Investigation into the effects of pesticides on amphibians

Abstract

Amphibian population decline is a recognised phenomenon spanning at least the last 40 years, and it is likely that a number of factors have contributed, including environmental contamination. Amphibians are vulnerable to agrochemical uptake as they must breed in water, and often spend the aquatic phase of their lifecycle in agricultural water bodies, which may contain a complex mixture of biologically active chemicals. Endocrine disrupting compounds may cause reproductive effects in humans and wildlife, although the link between pesticides and endocrine disruption is largely unknown. Therefore, in this study, the role of pesticides in endocrine disruption, in relation to amphibian metamorphosis and reproductive development, was investigated. To achieve this objective, population data were used to select suitable field sites, water from which was tested for endocrine activity using the yeast estrogen/androgen screen, hepatocyte culture (estrogenic response), and a transgenic Xenopus test (thyroid disruption). Toad (Bufo bufo) specimens from a subsample of these sites were used to compare morphology, thyroidal, and gonadal development of caged and wild-caught tadpoles/metamorphs, to their laboratory-raised counterparts. In addition, environmentally relevant pesticides were tested for endocrine effects in vitro, and a short-term in vivo exposure was used to assess the predictive ability of the in vitro screens in Xenopus. Mortality of Bufo bufo was high in both laboratory-reared and caged individuals, which hindered the interpretation of results due to low n values. However, laboratory-reared individuals from different sites had distinct morphology and gonadal differentiation, possibly suggesting maternal transfer, a latent effect of the pond environment, and/or genetic effects. In addition, caged and wild-caught individuals were smaller, metamorphosed later, and had retarded gonadal differentiation or increased incidence of intersex, compared to their laboratory-reared counterparts. Extracts of water samples from these sites were predominantly anti-estrogenic, and/or anti-androgenic in yeast based assays, and this was also the effect observed in response to environmentally relevant pesticides tested in the same assays. Pesticides also affected ovarian steroidogenesis in vitro, and pentachlorophenol had a reprotoxic effect on adult female Xenopus laevis. Data reported in this study suggest there may be endocrine disrupting effects in native amphibians in the agricultural landscape, although further investigation is needed to confirm these findings.