Electrical response to fertilization in ascidian oocytes

The fertilization potential of the ascidian oocyte has been studied using two intracellular electrodes. Two classes of oocyte were observed; low resting potential (RP) oocytes of ?20 to ?35 mV and high RP oocytes of ?80 to ?90 mV. The two types have comparable membrane resistance, falling in the range of 100–300 MΩ, and both may be fertilized and develop normally, although the fertilization potential (FP) is different in the two cases. High RP oocytes give rise to step-like regenerative potentials which attain positive values, whereas low RP oocytes give rise to slower depolarizations that reach zero level. In both cases the FP was sometimes preceded by a small-step depolarization as normally observed in the sea urchin. Irrespective of the original RP, the membrane resistance always decreased to 1–10% of its initial value during the first few minutes of the FP plateau. In contrast when the membrane was depolarized to a comparable potential by current injection the membrane resistance did not decrease. Polyspermic fertilization was induced by removing the extracellular coats, aging the oocytes and using high densities of sperm. The FP in monospermic and polyspermic oocytes were comparable and we could not correlate additional sperm interactions with additional electrical events. Our results suggest that the plasma membrane in ascidian oocytes lacks intrinsic mechanisms, electrical or otherwise, to prevent polyspermic fertilization.