Suppression of steroidogenic enzyme expression during androgen-induced sex reversal in Nile tilapia (Oreochromis niloticus)

Exogenous sex steroids markedly alter sex differentiation in fish. The endocrine and molecular mechanisms involved in these changes remain unclear. To further clarify the mechanism of androgen-induced testicular differentiation, we treated female tilapia Oreochromis niloticus with methyltestosterone (MT at a dose of 50 μg/g diet) and examined the expression of P450 cholesterol-side-chain-cleavage, 3β-hydroxysteroid dehydrogenase, and cytochrome P450 aromatase (P450arom) in the gonads. MT treatment resulted in 100% masculinization. Untreated fish showed normal ovarian differentiation with strong expression of all three steroidogenic enzymes. In gonads of MT-treated fish, expression of all three steroidogenic enzymes was attenuated within 15 days and completely disappeared within 30 days of treatment. Our results indicate that exogenous androgen treatment suppresses the expression of key steroidogenic enzymes, including P450arom throughout sex differentiation in tilapia, thus masculinizing the animal. Whether the absence of aromatase or the presence of androgens is responsible for testicular differentiation remains to be determined.