National Geographic Today
Atrazine, a top selling weed killer in the United States and the world,
has been found to dramatically affect the sexual development of male
frogs, turning them into hermaphrodites—creatures with both male
and female organs—at concentrations 30 times lower than those
deemed safe by the U.S. Environmental Protection Agency
(EPA).
"What struck us as unbelievable was that atrazine could
cause such dramatic effects at such low levels," says Tyrone Hayes, an
associate professor of integrative biology at the University of
California, Berkeley, who led the frog study.
"If you take five grains of salt, divide this weight by five thousand, that is the amount of atrazine that causes these abnormalities," added Hayes.
The EPA has set the limit for atrazine contamination for drinking water at 3 parts per billion (ppb). Hayes' and his colleagues found that concentrations of 0.1 ppb caused abnormalities.
The scientists exposed tadpoles of the African clawed frog, a popular research organism, to atrazine at concentrations as low as 0.1 ppb. When the tadpoles reached adulthood, Hayes found that between 16 and 20 percent had abnormal reproductive systems.
"Some had three ovaries and three testes, some had ovaries on one side and testes on the other, one animal even had six testes," says Hayes. The male voice box also shrunk, resembling the female version. And, when males were exposed to levels as high as 25 ppb of atrazine, the frogs showed a ten-fold decrease in testosterone levels.
Based on work done in other laboratories, Hayes' team suspects that atrazine feminizes the frogs by increasing production of an enzyme called aromatase, which converts testosterone to estrogen.
These results are published in the April 16 issue of Proceedings of the National Academy of Sciences.
Midwest Frogs Show Abnormalities
Hayes' team has also discovered that leopard frogs, native to the United States, living near atrazine-contaminated ponds in the Midwest show the same abnormalities as the atrazine-exposed animals in his lab. But these results must be viewed with caution until they have been peer reviewed, says Hayes.
The effects of atrazine on frogs had been examined prior to Hayes' work, but similar abnormalities had not been reported. The reason, according to Hayes: "They were looking for the wrong things."
"Most people were looking for external deformities, mortality or cancer," says Hayes. But at all the doses tested—0.01 to 200 ppb—atrazine did not effect mortality, alter metamorphosis or produce obvious lesions. "Atrazine-induced abnormalities are subtler—it took a year of experimentation before even we noticed the consequences," says Hayes.
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