U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

A Model Fish System to Test Chemical Effects on Sexual Differentiation and Development

By Diana M. Papoulias, Douglas B. Noltie, Donald E. Tillitt

This report is available in pdf format: Pappoulias.pdf

ABSTRACT

A model system was characterized which may be used as an in vivo screen for effects of chemicals or environmental mixtures on sexual differentiation and development of reproductive organs and gametes. We evaluated the effects of a model environmental estrogen, ethinyl estradiol (EE₂), on the d-rR strain of medaka, Oryzias latipes, using a nano-injection exposure. Gonad histopathology indicated that a single injection of 0.5-2.5 ng EE₂/egg can cause phenotypic sex-reversal of genetic males to females. Sex-reversals could be detected as early as seven days post-hatch. Sex-reversed males had female-typical duct development and the secondary sex characteristics we measured were generally consistent with phenotype, with the exception of a few EE₂-exposed XX and XY females which possessed ambiguous anal fins. Using discriminant analysis, we determined that the presence or absence of the secondary sex characteristic, a dorsal fin notch, was a very reliable indicator of gonadal sex. No instances of gonadal intersexes were observed. Ethinyl estradiol also appeared to reduce growth but not condition (weight at a given length) and exposed XX females appeared to have a higher incidence of atretic follicles relative to controls. Our results suggest that estrogenic chemicals may influence sexual differentiation and development and that the medaka model is well suited to assessing these effects. (The complete manuscript of this work was submitted to the journal Aquatic Toxicology, December, 1998).