The spike component of the fertilization potential in the toad, Bufo japonicus: changes during meiotic maturation and...

Immature oocytes of the toad, Bufo japonicus, inseminated between first- and second-meiotic metaphase, exhibited polyspermy. Monospermy occurred when the oocytes had reached second-meiotic metaphase. Electrical recording during insemination of the immature oocyte showed fast-rising and slow-rising spikes followed by a gradual shift to a positive membrane potential. The number of fast spikes in each oocyte corresponded well with the number of sperm observed in cytological sections. Mature oocytes elicited one fast spike followed by a rapid rise to a positive plateau. Ion-substitution experiments indicated that, like the plateau, the initial fast spike is mediated mainly by increased permeability of the oocyte plasma membrane to halides such as Cl- or I-. When inseminated with sperm of the newt, Cynops pyrrhogaster, mature Bufo oocytes exhibited polyspermy accompanied by a gradual hyperpolarization and a slowly developing positive plateau, without the fast spike that occurs in self-species fertilization. These results indicated that the spike component of the fertilization potential can be dissociated from the plateau component, and may be elicited by different mechanisms.