Voltage Noise Changes during Monospermic and Polyspermic Fertilization of Mature Eggs of the Anuran, Rana temporaria

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Abstract
Electrical response of mature anuran eggs to the fertilizing sperm consists of a rapid depolarization and a decrease in resistance of the plasma membrane (fertilization potential) and serves as a fast block to polyspermy. The fertilization potential, previously thought to be the earliest electrical response of the egg, is here shown to be preceded in R. temporaria by changes in voltage noise. Voltage noise was recorded after insemination and compared in monospermic and NaI-induced polyspermic eggs. Fertilization potential in monospermic eggs arised at 1 min 45 s-2 min 15 s-after insemination, and that in NaI-induced polyspermic eggs did at 3 min-3 min 30 s after insemination. The increase in voltage noise was detected at the similar time (1-2 min 30 s) after insemination in both the eggs. Duration of voltage noise increase before the fertilization potential was larger in polyspermic eggs (50-105 s) than in monospermic eggs (10-40 s). Polyspermic fertilization in R. temporaria induced by NaI was checked by visualizing multiple sperm entry sites with the scanning microscopy. The process of sperm entry and the development of the fertilization body are similar to those occurring with monospermic fertilization; all supernumerary sperm fuse only with the animal hemisphere of the egg. Although the physiological basis of the changes in voltage noise is unclear, these alterations appear to be the earliest electrical response to sperm yet reported.