| Abstract |
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.
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