Skip to main content
         

Vol. 07 Issue 2, Spring 2002

Dr. Tyrone Hayes' Research on the Effects of the Herbicide Atrazine on Sexual Development in Frogs
The Ribbon 

Suzanne Snedeker, BCERF Associate Director of Translational Research

Late in October of last year, on a rare, cool New Orleans day, I had the pleasure of hearing Dr. Tyrone Hayes, an Associate Professor of Integrative Biology at Berkeley, give an amazing talk on how low levels of the herbicide atrazine affect the sexual development of frogs. This talk was a part of a conference on endocrine disruption hosted by the Tulane University Center for Bioenvironmental Research. This is not a limb deformity, but rather permanent developmental changes in the sex organs of the frogs. You would not notice this change by just looking at the frog, because it is an internal change. Many of the frogs exposed to atrazine were hermaphrodites-they had both testes (male sex organs) and ovaries (female sex organs). Some of the frogs had several sets of sex organs. Not only is Dr. Hayes a dynamic speaker, he also is a thorough researcher. He explained how he had repeated his experiment dozens of times, always with the same results. Male tadpoles exposed to atrazine had abnormal sexual development. These effects were seen at very low levels of atrazine, as low as 0.1 part per billion (ppb), that is, 0.1 millionth of a gram in one liter (1000 grams) of water. These effects were seen at atrazine levels 30 times lower than the maximum contamination limit (MCL) set by the Environmental Protection Agency (EPA) for atrazine in drinking water (MCL for atrazine is 3 ppb). The tadpoles exposed to over 1 ppb atrazine had abnormal development of the laryngeal muscle, which normally develops in response to the male sex hormone testosterone.

I talked with Dr. Hayes the day after this presentation. He told me that while originally his research had been funded by the company that is the major manufacturer of atrazine, Syngenta (formerly Norvartis, formerly Ciba-Geigy), he had completely cut off ties with the company and secured independent research funding to repeat his experiments. He submitted his unpublished report on atrazine's effects on frogs to the EPA. Atrazine has been under Special Review by the EPA since the mid-1990s, and it is expected that the EPA will release a final risk assessment of atrazine this summer. What is unique about Dr. Hayes' research is that he observed effects on sexual development at very low levels of atrazine, levels commonly seen in the environment.

Atrazine is the most widely used herbicide in the US. 61 to 73 million pounds were used per year during the 1990s by the US alone. Atrazine is used to control weeds primarily in fields where corn and sorghum are grown. Atrazine is usually applied in the spring to fields during the planting season to prevent weeds from choking the new emerging plants. After application, atrazine can be washed from the fields by the heavy spring rains into streams, rivers and reservoirs, and levels can exceed 20 ppb. The time when atrazine levels are highest in rain runoff in the spring exactly coincides with the season when frog tadpoles are developing. While atrazine is frequently detected in surface and groundwater during other times of the year, levels are typically much lower than the 3 ppb MCL set for drinking water. But, it is not uncommon to see atrazine in drinking water in the range of 0.001 to 1.0 ppb; Dr. Hayes observed inter-sex frogs with both ovaries and testes at levels of atrazine as low as 0.1 ppb. Atrazine can also be transported in the air, and levels exceeding 1 ppb and as high as 40 ppb have been detected in rainwater in Midwestern states with high use of atrazine.

Dr. Hayes has recently published his research "Hermaphroditic, demasculinized frogs after exposure to the herbicide atrazine at low ecologically relevant doses" in the journal Proceedings of the National Academy of Sciences, USA (vol. 99, pp. 5476-5480). Dr. Hayes notes that about 20% of his atrazine-treated male frogs had multiple sex organs, or had both male and female sex organs. None of the control animals had any abnormalities. Dr. Hayes reported that he sampled the blood of adult frogs that had been exposed to atrazine as tadpoles. Exposed frogs had low levels of the male sex hormone testosterone. Testosterone levels were 10 times lower in the frogs raised in water with 25 ppb atrazine compared to untreated frogs. What could cause this abnormal hormonal change? This suggests there is a change in the way the frogs synthesized steroid hormones. Dr. Hayes thinks that atrazine may cause an increase in the levels of an important enzyme called aromatase which helps convert the male sex hormone testosterone to the female sex hormone, estrogen. Higher levels of estrogen with low levels of testosterone would explain the inter-sex frogs. However, Dr. Hayes has not tested this idea experimentally, and more work needs to be done to see if the aromatase pathway is disrupted.

Might this have implications for other species? The testosterone to estrogen conversion pathway is not just present in frogs. It is also important in mammals, including humans. After menopause when the ovaries no longer make estrogen, the major source of circulating estrogen is the conversion of testosterone in a series of steps to estrogen, via an aromatase pathway in body fat. Whether atrazine can have any effect on increasing the activity of aromatase in humans is not known. In other species atrazine does not have an effect on sexual development. For instance it does not appear to have a major effect on the sexual development of other wildlife like alligators, which are also very sensitive to changes in sex hormones. And, humans are certainly not frogs. We do not live in water nor do we drink water to the extent frogs would. However, because of other data on the endocrine disrupting effects of atrazine in laboratory animals, it is important that further research be done to rule out any possible disruptive effects on hormonal pathways in humans.

Dr. Hayes is continuing his studies, extending his research to frogs in the wild. However, his research is not without controversy. In an article written for the National Geographic, other researchers have not been able to duplicate the atrazine-effect at the very low levels of atrazine used by Dr. Hayes. However, they did see similar effects at higher levels of atrazine.

Currently, the importance of this study is finding that the sexual development of frogs can be adversely affected, resulting in multiple sex organs (gonads) or inter-sex frogs at levels of atrazine commonly found in the environment.

For more information on the EPA atrazine risk assessment (note: many documents listed on this page are hundreds of pages long) see: http://www.epa.gov/oppsrrd1/reregistration/atrazine/index.htm

More on Dr. Hayes and his research interests:
    http://www.berkeley.edu/news/media/releases/2002/04/15_frogs.html

Abstract of Dr. Hayes article published April 16, 2002:
    http://www.pnas.org/cgi/content/abstract/99/8/5476

More on Dr. Hayes and his research interests:
    http://www.exploratorium.edu/frogs/researcher/

On-line National Geographic article on Dr. Hayes' research:
    http://news.nationalgeographic.com/news/2002/04/0416_020416_TVfrog.html

Back to the top

Last Update 09.12.02