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| Hormone Disruption By Atrazine Pesticide |
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| SciMed - Biology | |||
| TS-Si News Service | |||
| Wednesday, 03 March 2010 10:00 | |||
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Fairfax, VA, USA. One of the world's most widely used pesticides, atrazine, wreaks havoc with the sex lives of adult male frogs, emasculating three-quarters of them and turning one in 10 into females, according to a new study. The 75 percent that are chemically castrated are essentially "dead" because of their inability to reproduce in the wild. Tyrone B. Hayes, professor of integrative biology, says "These male frogs are missing testosterone and all the things that testosterone controls, including sperm." The fertility of the males "is as low as 10 percent in some cases, and that is only if we isolate those animals and pair them with females." "In an environment where they are competing with unexposed animals, they have zero chance of reproducing." The 10 percent or more that turn from males into females — something not known to occur under natural conditions in amphibians — can successfully mate with male frogs but, because they are genetically male, all their offspring are male.
Research shows that atrazine interferes with endocrine hormones, such as estrogen and testosterone — in fish, amphibians, birds, reptiles, laboratory rodents and even human cell lines at levels of parts per billion. Professor Hayes of the University of California, Berkeley and his colleagues report their recent results in the Proceedings of the National Academy of Sciences (PNAS). [C1] The research team also published an extensive review of the pesticide's effects on amphibians in the Journal of Experimental Biology. The researchers concluded that atrazine is a likely contributor to documented worldwide declines in amphibian populations. [C2] "These kinds of problems, like sex-reversing animals skewing sex ratios, are much more dangerous than any chemical that would kill off a population of frogs," he said. "In exposed populations, it looks like there are frogs breeding but, in fact, the population is being very slowly degraded by the introduction of these altered animals." Some 80 million pounds of the herbicide atrazine are applied annually in the United States on corn and sorghum to control weeds and increase crop yield, but such widespread use also makes atrazine the most common pesticide contaminant of ground and surface water, according to various studies. More and more research, however, is showing that atrazine interferes with endocrine hormones, such as estrogen and testosterone — in fish, amphibians, birds, reptiles, laboratory rodents and even human cell lines at levels of parts per billion. Recent studies also found a possible link between human birth defects and low birth weight and atrazine exposure in the womb. As a result of these studies, the Environmental Protection Agency (EPA) is reviewing its regulations on use of the pesticide. Several states are considering banning atrazine, and six class action lawsuits have been filed seeking to eliminate its use. The European Union (EU) already bars the use of atrazine.
Subsequent studies showed that native leopard frogs (Rana pipiens) collected from atrazine-contaminated streams in the Midwest, including from areas up to 1,000 miles from where atrazine is applied, often had eggs in their testes. And many males had lower testosterone levels than normal females and smaller than normal voice boxes, presumably limiting their ability to call mates. Hayes' research also established that many frogs in Midwestern streams contaminated by atrazine and other pesticides have compromised immune systems, leading to increased mortality from bacterial disease. Those early studies were hampered by the inability to easily distinguish genetically male from genetically female frogs. Male frogs have two identical sex chromosomes (ZZ) while females have both a Z and a W — the opposite of XX female and XY male humans. But because all frog chromosomes look the same under a light microscope, it's not simple to distinguish male from female. To overcome this, Hayes' colleague Roger Liu developed a line of all-male frogs so that the genetics would be unequivocal. "Before, we knew we got fewer males than we should have, and we got hermaphrodites. Now, we have clearly shown that many of these animals are sex-reversed males," Hayes said. "We have animals that are females, in the sense that they behave like females: They have estrogen, lay eggs, they mate with other males. Atrazine has caused a hormonal imbalance that has made them develop into the wrong sex, in terms of their genetic constitution." Coincidentally, another lab in 2008 discovered a sex-linked genetic marker in Xenopus, which has allowed Hayes to confirm the genetic sex of his frogs. In Hayes' study, where 40 frogs lived for about three years after hatching in water with 2.5 ppb atrazine, about 10 percent of the frogs appeared to be resistant to the effects of the pesticide. In ongoing studies, Hayes is investigating whether this apparent resistance is inherited, as well as whether the sex-reversed males have more susceptible offspring. Syngenta, which manufactures atrazine, disputes many of these studies, including Hayes', that show adverse effects of the presticide. But Hayes said that "when you have studies all over the world showing problems with atrazine in every vertebrate that has been looked at — fish, frogs, reptiles, birds, mammals — all of them can't be wrong." "What people have to realize is that, just as with taking pharmaceuticals, they have to decide whether the benefits outweigh the costs," he said. "Not every frog or every human will be affected by atrazine, but do you want to take a chance, what with all the other things that we know atrazine does, not just to humans but to rodents and frogs and fish?" FundingThe work was funded by the Park Water Co., Mitch Kapor, Freada Klein, the Mitch Kapor Foundation, the David Foundation, the Cornell-Douglas Foundation, the Wallace Foundation, the UC Berkeley Class of '43 endowed chair and the Howard Hughes Biology Fellows Program.
