Sex reversal in medaka treated in vitro with 17alpha-methyldihydrotestosterone during oocyte maturation.

Using the S-rR strain of the medaka Oryzias latipes, we examined the effect of a non-aromatizable androgen on sex determination. Intrafollicular immature oocytes isolated before breakdown of the germinal vesicle were incubated in the presence of 17alpha-methyldihydrotestosterone (MDHT) for about 10 h during their maturational period. At the end of incubation, mature oocytes were rinsed and then artificially inseminated in regular saline. The fertilized eggs were then allowed to develop in tap water, and the fry were reared on a regular powdered diet until adulthood. Sex reversal of female to male was observed in a manner dependent on the dose of MDHT. In the solvent control group in which intrafollicular oocytes were matured in medium containing no exogenous androgen, no sex reversal was observed. The present finding, that the sex of medakas can be reversed by a single in vitro exposure of immature oocytes to androgen during the preovulatory period, suggests the existence in the oocyte of a sex determinant sensitive to sex steroids. This method for controlling the sex of eggs before fertilization may establish sex-determined eggs as potent material for investigating the mechanism of sex determination in the medaka.