Involvement of estrogens in the process of sex differentiation in two fishspecies: The rainbow trout (Oncorhynchus mykiss) and a Tilapia (Oreochromis niloticus)

in order to study the physiological implication of sex steroid hormones in gonadal sex differentiation in fish, we first investigated the potential ro le of estrogens using two fish models: the rainbow trout (Oncorhynchus myki ss) and a tilapia species (Oreochromis niloticus). All experiments were car ried out on genetically all-male (XY) and all-female (XX) populations. In v ivo treatments with an aromatase inhibitor (ATD, 1,4,6- androstatriene3-17- dione) result in 100% masculinization of an all-female population in rainbo w trout (dosage 50 mg/kg of food) and 75.3% in tilapia (dosage 150 mg/kg of food). In tilapia, the effectiveness of the aromatase inhibition by ATD is demonstrated by the marked decrease of the gonadal aromatase activity in t reated animals versus control. No masculinization is obtained following tre atment with an estrogen receptor antagonist (tamoxifen) in both species. Ar omatase and estrogen receptor gene expression was studied in rainbow trout by semi-quantitative RT-PCR in gonads sampled before, during and after sex- differentiation. Aromatase mRNA is specifically detected in female gonads, 3 weeks before the first sign of histological sex-differentiation, i.e., fi rst female meiosis. Aromatase expression in male gonads is at least a few h undred times less than in female gonads. Estrogen receptor gene is expresse d in both male and female gonads at all stages with no dimorphic expression between sexes. Specific aromatase gene expression before ovarian different iation was also demonstrated using virtual Northern blot, with no expressio n detected in male differentiating gonads. From these results it can be con cluded that estrogen synthesis is crucial for ovarian differentiation, and transcription of the aromatase gene can be proposed as a key step in that p rocess in fish. (C) 1999 Wiley-Liss, Inc.