Figure 1.9 Controlled Experiments Manipulate a Variable

Atrazine is an herbicide that is often applied to agricultural fields, eventually contaminating surface and groundwater. Previous laboratory research on the commonly studied African clawed frog (Xenopus laevis) indicated that treatment with atrazine induced reproductive abnormalities. To understand the reproductive system effects of atrazine on a frog species native to the United States, Hayes and colleagues collected leopard frogs (Rana pipiens) from a natural habitat in Wisconsin. Back in their lab at the University of California at Berkeley, eggs were hatched and tadpoles were assigned three treatments: 0, 0.1, or 25 parts per billion (ppb) of atrazine. After tadpoles had metamorphosed, frogs were euthanized and gonadal tissue sections were prepared. These sections were examined blind, such that the researchers did not know the atrazine treatment. Using these tissue sections, the number of individuals with testicular oocytes and retarded gonadal development (gonadal dysgenesis) was quantified. The results of this experiment showed that gonadal dysgenesis occurred in 36% of male frogs treated with 0.1 ppb atrazine and 12% of male frogs treated with 25 ppb atrazine. Testicular oocytes occurred in 29% of male frogs treated with 0.1 ppb atrazine and 8% of male frogs treated with 25 ppb atrazine. Control male frogs showed no testicular oocytes and only one individual had gonadal dysgenesis; female frogs were unaffected. Hayes and colleagues concluded that atrazine does indeed affect reproductive development, and that lower doses (i.e., 0.1 ppb) are more harmful than higher doses (i.e., 25 ppb). It is thought that the mechanism by which atrazine affects gonadal development is due to the induction of an enzyme that converts androgens, the male sex hormones, into estrogens, sex hormones that are mainly produced by female ovaries. The activity of atrazine as an endocrine distruptor of sex hormones has also been observed in other vertebrates such as fish, reptiles, and mammals.

 

Original Papers

Hayes, T., K. Haston, M. Tsui, A. Hoang, C. Haeffele, and A. Vonk. 2002. Herbicides: Feminization of male frogs in the wild. Nature 419: 895–896.
http://dx.doi.org/10.1038/419895a

Hayes, T., K. Haston, M. Tsui, A. Hoang, C. Haeffele, and A. Vonk. 2003. Atrazine-induced hermaphroditism at 0.1 ppb in American leopard frogs (Rana pipiens): laboratory and field evidence. Environmental Health Perspectives 111: 568–575.
http://dx.doi.org/10.1289/ehp.5932

 

Links

Site created by Dr. Tyrone Hayes with many useful links about atrazine and its effects
http://www.atrazinelovers.com/

New York Times: Debating How Much Weed Killer Is Safe in Your Water Glass
http://www.nytimes.com/2009/08/23/us/23water.html

Natural Resources Defense Council: August 2009 report on atrazine contamination
http://www.nrdc.org/health/atrazine/

Home page of Dr. Hayes and laboratory
http://ib.berkeley.edu/hayest

African reed frogs, color change, and contamination
http://www.nationalgeographic.com/field/explorers/tyrone-hayes.html

The Chronicle of Higher Education: Hayes research and atrazine manufacturer Syngenta
http://chronicle.com/free/v50/i10/10a02601.htm

Journal of Young Investigators: For the Love of Frogs Featuring Dr. Tyrone Hayes
http://www.jyi.org/volumes/volume10/issue1/features/lee.html

 

Figure 1.10 Comparative Experiments Look for Differences among Groups

In Figure 1.9, Hayes and colleagues took a laboratory approach to studying the effect of the herbicide atrazine on amphibian reproductive development. In the laboratory there was a clear effect of atrazine on the level of gonadal dysgenesis and testicular oocytes in male frogs. But what happens in the field? Is release of atrazine into the environment via herbicide application on crop fields actually detrimental to natural frog populations? To address this question Hayes and colleagues collected and euthanized 100 newly metamorphosed frogs from eight field locations from Utah to Iowa. Based on records of atrazine sales, the researchers were able to predict in advance the locations more likely to be contaminated with the herbicide. At each location, actual levels of atrazine were determined by water sampling and liquid chromatography/mass spectrometry analysis. All sites except site one had atrazine levels above the detection limit of 0.1 ppb. As with the laboratory experiments, Hayes and colleagues prepared and examined under the microscope gonadal tissue sections from 20 male frogs and a subset of female frogs per site. Male frogs from all sites except site one had testicular oocytes, and frogs from one site showed gonadal dysgenesis. In concordance with the laboratory experiments, reproductive abnormalities did not increase with higher levels of atrazine (above 0.2 ppb) at metamorphosis. Thus, both the laboratory and field studies taken together suggest a strong effect of atrazine at low levels. As a result of his research and work of other researchers suggesting a relationship between atrazine and human cancers of the breast and prostate, Hayes has become an outspoken advocate for restrictions on atrazine use. Although atrazine is now banned by the European Union, its use is still permitted in the United States, often with few controls. Despite roadblocks by powerful manufacturers, Hayes continues to reach out beyond the traditional realm of scientists to create awareness and push toward restrictions on the use of this herbicide.

 

Original Papers

Hayes, T., K. Haston, M. Tsui, A. Hoang, C. Haeffele, and A. Vonk. 2002. Herbicides: Feminization of male frogs in the wild. Nature 419: 895–896.
http://dx.doi.org/10.1038/419895a

Hayes, T., K. Haston, M. Tsui, A. Hoang, C. Haeffele, and A. Vonk. 2003. Atrazine-induced hermaphroditism at 0.1 ppb in American leopard frogs (Rana pipiens): laboratory and field evidence. Environmental Health Perspectives 111: 568–575.
http://dx.doi.org/10.1289/ehp.5932

 

Links

Site created by Dr. Tyrone Hayes with many useful links about atrazine and its effects
http://www.atrazinelovers.com/

New York Times: Debating How Much Weed Killer Is Safe in Your Water Glass
http://www.nytimes.com/2009/08/23/us/23water.html

Natural Resources Defense Council: August 2009 report on atrazine contamination
http://www.nrdc.org/health/atrazine/

Home page of Dr. Hayes and laboratory
http://ib.berkeley.edu/research/interests/research_profile.php?person=85

African reed frogs, color change, and contamination
http://www.nationalgeographic.com/field/explorers/tyrone-hayes.html

The Chronicle of Higher Education: Hayes research and atrazine manufacturer Syngenta
http://chronicle.com/free/v50/i10/10a02601.htm

Journal of Young Investigators: For the Love of Frogs Featuring Dr. Tyrone Hayes
http://www.jyi.org/volumes/volume10/issue1/features/lee.html