Relationships between gonadal and gonadotrophic differentiation during early gonadal sex differentiation in several strains from the Southern population of Japanese medaka (Oryzias latipes).

In this study, correlations between gonadal differentiation and gonadotrophic differentiation during early gonadal sex differentiation were examined in several strains from the Southern population of Japanese medaka, namely, Hd-rR, d-rR/Tokyo, d-rR/Shizuoka, and NIESR. Gonadal sex differentiation showed a similar pattern in all strains, that is, a sex-based difference in germ cell number was observed at hatching (XX > XY). In addition, a transition of XX germ cells into meiosis was observed at hatching in all strains. However, the ratio of meiotic transition of XX germ cells differed among strains, indicating that Hd-rR has fewer meiotic cells than other strains. Immunohistochemical analysis showed that glycoprotein α subunit (Gpa) and follicle-stimulating hormone β subunit (Fshb), but not luteinizing hormone β subunit (Lhb), localize to the pituitary at hatching in all strains examined. Although no sex differences were observed in the numbers of Gpa- and Fshb-positive cells in the pituitary at hatching in Hd-rR, d-rR/Tokyo, and NIESR strains, the d-rR/Shizuoka strain showed sex differences in the number of Fshb-positive cells at hatching (XX > XY); d-rR/Shizuoka is derived from d-rR/Tokyo. Even though d-rR/Shizuoka differs from its ancestral strain, d-rR/Tokyo, and Hd-rR in Fshb expression pattern, this suggests that d-rR/Shizuoka shows potential for use as a model to study the molecular regulatory mechanisms of fshb subunit gene expression and Fsh cell differentiation in the pituitary.