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Haplogroup X (mtDNA)

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« on: June 25, 2007, 01:02:37 am »

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.
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« Reply #1 on: June 25, 2007, 01:04:40 am »

North and South America

Haplogroup X is also one of the five haplogroups found in the indigenous peoples of the Americas.[1] Although it occurs only at a frequency of about 3% for the total current indigenous population of the Americas, it is a major haplogroup in northeastern North America, where among the Algonquian peoples it comprises up to 25% of mtDNA types.
It is also present in lesser percentages to the west and south of this area -- in North America among the Sioux (15%), the Nuu-Chah-Nulth (11%–13%), the Navajo (7%), and the Yakima (5%), and in South America among the Yanomami people (12%) in eight villages in Roraima in northwestern Brazil.

Unlike the four main Native American haplogroups (A, B, C, and D), X is not at all strongly associated with East Asia. The sole occurrence of X in Asia discovered so far is in Altaia in South Siberia (Derenko et al, 2001), and 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.

This absence of haplogroup X in Asia is one of the major factors causing the current rethinking of the peopling of the Americas. The New World haplogroup X DNA (now called subgroup X2a) is as different from any of the Old World X2 lineages as they are from each other, indicating a very ancient origin.

The Solutrean Hypothesis posits that haplogroup X reached North America with a wave of European migration about 20,000 BC by the Solutreans, a stone-age culture in south-western France and in Spain, by boat around the southern edge of the Arctic ice pack.

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« Reply #2 on: June 25, 2007, 01:05:39 am »

Title  mtDNA Haplogroup X: An Ancient Link between Europe/Western Asia and North America?
Author(s)  Michael D. Brown, Seyed H. Hosseini, Antonio Torroni, Hans-Jürgen Bandelt, Jon C. Allen, Theodore G. Schurr, Rosaria Scozzari, Fulvio Cruciani, and Douglas C. Wallace
Identifiers  The American Journal of Human Genetics, volume 63 (1998), pages 1852–1861
DOI: 10.1086/302155
PubMed ID: 9837837
 
Availability  This site:   PS  |  HTML  |  PDF (273.3k) 
Copyright  © 1998, The American Society of Human Genetics.
Abstract  On the basis of comprehensive RFLP analysis, it has been inferred that 97% of Native American mtDNAs belong to one of four major founding mtDNA lineages, designated haplogroups "A""D." It has been proposed that a fifth mtDNA haplogroup (haplogroup X) represents a minor founding lineage in Native Americans. Unlike haplogroups AD, haplogroup X is also found at low frequencies in modern European populations. To investigate the origins, diversity, and continental relationships of this haplogroup, we performed mtDNA high-resolution RFLP and complete control region (CR) sequence analysis on 22 putative Native American haplogroup X and 14 putative European haplogroup X mtDNAs. The results identified a consensus haplogroup X motif that characterizes our European and Native American samples. Among Native Americans, haplogroup X appears to be essentially restricted to northern Amerindian groups, including the Ojibwa, the Nuu-Chah-Nulth, the Sioux, and the Yakima, although we also observed this haplogroup in the Na-Denespeaking Navajo. Median network analysis indicated that European and Native American haplogroup X mtDNAs, although distinct, nevertheless are distantly related to each other. Time estimates for the arrival of X in North America are 12,00036,000 years ago, depending on the number of assumed founders, thus supporting the conclusion that the peoples harboring haplogroup X were among the original founders of Native American populations. To date, haplogroup X has not been unambiguously identified in Asia, raising the possibility that some Native American founders were of Caucasian ancestry

http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/302155&erFrom=2591904083650527317Guest
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« Reply #3 on: June 25, 2007, 01:09:02 am »

mtDNA Haplogroup X: An Ancient Link between Europe/Western Asia and North America?

Michael D. Brown,1 Seyed H. Hosseini,1 Antonio Torroni,2 Hans-Jürgen Bandelt,3 Jon C. Allen,1 Theodore G. Schurr,1 Rosaria Scozzari,2 Fulvio Cruciani,2 and Douglas C. Wallace1

1Center for Molecular Medicine, Emory University School of Medicine, Atlanta; 2Dipartimento di Genetica e Biologia Molecolare, Universita' di Roma "La Sapienza," Rome; and 3Mathematisches Seminar, University of Hamburg, Hamburg

Received July 13, 1998; accepted for publication October 6, 1998; electronically published November 25, 1998.

