The Origin of mtDNA X1 and X2 - Not Native American

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Origin and Diffusion of mtDNA Haplogroup X

Maere Reidla,(2003)



A maximum parsimony tree of 21 complete mitochondrial DNA (mtDNA) sequences belonging to haplogroup X

and the survey of the haplogroup-associated polymorphisms in 13,589 mtDNAs from Eurasia and Africa revealed

that haplogroup X is subdivided into two major branches, here defined as “X1” and “X2.” The first is restricted

to the populations of North and East Africa and the Near East, whereas X2 encompasses all X mtDNAs from

Europe, western and Central Asia, Siberia, and the great majority of the Near East, as well as some North African

samples. Subhaplogroup X1 diversity indicates an early coalescence time, whereas X2 has apparently undergone

a more recent population expansion in Eurasia, most likely around or after the last glacial maximum. It is notable

that X2 includes the two complete Native American X sequences that constitute the distinctive X2a clade, a clade

that lacks close relatives in the entire Old World, including Siberia. The position of X2a in the phylogenetic tree

suggests an early split from the other X2 clades, likely at the very beginning of their expansion and spread from

the Near East.

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Subhaplogroup X1 was found to be largely restricted to the Afro-Asiatic–speaking populations of North Africa

and neighboring areas, including Ethiopia, suggesting a possible geographic diffusion of X1 alongside the Mediterranean Sea and the Red Sea (table 1). This subhaplogroup

is subdivided into the two clades X1a and X1b,





. Virtually all (97.2%) haplogroup XmtDNAs from the Near East, the South Caucasus, and Europe were found

to belong to subhaplogroup X2, as did all (100%) of those from Siberia and Central Asia ……and some (36.8%)

of those from North Africa (table 2).



Thus, subhaplogroup X2 is characterized by a very wide geographic range but also by an infrequent occurrence. Indeed, it

generally comprises !5% of the mtDNAs in West Eurasian and North African populations (table 1). Three

exceptions include the Druze, the Georgians, and the Orkney Islanders, among whom the frequency of X2

reaches 11%, 8%, and 7%, respectively. The high frequencies of X2 in the Druze and the Orkney Islanders

are combined with a low haplotype diversity (0.400 and 0.473, respectively), and the relatively high frequency in

these populations is most likely due to genetic drift and

founder events.



Fourth, the split between “African” X1 and “Eurasian” X2 subhaplogroups

of X is phylogenetically as deep as that within the branches of haplogroup U that also differ

profoundly in their phylogeography. Thus, subhaplogroup U6 is largely restricted to North Africa (as X1),

whereas subhaplogroup U5 is widespread in West Eurasia (as X2). The phylogeographic patterns and the coalescence

times that we obtained here suggest that the basic phylogenetic structures of the mtDNA haplogroups

in West Eurasia and North Africa are as ancient as the beginning of the spread of anatomically modern

humans in this region. Finally, phylogeography of the subclades of haplogroup X suggests that the Near East

is the likely geographical source for the spread of subhaplogroup X2, and the associated population dispersal

occurred around, or after, the LGM when the climate ameliorated. The presence of a daughter clade in northern

Native Americans testifies to the range of this population expansion.