Doing the DNA shuffle

From San Diego, at a meeting of the American Society of Human Genetics

During the 6 million years since humans’ ancestral lineage diverged from the ancestors of the other great apes, DNA near the ends of human chromosomes has evolved more rapidly than scientists had previously realized.

A new comparison of macaque, orangutan, ape, chimpanzee, and human genomes shows a surprising amount of DNA reshuffling in these chromosome regions, called subtelomeres. These volatile areas of roughly 150,000 to 500,000 genetic units lie between the main body of a chromosome, which varies little among related species, and the caps on each end of the chromosome, called telomeres.

Previous studies had underestimated how much swapping and reordering of DNA segments has occurred within subtelomeres because scientists had looked only for segments in the apes’ genomes that matched parts of the human genome, says Katie Rudd of Emory University in Atlanta. That approach overlooked segments in the apes’ genomes that had been lost in the human genome.

To get a fuller picture of subtelomere evolution, Rudd and her colleagues closely examined the ends of chromosomes 14 and 15 in all the species studied. They found that chunks of subtelomere DNA had been thoroughly shuffled over evolutionary time. The chromosomes had occasionally lost or picked up chunks of DNA, often by swapping bits with the subtelomeres of neighboring chromosomes.

“This is crazy. This is off-the-charts different,” Rudd says. “No one knew that the subtelomeres [of related ape species] have totally different structures.” The cause of this high rate of rearrangement is still unclear, Rudd says.

The subtelomeres studied by Rudd’s team contained genes for smell receptors. A gene involved in a form of human muscular dystrophy is also found on a subtelomere. Scientists don’t yet know what other genes lie in these rapidly evolving areas, Rudd says.