Is Responsible for Androgen-Induced Masculinization in Nile Tilapia.

17α-Methyltestosterone (MT) is a widely used androgen for all-male fish production in aquaculture. However, the molecular mechanism underlying MT-induced masculinization remains unclear. In this study, we aim to identify the key gene responsible for MT-induced masculinization using the Nile tilapia () , , and mutants, which exhibit male-to-female sex reversal. Nile tilapia fry from these three mutant lines were treated with 50 μg/g MT from 5 to 30 days after hatching (dah). The results showed that and mutants, but not mutants, were masculinized by the MT treatment. Gonadal transcriptome analysis revealed that genes involved in steroidogenesis and germ cell development in MT-treated mutants exhibited a similar expression pattern to that of the wild type (WT) XX. In addition, the mutants cannot be masculinized by co-treatment with MT and the aromatase inhibitor fadrozole. The MT treatment completely blocked early steroidogenic enzyme (Star2, Cyp17a2, and Cyp19a1a) expression independent of , , and . A luciferase analysis showed that MT directly suppressed basal and Sf-1-activated promoter activity through and in cultured HEK293 cells. Furthermore, MT treatment inhibited germ cell proliferation in and mutants but not in mutants. Consistently, expression was induced in MT-treated WT XX, -, and - mutants. Taken together, these results suggest that is indispensable for MT-induced masculinization in Nile tilapia and that MT functions by inhibiting early steroid synthesis and activating to promote testis development.