| Steroids are suggested to be important in development of fishes, however, their specific roles are not completely understood. This study describes the levels of the steroids androstenedione (AD), testosterone (T), 11-ketotestosterone (KT) and estradiol (E2), and the activity of steroid metabolic enzymes in relation to early gonadal development in the tilapia Oreochromis niloticus. Further, these observations were conducted on populations of normally developing fry and on populations of monosex fry obtained by dietary treatment with methyltestosterone (MT) or tamoxifen (TX), a synthetic androgen or synthetic antiestrogen, respectively. During normal development, histological observations revealed the emergence of gonads at 15 days postfertilization (dpf) with ovaries appearing at 36 dpf. At 64 dpf, individuals exhibited either ovaries or indifferent gonads, presumably testes. The levels of steroids were determined by radioimmunoassay of organic extracts from whole body homogenates. Androgen levels decreased early in development with transient increases in T and KT occurring after the presence of the gonadal anlage. The levels of E2 were either low or non-detectable except after ovaries were present. The levels of T were distinctly bimodal from 57-64 dpf. Steroid metabolism was determined by incubations of whole body homogenates with radiolabelled AD. Steroid metabolites were isolated and identified by thin layer chromatography coupled with radioisotope image analysis. The results indicated a burst of steroid metabolic activity at 1 dpf followed by generally similar steroid metabolic profiles. Throughout development, the metabolites consisted of T, 5... |
