Immunocytochemical Identification of GtH1 and GtH2 Cells during the Temperature-Sensitive Period for Sex Determination in Pejerrey, Odontesthes bonariensis

Abstract

The ontogeny of gonadotropin 1 (GtH1) and 2 (GtH2) cells and its possible link to gonadal sex differentiation were studied in pejerrey, Odontesthes bonariensis, by immunocytochemistry using anti-chum salmon β-GtH1 and β-GtH2 antisera. In adults, GtH1 cells were found in the proximalis pars distalls (PPD) close to the neurohypophysis, whereas GtH2 cells were identified surrounding GtH1 cells, at the external layer of the PPD and in the pars intermedia. Essentially the same distribution was observed in larvae. In pejerrey, the phenotypic sex is governed by the temperature during the critical period of sex determination (temperature-dependent sex determination, TSD). Female proportions vary gradually from 100% at 15–19° to 0% at 29°, and the critical time of TSD has been estimated to be 3–5, 2–4, and 1–4 weeks after hatching at 17, 19, and 27°, respectively. Thus, the expression of both GtHs was examined weekly in larvae reared from hatching to week 11, at 17, 24, and 29°. The proportion of females at 17, 24, and 29° was, 100%, 78%, and 0%, respectively. Histological ovarian differentiation was noticed at 7 and 4 weeks at 17 and 24°, whereas testicular differentiation occurred at 7 and 4 weeks at 24 and 29°. GtH1 cells were first observed at week 3 at 29° and at week 4 at 17 and 24°. These cells increased in number until week 4 and then decreased, disappearing after week 6 at all temperature regimes. GtH2 cells appeared at week 2 at 24 and 29° and at week 3 at 17°. GtH2 cell number increased until week 3 at 29° and until week 4 at 17 and 24° and then temporarily decreased, thereafter increasing again. These results strongly suggest that GtH1 and GtH2 are expressed by different cells. The fact that GtH1 and GtH2 cells appear just before histological gonadal differentiation at all temperatures, together with the peak of GtH1 and GtH2 cell number during the temperature-sensitive period, suggests that GtHs are related to sex differentiation or TSD in O. bonariensis.