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One of the themes of this blog has been that virtually anything present in the environment can act as a selection factor influencing the course of evolution. A recent study on fish and PCB's and dioxins illustrates this point. Unfortunately for humans, the end result is not necessarily positive or beneficial.
New York's Hudson River is home to the Atlantic tomcod (Microgadus tomcod). From 1947 to 1976, two General Electric manufacturing plants along the Hudson River produced PCB's for a range of uses, including as insulating fluids in electrical transformers. Over the years, PCB and dioxin levels in the livers of the Hudson's tomcod rose to become "among the highest known in nature," according to researchers. Because these fish do not detoxify PCB's, it was a surprise to the researchers that they could accumulate such hefty contamination without becoming poisoned. The researchers have now found that the tomcod's protection traces to a single mutation in one gene; the gene is responsible for producing a protein needed to unleash the pollutants' toxicity.
All vertebrates contain molecules in their cells that will bind to dioxins and related compounds; these proteins (aryl hydrocarbon receptors, or AHR's) are often referred to as dioxin receptors. Once these poisons diffuse into an exposed cell, each molecule can mate with a receptor and together they eventually pick up a third molecule. This trio can then dock with select segments of DNA in the cell's nucleus to inappropriately turn on genes that can poison the host animal.
The tomcod actually has two types of AHR's, with AHR-2 offering the most effective binding to dioxin-like pollutants. But one naturally occurring AHR-2 variant, the result of a gene mutation, proves a very poor mate; it takes roughly five times more of the pollutants to get substantial binding than is needed with the conventional AHR-2.
In local rivers relatively free of dioxins and PCB's, 95 percent of tomcod possess AHR-2 only in the conventional form. But in the PCB-rich Hudson, the researchers found that the only kind of AHR-2 protein in 99 percent of tomcod is the poorly binding variant. Thus, the PCB's and dioxins have acted as a selection factor. The result is that the PCB's and dioxins are stored in the fat of the tomcod.
The bad news? As the tomcods are eaten by predators, the PCB's and dioxins move up the food-chain. The result is that species consumed by humans may have very high PCB and dioxin content even though they themselves have not been exposed directly to the contaminants.
The study can be found at http://www.sciencemag.org/content/early/2011/02/16/science.1197296.