mtDNA haplogroup X: An ancient link between Europe western Asia and North America?

On the basis of comprehensive RFLP analysis, it has been inferred that simi lar to 97% of Native American mtDNAs belong to one of four major founding m tDNA lineages, designated haplogroups "A"-"D." It has been proposed that a fifth mtDNA haplogroup (haplogroup X) represents a minor founding lineage i n Native Americans. Unlike haplogroups A-D, haplogroup X is also found at l ow frequencies in modern European populations. To investigate the origins, diversity, and continental relationships of this haplogroup, we performed m tDNA high-resolution RFLP and complete control region (CR) sequence analysi s on 22 putative Native American haplogroup X and 14 putative European hapl ogroup X mtDNAs. The results identified a consensus haplogroup X motif that characterizes our European and Native American samples. Among Native Ameri cans, haplogroup X appears to be essentially restricted to northern Amerind ian groups, including the Ojibwa, the Nuu-Chah-Nulth, the Sioux, and the Ya kima, although we also observed this haplogroup in the Na-Dene-speaking Nav ajo. Median network analysis indicated that European and Native American ha plogroup X mtDNAs, although distinct, nevertheless are distantly related to each other. Time estimates for the arrival of X in North America are 12,00 0-36,000 years ago, depending on the number of assumed founders, thus suppo rting the conclusion that the peoples harboring haplogroup X were among the original founders of Native American populations. To date, haplogroup X ha s not been unambiguously identified in Asia, raising the possibility that s ome Native American founders were of Caucasian ancestry.