The Truly Wild Life Around Chernobyl
Many animals are in evolutionary overdrive
The abundance of wildlife is a puzzle, given what is known about the biological effects of radiation. Indeed, high-tech probing of the cells of animals from the region challenges the conventional wisdom that animals cannot tolerate a high rate of genetic change. Here's why: Extensive studies from Hiroshima and cold war laboratories have shown that radiation breaks chromosomes and the strands of the DNA double helix, which contain the blueprints for making the body's proteins (graphic, above). Most of the time, genetic damage signals a cell to die a programmed death, or else it enlists repair enzymes to restore the genetic code. Problems arise when genetic mistakes aren't fixed or are repaired incorrectly, and persist as mutations. In the body, such genetic errors can lead to birth defects in offspring and cancer. More subtly, mutations can cause cells to produce faulty proteins, such as those important for immunity.
While Chernobyl mice don't look like "mutants," on closer inspection, they have many breaks in their DNA strands and a phenomenally high mutation rate. Baker and Ron Van Den Bussche at Texas Tech analyzed DNA from five voles--a type of field mouse--captured within the contaminated zone and compared it with DNA from voles living outside the zone. To search for signs of genetic mutation, they read the code of a gene called cytochrome b that, because it is passed down directly from mother to offspring and changes slowly, is used by scientists as a sort of "genetic clock" for estimating genetic relatedness. As expected, the voles from the area outside the zone had essentially the same cytochrome b gene. But among the Chernobyl voles, the gene sequences as well as the proteins from all five animals were different. Indeed, the differences in the genes between two Chernobyl voles were greater than those normally found between mice and rats, two species that diverged about 15 million years ago.
How adaptable? During mammalian evolution, the rate of spontaneous mutation of one letter in the genetic code has been estimated at 1 in a billion per generation, says David Hillis, who studies molecular evolution at the University of Texas at Austin. But at Chernobyl, the mutation rate in the cytochrome b gene is 1 in 10,000. The important question now is whether rapid mutation observed in the cytochrome b gene is also occurring in other genes. "If that kind of high mutation rate can be tolerated across the entire genome," says Hillis, "it would indicate that mammals in particular are a whole lot more resilient than anyone ever guessed."
Still, the full impact on the mice of this rapid rate of mutation is far from clear. While populations seem to thrive in the Chernobyl environment, individual mice may be living on the edge of their ability to adapt to stress or paying a price with shorter life spans, says Chesser. His lab is now investigating whether Chernobyl mice have had to maximize their resistance to cancer by keeping a well-known tumor suppressor gene called p53 turned on full throttle.
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