Zinc Protection of Fertilized Eggs is Conserved in Non-Mammalian Species

Abstract

Polyspermy, the fertilization of an egg by multiple sperm, is embryonic lethal in most sexually reproducing species. As such, most eggs utilize strategies to protect zygotes from further fertilizations. One of these mechanisms is the slow block to polyspermy, which involves the release of cortical granules from the egg. This exocytosis event modifies the extracellular matrix of the eggs to become impermeable to sperm. In mammals, zinc is released as part of the slow block and is hypothesized to contribute to inhibition of fertilization. We demonstrate that zinc release also occurs during activation of eggs in the frog Xenopus laevis, the axolotl Ambystoma mexicanum, and the fish Danio rerio. By assaying for embryonic development from eggs inseminated in varying concentrations of zinc, we found that zinc inhibits fertilization in X. laevis, as well as the sea urchin Strongylocentrotus purpuratus, and in cnidarian Hydractinia symbiolonigarpus. In X. laevis, inhibition of fertilization by various transition metals suggests that zinc coordinates proteins in the extracellular matrix to prevent sperm penetration. Also in X. laevis, whole cell recording data from eggs treated with zinc suggests that the cation slowly depolarizes the cell. Our data represents evidence of zinc release acting as a well conserved strategy to prevent supernumerary sperm entry in eggs of sexually reproducing organisms from mammals to cnidarians.