Summary

On the basis of comprehensive RFLP analysis, it has been inferred that 97% of Native American mtDNAs belong to one of four major founding mtDNA lineages, designated haplogroups "A""D." It has been proposed that a fifth mtDNA haplogroup (haplogroup X) represents a minor founding lineage in Native Americans. Unlike haplogroups AD, haplogroup X is also found at low frequencies in modern European populations. To investigate the origins, diversity, and continental relationships of this haplogroup, we performed mtDNA high-resolution RFLP and complete control region (CR) sequence analysis on 22 putative Native American haplogroup X and 14 putative European haplogroup X mtDNAs. The results identified a consensus haplogroup X motif that characterizes our European and Native American samples. Among Native Americans, haplogroup X appears to be essentially restricted to northern Amerindian groups, including the Ojibwa, the Nuu-Chah-Nulth, the Sioux, and the Yakima, although we also observed this haplogroup in the Na-Denespeaking Navajo. Median network analysis indicated that European and Native American haplogroup X mtDNAs, although distinct, nevertheless are distantly related to each other. Time estimates for the arrival of X in North America are 12,00036,000 years ago, depending on the number of assumed founders, thus supporting the conclusion that the peoples harboring haplogroup X were among the original founders of Native American populations. To date, haplogroup X has not been unambiguously identified in Asia, raising the possibility that some Native American founders were of Caucasian ancestry.



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Address for correspondence and reprints: Dr. Michael D. Brown, Center for Molecular Medicine, Emory University School of Medicine, 420B Dental School, 1462 Clifton Road, Atlanta, GA. E-mail: mdbrown@gmm.gen.emory.edu
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Introduction

     For Native Americans, extensive RFLP and control region (CR; also known as the "D-loop") sequence analysis has unambiguously identified four major founding mtDNA haplogroups, designated "A""D" (Torroni et al. 1992; 1993a). Together, these haplogroups account for 97% of modern Native American mtDNAs surveyed to date (Torroni and Wallace 1994; Merriwether et al. 1995). Apparent nonhaplogroup AD mtDNAs can result from reversion of key AD markers, recent admixture with nonNative Americans, or represent additional Native American founding mtDNA lineages. A striking example of the presence of nonhaplogroup AD genotypes in Native Americans can be seen in the Ojibwa, an Amerindian population from the Great Lakes region of North America. Using high-resolution RFLP analysis, Torroni et al. (1993a) found that 25% of the northern Ojibwa mtDNAs did not belong to haplogroups AD and that nearly all of these "other" mtDNAs encompassed four distinct but related haplotypes characterized by the RFLP motif -1715 DdeI and +16517 HaeIII. This motif was also present in 4% of the Navajo, but it was not observed in 18 other tribes from North, South, and Central America. The high incidence of this motif in the Ojibwa has been confirmed recently by Scozzari et al. (1997), who reported its presence in 26% of the southeastern Ojibwa from Manitoulin Island, Canada.

     A recent survey of European mtDNAs has demonstrated the presence of the same "other" haplotype motif in modern European populations, in which it is called "haplogroup X." Haplogroup X represents 4% of European mtDNAs and has been found to be further characterized by CT transitions at nucleotide position (np) 16223 and np 16278 of the CR (Torroni et al. 1996). Thus, haplogroup X mtDNAs are minimally characterized by a combination of RFLP and CR markers including -1715 DdeI, +16517 HaeIII, and the 16223T and 16278T mutations.

     On the basis of the presence of the 16223T-and-16278T motif in nine nonhaplogroup AD mtDNAs from the Nuu-Chah-Nulth and Yakima initially reported by Ward et al. (1991, 1993), Bailliet et al. (1994) suggested the existence of a fifth haplogroup in the Americas. More recently, Forster et al. (1996) analyzed the same data set and proposed that this haplogroup (haplogroup X) constituted an additional founding Native American lineage. However, this conclusion was based on such a limited number of samples that it was not possible to address important issues, such as the origin and diversity of the Native American haplogroup X mtDNAs, as well as the relationship between Old World and Native American haplogroup X mtDNAs.

