Atlantic tomcod: one of the fastest evolving populations

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Wedge
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18 Feb 2011, 10:06 am

Hudson River Fish Evolve Toxic PCB Immunity


Image
A mature Atlantic tomcod collected from the Hudson River.
Photograph courtesy Mark Mattson, Normandeau Associates via Science/AAAS


Anne Minard
for National Geographic News
Published February 17, 2011

This story is part of a special National Geographic News series on global water issues.

"Bottom-feeding fish in the Hudson River have developed a gene that renders them immune to the toxic effects of PCBs, researchers say.

A genetic variant allows the fish to live in waters notoriously polluted by the now-banned industrial chemicals, and distinguishes the fish—Atlantic tomcod (Microgadus tomcod)—as one of the world’s fastest evolving populations.

"This is very, very ra­­­­­­­­­­­­pid evolutionary change," said Isaac Wirgin, an environmental toxicologist at New York University’s School of Medicine, and the study's lead investigator. "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.”

The study appears in the Feb. 18 online issue of Science.

Toxic River, Oblivious Fish

PCBs, or polychlorinated biphenyls, were first introduced in 1929 and were used in hundreds of industrial and commercial applications, mostly as electrical insulators. They were banned 50 years later, but they don’t simply degrade. Partly because of PCB contamination, a 200-mile stretch of the Hudson River is the nation’s largest Superfund site.

The 10-inch Atlantic tomcod has thrived despite the exposure to PCBs, and levels of the chemical in the livers of these fish are among the highest reported in nature. But until now, scientists have never understood how they survived PCB exposures that kill most other fish.

“Exposure of fish embryos to PCBs in the lab causes the heart to be smaller, to not beat properly,” Wirgin said. He and his colleagues suspected the fish harbored some sort of protection. They spent four years capturing tomcod from contaminated and relatively clean areas of the Hudson River during the winter spawning season.

Lightning-Fast Evolution

It turns out the fish sport a handy modification to a gene encoding a protein known to regulate the toxic effects of PCBs and related chemicals, called the aryl hydrocarbon receptor2, or AHR2.

The fish are missing six base pairs of DNA of the AHR2 gene, and the two amino acids each triplet would code for. PCBs bind poorly to the mutated receptors, apparently blunting the chemicals' effects.

The adaptation occurs almost universally in Hudson River tomcod, but crops up only infrequently in two other tomcod populations—in Connecticut’s Niantic River and the Shinnecock Bay at Long Island’s south shore. The fact that it exists at all in those nearby populations leads the researchers to believe the Hudson Bay tomcod had the mutation at least to a low degree before the PCB onslaught. In a classic case of natural selection, the fish with the mutated genes survived.

“They were getting blasted with chemicals all of a sudden,” Wirgin said, “and the early life stages are so sensitive. If they didn’t have a mechanism to deal with this, it’s likely the population would have been extirpated.”

Achilles’ Heel?

General Electric released about 1.3 million pounds of PCBs into the Hudson River from 1947 to 1976, and bears most of the responsibility for the cleanup.

Following highly controversial wrangling throughout the past decade, GE conducted a year’s worth of experimental dredging in 2009. The EPA studied the risks from resuspended contaminants and decided cleanup is the best option. Dredging will resume this spring and will last for at least six years.

Cleanup might not be best for tomcod, Wirgin said. That’s because evolutionary theory predicts a genetic mutation like theirs could render them compromised in some other area of their biology, and perhaps not well adapted to life without PCBs.

But it’s likely to be a boon for the Hudson’s predatory fish that are less likely to have an adaptation to PCBs—and are therefore gravely at risk from a diet of tomcod."


From National Geographic Daily News: http://news.nationalgeographic.com/news ... ion-water/


Also I read from other source that Isaac Wirgin from the Department of Environmental Medicine, School of Medicine, University of New York said that this kind of response was already known in insects that developed resistence to pesticides and bacteria that are immune to antibiotic. But this is the first time that this mechanism of defense is developed in a vertabrate population.



Philologos
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18 Feb 2011, 10:12 am

I am not convinced this belongs here rather than in the science forum. Again, intriguing if so. I see no reason why the time span should be any objection. A tweak to the biochemistry surely need not require as long a span as growing horns, let alone new limb shapes.

Do you know [not close enough to my line that I would be able to evaluate the paper] how much of a shift this represents in DNA terms?



Dantac
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18 Feb 2011, 10:37 am

Most of our DNA sequences are silent 'junk' genes that never do anything... but in reality they are millions of years worth of genetic evolutionary archives. Many adaptations to hostile environments are nothing more than one of those ancient genes becoming activated under the right conditions.



Philologos
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18 Feb 2011, 11:58 am

Dantac wrote:
Most of our DNA sequences are silent 'junk' genes that never do anything... but in reality they are millions of years worth of genetic evolutionary archives. Many adaptations to hostile environments are nothing more than one of those ancient genes becoming activated under the right conditions.


Right - do the cave creatures that on reaching the light reactivate eyes come in here, dpo you know, or is that a different mechanism?



Dantac
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18 Feb 2011, 12:17 pm

sorry I cant make sense of what you're asking there :(



Philologos
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18 Feb 2011, 12:26 pm

I just took a quick scan and found this:

http://www.cbc.ca/technology/story/2008 ... brids.html

Obviously this is a totally different thing. I thought I had run across a claim of cave fish in a generation or do becoming sighted when moved out into the light - but I may have misreadit.

If I DO spot it, I will pass it on.



Dantac
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18 Feb 2011, 12:43 pm

I see what you mean.

I think that the fish that were in the dark 1 million years didn't lose their eyesight genes.. they were switched off. Case in point: they still had eyes but no eyesight. Best guess is if they had taken those fish and moved them to a location where there was light .. in a couple dozen generations they would've regained use of their eyes.



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18 Feb 2011, 1:09 pm

Dantac wrote:
I see what you mean.

I think that the fish that were in the dark 1 million years didn't lose their eyesight genes.. they were switched off. Case in point: they still had eyes but no eyesight. Best guess is if they had taken those fish and moved them to a location where there was light .. in a couple dozen generations they would've regained use of their eyes.


Okay in which case that could be pretty comparable to [if that is what happened] activating an archived gene protecting feom chemicals.