Steroid levels and steroid metabolism in relation to early gonadal development in the tilapia Oreochromis niloticus (Teleostei : Cyprinoidei)

Sex steroid levels and steroid metabolism were investigated in relation to early gonadal development in a mixed sex population of the tilapia Oreochro mis niloticus. Androstenedione (AD), testosterone (T), 11-ketotestosterone (KT), and estradiol (E-2) were quantified by radioimmunoassay (RIA) of whol e body extracts. Androstenedione metabolism was assessed by incubations in vitro with H-3-AD and metabolites were identified by thin-layer chromatogra phy coupled with radioisotope image analysis. Histology revealed the presen ce of gonadal structures at 15 days postfertilization (dpf) and ovaries at 36 dpf, with other individuals exhibiting undifferentiated gonads containin g germinal cells, presumably eventual testes. Androgen levels were initiall y high in eggs then decreased severalfold prior to the emergence of gonads. A transient increase in the levels of T and KT occurred at 22 dpf. Levels of E2 were either low or undetectable except for a transient increase (43 d pf) after ovaries were present. Levels of T approached bimodality from 57 t o 64 dpf. Steroid metabolism generally increased throughout development. Me tabolites were generally similar, consisting of T predominantly as well as 5 beta-reduced androgen derivatives and 11-oyxgenated derivatives. Estriol was tentatively identified. Conjugated steroids were not formed. Two types of steroid metabolic profiles occurred at 50 dpf These results demonstrate that changes in the steroidogenic profile occur during early transitions of gonadal development. Notably, (1) steroid biosynthetic capacity preceeds g onadal differentiation, (2) evidence for estrogens occurs after ovarian dev elopment has begun, and (3) bimodality of levels of T and differential ster oid metabolism later in development may reflect the onset of sexual diverge nce. (C) 1999 Academic Press.