     To investigate these issues, we performed high-resolution RFLP and complete CR sequence analysis for available Native American and Old World mtDNAs and found 22 Native American and 14 European mtDNAs that belonged to haplogroup X. Despite a shared consensus RFLP haplotype, substantial genetic differences exist between the Native American and European mtDNAs. Phylogenetic analysis indicates that the two groups are relatedbut only distantlyto each other and that considerable genetic substructure exists within both groups. Further, coalescence-age estimates for haplogroup X in the Americas, based on either RFLP or CR sequence data, clearly indicate the antiquity of this haplogroup in the New World. Overall, these data exclude the possibility that the occurrence of haplogroup X in Native Americans is due to recent European admixture and, instead, provide a rigorous demonstration that this haplogroup represents an additional founding mtDNA lineage in Native Americans.

http://www.journals.uchicago.edu/AJHG/journal/issues/v63n6/980554/980554.html
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« Reply #4 on: June 25, 2007, 01:09:49 am »

Subjects and Methods

Population Samples


     A total of 36 (22 Native American and 14 European) individuals were available in our collection, as putative haplogroup X samples, and were used for high-resolution RFLP and CR sequence analysis. The 22 Native Americans (designated "NA1""NA22") consisted of 7 northern Ojibwa (NA1, NA2, NA7NA10, and NA20) from the northwestern region of Ontario, 2 southwestern Ojibwa samples (NA3 and NA21) from Wisconsin, 5 southeastern Ojibwa (NA4NA6, NA11, and NA22) from Manitoulin Island in Lake Huron, 2 Navajo (NA14 and NA15) from New Mexico, 2 Nuu-Chah-Nulth (NA12 and NA13) from Vancouver Island, British Columbia (Torroni et al. 1993a; Scozzari et al. 1997), and 4 Navajo (NA16NA19) collected from New Mexico by Dr. Rene Herrera. The 14 Caucasian-European haplogroup X samples (designated "CE1""CE14") included 2 Caucasians of European ancestry (CE1 and CE4) from the United States and 1 French Canadian (CE5), (Torroni et al. 1994a), 1 Finn (CE8) (Torroni et al. 1996), 5 Israeli Druze (CE2 and CE11CE14), and 5 Italians (CE3, CE6, CE7, CE9, and CE10).

mtDNA Analysis

     High-resolution RFLP analysis was performed as described elsewhere (Torroni et al. 1996). AccI was also used to detect the haplogroup Xassociated AccI site gain at np 14465. RFLP analysis had previously been performed for samples NA1 and NA2 (haplotype AM76), NA7NA9 (haplotype AM75), NA10 (haplotype AM74), and NA14 and NA15 (haplotype AM29) (Torroni et al. 1993a), CE1, CE4, and CE5 (haplotypes 71, 7, and 101, respectively) (Torroni et al. 1994a). Partial RFLP analysis was performed on eight mtDNAs (NA3, NA12, NA13, NA16NA19, and NA21), because of limitations of sample material, and included assays for the haplogroup X RFLP markers (-1715 DdeI, -10394 DdeI, +14465 AccI, and +16517 HaeIII), as well as for the restriction-site changes (-1413 TaqI, -8150 MspI, -10254 MboI, and +13367 BamHI) previously found in the high-resolution screening of other Native American haplogroup X samples (see table 1). For the Nuu-Chah-Nulth samples (NA12 and NA13), it has been shown elsewhere that these nonhaplogroup AD mtDNAs are -10394 DdeI and +16517 HaeIII (Torroni et al. 1993a). These mtDNAs were further tested for the 1715 DdeI and 14465 AccI sites, but a complete haplotype assignment was not possible.

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« Reply #5 on: June 25, 2007, 01:11:19 am »

  The entire CR was sequenced for all 36 putative haplogroup X samples. The target sequence encompassed 1,349 nucleotides (np 15880660), which included both CR hypervariable segmentsHVS-I and HVS-IIalong with some flanking coding region. Sequence analysis was performed by means of standard cycle sequencing with fluorescent dideoxy nucleotides. Both strands of the mtDNA were sequenced, and ambiguities were resolved by repeated sequencing reactions using different deoxyoligonucleotide primers.

