An in vivo model fish system to test chemical effects on sexual differentiation and development: exposure to ethinyl estradiol

A model system was characterized which may be used as an in vivo screen for effects of chemicals or environmental mixtures on sexual differentiation a nd development of reproductive organs and gametes, We evaluated the effects of a model environmental estrogen, ethinyl estradiol (EE,), on the d-rR st rain of medaka, Oryzias latipes, using a nana-injection exposure. Gonad his topathology indicated that a-single injection of 0.5-2.5 ng EE2/egg can cau se phenotypic sex-reversal of genetic males to females. Sex-reversals could be detected as early as 7 days post-hatch, Sex-reversed males had female-t ypical duct development and the secondary:sex characteristics we measured w ere generally consistent with phenotype, with the exception of a few EE2-ex posed XX and XY females which possessed ambiguous anal fins. Using discrimi nant analysis, we determined that the presence or absence of the secondary sex characteristic, a dorsal fin notch, was a very reliable indicator of go nadal sex. No instances of gonadal intersexes were observed, Ethinyl estrad iol also appeared to reduce growth but not condition (weight-at-length) and exposed XX females appeared to have a higher incidence of atretic follicle s relative to controls. Our results suggest that estrogenic chemicals may i nfluence sexual differentiation and development:and that the medaka model i s well suited to assessing these effects. (C) 2000 Elsevier Science B.V. Al l rights reserved.