A pigmentation gene first identified in zebrafish helps explain skin color differences between human European and African populations, a new
While scientists had previously found pigmentation genes that contribute to variations within populations, said senior author Keith Cheng, "it's been a complete mystery" as to what drives major variations in human skin color. "It's remarkable that this difference in skin color that has historically been partly responsible for a great deal of problems in our civilization is due to this one nucleotide out of 3 billion," Cheng told
"What these findings speak to is the molecular differences that explain differences in skin color. That is very different than what people refer to as race," said Greg Barsh at Stanford University, who did not participate in this study.
Cheng's team at the Pennsylvania State University College of Medicine in Hershey was studying the
Using positional cloning, the researchers isolated
When the researchers injected human
The scientists found
The team then turned to genomics to see how the protein might be important in humans. When they consulted the recently published HapMap, they discovered that there were two primary alleles, varying at only one locus. And while nearly all East Asian and African genomes had a site containing alanine, the ancestral allele shared by other vertebrates, 99% of the Europeans had threonine, representing a derived allele. This striking bifurcation, coupled with a marked decrease in heterozygosity in nearby genes within the European genomes, led the group to conclude that the threonine variant has been the target of strong natural or sexual selection in European populations.
As a functional test of their findings, Cheng's group was able to correlate
While Cheng said they have "identified the probable largest impact gene explaining the difference between Europeans and Africans," they are curious about other genes in play that would explain pigmentation differences between East Asian and African populations. On a biochemical level, Barsh said, other proteins that have been implicated in pigmentation seem to have similar biochemical mechanisms to SLC24A5, highlighting the need to determine how SLC24A5 interacts with these proteins and with ones that have yet to be identified.
Cheng's findings are consistent with what he said is the prevailing evolutionary wisdom: melanin blocks UV light, and while darker skin is advantageous under strong sunlight because it reduces the destructive effects of UV rays, lighter skin is adaptive in less sunny climates since it allows more sunlight absorption for the production of vitamin D.