     Phylogenetic networks, which retain ambiguities in the branching structure by incorporation of reticulations, were constructed by means of the median algorithm of Bandelt et al. (1995), with two different parameter settings. The implicit parameter governing the decision about prediction of recurrent mutations is hidden in equation (5) of that paper: it equals 2 by default, but, instead, we used 3, to capture all most parsimonious (MP) trees here. The elucidation of all MP trees could be done by hand in this case, by means of the mathematical techniques of Foulds et al. (1979), Hendy and Penny (1982), and Bandelt et al. (1995).

     Coalescence times for haplogroup X in North America were calculated from HVS-I sequences (with either one or two founder sequences being assumed) and also were estimated from RFLP haplotypes. For HVS-I sequence data, coalescence ages were estimated by means of HVS-I, the mean transitional distance from inferred founder root sequence(s), in which one transition within the np 1609016365 sequence was estimated to correspond to 20,180 years (Forster et al. 1996, Watson et al. 1997). To estimate the sampling error for , we assumed a Poisson distribution for the observed starlike network, with SD calculated as , where n denotes the sample size. Transversions, insertions/deletions, and variation in the number of C's in the unstable region np 1618216193 were not included in the calculations of HVS-I values. For RFLP data, we used the estimated mean distance, RFLP, from the inferred root sequence, with one restriction-site change being estimated to correspond to 24,420 years (for RFLP calibration, see Torroni et al. 1998). Major insertion/deletions, as well as variation at the hypervariable +16517 HaeIII site, were not considered in this analysis. This calibration, which was based on a direct comparison of Siberian and Native American haplogroups AD RFLP data and HVS-I data, would correspond to an mtDNA evolutionary rate of 2.7%/million years (Myr), which fits with the 2.2%2.9%/Myr estimate (Torroni et al. 1994c) used for divergence-based time estimates. The average nucleotide sequence divergence was calculated from complete RFLP haplotypes, by means of the maximum-likelihood procedure of Nei and Tajima (1983).

Results

     RFLP and sequencing results confirm that all 22 Native American and 14 European mtDNAs analyzed belong to haplogroup X (table 1). Figure 1 illustrates the reduced median network encompassing both the RFLP and the CR variation observed in the Native American and European haplogroup X mtDNAs. For easy reference, we have highlighted clusters (subclusters IV) of closely related mtDNAs from Native American populations. The network suggests that European and Native American haplogroup X mtDNAs are separated into two major branches. Nearly all Native American mtDNAs are encompassed by the branch harboring the 16213A and 200G variants, whereas all European mtDNAs are included within the branch that lacks these mutations (table 1 and fig. 1).

http://www.journals.uchicago.edu/AJHG/journal/issues/v63n6/980554/980554.html
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« Reply #6 on: June 25, 2007, 01:14:24 am »



Figure 1     Reduced median network of 22 Native American (NA) and 14 Caucasian (CE) haplogroup X mtDNAs as defined by both RFLP and CR variation. This network represents all of the MP trees. The boldface lines indicate the more plausible MP trees, and the dashed link is not found in any MP tree. mtDNA types are represented by circles, with areas proportional to number of individuals. Lines are labeled by restriction site changes and CR mutations, with restriction site changes indicated by their enzyme letter code listed in table 1. CR nucleotide variants correspond to transitions, while transversions are further specified, and insertions/deletions are indicated, in terms of np, within parentheses. Variations of C's in the unstable regions between np 1618216193 and np 309315 were not included. The node marked with a large asterisk matches the haplogroup X basic motif: -1715c, +14465s, 16189C, 16223T, 16278T, +16517e/16519C, 153A, 195C, and 225A. The order of mutations on a path not interrupted by branches or nodes is arbitrary. Underlining indicates nucleotide positions which have mutated more than once, and reticulations indicate ambiguity in the topology. Parallel lines in a reticulation represent the same mutation. Ellipses indicate Native American haplogroup X subdivided into clusters (IV) of closely related individuals.

