Pesticide Turns Male Frogs into Females

Rachael Rettner

Date: 01 March 2010 Time: 10:04 AM ET

The pesticide atrazine can turn male frogs into females that are able to mate and successfully reproduce. Here, two male frogs mating. The larger animal on the bottom has been completely feminized by atrazine exposure and can produce viable eggs.
CREDIT: Tyrone B. Hayes, the University of California, Berkeley.

A commonly used pesticide known as atrazine can turn male frogs into females that are successfully able to reproduce, a new study finds.

While previous work has shown atrazine can cause sexual abnormalities in frogs, such as hermaphroditism (having both male and female sex organs), this study is the first to find that atrazine’s effects are long-lasting and can influence reproduction in amphibians.

The results suggest that atrazine, which is a weed killer used primarily on corn crops, could have potentially harmful effects on populations of amphibians, animals that are already experiencing a global decline, said study author Tyrone B. Hayes of the University of California, Berkeley. Atrazine is banned in Europe.

And since atrazine interferes with the production of the sex hormone estrogen, present in people and frogs, the findings could have implications for humans as well. "If you have problems in amphibians, you can anticipate problems in other animals," Hayes said.

Sex change

Hayes and his colleagues raised 40 male African clawed frogs in water containing atrazine, from when they were larvae all the way up until sexual maturity. The atrazine levels were about what the frogs would experience in environments where the pesticide is used, and below levels that the U.S. Environmental Protection Agency considers safe for drinking water.

They compared this atrazine-exposed group with 40 other male frogs reared in atrazine-free water.

At the end of the experiment, all frogs in the atrazine-free group remained male, while 10 percent of the frogs exposed to atrazine were completely feminized — their genes said they should be male, but they had female anatomy, including ovaries. The feminized frogs were able to mate with males and produce viable eggs.

In both frogs and humans, sex is genetic. In people, females have two X (sex) chromosomes, while males have one X and one Y. For frogs, the sex chromosomes are labeled as Z or W and females have dissimilar chromosomes (ZW), while males have matching ones (ZZ).

Frogs exposed to atrazine also had reduced testosterone levels, decreased fertility, and showed less mating behavior.

Implications of feminized males

The results indicate atrazine could contribute to amphibian population declines, along with climate change, habitat loss and invasive species, Hayes said.

Hayes notes that if the feminized males do reproduce as females, they can only produce male offspring, which further skews the population sex ratio.

The pesticide could also cause problems for other species, including our own, Hayes said.

Atrazine mimics a biological compound and increases the production of estrogen. It has been shown to disrupt hormone levels in other animals as well as in human cells. It has also been found to induce breast cancer in rats, Hayes said.

The good news is that humans don't live in water, and so we aren't exposed to atrazine constantly.

But problems with wildlife should still raise concern for us, Hayes said. "Anytime you see dramatic declines like we're seeing in amphibians and fish…we should recognize that we drink and swim and bathe in that same water," Hayes said.

The results will be published this week in the journal Proceedings of the National Academy of Sciences.