A comparative study of gonadal development and plasma sex steroid levels over the seasonal reproductive cycle of farm-raised channel catfish Ictalurus punctatus.

摘要

The objective of this study was to test the hypothesis that plasma levels of the sex steroids, estradiol-17β (E₂), testosterone (T), and 17-hydroxyprogesterone (17HP), varied predictably with the histological changes observed in the gonads during the annual reproductive cycle of the channel catfish Ictalurus punctatus. This seasonal relationship was tested by characterizing the changes in gonadal development and plasma concentration of these sex steroids under natural environmental influences. Adult channel catfish were collected from commercial ponds between March 2002 and February 2003. Of the 131 fish collected, 65 were males, and 66 were females. Fish age was determined by the total length. Gonads were removed and prepared for histological analysis. Gonadal growth was measured by gonadosomatic index and oocyte's diameter. Blood samples were centrifuged, and plasma was removed and stored at −20°C. Plasma levels of the gonadal steroids were measured by radioimmunoassay.; The female's gonadosomatic index (GSI) was initiated in September and continued to increase until it peaked in fully mature ovaries during spawning in May. Immediately following spawning, GSI declined. Plasma levels of E ₂ and T were elevated in females during the previtellogenic and vitellogenic stages. The highest level of E₂ was observed in February, whereas T levels were highest in May prior to ovulation. Postspawning females collected after May possessed much lower levels of E₂ and T. The 17-hydroxyprogesterone only increased significantly in May, although it remained low and unrelated to ovarian development for the rest of the year.; Males' GSI fluctuated through the year, indicating multiple spawning periods. Plasma levels of T were significantly driven by gonad growth. Males during spermatogenesis had the highest level of T and 17HP. Testosterone levels declined in ripe males and were lowest in postspawning males. The level of 17-hydroxyprogesterone stayed relatively constant during ripe and postspawning stages. Estradiol levels were low and remained so throughout the annual reproductive cycle; no observable link to testes growth and maturation were observed. The presence of Leydig cells was indicated, and the secreting function of the posterior region of testes was determined.