Genetic Chaos

Friday, May 20, 2005

mtDNA Variation among Greenland Eskimos: The Edge of the Beringian Expansion

The Eskimo-Aleut language phylum is distributed from coastal Siberia across Alaska and Canada to Greenland and is well distinguished from the neighboring Na Dene languages. Genetically, however, the distinction between Na Dene and Eskimo-Aleut speakers is less clear. In order to improve the genetic characterization of Eskimos in general and Greenlanders in particular, we have sequenced hypervariable segment I (HVS-I) of the mitochondrial DNA (mtDNA) control region and typed relevant RFLP sites in the mtDNA of 82 Eskimos from Greenland. A comparison of our data with published sequences demonstrates major mtDNA types shared between Na Dene and Eskimo, indicating a common Beringian history within the Holocene. We further confirm the presence of an Eskimo-specific mtDNA subgroup characterized by nucleotide position 16265G within mtDNA group A2. This subgroup is found in all Eskimo groups analyzed so far and is estimated to have originated <3,000 years ago. A founder analysis of all Eskimo and Chukchi A2 types indicates that the Siberian and Greenland ancestral mtDNA pools separated around the time when the Neo-Eskimo culture emerged. The Greenland mtDNA types are a subset of the Alaskan mtDNA variation: they lack the groups D2 and D3 found in Siberia and Alaska and are exclusively A2 but at the same time lack the A2 root type. The data are in agreement with the view that the present Greenland Eskimos essentially descend from Alaskan Neo-Eskimos. European mtDNA types are absent in our Eskimo sample.

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High level of male-biased Scandinavian admixture in Greenlandic Inuit shown by Y-chromosomal analysis

We have used binary markers and microsatellites on the Y chromosome to analyse diversity in a sample of Greenlandic Inuit males. This sample contains Y chromosomes typical of those found in European populations. Because the Y chromosome has a unique and robust phylogeny of a time depth that precedes the split between European and Native American populations, it is possible to assign chromosomes in an admixed population to either continental source. On this basis, 58+/-6% of these Y chromosomes have been assigned to a European origin. The high proportion of European Y chromosomes contrasts with a complete absence of European mitochondrial DNA and indicates strongly male-biased European admixture into Inuit. Comparison of the European component of Inuit Y chromosomes with European population data suggests that they have their origins in Scandinavia. There are two potential source populations: Norse settlers from Iceland, who may have been assimilated 500 years ago, and the Danish-Norwegian colonists of the eighteenth century. Insufficient differentiation between modern Icelandic and Danish Y chromosomes means that a choice between these cannot be made on the basis of diversity analysis. However, the extreme sex bias in the admixture makes the later event more likely as the source.

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A common inversion under selection in Europeans

A refined physical map of chromosome 17q21.31 uncovered a 900-kb inversion polymorphism. Chromosomes with the inverted segment in different orientations represent two distinct lineages, H1 and H2, that have diverged for as much as 3 million years and show no evidence of having recombined. The H2 lineage is rare in Africans, almost absent in East Asians but found at a frequency of 20% in Europeans, in whom the haplotype structure is indicative of a history of positive selection. Here we show that the H2 lineage is undergoing positive selection in the Icelandic population, such that carrier females have more children and have higher recombination rates than noncarriers.

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