マダイ雌の性成熟に伴う血中ステロイドホルモン量の変動

Abstract

Changes in estradiol-17β, testosterone and 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-diOHprog) levels were correlated with changes in gonadosomatic index and ovarian histology during the sexual maturation of female red seabream Pagrus major.Between January and March, plasma estradiol-17β levels were very low (below 200pg/ml). A slight increase was observed in vitellogenic females (about 300pg/ml) collected in April. A dramatic increase in estradiol-17β levels occurred in females collected in May, which contained mature oocytes (i.e., the germinal vesicle had disappeared and the ooplasm was completely trans-lucent) as well as oocytes at various stages of vitellogenesis. Although plasma testosterone levels also increased in April and May, the magnitude of elevation was much smaller than that of estra-diol-17β. Testosterone levels were less than 100pg/ml throughout the remainder of the sampling periods. 17α, 20β-diOHprog was present in plasma throughout the sampling periods, but the levels (about 200pg/ml) were lower than those in salmonids. A slight but significant increase (about 400pg/ml) was found in prespawning females collected in April and spawning females in May. These results are discussed in relation to previous works on seasonal changes in plasma steroid levels of salmonids and other species which have ovaries of the total synchronous or group synchronous type.