Persistent sex-reversal and oviducal agenesis in adult Xenopus (Silurana) tropicalis frogs following larval exposure to the environmental pollutant ethynylestradiol

It is known that estrogen-like environmental pollutants can feminise gonadal differentiation in frogs resulting in female-biased sex-ratios at metamorphosis. The long-term effects on reproductive function in frogs following larval exposure to pollutants are less known. Amphibian test systems which allow life-cycle studies are therefore needed. The aim of the present study was to characterise long-term estrogenic effects on the reproductive system of the emerging model species Xenopus (Silurana) tropicalis following larval exposure to ethynylestradiol (EE2). EE2 is a synthetic estrogen that has been detected in sewage effluents and in surface waters. Newly hatched tadpoles (Niewkoop Faber (NF) stage 48) were exposed to the nominal EE2 concentrations 0 (control), 1, 10, and 100nM (with analytical chemistry support) until complete metamorphosis (NF stage 66). Effects on the reproductive organs were determined in juveniles (1 month after metamorphosis) and in 9-month-old frogs. Larval exposure to EE2 caused female-biased phenotypic sex-ratios in both juvenile and adult frogs, which is in agreement with previous work on other frog species. Nearly all (97%) of the 63 EE2-exposed 9-month-old frogs had ovaries. Histological evaluation of the gonads of the 9-month-old frogs showed that they were sexually mature. Among the adult frogs with ovaries there was a dose-dependent increase in the frequency of individuals lacking oviducts. Adult frogs exposed to 100nM EE2 that had ovaries but no oviducts had lower levels of estrogen receptor alpha (ER ) mRNA in the brain than control animals and those exposed to 100nM EE2 that had ovaries as well as oviducts. EE2 exposure did not cause any significant changes in ER mRNA levels in the ovaries of the adult frogs. The reduced level of ER mRNA in the brain of individuals with ovaries lacking oviducts suggests an organizing effect of EE2 on the central nervous system. The results show that transient early life-stage exposure to an environmental pollutant can induce effects on the reproductive organs and the central nervous system that persist into adulthood. Overall, our data suggest that X. tropicalis, which has a shorter generation time than the well-established model species Xenopus laevis, is a suitable model organism for research on developmental reproductive toxicity in anuran species.