Prior posts have noted various theories, and the support for such, related to the decreasing presence of frogs in riparian habitats. These have included various pesticides and infections from viruses and fungi. Now, Atrazine joins the parade of possibilities.
Past research reports have noted evidence that Atrazine can demasculinize (and in some instances feminize) exposed animals. But frogs do not have readily visible sex chromosomes, so it was difficult to ascertain if feminized males were actually masculinized females, or vice versa.
In order to assess the feminization potential, researchers reared African clawed frogs (the amphibian equivalent of lab rats) from colonies that allegedly never contained a "real female" (their DNA does not contain the gene to develop into a female) and cross-bred them in order to yield a line of pure genetic males. Then he raised some in clean water. Others were grown from the larval stage to three-year post metamorphosis in tanks laced with trace concentrations of Atrazine. [To put the concentrations in context, EPA permits 3 ppb of Atrazine in drinking water. The researchers conducted experiments using 2.5 ppb, a concentration equal to that found in some surface waters downstream from agricultural operations that apply Atrazine to fields.]
Compared to males grown in clean water, those chronically exposed to the Atrazine developed few sperm, produced very low testosterone levels, and failed to "sing" the calls that should invite females or repel competing would-be suitors. But some of the males did not just have diminished levels of the male sex hormone. Four out of 40 also produced elevated levels of estrogen, the primary female sex hormone (not as high as true females would develop, but far higher concentrations than a male should have). These animals also developed external reproductive organs typical of females and exhibited female behaviors.
Of these four, two were examined; their internal reproductive organs were characteristic of females as well. The remaining two transgender animals were introduced to males who had grown up in tanks of clean water. The would-be females accepted the advances of the males, allowing them to fertilize their eggs, which grew into healthy frogs. The offspring were, of course, all male. If housed in clean water, the offspring developed into robust normal males. If reared in Atrazine-laced water, however, they showed the same demasculinization of the earlier generation. Some of the offspring reared in Atrazine-laced water also mirrored their "mother", becoming a fertile female able to bear viable eggs.
Why did this effect appear in only 10%? The researchers suspected a genetic susceptibility, which appears to have been confirmed. In subsequent studies, they found that the proportion of completely feminized males varied from family to family. In some cases, up to 45 percent of the animals come up as females; in others, more than half will be become female.
One confirmatory aspect of the study was the use of a genetic marker. Female frogs carry a gene known as DMW. The modified "female" frogs in the study lacked this gene, confirming they were genetic males whose bodily structures and physiology had been modified.
The study can be found at http://www.pnas.org/content/107/10/4612.abstract.
In a separate study, researchers collected fertilized eggs in the wild and then brought them to the lab for exposure to Atrazine at levels between 1.2 and 1.8 ppd. They found that the animals so exposed had a 2.5-fold increase in estrogen receptors in their brains. The researchers note that this may increase the feminizing effects of the frogs own estrogens as well as environmental mimics of estrogen.
More importantly to the survival of frog populations, the researchers found that the Atrazine-exposed pollywogs were only half as likely to metamorphosize as those raised in clean water. This greatly reduces the population that can be lost without impacting overall population levels in an area.
The study can be found at http://ehp03.niehs.nih.gov/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1289%2Fehp.0901418.