マダイ雄の性成熟に伴う血中ステロイドホルモン量の変動

Abstract

Changes of plasma testosterone, 11-ketotestosterone and 17, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-diOHprog) levels were correlated with changes in gonadosomatic index (GSI) and testi-cular histology during the sexual maturation of male red seabream Pagrus major.A sharp increase in the GSI occurred during April, with the increased formation of sperma-tozoa. High GSI values were maintained in May, followed by a sharp drop in June. The values were consistenily low during the remainder of the sampling periods. Plasma testosterone levels were maintained at low levels (below 200pg/ml) from January to March, rapidly increased to 500pg/ml in April and reached a peak of 870pg/ml in May. The levels showed a distinct drop in June, and remained low throughout the remainder of the sampling periods. The profiles and absolute levels of plasma 11-ketotestosterone were almost similar to those of testosterone. 17α, 20β-diOHprog could be detected in plasma throughout the sampling periods, but the levels (about 200pg/ml) were much lower than those of salmonids. There was a slight but significant increase in l7α, 20β-diOHprog levels (350pg/ml) in males collected during early spermiating period (April). However, the levels decreased even during the period of active spermiation (May).