Cell Adhesion and Fusion Induced by Insemination in Zona-Free Hamster Eggs: Electrophysiological Monitoring of the Fusion Process

Cell adhesion and fusion were found to occur in zona-free hamster eggs placed in contact and then inseminated. A cytoplasmic fusion occurred in 7% of paired eggs mainly between 10 and 50 min after insemination. All other eggs that failed to fuse adhered tightly to one another within about 1 hr. Electrophysiological monitoring of the fusion process revealed that an electrical coupling (EC) suddenly appears between apposed eggs and completed within 2–15 min as the first step of cell fusion. Periodic hyperpolarizing responses (HRs), which have been found previously in fertilized hamster eggs to reflect a periodic increase in cytoplasmic Ca²⁺ ions, become gradually synchronized and completely coincide in paired eggs 10–30 min after the establishment of EC. Thus the paired eggs become an electrically single cell as the second step. Then the boundary between the eggs breaks down, resulting in formation of a single, spherical egg in several minutes. On the other hand, most of adhered eggs showed neither EC nor synchronization of HRs. Some adhered eggs showed only a low efficiency EC.     

SOME ASPECTS OF THE BREEDING BIOLOGY OF PLANAXIS SULCATUS (BORN) (GASTROPODA: PLANAXIDAE)

Sex ratio and breeding habits of Planaxis sulcatus were studiedfor Japanese and other Indo-Pacific populations. On the coastof the Kii Peninsula, Japan, the gonad of P. sulcatus is developedfrom June through August, when pairing of the snails is observed.The females contain embryos in their brood pouches and releaseveligers. New recruits appear in autumn, grow during the nextspring and summer, and merge with the older-year cohort in sizein early autumn. The position on the shore of the first-yearand the older-year snails is lower in spring and early summerthan in other seasons. Sex ratio was significantly differentbetween populations, seemingly due to different percentagesof large snails which were mostly females. Inspection of thesamples from a wider geographic range, from Japan to India,detected various sex ratios among the localities; females weredominant in the samples composed exclusively of large individuals.The most developed embryos in the female brood pouches wereveligers. Parthenogenetic development and rearing of embryosup to crawling juveniles, which have been reported for the populationsin the inner Arabian Sea, does not seem to hold for these otherIndo-Pacific populations. (Received 20 February 1996; accepted 8 July 1996)