The electrical block to polyspermy induced by an intracellular Ca2+ increase at fertilization of the clawed frogs, Xenopus laevis and Xenopus tropicalis.

Abstract

Polyspermy blocking, to ensure monospermic fertilization, is necessary for normal diploid development in most animals. We have demonstrated here that monospermy in the clawed frog, Xenopus tropicalis, as well as in X. laevis, is ensured by a fast, electrical block to polyspermy on the egg plasma membrane after the entry of the first sperm, which is mediated by the positive-going fertilization potential. An intracellular Ca2+ concentration ([Ca2+ ]i ) at the sperm entry site was propagated as a Ca2+ wave over the whole egg cytoplasm. In the X. tropicalis eggs fertilized in 10% Steinberg's solution, the positive-going fertilization potential of +27 mV was generated by opening of Ca2+ -activated Cl- -channels (CaCCs). The fertilization was completely inhibited when the egg's membrane potential was clamped at +10 mV and 0 mV in X. tropicalis and X. laevis, respectively. In X. tropicalis, a small number of eggs were fertilized at 0 mV. In the eggs whose membrane potential was clamped below -10 mV, a large increase in inward current, the fertilization current, was recorded and allowed polyspermy to occur. A small initial step-like current (IS current) was observed at the beginning of the increase in the fertilization current. As the IS current was elicited soon after a small increase in [Ca2+ ]i , this is probably mediated by the opening of CaCCs. This study not only characterized the fast and electrical polyspermy in X. tropicalis, but also explained that the initial phase of [Ca2+ ]i increase causes IS current during the early phase of egg activation of Xenopus fertilization.