Role of follicle-stimulating hormone in gonadal sex differentiation via expression of steroidogenic enzymes in Nile tilapia, Oreochromis niloticus

Estradiol-17β (E2) production is the key event for the initiation of ovarian differentiation in Nile tilapia. The enzyme aromatase, encoded by the cyp19a1a, converts testosterone into E2. Along with the transcription factors foxl2 and ad4bp/sf1, follicle-stimulating hormone (FSH) is also suggested to be involved in cyp19a1a expression. Although fsh is expressed equally in genetic female (XX) and genetic male (XY) pituitaries, expression of the FSH receptor (fshr) is significantly higher in XX undifferentiated gonads than in XY undifferentiated gonads during early sex differentiation, possibly indicating that FSH may play a role in ovarian differentiation via cyp19a1a expression. To investigate whether FSH signaling influences E2 production through the expression of steroidogenic enzymes, we injected recombinant FSH (rFsh) into XX larvae during early sex differentiation and then assessed the expression of various sex differentiation–related genes. We found that after the injections, hsd3b levels were significantly higher in the rFsh injection group than in the control groups, and that the levels of cyp11a1, cyp17a1, and cyp19a1a tended to be higher in the rFsh injection group. These results suggest that FSH may induce the expression of cyp11a1, hsd3b, cyp17a1, and cyp19a1a, thereby contributing to E2 production. However, FSH stimulation of the expression of these enzymes does not appear to be strong, indicating that the role of FSH in E2 production may be limited and suggesting a compensatory role in ovarian differentiation.