Reproductive toxicity in Xenopus tropicalis after developmental exposure to environmental concentrations of ethynylestradiol

Reproductive disorders in wildlife and humans have been linked to developmental exposure to endocrine disrupting chemicals. In frog tadpoles, environmental concentrations of ethynylestradiol (EE2) disrupt gonadal differentiation which results in female-biased sex ratios at metamorphosis indicating sex-reversal of genotypic males. It is not known if developmental exposure to estrogens results in reduced reproductive success in amphibians. The objective of this work was to investigate if exposure to environmentally relevant concentrations of EE2 during sex differentiation impairs reproductive organ development, fertility, and sexual behavior in adult frogs. A specific aim was to evaluate if testicular structure and function was affected in males that were not sex-reversed. Xenopus tropicalis tadpoles were exposed until metamorphosis to 6, 60, and 600 pM EE2. Eight months after metamorphosis, reproductive organ morphology and fertility were evaluated. Larval EE2-exposure caused an increased proportion of phenotypic females indicating that sex-reversal of genotypic males is persistent. Sex-reversal was implied at concentrations as low as 6 pM (1.8 ng/l), which is comparable to levels observed in the environment. EE2-exposed males that were not sex-reversed had a significantly reduced fertilization rate compared with control males. Histological evaluation revealed that EE2-exposed males had a reduced amount of spermatozoa in the testis. Among frogs with ovaries there was a significantly higher percentage that lacked oviducts in the group exposed to 600 pM EE2 compared with control females. No effect of EE2 on sexual behavior was noted. The results indicate that reproduction in wild frogs might be impaired by estrogenic environmental pollutants. Similarities between the present effects and those reported in fish, birds and mammals after developmental exposure to estrogens suggest that X. tropicalis is a promising animal model for research on developmental reproductive toxicity.