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« Reply #7 on: June 25, 2007, 01:15:30 am »

The reduced median network further suggests that np 153 mutated more than twice (possibly even five times). This mutation has indeed arisen multiple times in human mtDNA, being found in Caucasians (Stoneking et al. 1991; Piercy et al. 1993) and Asians (Stoneking et al. 1991; Lee et al. 1997). The question of whether either np 225, np 16213, and np 200 underwent recurrent mutation cannot be unequivocally decided. However, np 225 appears to be stable, since this variant has never been seen in populations from Asia (Stoneking et al. 1991; Redd et al. 1995; Lee et al. 1997), Central and South America (Ginther et al. 1993; Batista et al. 1995; Easton et al. 1996), or Africa (Stoneking et al. 1991; Graven et al. 1995; Y.-S. Chen and D. C. Wallace, unpublished data). It has been detected in 4/167 European mtDNAs (Stoneking et al. 1991; Piercy et al. 1993; Torroni et al. 1996), each time on a haplogroup X background including the 16223T, 16278T, 153G, and 195C mutations, which renders 225A a marker for a major part of haplogroup X.

     The consensus motif of our total sample of 36 haplogroup X mtDNAs is -1715 DdeI, +14465 AccI, +16517 HaeIII/16519C, 16189C, 16223T, 16278T, 73G, 153G, 195C, 225A, and 263G, relative to the human reference sequence (Anderson et al. 1981). However, it is not clear whether the root haplotype for haplogroup X would also have 153G, and/or 225A, since these polymorphisms are found in the majority, but not all, of the haplogroup X mtDNAs. Overall, the sequence data and phylogenetic analysis suggest that the Native American and the European haplogroup X mtDNAs share a common maternal ancestor but also suggest that they diverged from each other long ago.

     Previously, both Ward et al. (1991) and Shields et al. (1993) reported CR sequences for seven Nuu-Chah-Nulth and two Yakima that contained the 16223T and 16278T mutations and did not belong to haplogroups AD. Hence, these mtDNAs likely belong to haplogroup X. Because these data encompass only HVS-I, these samples could not be included within figure 1. Five of these Nuu-Chah-Nulth and both Yakima mtDNAs also harbored 16213A, thus indicating that they probably belong to the 16213A/200G branch. However, none of these harbored either the 16093C variant of subclusters III and IV or the 16254C variant of subcluster V, indicating that these are subcluster II mtDNAs, which also encompasses our two Nuu-Chah-Nulth samples. The remaining two Nuu-Chah-Nulth reported by Ward et al. (1991) did not harbor either the 16213A variant or any of the other HVS-I variants observed in the Native American 16213A/200G branch. Thus, they appear to be very similar to our Ojibwa sample NA22 (subcluster I). One of these mtDNAs most closely represents the consensus haplogroup X motif (indicated by an asterisk in fig. 1), whereas the other differs from the first only by the presence of the variant 16362C.

     The coalescence time for haplogroup X mtDNAs in the New World was calculated by means of either RFLP or CR sequence data. RFLP data suggest only one possible founder haplotype, defined by -1715 DdeI, +14465 AccI, and +16517 HaeIII, which is shared by Native Americans and Europeans (table 1) and is central in the phylogeny of Native American haplogroup X mtDNAs. Using RFLP data, we calculated the mean distance from the putative founder haplotype (), for all Native American mtDNAs, using both our high-resolution RFLP haplotype data set (n = 14 samples) and our total RFLP data set (n = 22 samples) (table 2). Using the calibration of  = 1 as corresponding to 24,420 years, for RFLP data (Torroni et al. 1998), we estimate that the age of haplogroup X is 23,000 years. For CR HVS-I data, the presence, in both Europe and the Americas, of the basic HVS-I motif of 16189C, 16223T, and 16278T (subcluster I in fig. 1) suggests that this is the founder motif that originated in the Old World and moved to the Americas. When this motif was used as the root sequence, the coalescence time was estimated to be 31,00036,000 years ago
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« Reply #8 on: June 25, 2007, 01:17:27 am »

Discussion

     Our analysis confirmed that haplogroup X is present in both modern Native American and European populations. For the Native Americans, this haplogroup encompasses 25% of the Ojibwa, 15% of the Sioux, 11%13% of the Nuu-Chah-Nulth, 7% of the Navajo, and 5% of the Yakima (table 3). Thus, with the exception (see below) of the Na-Denespeaking Navajo, the distribution of this haplogroup among the Native Americans appears to be restricted to northern Amerindian populations.

