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AMERICAN colleagues have warned Dr. Ron Chesser, only half jokingly, that radiation exposure might kill him if he keeps going back to Chernobyl to collect rodents for genetic studies. Around Chernobyl itself, local people simply recommend vodka to flush the radioactivity out of one's system. But Dr. Chesser is not fond of vodka, and to his mind, what can be learned at the site of the world's worst nuclear accident is well worth any theoretical risk from radiation.
"Chernobyl represents a huge mystery," he said, "and scientists love mystery."
Dr. Chesser, a geneticist at the University of Georgia, and Dr. Robert Baker, from Texas Tech University, periodically venture into the most radioactive areas they can find inside the six-mile "exclusion zone" in Ukraine around the reactor that melted down in 1986. Wearing no special protective gear other than shoe covers, and sometimes respirators, they go in quest of the mouse-like rodents known as voles.
The mystery that keeps luring the two back is that voles and other rodents are thriving in the zone, an environment so contaminated that the animals themselves become radioactive. The research team considers them safe enough to handle, but Dr. Chesser said, "You wouldn't want to keep one of those voles in your pocket for any length of time."
Despite their hardy appearance, the voles sustain extraordinary amounts of genetic damage, Dr. Chesser and Dr. Baker reported last month in the journal Nature. "The mutation rate in these animals is hundreds and probably thousands of times greater than normal," Dr. Baker said.
The Chernobyl accident has, in essence, compressed several thousand years of evolution into a decade, according to Dr. David Hillis, a molecular biologist at the University of Texas who wrote an editorial accompanying the Nature paper.
Dr. Baker and Dr. Chesser measured mutations by studying the DNA in a cellular structure known as the mitochondrion, which processes energy. They chose that structure rather than nuclear DNA because the mitochondrial genes are smaller and better known than nuclear DNA. In addition, the mitochondria cannot repair their DNA, making tracking genetic damage easier.
Despite the mutation rate, the vole population was booming. There were more animals inside the exclusion zone than outside it, probably because people had been evacuated, improving the habitat for animals. Except for enlarged spleens, which may signal infection or the onset of cancer, the voles seemed healthy.
News reports have described the Chernobyl rodents as "nuclear supervoles." But the researchers take exception to that idea, even though the voles appear to have adapted to their radioactive environment.
"Adaptation always involves a cost," Dr. Chesser said. "For there to be evolution or adaptation, one form must succeed at the expense of another."
A close examination of the genetic changes in the animals suggested one possible cost of adaptation. As many as a third of the mutations that the researchers expected to find were not detected. "We think they were lethal," Dr. Baker said. "It could be that the animals were never even born." He said,the researchers also did not know whether the animals were living a normal life span.
Another ominous finding was that the vole mutations were cumulative, increasing with each succeeding generation. Both researchers doubt that any species could sustain such a mutation rate indefinitely.
A major question that is still unanswered is whether the changes seen in the mitochondrial DNA mean that the mutation rate has also increased in the nuclear DNA.
"But if it's anywhere close," Dr. Baker said, "it's ultimately going to be ugly." He said that even if the changes were only in the mitochondria, that could have serious consequences for the voles, because mitochondria play an essential role in metabolism.