Egg Activation in Polyspermy: Its Molecular Mechanisms and Evolution in Vertebrates

Abstract

In amphibians, most urodeles (newts) exhibit polyspermy physiologically, but primitive urodeles (Hynobius) and anurans (frogs) exhibit monospermy. Several fertilizing sperm induce multiple small Ca2+ waves in the polyspermic egg, but a single large Ca2+ wave occurs in the monospermic egg. The Ca2+ waves in newt eggs are caused by a sperm-specific citrate synthase localized outside the mitochondria. The single Ca2+ wave at monospermy is necessary for eliciting a fast block to polyspermy, whereas the small multiple Ca2+ waves provide slower egg activation to permit the entry of several sperm at polyspermy. Physiological polyspermy seems to be evolved in association with the increase in size of eggs in urodeles, reptiles, and birds laying larger yolky eggs. The sperm factor (citrate synthase) operating in slower egg activation in polyspermic eggs is already prepared in the monospermic urodele Hynobius. We have focused on comparative studies in fertilization among amphibians to understand the role of egg activation in establishment of polyspermy with discussion of the evolution in vertebrates.