Egg activation in physiological polyspermy

Fertilization is indispensable not only for restoring diploid genomes but also for the initiation of early embryonic cell cycles in sexual reproduction. While most animals exhibit monospermy, which is ensured by polyspermy blocks to prevent the entry of extra sperm into the egg at fertilization, several animals exhibit physiological polyspermy, in which the entry of several sperm is permitted but only one sperm nucleus participates in the formation of a zygote nucleus. Polyspermy requires that the sperm transmit the egg activation signal more slowly, thus allowing the egg to accept several sperm. An increase in intracellular Ca2+ concentration induced by the fertilizing sperm is both necessary and sufficient for egg activation in polyspermy. Multiple small Ca2+ waves induced by several fertilizing sperm result in a long-lasting Ca2+ rise, which is a characteristic of polyspermic amphibian eggs. We introduced a novel soluble sperm factor for egg activation, sperm-specific citrate synthase, into polyspermic newt eggs to cause Ca2+ waves. Citrate synthase may perform dual functions: as an enzyme in mitochondria and as a Ca2+-inducing factor in egg cytoplasm. We also discuss the close relationship between the mode of fertilization and the Ca2+ rise at egg activation and consider changes in this process through evolution in vertebrates.