MADISON, WISCONSIN--Larger bodies may come with larger brains, but size means little when it comes to how much DNA an organism can pack in each cell. Researchers have discovered the first mammal--a mouse species in Argentina--that carries an extra set of chromosomes and can have offspring. The finding, reported here on 25 June at the Evolution '99 meeting, overturns the long-held notion that a double complement of chromosomes in mammals results in either sterile or dead individuals.
Plenty of plants, and a fish and a reptile or two, sport four copies of each chromosome instead of the usual two, a phenomenon called tetraploidy, which gives an organism lots or genes to play with in adapting to new environments. Thus while trying to reclassify species in a family of South American rodents called Octodontoidea, evolutionary biologist Milton Gallardo of the Universidad Austral de Chile was surprised to find in 1990 that a 100-gram mouse, called Tympanoctomys barrerae, had 51 pairs of chromosomes. By 1997, Gallardo and his colleagues had established that species in this family had about 26 pairs of chromosomes each, leaving him to suspect that the only way Tympanoctomys could wind up with twice that many pairs was through some sort of doubling of the genome.
Gallardo has now found several signs that indeed there is something unusual about the amount of DNA carried by this rodent. After obtaining cells from 31 species of nine rodent families, Gallardo's group used a stain to estimate the amount of nuclear DNA in each cell. Typically, rodents, like nearly all mammals, have 6 to 7 picograms of DNA per cell. The other members of Tympanoctomys's family averaged around 8, while it had a whopping 17 picograms. In addition, the sperm heads of this mouse "are huge," suggesting that they pack much more DNA than typical rodent sperm does, Gallardo notes. "It's one of the largest in mammals," he says. Animal breeders have sometimes seen such large sperm from livestock, but those turned out to have twice the number of chromosomes they were supposed to and were likely unable to fertilize eggs.
Gallardo suspects this mouse species arose when an ancestor doubled its chromosomes--yielding four, rather than two, copies of each. It's not clear how that may have happened, because large deviations in the amount of DNA usually kills fetuses or renders them infertile because of abnormalities in the amounts of various proteins produced or because organisms cannot tolerate having extra copies of sex chromosomes. But if the sex chromosomes didn't double--which seems to be the case--then these animals may still have been able to reproduce despite all this extra DNA, Gallardo says. He doesn't know why these particular chromosomes, nor another one with a unique shape, are not doubled in these animals.
If Gallardo 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. Toward that end, Gallardo plans to label the chromosomes and tally the copies.