WASHINGTON — Most people think of evolution occurring gradually over thousands of years, but apparently no one told the Atlantic tomcod.
In just 50 years or so, the Hudson River fish has evolved to become resistant to toxic PCBs that polluted the river, researchers reported Thursday. Their secret is a gene variant.
“You’re talking about very rapid evolution,” said Isaac Wirgin, an associate professor of environmental medicine at New York University School of Medicine.
The speed of evolution depends on characteristics of the population and how strong selection pressure is, Wirgin said in a telephone interview. “Pretty strong here, I think.”
Long-term evolution is a result of natural selection, he noted, “these were not natural factors.”
Because the tomcod is resistant to the toxic effects of PCBs they are able to accumulate the industrial chemical in larger amounts than nonresistant creatures without becoming ill or dying, explained Wirgin. His findings were reported Thursday in the online edition of the journal Science.
The resistance is provided by a variant in a single gene that prevents the chemical from binding onto cells in the fish, Wirgin explained.
That variant, he said, is found in about 95 percent of the tomcod in the Hudson. It appears in about 5 percent of tomcod in two smaller streams in Connecticut and on Long Island, and “if you go further from the Hudson you don’t see it at all.”
Adria Elskus, a U.S. Geological Survey fish toxicologist, said several research teams have been studying fish resistance to PCBs, mercury and dioxins.
“The obvious question is how they are doing it,” she said in a telephone interview. Wirgin’s finding that it involved a gene controlling the AHR receptor was a logical place to look, she said.
The next question is what is the tradeoff for the fish? asked Elskus, who is based at the University of Maine.
Pollution of the Hudson by PCBs is traced to 1947 and continued for 30 years before being banned. During that period, General Electric Co. plants discharged an estimated 1.3 million pounds of PCBs into the river. Cleanup is continuing.
The bottom-feeding tomcod grows to about 10 inches long and lives primarily in rivers.
There is no commercial fishing for tomcod, Wirgin said, though some individuals do fish it for sport and it is a popular recreational fish in parts of Canada.
And young tomcod are often prey for larger fish, he said, “so certainly there is transfer of PCBs up the food chain.”
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Online: http://www.sciencemag.org
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