The Japanese medaka (Oryzias latipes) was used as an in vivo model to evaluate the effects of exposure to the synthetic estrogen 17alpha-ethinylestradiol (EE2) on reproductive behavior and reproductive success in fish. Exposures to EE2 began within 2 to 5 d posthatch and continued until medaka were sexually mature, between four and six months of age. Among male and female medaka exposed to EE2 at nominal concentrations of 0.2 and 2 ng/L, mating behavior and reproductive success were normal in reproductive trials. However, reproductive behavior (i.e., copulations) was suppressed in the treatment with 10 ng/L EE2. Among 19 males exposed to 10 ng/L EE2 and placed with unexposed females in reproductive trials, 16 males did not copulate and reproductive success was very low. None of the females exposed to 10 ng/L of EE2 participated in reproductive behavior with unexposed males. The reproductive trials in combination with a histological survey indicated that male fish with gonadal intersex (i.e., testis-ova) were still capable of reproductive behavior and could fertilize eggs. Even though females exposed to 10 ng/L EE2 had poor reproductive success, their ovaries showed normal development and oogenesis. These data are relevant to observations of intersex in feral fish populations. Although intersex in male fish may be an indicator of exposure to estrogenic compounds, it appears that the presence of oocytes in testicular tissue may not directly impact the reproductive capability of the male fish. However, it is clear that concentrations of endocrine disrupting chemicals (EDCs) that cause gonadal intersex are sufficient to reduce reproductive performance.

Exposure of amphibians to endocrine disrupting compounds (EDCs) may alter differentiation of gonads, especially when exposures begin during early life stages. Gonadal differentiation was observed in leopard frogs (Rana pipiens) and wood frogs (Rana sylvatica) exposed as tadpoles to estrogenic (estradiol, ethinylestradiol, nonylphenol) and antiestrogenic compounds (an aromatase inhibitor, flavone, and an antiestrogen, ICI 182780). Exposure to all compounds at micrograms/L concentrations altered gonadal differentiation in some animals by inducing either complete feminization or an intersex condition, and altered testicular tubule morphology, increased germ cell maturation (vitellogenesis), and oocyte atresia. Comparisons between the two species indicate that R. pipiens are more susceptible to sex reversal and development of intersex gonads. However, R. sylvatica also showed alterations to testicular morphology, germ cell maturation, and ooctye atresia. These laboratory results indicate that amphibians could be susceptible to altered gonadal differentiation and development when exposed to estrogenic and antiestrogenic compounds in aquatic environments, such as those impacted by agricultural, industrial, and municipal runoff.

Members of the Aamjiwnaang First Nation community near Sarnia, Ontario, Canada, voiced concerns that there appeared to be fewer male children in their community in recent years. In response to these concerns, we assessed the sex ratio (proportion of male births) of the Aamjiwnaang First Nation over the period 1984–2003 as part of a community-based participatory research project. The trend in the proportion of male live births of the Aamjiwnaang First Nation has been declining continuously from the early 1990s to 2003, from an apparently stable sex ratio prior to this time. The proportion of male births (m) showed a statistically significant decline over the most recent 10-year period (1994–2003) (m = 0.412, p = 0.008) with the most pronounced decrease observed during the most recent 5 years (1999–2003) (m = 0.348, p = 0.006). Numerous factors have been associated with a decrease in the proportion of male births in a population, including a number of environmental and occupational chemical exposures. This community is located within the Great Lakes St. Clair River Area of Concern and is situated immediately adjacent to several large petrochemical, polymer, and chemical industrial plants. Although there are several potential factors that could be contributing to the observed decrease in sex ratio of the Aamjiwnaang First Nation, the close proximity of this community to a large aggregation of industries and potential exposures to compounds that may influence sex ratios warrants further assessment into the types of chemical exposures for this population. A community health survey is currently under way to gather more information about the health of the Aamjiwnaang community and to provide additional information about the factors that could be contributing to the observed decrease in the proportion of male births in recent years.

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