Researchers in the United States say they have turned male frogs into females by exposing the amphibians to tiny amounts of atrazine, a weed killer widely used on corn fields in Canada and often found in water supplies in agricultural areas.

The chemically induced sex change occurred by dosing frogs at concentrations of the herbicide 50 per cent below Health Canada's guideline for drinking water.

When the amphibians that had been chemically turned into females copulated with other male frogs that weren't given the herbicide, they laid eggs and all the resulting tadpoles were males.

The discovery that in an experimental setting atrazine induces sex changes in frogs is likely to further increase the controversy over the chemical, which has been banned in Europe because it contaminates ground water but is one of the most commonly used herbicides in North America.

The finding was based on a study led by researchers at the University of California Berkeley and released today online in the , a major peer-reviewed U.S. journal. The study concluded that the males had been “chemically castrated” and “completely feminized.”

“There is no question that atrazine completely sex-reversed genetic [chromosome] males, resulting in reproductively functional females,” it said.

The lead researcher, Tyrone Hayes, a professor of developmental endocrinology at the university, said the possibility that a herbicide can skew reproduction in frogs represents a potential new threat to the amphibians, whose populations are dwindling in many parts of the world.

“Everybody is focusing on things that kill frogs in terms of global amphibian decline, but you can easily imagine a population going extinct just because they don't breed properly,” Dr. Hayes said.

All the atrazine sold in Canada is made by Syngenta AG, a Swiss-based seed and pesticide producer that has previously disputed findings that the herbicide hazardous. “We stand behind the safety of our product,” said Judy Shaw, a company spokesperson.

She said Syngenta hasn't seen the latest research and isn't yet in a position to comment on it.

Atrazine has been dogged by controversy since the late 1980s, when it was found to cause mammary tumours in one strain of laboratory rats, a discovery followed by contested research indicating it may have gender bending impacts on amphibians and other types of animals.

The U.S. is currently re-evaluating the safety of the weed killer based on concerns it is polluting groundwater and may be a human health hazard. But in 2007 it rejected the view that the chemical can alter the sexual development of amphibians, in part because research hasn't consistently detected these effects.

Health Canada also conducted a review of atrazine released in 2007 that concluded the chemical doesn't “entail an unacceptable risk to the environment.”

But it is looking at the new research. “Health Canada is monitoring new findings on atrazine and will take additional regulatory action if necessary to protect human health and the environment,” it said in a response to the study.

Regulators typically don't react immediately to new studies until their effects are confirmed by other scientists, which would give the results added weight.

Some researchers are skeptical about the sex change finding. The study speculated atrazine causes frogs to increase the amount of a key enzyme known as aromatase, that converts testosterone to estrogen.

“That theory doesn't stand up to closer examination,” contends Keith Solomon, a professor of environmental biology at the University of Guelph. He said that previous testing hasn't found aromatase increases “in well-conducted laboratory studies.”

In the new study, the researchers exposed a group of male African clawed frogs, a commonly used laboratory species, to 2.5 parts per billion of atrazine from hatching onward, and another male group to none of the chemical.

Of the 40 exposed male frogs, four were turned into females, four were normal males, and the rest were emasculated, with decreased testosterone levels, feminized larynxes, and decreased sperm production. None of the so-called control animals experienced gender changing impacts.

A part per billion is an extremely small amount, the equivalent of one second or elapsed time over a 32-year period. The experimental dose used was below Canada's drinking water guideline of 5 ppb, but above the safe wildlife exposure standard of 1.8 ppb. Figures contained in Health Canada's 2007 atrazine evaluation found concentrations in groundwater of up to 1.2 ppb.

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