A witches' brew of controversy is bubbling up over the potential link between atrazine, one of the most widely used herbicides in the United States, and the decline of amphibians. The latest additions to the pot are new findings, published in the 31 October issue of Nature, suggesting that exposure to very low levels of atrazine in the wild is turning male frogs into hermaphrodites. But other results in a different frog species cast doubt on such low-dose effects.
Earlier this year developmental endocrinologist Tyrone Hayes and colleagues at the University of California, Berkeley, set the kettle boiling when they reported that male tadpoles exposed to low levels of atrazine in the lab developed into hermaphrodites or had other deformities in their reproductive organs (Science, 19 April, p. 447). The study used African clawed frogs (Xenopus laevis), known as the "lab rat" of amphibian toxicology studies.
The new study extends the finding to a native species, the leopard frog (Rana pipiens). When Hayes's team exposed Rana tadpoles in the lab to atrazine, they found that the testes of some males were underdeveloped or contained oocytes--eggs that should only be found in female frogs. The team also examined 800 newly metamorphosed Rana frogs from eight sites across the western and midwestern United States. They found hermaphrodite males at the seven sites with waterborne atrazine contamination higher than 0.2 parts per billion (ppb). (Drinking water in the United States is allowed to have up to 3 ppb atrazine.) The one site with no measurable amounts of atrazine hosted healthy frogs.
But other teams, including one led by experimental toxicologist James Carr of Texas Tech University in Lubbock, have been unable to replicate Hayes's original Xenopus findings and are skeptical of the current findings. Some of these researchers say that their most recent experiments--including a field study of Xenopus in South Africa, the frog's native habitat--do not support a link between low doses of atrazine and amphibian abnormalities. Several of these studies will be presented at the Society of Environmental Toxicology and Chemistry annual meeting in Salt Lake City, Utah, 16 to 20 November.
The debate could have a bearing on continued regulatory approval for atrazine. The U.S. Environmental Protection Agency (EPA) is reevaluating the risk that atrazine affects human or ecological health at environmental levels. The possible link between atrazine and reproductive-organ abnormalities in frogs is so important to this reevaluation that the agency is delaying the process to convene a panel of scientists to evaluate the available data, according to an EPA spokesperson.