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Hiromi Ohtani,¹ Ikuo Miura,¹ and Youko Ichikawa²

¹Laboratory for Amphibian Biology, Faculty of Science, Hiroshima University, Higashihiroshima, Japan; ²Department of Health Science, Faculty of Human Life and Environmental Science, Hiroshima Prefectural Women's University, Hiroshima, Japan

Abstract

To examine the effects of dibutyl phthalate (DBP) on gonadal sex differentiation, genetically male tadpoles of Rana rugosa were exposed to dilute solutions of DBP at concentrations of 0.1, 1, or 10 µM during days 19-23 after fertilization, which is the critical period of gonadal sex differentiation in R. rugosa. Tadpoles were necropsied on day 40. The genetically male tadpoles were produced from crossings between males (ZZ) of one local population, in which females are the heterogametic sex, and females (XX) of another local population, in which males are the heterogametic sex. As positive control groups, tadpoles were exposed to dilute solutions of 17ß-estradiol (E₂) at concentrations of 0.01, 0.1, or 1 µM during the same period. The internal structure of the gonads was histologically examined in a total of 30 control tadpoles, 86 E₂-treated tadpoles, and 90 DBP-treated tadpoles. The gonads of the control tadpoles all showed the typical structure of testes. In contrast, 0.01, 0.1, and 1 µM E₂ treatments caused the undifferentiated gonads of 18, 63, and 100% of the tadpoles, respectively, to develop into gonads of complete or partial ovarian structure. After 0.1, 1, and 10 µM DBP treatment, 0, 7, and 17% of tadpoles, respectively, were similarly affected. These findings suggest that DBP was about 1,000-fold less potent than E₂. Nevertheless, DBP is an environmentally dangerous hormone that disrupts the pathways of testicular differentiation in genetically male animals. Key words: environmental endocrine disruptors, estrogens, frogs, gonadal sex differentiation, phthalate ester toxicity, plasticizers, sex chromosomes, sex-determining systems. Environ Health Perspect 108:1189-1193 (2000) . [Online 15 November 2000]

http://ehpnet1.niehs.nih.gov/docs/2000/108p1189-1193ohtani/ abstract.html

Address correspondence to Y. Ichikawa, Department of Health Science, Faculty of Human Life and Environmental Science, Hiroshima Women's University, Hiroshima 734-8558, Japan. Telephone: +81-82-251-9838. Fax: +81-82-251-9405. E-mail: ichikawa@hirojo-u.ac.jp

We thank J.N. Raybould for his corrections to the manuscript.

We are grateful for the support of the Extensive Research Program of Hiroshima Prefectural Government.

Received 22 March 2000 ; accepted 15 August 2000.