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 sti mulates intracellular Ca2+ release that is spatially and temporally like th at occurring at fertilization, suggesting that Ca2+ release at fertilizatio n 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 sp erm extract injection requires the same signal transduction molecules as ar e required at fertilization. Eggs of the ascidian Ciona intestinalis were i njected with the Src-homology 2 domains of phospholipase C gamma or of the Src family kinase Fyn (which act as specific dominant negative inhibitors o f the activation of these enzymes), and the effects on Ca2+ release at fert ilization or in response to injection of a sperm extract were compared. Our findings indicate that both fertilization and sperm extract injection init iate Ca2+ release by a pathway requiring phospholipase C gamma and a Src fa mily kinase. These results support the hypothesis that, in ascidians, a sol uble factor from the sperm cytoplasm initiates Ca2+ release at fertilizatio n, and indicate that the activating factor from the sperm may be a regulato r, directly or indirectly, of a Src family kinase in the egg.