Estradiol‐17β content in developing eggs and induced sex reversal of the medaka (<i>Oryzias latipes</i>)

Abstract

To clarify the effect of exogenous estradiol-17beta (E2) on sex differentiation, the E2 content of developing eggs of Oryzias latipes was measured by radioimmunoassay. Endogenous E2 was present in lower concentrations in ovulated, mature eggs in the ovarian cavity than in intrafollicular oocytes. The E2 content of eggs precipitously declined to a minimum level by 2 days post-fertilization. The E2 content of eggs was affected by 24 hr of incubation in medium containing exogenous E2 at concentrations above 10 ng/ml. Short (24 hr) exposure of fertilized eggs in the early developmental stage to exogenous E2 at concentrations of 10 ng/ml induced sex reversal of some genotypic males to functional females. However, endogenous E2 levels in fertilized eggs might not influence sexual differentiation in embryogenesis. The present results suggest the possibility that concentrations of exogenous E2 higher than that of endogenous E2 triggers a priming step in the cascade of sex differentiation toward the female, and this effect is maintained.