The Ca2+ increase by the sperm factor in physiologically polyspermic newt fertilization: Its signaling mechanism in egg cytoplasm and the species-specificity

► A Ca2+ wave occurs at each sperm entry site. ► A spike-like Ca2+ increase is caused by a sperm protease on egg surface. ► The Ca2+ wave is induced by a sperm factor after sperm-egg fusion. ► Citrate synthase activity is necessary for egg activation by the sperm factor. ► The mechanism of egg activation is quite different between monospermic and polyspermic species.