In studies of Native American mtDNA diversity, the co-occurrence of the same haplogroup at significant frequencies in both the modern Native American and European populations is unique. Recent European genetic admixture cannot explain the presence of haplogroup X in the Amerindians. First, if the occurrence of haplogroup X were the result of female gene flow from Europeans, then other, more common European mtDNA haplogroups should also be present in the northern Native Americans, and they are not. Second, the Native American and European mtDNAs are very different and are connected only through an ancient common ancestor. Hence, Native American and European haplogroup X mtDNAs diverged long ago. Finally, Native American haplogroup X mtDNAs encompass substantial continent-specific diversity, implying an ancient arrival in America. Thus, haplogroup X represents a fifth founding mtDNA haplogroup for the Native Americans, supporting the conclusions of Bailliet et al. (1994), Forster et al. (1996), and Scozzari et al. (1997).

     An ancient arrival of haplogroup X in the Americas could be corroborated by the presence of haplogroup X in pre-Columbian human remains. Two studies on mtDNA variation in pre-Columbian samples have reported partial CR sequences that include the 16223T-and-16278T motif (Hauswirth et al. 1994; Ribeiro-Dos-Santos et al. 1996). However, in the absence of either more-complete CR sequence or RFLP data, it is not possible to definitively assign these pre-Columbian mtDNAs to haplogroup X. In a third study, Stone and Stoneking (1993, 1998) analyzed mtDNA variation in skeletal remains of 52 individuals from an Amerindian (Oneota culture) population, from the Illinois River valley, which dates to 1300 A.D. Two of these mtDNAs were not subsumed within Native American haplogroups AD, and partial CR sequence (essentially HVS-I) analysis indicated that these samples contained not only the 16223T and 16278T mutations but also the 16093C, 16189C, 16227G, and 16357C mutations (Stone and Stoneking 1998). These sequences are essentially identical to those of Ojibwa mtDNAs NA1 and NA2 (table 1 and subcluster IV in fig. 1), demonstrating the presence of haplogroup X in the Americas prior to European introgression.

     Since haplogroup X appears to be a pre-Columbian, founding Native American mtDNA lineage, the question remains: Where did this haplogroup originate? Thus far, haplogroup X has not been detected in numerous Asian/Siberian populations analyzed by high-resolution restriction analysis or CR sequencing (table 3). Three (2.9%) of 103 Mongolians harbored both the 16223T and 16278T mutations, but they also contained the +10394 DdeI and +10397 AluI markers (Kolman et al. 1996), placing them in Asian superhaplogroup M (Ballinger et al. 1992) and excluding them from haplogroup X. Among Koreans, analysis of CR HVS-I and II sequences showed that 13 (4.3%) of 306 contained the 16223T and 16278T mutations (Lee et al. 1997), but none harbored the consensus haplogroup X motif, suggesting that they are also members of superhaplogroup M. Finally, partial CR sequences of a large group of eastern Asians (Han from Taiwan, Japanese, Ainu, Ryukyans, and Koreans) revealed that 14 (4.8%) of 293 had both the 16223T and 16278T mutations, but none of these contained the 16093C, 16213A, or 16254C mutations (Horai et al. 1996). However, since only part of the CR was analyzed and no RFLP data were obtained, it is not possible to definitively classify these mtDNAs as members of either haplogroup X or superhaplogroup M. Thus, to date, haplogroup X mtDNAs have not been unambiguously identified in Asians.

     The apparent absence of haplogroup X in northern Asia parallels, in part, the situation already seen for Native American mtDNA haplogroup B. However, unlike haplogroup X, haplogroup B is present in Tibetans, Koreans, Japanese, and Mongolians. Moreover, haplogroup B is widely distributed among North, Central, and South American Amerindian populations, whereas haplogroup X is restricted to some of the most-northern Amerindian populations. Its presence in the Navajo but not in other Na-Dene populations suggests that, in a manner similar to that characterizing some nuclear-gene markers (Schell and Blumberg 1988), the Navajo have acquired haplogroup X through admixture with northern Amerindian populations. This could have occurred during or after the recent migration (1,000 years ago) that brought the ancestors of the Navajo from the Athapaskan homeland (Alaska and western Canada) to the southwestern United States (Haskell 1987). An Amerindian origin of the Navajo haplogroup X mtDNAs is also supported by the fact that the Navajo sequences are very similar, if not identical, to those observed in some Ojibwa (fig. 1). Also, the homogeneity of the Navajo sequences (table 1 and fig. 1) suggests that the Navajo acquired haplogroup X very recently.

