A long-lasting electrically mediated block, due to the egg membrane hyperpolarization at fertilization, ensures physiological monospermy in eggs of the crab Maia squinado

Abstract

In response to fertilization, the membrane potential ( E m) of the crab egg hyperpolarizes from about −50 mV to about −80 mV in 400 msec. To establish whether this fast hyperpolarization is correlated with physiological polyspermy or conversely mediates an electrical block to polyspermy, we examined the morphological and electrophysiological characteristics of eggs from the crab Maia squinado. Fertilized naturally spawned eggs were found to be physiologically monospermic and their average E m was constant at −77 ± 0.5 mV. To examine a possible electrical block ensuring this monospermy, unfertilized eggs were voltage clamped at various E m values ranging from +20 to −90 mV, inseminated, and examined morphologically. All eggs clamped at +20 to −65 mV responded by developing a fertilization current, I f. It consisted of an outwardly directed K + current in one or several steps, each caused by a single spermatozoon interacting with the egg membrane. The percentage of eggs clamped at values more negative than −65 mV, which responded at insemination by developing an I f, decreased and dropped to 0 at −80 mV. This indicated that the membrane processes occurring during the contact between gametes and eliciting an electrical response by the egg membrane are voltage dependent. Further, the spermatozoon never penetrated into eggs voltage clamped at a E m between +20 and −60 mV and at voltages more negative than −75 mV. E m values between −65 and −75 mV were required for spermatozoon incorporation into the egg, indicating that sperm entry is also voltage dependent. It is proposed that the hyperpolarization of the egg membrane in response to fertilization constitutes a long-lasting electrical block to polyspermy in crab eggs.