Haplogroup X (mtDNA)

In human mitochondrial genetics, Haplogroup X is a human mitochondrial DNA (mtDNA) haplogroup which can be used to define genetic populations. The genetic sequences of haplogroup X diverged originally from haplogroup N, and subsequently further diverged about 20,000 to 30,000 years ago to give two sub-groups, X1 and X2. Overall haplogroup X accounts for about 2% of the population of Europe, the Near East and North Africa. Sub-group X1 is much less numerous, and restricted to North and East Africa, and also the Near East. Sub-group X2 appears to have undergone extensive population expansion and dispersal around or soon after the last glacial maximum, about 21,000 years ago. It is more strongly present in the Near East, the Caucasus, and Mediterranean Europe; and somewhat less strongly present in the rest of Europe. Particular concentrations appear in Georgia (8%), the Orkney Islands (7%) and amongst the Israeli Druze (26%); the latter are presumably due to a founder effect.

North and South America

Haplogroup X is also one of the five haplogroups found in the indigenous peoples of the Americas, occurring at a frequency of about 3%. It is found with particular prevalence in the Ojibwa (25%) from the Great Lakes, the Sioux (15%), the Nuu-Chah-Nulth (11%–13%), the Navajo (7%), and the Yakima (5%). Haplogroup X has been registered in the isolated Yanomami people (12%) from eight villages of Roraima in northwestern Brazil and also in a pre-Columbian native sample of Sambaqui Pirabas in the Amazon Region.

Unlike the four main Native American haplogroups (A, B, C, and D), X is not at all associated with East Asia. This has led to speculation that the haplogroup X occurrences might indicate a minority European ancestry for some Native Americans. A particularly concrete suggestion was that the genetic inheritance might reflect transatlantic links perhaps made in about 20,000 BC by the Solutreans, a stone-age culture excavated in south-west France and Spain, who it was suggested might have carried on an Atlantic-spanning maritime life around the margins of the retreating ice-age Atlantic ice fields, similar to the existence of the present-day Inuit.

The speculation abated somewhat when haplogroup X individuals were discovered in Altaia in South Siberia (Derenko et al, 2001). However, more detailed examination (Reidla et al, 2003) has shown that the Altaian sequences are all almost identical, suggesting that they arrived in the area probably from the South Caucasus more recently than 5000 BC. On the other hand, the North American haplogroup X DNA (now called subgroup X2a) is as different from any of the old world X2 lineages as they are from each other. In a parsimony tree, the Iranian mtDNA would share a common ancestor with the Native American clade. This suggests that the ancestors of the X2a population presumably separated very early from all of the other X2 lineages, but gives little clue as yet to the true path of their paleolithic migration from Iran to North America.

See also

• Human mitochondrial DNA haplogroups
• Human mitochondrial genetics
• Models of migration to the New World
• In his popular book The Seven Daughters of Eve, Bryan Sykes named the originator of this mtDNA haplogroup "Xenia".

External links

• Spread of Haplogroup X, from National Geographic
• Coming into America: Tracing the Genes, PBS, popular presentation of the Solutrean hypothesis

References

• Andrea K. C. Ribeiro-Dos Santos; S. E. B. Santos; A. L. Machado; V. Guapindaia; and M. A. Zago 1996. Heterogeneity of Mitochondrial DNA haplotypes in pre-Columbian Natives of the Amazon Region. American Journal of Physical Anthropology 101:29-37 (1996).
• RD Easton, DA Merriwether et al, mtDNA variation in the Yanomami: evidence for additional New World founding lineages. American Journal of Human Genetics, 59(1):213-225 (1996)
• Michael D. Brown et al, mtDNA Haplogroup X: An Ancient Link between Europe/Western Asia and North America?, American Journal of Human Genetics, 63:1852-1861 (1998)
• David Glenn Smith et al, Distribution of mtDNA Haplogroup X among Native North Americans, American Journal of Physical Anthrolopogy 110:271-284 (1999)
• Miroslava V. Derenko et al, The Presence of Mitochondrial Haplogroup X in Altaians from South Siberia American Journal of Human Genetics, 69(1):237–241 (2001)
• Maere Reidla et al, Origin and Diffusion of mtDNA Haplogroup X, American Journal of Human Genetics, 73:1178–1190 (2003)
• Ilia A. Zaharov et al, Mitochondrial DNA Variation in the Aboriginal Populations of the Altai-Baikal Region: Implications for the Genetic History of North Asia and America Annals of the New York Academy of Sciences 1011: 21 (2004)

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