EVIDENCE BY A VOLTAGE-CLAMP STUDY OF AN ELECTRICALLY MEDIATED BLOCK TO POLYSPERMY IN THE EGG OF THE ASCIDIAN PHALLUSIA-MAMMILLATA

Eggs of the ascidian Phallusia mammillata were voltage clamped (from - 100 to +60 mV) and inseminated with a low or heavy sperm concentration . From inseminations with low sperm concentration (1 X 10(6) sp/ml), w e found that fertilization currents occurred between -100 and +40 mV: they were always inward and displayed an analogous pattern whatever th e clamped voltage. We established that the percentages of inseminated eggs that produced a fertilization current varied as a function of the clamped voltage. These percentages were not statistically different f rom 100% at clamped voltages between -100 and -30 mV, they decreased t o 68 and 56% at clamped V-m of -10 and 0 mV, respectively, but were no t statistically different from 0% at clamped V-m between +10 and +40 m V. Tire never obtained any egg electrical response at a clamped voltag e of +50 mV. Almost all eggs (96%) which responded electrically were p enetrated by one or several spermatozoa. These eggs were resuming meio sis (81 to 50%) at values of clamped V-m between -100 and 0 mV, respec tively. At clamped V-m between +10 and +50 mV, the percentages of eggs resuming meiosis were not statistically different from 0. These resul ts indicate that in P. mammillata eggs, the occurrence of an electrica l response is voltage dependent and consequently that the initial depo larizing shift of the fertilization potential constitutes a fast block to polyspermy. However, in this species, the sperm penetration is not voltage dependent, since it occurred at clamped V-m from -100 to +40 mV. On the other hand, when eggs were clamped from -100 to +60 mV and inseminated with a heavy sperm concentration (2 X 10(7) sp/ml), the cu rves expressing, respectively, the percentages of eggs which responded electrically, the percentages of eggs which were penetrated by one or several spermatozoa, and the percentages of eggs resuming meiosis, as functions of the clamped V-m, were shifted by similar to 35 mV toward more positive voltages, compared to the corresponding curves obtained from eggs inseminated with a low sperm concentration. This last resul t means that the critical value of the membrane potential which charac terizes the electrical block to polyspermy is dependent on the sperm c oncentrations used for inseminations. (C) 1994 Academic Press, Inc.