Male frogs exposed to the herbicide atrazine can become female, produce eggs and mate with other males, scientists said in a study published online yesterday in the Proceedings of the National Academy of Sciences.
The report comes amid heated debate on atrazine's safety.
U.S. EPA is currently seeking feedback from an science advisory panel on its risk assessment for the herbicide. U.S. farmers apply 60 million to 70 million pounds of atrazine each year, and it is one of the most common contaminants found in U.S. drinking water.
Syngenta AG, the leading atrazine manufacturer, in an interview said it stands behind the safety of its product and that it expects EPA to make a positive decision based on sound science.
The study released yesterday was the latest to link the herbicide to sexual abnormalities in frogs and fish.
The research on the NAS Web site examined long-term effects of atrazine on reproductive development and function in 40 male African clawed frogs.
Researchers placed tadpoles in water containing 2.5 parts per billion of atrazine, an amount within EPA's drinking water standards. They found 10 percent of male frogs exposed to the herbicide turned into completely functioning females that mated with males and produced eggs.
The rest were also "chemically castrated," said lead researcher Tyrone Hayes, a professor at the University of California, Berkeley. They had lower sperm counts and were less able to compete with nonexposed males, he said.
"In previous studies, all people have looked at is what the frogs looked like," Hayes said. "With juveniles, we didn't know whether hermaphrodites were males with ovaries or females with testes. It's different here because we have a molecular marker, so we know who's chromosomally male. If they're females, we know they were genetic males that have been converted."
Hayes has published previous studies looking at atrazine's effects on amphibians. In 2002, he released research showing very low levels of exposure to atrazine can disrupt hormones and cause aberrant sexual development in male African clawed frogs, and that atrazine appears to make leopard frogs -- the most common, widely distributed native American frog -- hermaphrodites in the wild.
Several industry-backed studies have failed to replicate Hayes' findings, including research finding no hermaphrodites among wild-caught males of the clawed frog in Africa (Greenwire, Nov. 19, 2002).
Syngenta points to the work of Werner Kloas, a professor at the Humboldt University in Berlin, who found atrazine did not affect frogs at concentrations up to much higher levels than observed by Hayes. Kloas also tried to replicate Hayes' previous work and found no change resulting from atrazine exposure.
In a statement, Kloas criticized Hayes' new study for having "obviously big flaws concerning experimental design and inadequate statistics."
Keith Solomon, a professor emeritus at the University of Guelph in Canada, also questioned Hayes' findings.
"When you look at the big picture, everything is against all the literature that's out there," Solomon said. "I'm not rushing off to say one thing or another -- this is another small set of data that needs to be analyzed. I'll be interested to see if others can repeat these observations because I don't know if it's repeatable; there's less evidence of causality."
Syngenta also points to EPA's finding in 2007 that atrazine does not adversely affect amphibian gonadal development.
"All of the scientific issues have been reviewed thoroughly by EPA," said Tim Pastoor, a Syngenta scientist. "EPA's response declared that the issue of frog sexual development had been resolved; atrazine does not cause adverse effects. This next study that was recently published did not add to that at all; it's based on the same shaky foundation and does not add to the scientific literature."
Pastoor criticized Hayes' decision to use just one dose rather than a range of doses, as well as the study's lack of a positive control group, which he called the study's "fundamental fatal flaw."
But Jason Rohr, an assistant professor at the University of South Florida, said there is growing data suggesting atrazine adversely affects amphibians, even without Hayes' work.
Rohr last year examined more than 100 studies of the environmental effects of atrazine, finding that while the weed killer does not kill fish or amphibians, it can lead to changes in their reproductive, immune and other systems (Greenwire, Sept. 30, 2009).
"They [Syngenta] are partially correct -- there hasn't been a study that's been able to replicate Tyrone's results at given concentrations," Rohr said. "But there are studies that show evidence of endocrine disruption and gonadal abnormalities."
He added that while there are statistical anomalies in Hayes' research, the science behind it is strong.
"There are some issues, but this is a fantastic piece of science," Rohr said. "Regardless of what sort of statistical violations there are, nothing can account for having a colony of 100 percent genetic males, and in the end resulting in animals that only have female gonads.
"This is an impressive and important discovery in my mind because it suggests that endocrine disrupting chemicals have the potential to cause a complete sex reversal," Rohr added.
What the research means for human health is unclear, Rohr said.
"This," he added, "definitely warrants some future research."