GONADOTROPIN DEVELOPS SENSITIVITY TO MATURATION-INDUCING STEROID IN THE OOCYTES OF DAILY SPAWNING TELEOSTS, TOBINUMERI-DRAGONET REPOMUCENUS-BENITEGURI AND KISU SILLAGO-JAPONICA

The postvitellogenic oocytes of daily spawning marine teleosts, tobinumeri-dragonet Repomucenus beniteguri and kisu Sillago japonica, undergo germinal vesicle breakdown (GVBD) in vitro in the presence of maturation-inducing steroids (MIS). The responsiveness of the occytes to MIS appears a few hours before ovulation, and the oocytes collected earlier than this time are insensitive to MIS. In the present study, the effects of gonadotropin (GTH) on the development of sensitivity to MIS were examined in vitro in the oocytes of these two species. The oocytes which were collected 13-16 h before ovulation and insensitive to MIS were first incubated in medium containing GTH for a short period (10 to 300 min in dragonet and 1.5 h in kisu) and subsequently incubated for a longer period (up to 24 h in dragonet and 18 h in kisu) in medium with or without 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20-P). Following short-term treatment with GTH, the oocytes became sensitive to 17,20-P and underwent GVBD. A short-term GTH treatment only did not induce GVBD in these oocytes. The present results indicate that GTH develops sensitivity to MIS in oocytes in dragonet and kisu.