MSNBC, NPR, and BBC News have run articles about a publication in Science run nearly two weeks ago about one of the genes that control skin pigmentation in humans. The source article is titled, “SLC24A5, a Putative Cation Exchanger, Affects Pigmentation in Zebrafish and Humans” and is led by a team of pathologists, molecular biologists, geneticists, and biochemists primarily from the Penn State College of Medicine.
Actually, finding SLC24A5 was an accident. The group of researchers were seeking cancer genes and stumbled upon pigment cells in zebrafish that resembled human pigment cells. The researchers were using zebrafish as the model organism for cancer genes, primarily because of their fast developmental cycle and the similarities in vertebrate genome.
SLC24A5 was first identified by the Penn State team and was not identified initially as a pigmentation gene, since they were on the prowl for cancer genes. As a gene, SLC24A5 is an acronym for solute carrier family 24, member 5, which lies in on the long (q) arm of chromosome 15 on postion 21.1, from base pair 46,200,461 to base pair 46,221,881 (Source), Entrez Gene has more on the specifics of the gene, however it should be noted that the gene is conserved throughout the lineage of Eukaryota to Metazoa, Vertebrata, Mammalia, Primates, Catarrhini, Hominidae, and onto Homo. Remarkably, the human and fish versions of the gene share nearly 70 percent of the same protein sequence!
It was not until the the scientists drew tangents between the similarities in zebrafish and human pigment cells, which both contain granules called melanosomes. In specific strain of zebrafish, named golden for its ligher appearance, there are less melanosomes. Similarly, in lighter skin toned humans there are less melansomes which is due to a slightly different variation of the gene — one that codes the amino acid threonine. The reverse is for the dark, or wild-type, strain of zebrafish and humans with darker skin tones, which codes for the amino acid alanine — there are more melanosomes and a higher expression level of SLC24A5, as high as 38% in people of African origin. Whereas there is a 25% expression level of SLC24A5 in people of European origin. The image to your right, displays the wild-type zebrafish above has darker stripes than the golden zebrafish below. The insets show that the golden zebrafish has fewer, smaller and less dense pigment-filled compartments called melanosomes than the wild-type zebrafish.
The researchers then looked at two different human populations in which people with European and African ancestors had mixed relatively recently — African-Americans and African-Caribbeans. They found that, on average, people with two copies of the European version of the gene had the lightest skin. People with two copies of the non-European version of the gene had darker skin, and people with one copy of each version of the gene had skin color somewhere in between.
Ultimately this gene has deep roots into the debate over the validity of race. Personally, I was taught during my undergraduate career, that race is completely a social construct. And this is true for most anthropologists, who live by the mantra that there are more ‘differences’ amongst described races than between the races… meaning there is more prey area and things are not simply black and white. This opinion can be traced back to a 1972 paper by Richard Lewontin.
But this gene changes the foundation of that mantra, in my opinion. I wonder does a 7% difference in expression levels between SLC24A5 genes of peoples of African and European decent indicate somewhat a molecular indication different races? Especially when the difference is between 1 amino acid. Though, I must state that my curiousity will not go as far to follow conservative voice of Vincent Sarich and Frank Miele, in Race: The Reality of Human Differences, who say,
“racial differences in humans exceed the differences that separate subspecies or even species in such other primates as gorillas and chimpanzees” and that “race is a biologically real phenomenon with important consequences”
I still wonder the how this finding shakes the validity of race debate?
I also wonder how the identification of this gene also relates to the theory of human skin color varies with the amount of exposure to the sun’s ultraviolet (UV) radiation, or rather the more sunlight you receive the darker your skin will become. I was once taught in my Human Adaptability and Variation class, that
genotype + environment = phenotype
and won’t argue that if you are genetically predispositioned to be black that environment can or will change your phenotype, but I ask what role does environment have in the expression of this gene? Does skin not react to amount of sunlight to produce more melanin and therefor more melanosomes? Won’t people around the equator, and exposed to more sun, have darker skin tones than those farther from the equator and/or exposed to less sun regardless of how much this gene is expressed or regulated? How does acclimatization affect SLC24A5?
I guess this all boils down to what is race? Simply put is race solely a social construct, or is it biologically driven, or both? And why? Maybe it is another one of my rhetorical questions that can never be answered, but I’d like to know what y’all think about race, especially after reading about this gene.