Permanent and functional male-to-female sex reversal in d-rR strain medaka(Oryzias latipes) following egg microinjection of o,p '-DDT

Complete sex reversal of fish is accomplished routinely in aquaculture prac tices by exposing fish to exogenous sex steroids during gonadal differentia tion. A variety of environmental chemicals are also active at sex steroid r eceptors and theoretically possess the potential to alter normal sexual dif ferentiation in fish. However, in controlled environmental chemical exposur es to date, only partial alterations of fish sexual phenotype have been obs erved. Here we report complete, permanent, and functional male-to-female se x reversal in the Japanese medaka (Oryzias latipes, d-rR strain) after a on etime embryonic exposure to the xenoestrogen o,p'-DDT. d-rR strain medaka a re strict gonochorists that possesses both sex-linked pigmentation, which d istinguishes genotypic sex, and sexually dimorphic external secondary sexua l characteristics, which distinguish phenotypic sex. We directly microinjec ted the xenoestrogen o,p'-DDT into the egg yolks of medaka at fertilization to parallel the maternal transfer of lipophilic contaminants to the embryo . At 10 weeks of age, microinjected medaka were examined for mortality and sex reversal. A calculated embryonic dose of 511 +/- 22 ng/egg o,p'-DDT (me an +/- standard error) resulted in 50% mortality. An embryonic exposure of 227 +/- 22 ng/egg o,p'-DDT resulted in 86% (6 of 7) sex reversal of genetic males to a female phenotype (XY females). XY females were distinguished by sex-linked male pigmentation accompanying female secondary sexual characte ristics. Histologic examination of the gonads confirmed active ovaries in 1 00% of the XY females. In 10-day breeding trials in which XY females were p aired with normal XY males, 50% of the XY females produced Fertilized embry os; this represents a comparable breeding success rate to normal XX females . fertilized eggs produced from XY females hatched to viable larvae. These results clearly indicate that a weakly estrogenic pesticide, o,p'-DDT, when presented during the critical period of gonadal development, can profoundl y alter sexual differentiation.