Successive Electrical Responses to Insemination and Concurrent Sperm Entries in the Polyspermic Egg of the Ctenophore Beroe ovata

Abstract

In the ctenophore Beroe ovata, action potentials and sperm-induced egg electrical responses were recorded, in unfertilized and in vitro inseminated eggs, respectively. The unfertilized egg had a resting membrane potential of -65.6 ± 1.3 mV and a membrane resistance of 1.45 ± 0.09 MΩ. When depolarized beyond a threshold of about -20 mV, the membrane displayed an action potential with a fast phase and then a peak reaching 38 ± 0.6 mV, followed by a positive declining plateau which lasted about 20 sec and ended by a biphasic repolarization of the membrane. The action potential was mainly Ca2+-dependent and to a lesser extent, Na+-dependent. Upon one or several inseminations, monospermic eggs generated one fertilization potential and polyspermic eggs generated several sperm induced egg electrical responses. These sperm-induced egg electrical responses were composed of a voltage-gated action potential elicited by sperm-egg contact, resembling the action potential triggered by a depolarizing current, followed by a true spermgated depolarizing plateau which lasted about 60 sec and was mainly Na+-dependent. The number of electrical responses displayed by each egg corresponded to the number of male pronuclei detected in its cortex. An electrical response occurred for all egg membrane potentials ranging from -78 to +16 reV. In addition, the histogram giving the number of eggs in each class of egg characterized by a number of sperm entries indicated that each sperm entry constituted an independent process. However, when we compared the effectiveness of the in vitro inseminations during electrophysiological measurements and the in vivo inseminations under simulated natural conditions, the results showed two predominant classes of fertilized eggs, with one and two male pronuclei in the cortex respectively. Consequently, although our overall results exhibit features characteristic of polyspermy, they do not yet preclude the possibility that the eggs of the ctenophore B. ovata might be monospermic, or might, under natural planktonic conditions, display a small number of sperm entries.