Development of sensitivity to maturation-inducing steroids in the oocytes of the daily spawning teleost, the kisu Sillago japonica.

The responsiveness of oocytes to 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-diOH-P), 17 alpha-hydroxyprogesterone (17 alpha-OH-P), and silver carp gonadotropin (GtH) was examined in vitro at seven different times of the day (0100, 0500, 0800, 1000, 1300, 1700, 2100 hr) in a daily spawning teleost, the kisu Sillago japonica. 17 alpha,20 beta-DiOH-P, 17 alpha-OH-P, and GtH were effective in inducing germinal vesicle breakdown (GVBD) and ovulation in kisu oocytes. However, the oocytes of the kisu responded to these hormones during a limited time of the day (2100-1000 hr), and their responsiveness to steroids and GtH was different with the sampling time. After a 20-hr incubation, the oocytes with yolk globules were classified into four stages based on their responses (GVBD) to GtH and steroids. Stage A, the oocytes did not respond to either GtH or steroids. These unresponsive oocytes were observed in all the ovaries examined. Stage B, GVBD could be induced by GtH but not by steroids. These oocytes were found in the ovaries collected from 2100 to 1000 hr. Stage C, GVBD could be induced by both GtH and steroids. These oocytes were found only between 0800 and 1000 hr. Stage D, oocytes spontaneously underwent GVBD without hormones. These oocytes were found between 1300 and 1700 hr. Ovulated oocytes were found in the ovaries collected during 1700-0500 hr. These results indicate that the kisu possesses a daily rhythm in oocyte development from stage A to stage D, and that the sensitivity to 17 alpha,20 beta-diOH-P appears at stage C.(ABSTRACT TRUNCATED AT 250 WORDS)