Abstract
The process of gonadal sex differentiation in the F2 hybrid sturgeon, the bester (Huso huso female × Acipenser ruthenus male), was investigated histologically. In fish more than 6 months old, two types of gonads were distinguishable. In the putative ovary, the germinal epithelium had invaginations in the stroma of the gonad, and a few germ cells were localized immediately below the epithelium. In putative testis, invaginations were lacking and germ cells were observed deep in the stroma. However, active mitotic and meiotic germ cells were not observable even by 16 months of age in ovaries, and only some of the testes showed initiation of spermatogenesis by 21 months of age. The invaginations in the stroma of putative ovaries developed into ovarian lamella-like structures, whereas the gonadal surface of putative testes remained smooth. These results suggest that the formation of invaginations in the stroma of gonads is the morphological index of ovarian differentiation and that it is necessary to start hormone treatment for functional sex reversal before 6 months of age in these fish.
