Estradiol-17β content in developing eggs and induced sex reversal of the medaka (Oryzias latipes)

To clarify the effect of exogenous estradiol-17beta (E-2) on sex differentiation, the E-2 content of developing eggs of Oryzias latipes was measured by radioimmunoassay. Endogenous E-2 was present in lower concentrations in ovulated, mature eggs in the ovarian cavity than in intrafollicular oocytes. The E-2 content of eggs precipitously declined to a minimum level by 2 days post-fertilization. The E-2 content of eggs was affected by 24 hr of incubation in medium containing exogenous E-2 at concentrations above 10 ng/ml. Short (24 hr) exposure of fertilized eggs in the early developmental stage to exogenous E-2 at concentrations of 10 ng/ml induced sex reversal of some genotypic males to functional females. However, endogenous E-2 levels in fertilized eggs might not influence sexual differentiation in embryogenesis. The present results suggest the possibility that concentrations of exogenous E-2 higher than that of endogenous E-2 triggers a priming step in the cascade of sex differentiation toward the female, and this effect is maintained (C) 2005 Wiley-Liss, Inc.