Evolution of the ABO blood group locus in pre-Columbian Native Americans

Fernando Alberto Villanea

Among all studied genes in humans, the locus coding for ABO blood group alleles is the single best studied genetic trait. This makes this gene a prime marker to address Native American population history. Across loci, Native Americans have the lowest genetic diversity of all continental populations. While some Native American populations exhibit A, B and O alleles, such as some Eskimo groups, and some North American populations exhibit A and O alleles, all other Native American populations from North, Central, and South America are nearly fixed for the O allele. Contrastingly, most other human populations outside of America seem to always maintain all three alleles and even orbit around similar allelic frequencies. This has been attributed to frequency dependent selection, mediated by our immune response to bacteria and virus, pressure which is consistent across the world. The high variance in ABO allele frequencies in Native Americans stands as an oddity. In this study, four hypotheses were tested to explain the low overall Native American diversity and the high within Native American variance of ABO allele frequencies: a) Founder’s effect in the original population, b) Post-European contact reduction of diversity, c) PostEuropean contact natural selection and d) High population structuring following subsequent migrations into the continent. To address the possibility of ABO allele frequencies changing after European contact, ABO allele diversity data was recovered from pre-contact Muwekma-Ohlone samples. This ancient population, referenced as CA SCL-38, exhibits similar mitochondrial haplogroup and ABO allelic diversity as contemporary Native American populations from the same geographic area. Additionally a model was created to simulate loss of allelic diversity under the effect of genetic drift as a consequence of a reduced effective population size. Modeled data sets a lower limit to the effective population size a group would require in order to maintain diversity. Contrasted against ABO diversity data from extant populations, my results are consistent with ABO diversity being lost as a result of isolation as populations migrated deeper into the continent and not as a result of a bottleneck in the original founder population.

Evolution of the ABO blood group locus in pre-Columbian Native Americans

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