A new paper shows that a polymorphism on the AIM1 locus which is associated with human pigmentation has been under strong positive selection in Europeans, reaching almost fixation in tested European populations (0.89 in South Africans and 0.96 in Germans), while being rare elsewhere. The derived allele is associated with lighter overall pigmentation. This contrasts to the situation with the MC1R locus in which the ancestral variant is maintained by selection in Negroids, but multiple unrelated mutations outside Africa have resulted in lighter-skinned phenotypes. Unlike the MC1R where relaxation of selection constraints were observed in non-Africans, the new AIM1 polymorphism has been positively selected.
The time of the common ancestor of alleles bearing the haplotype is estimated to be 10,965 years, although the 95% confidence interval is wide from 1,328 to 39,609 years. We should probably not speculate on what triggered the selection based on this very uncertain dating, but the repopulation of Europe after the last glaciation may be a candidate. As humans spread to higher latitudes, they may have been subjected to higher selective pressures for light pigmentation. It would be interesting to determine the frequency of the polymorphism in different Caucasoid populations and determine the most likely ancestral populations.
Molecular Biology and Evolution (published online)
Evidence for Recent Positive Selection at the Human AIM1 Locus in a European Population
Mikiko Soejima et al.
Two missense polymorphisms (E272K and L374F) of the AIM1 locus, encoding a melanocyte differentiation antigen, were shown to have a clear association with human ethnicities. These two nonpathogenic SNPs may be associated with human pigmentation variation. In this study, we investigated sequence variation in the coding region and exon-flanking sequence and found low genetic variation only in subjects of European descent. All four statistical tests applied to the 7.55-kb region surrounding the L374F polymorphism detected statistically significant deviations from selective neutrality in Europeans. In addition, haplotype analysis revealed that one haplotype carrying 374F was overrepresented in this population, and the low rate of variation, with some features of selective sweeps, was shown to be statistically significant. These results suggest that positive selection recently has been acting or has acted on at least this region of the melanogenic gene and that an advantageous haplotype spread rapidly in Europe.