Atrazine is one of the world's most commonly used herbicides. It is typically used to control weeds in agriculture, such as in the production of maize crops. Atrazine can be carried up to 1000km by rainfall from where it was originally applied, and can contaminate environments that would otherwise be clear of the herbicide. The herbicide can persist in the environment and is a significant pollutant of ground and surface water. The EU banned the use of atrazine with effect from 2005 under council directive 91/414/EEC 1.
It is thought that atrazine acts as an endocrine disruptor (disrupting the hormonal system), affecting the reproductive development of fish, amphibians, reptiles and human cell lines in concentrations of ppb (parts per billion) and at higher doses of ppm (parts per million) in reptiles, birds and laboratory rodents. Amphibians, such as frogs and toads, appear to be particularly sensitive to atrazine in concentrations as low as 0.1 ppb.
In this study, the researchers investigated the long-term effects of atrazine on the sexual development of an all-male population of African clawed frogs. The frogs were exposed to 2.5 ppb atrazine throughout larval development and up to 3 years after changing into adult animals.
The research demonstrated that atrazine-exposed frogs suffered from feminisation and demasculinisation (chemical castration) when they became adults. Testosterone levels were diminished; nuptial pads (thumb pads) and breeding glands on the forearms were smaller; the larynx (voice box) was feminised and sperm development was impaired in 75 per cent of the exposed frogs. In addition, when competing for females during mating, emasculated males were out-competed by normal males.
More alarmingly, 10 per cent of atrazine-exposed frogs appeared to be female at sexual maturity. These frogs displayed external feminine features, and had ovaries internally. In addition, atrazine-induced females produced viable eggs when allowed to mate with normal male frogs. This implies the genetic males that had been exposed to atrazine had been completely feminised. All the resulting offspring were male when raised to maturity, because the sex-reversed males are genetic males (unlike humans, it is male frogs that carry two identical male sex chromosomes). In the wild, sex-reversed animals could distort sex ratios in populations to the point where populations are driven to extinction.
The impact of atrazine on amphibians and other wildlife is potentially of huge concern. The 2.5 ppb level of exposure in this study is within the range amphibians might encounter in areas where atrazine is used, and is also lower than the current US Environmental Protection Agency drinking water standard of 3 ppb. Recent studies suggest amphibians absorb atrazine through the skin at higher rates than mammals and the study suggests atrazine and other hormone-disrupting chemicals are likely to contribute to the global decline of amphibians.
Source: Hayes, T.B., Khoury, V., Narayan, A., et al. (2010). Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). Proceedings of the National Academy of Sciences. 107(10): 4612-4617.