Toxic chemicals are speeding up evolution

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Blinky the three-eyed fish may come along sooner than you expect, according to research to be published in an upcoming issue of the journal Science.

A team of scientists led by New York University School of Medicine’s Isaac Wirgin has found that a population of the Atlantic tomcod (Microgadus tomcod) in the Hudson River has apparently evolved in the space of only a few decades in response to the presence of toxic chemicals.

Polychlorinated biphenyls (PCBs) were first introduced in 1929, seeing wide industrial and commercial usage (particularly as insulators).  They were banned about 50 years later after their toxic effects were well documented.  

A 200-mile stretch of the Hudson River is heavily contaminated with PCBs, after General Electric plants upriver dumped 1.3 million pounds of it from 1947 to 1976.  

The research team spent four years capturing tomcod from contaminated and uncontaminated stretches of the Hudson River, screening the fish for a gene encoding for a protein known as aryl hydrocarbon receptor2 (AHR2) that regulates the toxic effects of PCBs.

The scientists found that genetic alterations as slight as the deletion of only six base pairs in DNA of the AHR2 gene protect the tomcod from PCBs.

The regular form of the AHR2 protein binds to PCBs to set off a cascade of reactions that transmit the toxic effects of the compound. However, the study found that PCBs bind very poorly to the modified AHR2 protein (by a fivefold reduction), which greatly reduces their toxicity.  

In the polluted stretches of the Hudson River, the research team found that every tomcod they tested had at least one copy of the mutated AHR2 gene, with 95% carrying two copies of the gene. In comparison, only 5% of the tomcods from nearby Long Island and Connecticut carried one copy of the mutant gene.

The changes in gene frequencies that the researchers observed are considered very rapid. According to Isaac Wirgin: "Normally you think of evolution occurring in thousands to millions of years. You’re talking about all this occurring in 20 to 50 generations maybe."

Although it seems good that the tomcods are able to evolve resistance to PCBs in such a short time, researchers warn against taking solace in this fact, because the fish are robbing Peter to pay Paul for this resistance.  

The price of carrying this mutation might be slower growth or lower resistance to other stresses.  

Surviving the toxic chemicals is also bad news for other organisms, as it means that they can now be introduced much more easily into the food chain.  " serve as a prime prey for striped bass," Wirgin said. "You've got this fish that would normally be dead from PCBs or dioxin. It's alive and it's carrying around all this PCB and dioxin and it gets eaten."

For more information, see the paper: Wirgin I, NK Roy, M Loftus, RC Chambers, DG Franks and ME Hahn (2011) Mechanistic basis of resistance to PCBs in Atlantic tomcod from the Hudson River. Science DOI:10.1126/science.1197296