ParticipantsHayes' long-term studies of the effects of atrazine on frogs have been assisted by many UC Berkeley undergraduate students, including co-authors on the current paper: Vicky Khoury, Anne Narayan, Mariam Nazir, Andrew Park, Lillian Adame, Elton Chan, and graduate students Travis Brown, Daniel Buchholz, Sherrie Gallipeau and Theresa Stueve.
Citations[C1] Atrazine induces complete feminization and chemical castration in male African clawed frogs (Xenopus laevis). Tyrone B. Hayes, Vicky Khoury, Anne Narayan, Mariam Nazir, Andrew Park, Travis Brown, Lillian Adame, Elton Chan, Daniel Buchholz, Theresa Stueve, and Sherrie Gallipeau.PNAS 2010. doi:10.1073/pnas.0909519107
Abstract The herbicide atrazine is one of the most commonly applied pesticides in the world. As a result, atrazine is the most commonly detected pesticide contaminant of ground, surface, and drinking water. Atrazine is also a potent endocrine disruptor that is active at low, ecologically relevant concentrations. Previous studies showed that atrazine adversely affects amphibian larval development. The present study demonstrates the reproductive consequences of atrazine exposure in adult amphibians. Atrazine-exposed males were both demasculinized (chemically castrated) and completely feminized as adults. Ten percent of the exposed genetic males developed into functional females that copulated with unexposed males and produced viable eggs. Atrazine-exposed males suffered from depressed testosterone, decreased breeding gland size, demasculinized/feminized laryngeal development, suppressed mating behavior, reduced spermatogenesis, and decreased fertility. These data are consistent with effects of atrazine observed in other vertebrate classes. The present findings exemplify the role that atrazine and other endocrine-disrupting pesticides likely play in global amphibian declines. Keywords: amphibian decline, endocrine disruption, pesticide, sex reversal. [C2] The cause of global amphibian declines: a developmental endocrinologist's perspective. T. B. Hayes, P. Falso, S. Gallipeau and M. Stice. Journal of Experimental Biology 2010; 213(6): 921-933. doi:10.1242/jeb.040865 Download PDF Abstract Greater than 70% of the world's amphibian species are in decline. We propose that there is probably not a single cause for global amphibian declines and present a three-tiered hierarchical approach that addresses interactions among and between ultimate and proximate factors that contribute to amphibian declines. There are two immediate (proximate) causes of amphibian declines: death and decreased recruitment (reproductive failure). Although much attention has focused on death, few studies have addressed factors that contribute to declines as a result of failed recruitment. Further, a great deal of attention has focused on the role of pathogens in inducing diseases that cause death, but we suggest that pathogen success is profoundly affected by four other ultimate factors: atmospheric change, environmental pollutants, habitat modification and invasive species. Environmental pollutants arise as likely important factors in amphibian declines because they have realized potential to affect recruitment. Further, many studies have documented immunosuppressive effects of pesticides, suggesting a role for environmental contaminants in increased pathogen virulence and disease rates. Increased attention to recruitment and ultimate factors that interact with pathogens is important in addressing this global crisis. Keywords: amphibian, decline, stress.
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