According to a new study by University of California, Berkeley biologists, the pesticide Atrazine emasculates three-quarters of adult male frogs and turns one in 10 into females.
UC Berkeley integrative biology professor Tyrone B. Hayes calls the 75 percent chemically castrated “dead” because they cannot reproduce in the wild.
Testosterone
These male frogs lack testosterone and its effects, including sperm. “If we separate those animals and couple them with females, their fertility is as low as 10%,” he said. “They cannot reproduce in a competitive environment.”
The 10 percent or more that turn from males to females, a rare occurrence in amphibians, can mate with male frogs but have male offspring because they are genetically male.
Hayes said, “When we grow these guys up, based on the family, we will get anywhere from 10 to 50 percent males. Because they skew sex ratios, genetically male females can wipe out a population.
Though the experiments were done on a common lab frog, the African clawed frog (Xenopus laevis), field studies show that Atrazine, a potent endocrine disruptor, affects wild frogs and may be one of the causes of amphibian declines worldwide, Hayes said.
Findings
The online edition of Proceedings of the National Academy of Sciences features Hayes and his UC Berkeley colleagues’ findings. Hayes and colleagues reviewed the potential causes of a worldwide decline in amphibian populations in last week’s Journal of Experimental Biology and found that Atrazine and other hormone-disrupting pollutants affect recruitment and make amphibians more disease-prone.
He said that sex-reversing animals skewing sex ratios are more dangerous than any chemical that would kill frogs. In exposed populations, it appears that frogs are breeding, but these altered animals are slowly degrading the population.
According to studies, Atrazine is the most common pesticide contaminant of ground and surface water in the US, where 80 million pounds are applied to maize and sorghum to control weeds and increase crop yield (in South Africa, Atrazine it is also used on sugar & pineapple crop).
However, more research shows that atrazine interferes with endocrine hormones like estrogen and testosterone in fish, amphibians, birds, reptiles, laboratory rodents, and even human cell lines at parts per billion. Recent studies also linked low birth weight and atrazine exposure to human birth defects.
These studies prompted the EPA to review pesticide regulations. Six class action lawsuits seek to ban atrazine in several states. The EU bans atrazine.
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Atrazine’s hormonal effects on amphibians’ sexual development were first shown by Hayes in the early 2000s. In 2002, Hayes and colleagues found that African clawed frog tadpoles raised in atrazine-contaminated water develop both female (ovaries) and male (testes) gonads. Atrazine levels as low as 0.1 ppb, 30 times lower than EPA drinking water limits, caused this (3 ppb).
Subsequent studies found that native leopard frogs (Rana pipiens) from atrazine-contaminated Midwest streams, even from 1,000 miles away, often had eggs in their testes. Many males had lower testosterone than females and smaller voice boxes, limiting their ability to call mates.
Hayes found that many frogs in Midwestern streams contaminated by atrazine and other pesticides have compromised immune systems, which increases bacterial disease mortality.
The inability to distinguish genetically male and female frogs hampered early studies. Female frogs have both a Z and a W sex chromosome, unlike XX female and XY male humans. Under a light microscope, all frog chromosomes look the same, making gender identification difficult.
Haemophrodite frogs?
Roger Liu, Hayes’ colleague, created an all-male frog line to clarify the genetics.
We had hermaphrodites and fewer males than expected. “We have shown that many of these animals are sex-reversed males,” Hayes said. “We have animals that behave like females—they have estrogen, lay eggs, and mate with other males. Atrazine caused a hormonal imbalance that made them genetically the wrong sex.
Coincidentally, another lab discovered a sex-linked genetic marker in Xenopus in 2008, allowing Hayes to confirm his frogs’ genetic sex.
Hayes found that 10% of 40 frogs that lived for three years in 2.5 ppb atrazine water were resistant to the pesticide. Hayes is studying whether this resistance is inherited and whether sex-reversed males have more susceptible offspring.
Syngenta, which makes atrazine, disputes many of these studies, including Hayes’, that show adverse effects. “Fish, frogs, reptiles, birds, and mammals have all been shown to be adversely affected by atrazine in studies conducted around the world. Therefore, none of the studies can be incorrect.” Hayes said.
“Just like with pharmaceuticals, people have to decide whether the benefits outweigh the costs,” he said. “Not every frog or 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, frogs, and fish?”
