Hermaphrodite Frogs Caused By Popular Weed Killer?
By Bijal P. Trivedi
National Geographic Today
|April 16, 2002|
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 hermaphroditescreatures with both male
and female organsat concentrations 30 times lower than those
deemed safe by the U.S. Environmental Protection Agency
"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 tested0.01 to 200 ppbatrazine did not effect mortality, alter metamorphosis or produce obvious lesions. "Atrazine-induced abnormalities are subtlerit took a year of experimentation before even we noticed the consequences," says Hayes.
"Tyrone's work shows the lack of sensitivity of current testing protocols," says Theo Colborn, a senior scientist at the World Wildlife Fund. "We really don't know how to test chemicals properly. Tyrone's work shows that low dose exposure during development can have insidious hidden effects that can change the chemistry of the animal and significantly affects its development."
"Frogs are vertebrates like us and have similar endocrine systemsthat is they use the same hormones to control development," says David Wake, an amphibian biologist at Berkeley. "Hayes' work is excellent. We must assume that his findings have relevance for us."
It is too early to say whether atrazine is responsible for the global decline in amphibian populations, but both Colborn and Wake agree that it is an angle that needs to be pursued.
"What is most disturbing," adds Wake, "is that the effects are manifested at extraordinarily low concentrations of atrazinealmost what you get from rainwater."
Atrazine can be as high as 21 ppb in groundwater, 102 ppb in river basins in agricultural areas, and 224 ppb in streams in the Midwest. There is virtually no atrazine-free environment, says Hayes, who adds that the herbicide has been used for 40 years in over 80 countries.
But not everyone agrees with Hayes' results.
"We have repeated Hayes' work and have seen effects only at levels of 25 ppb," says Ernest Smith, a biologist at the Institute of Environmental and Human Health at Texas Tech University. Smith says that at concentrations below 25 ppb neither he nor his colleagues observed abnormal numbers of testes or ovaries. And at no levels did they see any effects on the larynx.
In contrast with Hayes' study, Smith found that tadpoles exposed to 25 ppb atrazine caused abnormalities in only 4 or 5 percent of adults.
"Dr. Hayes' study is what it is," says John Giesy, an aquatic toxicologist at Michigan State University. Giesy is currently examining the effects of atrazine on adult frogs and investigating whether the pesticide affects their survival, growth and reproductive capabilities.
Both Smith and Giesy's studies were partially funded by Syngenta, the manufacturer of the herbicide. Syngenta also funded Hayes' studies until November 2000, when he decided to pursue the work independently.
Field studies have begun in regions of South Africa where atrazine has been used, and also planned for Michigan and Iowa where the herbicide is used extensively on corn and sorghum crops.
Both Giesy and Smith were present at a panel discussion held today by the EPA to review recent research on atrazine and re-evaluate allowable levels in drinking water. The EPA is required to make a final decision on the regulation of atrazine by August 5, 2002.
This story will be reported on National Geographic Today tonight.
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