     The time of entry of haplogroup X into the Americas, calculated from both RFLP and CR HVS-I sequence data and on the assumption that there is a single founder root for Native American mtDNAs, yielded a coalescence age, in the New World, of 23,00036,000 years ago. However, it is possible that there was more than one founder motif. A possible additional founder mtDNA is represented by the HVS-I motif 16189C, 16213A, 16223T, and 16278T, which is supported by the variant 200A in HVS-II. This motif represents 86% of our Native American haplogroup X samples and is found in all of the tribes studied. If two haplogroup X founders are proposed for our 22 Native Americans, then the HVS-I value would be .818, corresponding to a coalescence time of 17,000 ± 4,000 years ago. If the seven Nuu-Chah-Nulth and two Yakima sequences from the literature are added to our data, a HVS-I value of .613 is obtained, corresponding to a coalescence time of 12,000 ± 3,000 years ago. A similar coalescence time, 13,00017,000 years ago, is obtained when the average nucleotide sequence diversity is calculated from complete RFLP data ( = .0377%) and a mtDNA sequence evolution rate of 2.2%2.9%/Myr (Torroni et al. 1994c).

     A coalescence time of 23,00036,000 years ago would suggest that haplogroup X arrived in the Americas during the initial major Amerindian migration 20,00030,000 years ago. A coalescence time of 12,00017,000 years ago could be interpreted as a rapid reexpansion of haplogroup X mtDNAs near the time of the Na-Dene expansion, or, alternatively, as an independent and late arrival of haplogroup X mtDNAs into the Americas.

     Given the apparent absence of haplogroup X in modern eastern and northern Asia, it is difficult to define a source population for haplogroup X in the Americas. The similarity between the western Asian/European and Native American haplogroup X mtDNAs appears to indicate a western Asia origin of this haplogroup. Indeed, on the basis of limited RFLP data, the coalescence time for haplogroup X in Caucasians is estimated to be 30,00040,000 years ago (data not shown), compatible with both a Near Eastern origin of haplogroup X and its subsequent spread, probably at a low frequency, into Europe and Asia. If this is the case, then it is possible that this mtDNA was brought to Beringia/America by the eastward migration of an ancestral Caucasian population, of which no trace has so far been found in the mtDNA gene pool of modern Siberian/eastern Asian populations.

     In conclusion, we have described the occurrence, variation within, and population distribution of haplogroup X mtDNAs in Native Americans. This haplogroup appears, on the basis of archaeological data, to be pre-Columbian and may have arrived in the Americas either 12,00017,000 years ago or 23,00036,000 years ago. Haplogroup X is remarkable in that it has not been found in Asians, including Siberians, suggesting that it may have come to the Americas via a Eurasian migration. However, a more extensive survey of Asian mtDNAs, as well as additional characterization of European and Native American haplogroup X mtDNAs, will be necessary to fully deduce the origin of haplogroup X in North America.

Acknowledgments

     The authors would like to thank the following individuals for providing samples for this study: Drs. R. Herrera, K. Kidd, J. Kidd, B. Bonne-Tamir, D. Cole, D. Labuda, R. Fourney, C. Fregeau, G. Troup, D. Smith, M.-L. Savontaus, and R. Sukernik. This work was supported by National Institutes of Health (NIH) awards NS21328, HL45572, AG10130, and AG13154 (all to D.C.W.); NIH Clinical Research Center grant RR00039 (to Emory University School of Medicine); P.F. Beni Culturali contract 96.01182.PF36 (to R.S.); and Italian Consiglio Nazionale delle Ricerche grant 97.04297.CT04 and Ministero dell Università e Ricerca Scientifica e Tecnologica (40%) (to A.T.).

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« Reply #9 on: June 25, 2007, 10:41:08 pm »

Well done!  Smiley

This comparative discovery just arrived in the news;
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