Weed killer linked to frog mutations

Males exposed to low doses can develop multiple sex organs, Calif. scientists find


WASHINGTON - Male frogs exposed to very low doses of a common weed killer can develop multiple sex organs - sometimes both male and female - researchers in California have discovered.

"I was very much surprised," at the effect on developing frogs of atrazine, an herbicide used on crops and city weeds, said Tyrone B. Hayes of the University of California at Berkeley.

Atrazine is the most commonly used weed killer in North America. Farmers on the Eastern Shore and across the country spray it on corn. Road crews and property managers use it to control weeds. It can be found in rainwater, snow runoff and ground water.

"There is virtually no atrazine-free environment," Hayes said.

In the mid-Atlantic, the weed killer was found in 72 percent of 2,000 water samples taken by the U.S. Geological Survey, including in the Potomac and Susquehanna rivers and on the Eastern Shore. It was found in urban and rural waterways, said the Survey's Scott Phillips. Peak levels occurred in May, June and July - the season when frogs and other amphibians have emerged from winter hibernation and are breeding.

Almost all of the samples taken in the region were lower than the maximum level of 3 parts per billion that the Environmental Protection Agency permits in drinking water.

But Hayes' team found atrazine affected frogs at doses as small as 0.1 part per billion. As the amount of atrazine increased as many as 20 percent of frogs exposed during their early development produced multiple sex organs or had both male and female organs. Many had small, feminized larynxes.

Scientists have been trying to solve the mystery of widespread frog deformities since 1995, when a group of Minnesota schoolchildren on a field trip found frogs with missing legs, extra legs or other problems. Since then, a variety of deformities has been found in different parts of the country and different frog species. Some studies have pointed the finger at parasites, pesticides or high levels of ultraviolet light. Most researchers think there may be no single cause for the deformities.

Frogs are also disappearing from places where they once were common, in the United States, Australia and other countries. Because they spend most of their lives in water, scientists think they may be especially sensitive to environmental changes such as pollution or the loss of ponds and marshes.

Hayes' research team concluded that atrazine causes the frogs' cells to produce an enzyme called aromatase, which is present in all vertebrates, including humans. Aromatase converts the male hormone testosterone to the female hormone estrogen.

The effects on frogs in Hayes' study occurred at exposure levels more than 600 times lower than the dose that has been seen to trigger aromatase production in human cells.

The research is reported in today's issue of Proceedings of the National Academy of Science.

Hayes said he did not know whether atrazine might also be a threat to people at low levels. Unlike frogs, "we're not in the water all the time," he said. "I'm not saying it's safe for humans. I'm not saying its unsafe for humans. All I'm saying is it makes hermaphrodites of frogs," Hayes said.

Stanley I. Dodson of the University of Wisconsin at Madison called the work "the most important paper in environmental toxicology in decades."

"It shows the effect of the most commonly used herbicide on amphibians in environmentally relevant concentrations," he said.

Asked whether people should be worried, he also said: "We don't know. It's like a canary in the mine shaft sort of thing." He was referring to a traditional practice among miners, who brought canaries with them as warnings of dangerous gases. The sensitive birds would die before the concentration of the gas was enough to harm the miners.

In addition to its effects on developing frogs, the Berkeley researchers found that male frogs exposed to atrazine after reaching maturity had a decrease in testosterone to levels equivalent to that found in females.

Sun staff writer Heather Dewar contributed to this article.

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