Researchers have discovered the first mammal--a rat species in Argentina--that seems to thrive despite carrying an extra set of chromosomes. The finding, reported last month at the Evolution '99 meeting in Madison, Wisconsin, overturns the long-held notion that for a mammal, a double complement of chromosomes spells death or, at best, a reproductive dead end.
Scientists knew that some fish and amphibians sport four copies of each chromosome, instead of the usual two. The phenomenon, called tetraploidy, gives an animal lots of genes to play with in adapting to new environments. Then, in 1990, while evolutionary biologist Milton Gallardo of the Universidad Austral de Chile was reclassifying a rodent family called Octodontoidea, he stumbled upon a species, the red viscach rat (Tympanoctomys barrerae), with 51 pairs of chromosomes. By 1997, Gallardo and colleagues had established that the other members of the family had only about 26 pairs.
Chock full of chromosomes.
Credit: S. NIHUIL AND M. GALLARDO
Gallardo's group now has more evidence that the viscach rat is a strange breed. Using a stain to estimate the amount of nuclear DNA in the cells of 31 different rodent species, the group found that, like most mammals, rodents average 6 to 8 picograms of DNA per cell. But not Tympanoctomys: It had a whopping 17 picograms. What's more, says Gallardo, its sperm heads "are huge."
Gallardo suspects this species arose when an ancestor somehow doubled its chromosomes. Reproduction is apparently still possible if the sex chromosomes fail to double--as appears to be the case in this rat. Gallardo plans to tally and label all the chromosomes. If he can prove that Tympanoctomys really has four copies of each gene, the find will be "remarkable," says Stephen O'Brien, a geneticist at the National Cancer Institute, as once again evolution will have proved it can do what most biologists had considered impossible.
