Loss of growth differentiation factor 9 causes an arrest of early folliculogenesis in zebrafish – a novel insight into its action mechanism

Growth differentiation factor 9 (GDF9) is the best characterized growth factor released by the oocyte; however, most information about GDF9 functions comes from studies in the mouse model. In this study, we created a mutant for Gdf9 gene ( gdf9-/- ) in zebrafish using TALEN approach. The loss of Gdf9 caused a complete arrest of follicle development at primary growth (PG) stage. These follicles eventually degenerated, and all mutant females gradually changed to males through sex reversal, which could be prevented by mutation of the male-promoting gene dmrt1 . Interestingly, the phenotypes of gdf9-/- could be rescued by mutation of inhibin α ( inha -/-) but not estradiol, suggesting a potential role for the activin-inhibin system in Gdf9 actions. In gdf9 null follicles, activin βAa ( inhbaa ) expression decreased dramatically; however, its expression rebounded in the double mutant ( gdf9 -/- ;inha-/- ). These results clearly indicate that although endocrine hormones such as follicle-stimulating hormone (FSH) are important for folliculogenesis, the activation of PG follicles requires intrinsic factors from the oocyte, such as Gdf9, which in turn works on the neighboring follicle cells to trigger follicle activation, probably via activins.