Researchers peering into the DNA toolbox have found yet another instrument of evolution. Simply replicating a piece of a particular gene--from one copy in mice to more than 200 in humans--may have prompted some of the changes in the brain that define us as human, according to a new study.
Evolution occurs when genes mutate, or when they alter where, when, and how strongly they are active. In addition, hiccups in DNA replication can foster change by causing some parts of genes to be repeated as they are copied. Twin genes or duplicated regulatory regions result, and although one in the pair usually has to keep doing its original job, the other is free to mutate and take on new roles that can enhance an organism's survival.
In earlier work, James Sikela, a genome researcher at the University of Colorado, Denver, and Jonathan Pollack from Stanford University and colleagues found 134 genes that had been duplicated primarily after human ancestors split off from other primates. In the new study, Sikela, Gerald Wyckoff from the University of Missouri, Kansas City, and their colleagues compared the sequences of these genes in primates, as well from mice and rats, to reconstruct the history of each duplicated gene.
They found 44 genes with more than five copies each in the human genome. One in particular, called MGC8902, caught their eye. Humans have 49 copies of this gene, while chimps have 10 and macaques have four, the team reports tomorrow in Science. With so many copies, "MGC8902 stands out as a very good candidate to be important to a human specific trait," says Sikela. A closer look revealed that the heavily-duplicated gene contained its own duplications: six copies of a domain called DUF1220.
The domains exist in other primates, but are most common in humans, says Sikela. The researchers discovered that the genes with the DUF1220 domains are expressed in the heart, spleen, skeletal muscle, and small intestine, and are particularly active in the brain's neocortex. Thus, they may play a role in higher cognition.
"The exciting thing here is the expansion of a gene family associated with expression in specific neurons," says Evan Eichler, a geneticist at the University of Washington, Seattle. But, he adds, "I would be cautious about overextrapolating these observations to brain enlargement."