The membrane potential changes of amphibian eggs during species- and cross-fertilization

Abstract

The mature eggs of the newt Cynops pyrrhogaster, which exhibit physiological polyspermy, had a resting membrane potential of about −16 mV, an input resistance of about 64 MΩ, and a time constant of 5.7 sec. Upon insemination with homologous sperm, the eggs generated several small hyperpolarizations (2–10 mV) lasting for 30–90 sec at 2- to 3-min intervals. The number of these hyperpolarizations in each egg corresponded well to the number of the sperm in those eggs that were observed in cytological sections. When the eggs of the frog Xenopus laevis were inseminated with the Cynops sperm, polyspermic fertilization occurred instead of monospermy with the homologous sperm. In these cross-fertilized eggs, the cortical reaction occurred normally but in an apparent delay when compared with that during the self species fertilization. Upon insemination with the Cynops sperm, the Xenopus eggs having a resting membrane potential of −10 mV underwent a gradual hyperpolarization to −35 mV, which was never observed during the self species fertilization. The eggs then generated a rapid depolarization amounting to +30 mV. The experiments on various ionic conditions indicate that the former hyperpolarization and the latter depolarization are mediated by different ionic permeability of the egg plasma membrane.