New study on pesticide's possible role in worldwide frog decline
Suzanne Bohan
Oakland Tribune
03/01/2010
A common pesticide can turn one in 10 male frogs into females, and leave most other males infertile, according to a UC Berkeley study published Monday online.
Previous studies linked the weedkiller atrazine with the development of hermaphrodite frogs — which have both male and female sex organs. Scientists also suspected the pesticide tinkered with cellular machinery enough to cause male frogs to switch sexes, but they couldn't be certain without a genetic test to show that a frog bearing young was in fact a male.
UC Berkeley scientists developed such a test, and found that of 40 African clawed frogs born and raised in water containing 2.5 parts per billion of atrazine, 10 percent of the males became females in all ways except genetically.
In previous studies, "we got fewer males than we should have, and we got hermaphrodites," said Tyrone Hayes, a UC Berkeley biology professor and lead author of the study, published in this week's online early edition of the Proceedings of the National Academy of Sciences.
"Now, we have clearly shown that many of these animals are sex-reversed males," Hayes said. "They have estrogen, lay eggs, they mate with other males."
The male-turned-female frogs also only bore male offspring, and such skewing of sex ratios would endanger the long-term viability of a wild population of frogs, Hayes said.
In addition, three-quarters of the frogs studied had such reduced testosterone levels, and hence sperm production, that they were considered "chemically castrated," he said.
Sherry Ford, a spokeswoman with Syngenta, which manufactures atrazine, strongly criticized the study, saying it didn't test various levels of the pesticide, and that studies over five decades have shown that it has no adverse effects on wildlife at levels found in the environment. She said Hayes also misrepresented the findings of another scientist to make his case.
Hayes said he would have liked to test various levels of exposure to the pesticide. "More doses would have been better. You can't do everything in one study," he said.
He adamantly disagreed that he misstated another scientist's conclusions, and that his findings were taken exactly as is from the scientist's paper. Hayes also said numerous studies have indeed shown reproductive harm to various animal species from atrazine at levels below the Environmental Protection Agency's allowable level in drinking water of 3 parts per billion.
He said numerous studies, ranging from those on human cell lines and rats to fish and frogs, show that atrazine affects the gene that produces an enzyme called aromatase. This enzyme is critical in the development of the female hormone estrogen across a range of species.
"The enzyme aromatase is the only way to make estrogen, and it's regulated the same way in everything from fish to farmers," said Hayes, who first began studying the pesticide's affect on frog development in 1997.
When male frogs are exposed to atrazine at levels below the EPA's allowable level, their testosterone is converted into estrogen during development, which leads to their permanent transformation into females, at least functionally.
