Role of gonadal soma-derived growth factor (gsdf) on sex-differentiation in Nile tilapia, Oreochromis niloticus

Molecular sex differentiation is regulated by complex transcriptional dynamics. In Nile tilapia, ovarian differentiation requires the synthesis of estradiol-17β (E₂), in which forkhead box L2 (foxl2) and ovarian aromatase (cytochrome P450, family 19, subfamily A, polypeptide 1a; cyp19a1a) play critical roles, whereas gonadal soma-derived growth factor (gsdf) and doublesex and mab-3-related transcription factor 1 (dmrt1) are implicated in testicular differentiation. During the early phase of molecular sex differentiation, gsdf mRNA is expressed to a significantly higher degree in the undifferentiated gonads of XY males than in XX females. However, the molecular mechanisms by which gsdf promotes testicular differentiation remain poorly understood. We hypothesized that gsdf acts by suppressing ovarian differentiation or promoting testicular differentiation through the upregulation of dmrt1. To clarify the relationship with dmrt1, we performed fluorescent double in situ hybridization on undifferentiated gonads and testes. Furthermore, recombinant Gsdf (rGsdf) was produced and microinjected into the body cavity of XX larvae to investigate its effects on the expression of sex differentiation-related genes in undifferentiated gonads. We demonstrated that gsdf and dmrt1 were expressed in distinct cell populations in undifferentiated gonads, whereas partial co-localization occurred in certain cells within the testes. Administration of rGsdf led to a significant decrease in the expression of E₂ synthesis-related genes, including foxl2, hydroxysteroid 17-beta dehydrogenase 1 (hsd17b1), and cyp19a1a. These findings suggest that gsdf suppresses E₂ production and that its expression is regulated not only by dmrt1 but also by other factors during the molecular sex differentiation phase in Nile tilapia.