalis were injected with the Src-homology 2 domains of phospholipase C gamma or of the Src family kinase Fyn (which act as specific dominant negative inhibitors of the activation of these enzymes), and the effects on Ca2+ release at fertilization or in response to injection of a sperm extract were compared. Our findings indicate that both fertilization and sperm extract injection initiate Ca2+ release by a pathway requiring phospholipase C gamma and a Src family kinase. These results support the hypothesis that, in ascidians, a soluble factor from the sperm cytoplasm initiates Ca2+ release at fertilization, and indicate that the activating factor from the sperm may be a regulator, directly or indirectly, of a Src family kinase in the egg.