* Laboratory of Cellular and Molecular Evolution, and Molecular Biology of Domestic Animals, Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, China; Laboratory for Conservation and Utilization of Bio-resource, Yunnan University, Kunming, China; and
Graduate School of the Chinese Academy of Sciences, Beijing, China
Correspondence: E-mail: zhangyp{at}public.km.yn.cn.
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Abstract |
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Key Words: mtDNA haplogroup Silk Road ethnic group migration admixture
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Introduction |
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Xinjiang Province, located in northwest China, is in the vicinity of and sometimes has been regarded as part of Central Asia. This region is famous for being home to part of the ancient Silk Road and has undergone unceasing migration events through present day, maintaining various cultures (Ge, Wu, and Chao 1997; Chen 1999). However, few mtDNA studies have been carried out for the ethnic populations residing in this region (Yao et al. 2000). A finer dissection of matrilineal components for more samples from this region will undoubtedly be helpful in better understanding the origin of Central Asians and in providing a clearer insight into the genetic structure of the ethnic populations that have undergone different demographic histories; thus serving as a good model for understanding whether the matrilineal genetic structure of the populations reflect the recent demographic episodes that have occurred and/or the different cultures they practiced.
In this study, we compared the matrilineal genetic components of six ethnic populations (Uygur, Uzbek, Kazak, Mongolian, Hui, and Han) from Xinjiang, China, and the reported samples from the adjacent regions (Comas et al. 1998, 2004), as well as a Mongolian population from northeast China (Kong et al. 2003a). Our results showed that the extent of genetic admixtures of the eastern and western Eurasian lineages varied among these populations from Xinjiang. There was no central heartland in mtDNA distribution pattern in this region.
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Materials and Methods |
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Data Collection
Genomic DNA was isolated from the whole blood by standard phenol/chloroform method. The mtDNA control region hypervariable segment I (HVS-I) sequences of Uzbek (N = 58), Hui (N = 45), Kazak (N = 23), and Mongolian (N = 49) samples were amplified and sequenced as described in our previous studies (Yao et al. 2002a, 2003; Kong et al. 2003a). Some samples were further selected for the hypervariable segment II (HVS-II) region sequencing and/or coding region motif(s) typing by using the same strategy and condition described in our previous studies (Yao et al. 2002a, 2003; Kong et al. 2003a). The reported Uygur (N = 47) and Kazak (N = 30) samples with only HVS-I information available (Yao et al. 2000) were also subjected to haplogroup-specific HVS-II and/or coding region polymorphisms typing in this study. Other reported mtDNA data sets considered here included 47 Hans from Xinjiang (with HVS-I and II and coding region information available; Yao et al. 2002a); 55 Uighurs and 55 Kazakhs from Kazakhstan; 47 Sary-Tash Kirghizs and 48 Talas Kirghizs from Kirgizistan (with HVS-I information; Comas et al. 1998); and 232 mtDNAs from 12 Central Asian populations (with HVS-I and II information; Comas et al. 2004). Because the sample size of each population in Comas et al. (2004) was relatively small (20), we aggregated all the samples together as one population in our analysis. We also included 48 Mongolians from Inner Mongolia (Kong et al. 2003a) to compare with the Mongolian sample from Xinjiang. The sequence data of the mtDNA control region in this study can be retrieved from DDBJ/EMBL/GenBank by accession numbers AY677733-AY678071.
MtDNA Haplogroup Classification
We used the strategy described in recent studies (Yao et al. 2002a, 2003; Kong et al. 2003a, 2004) to classify the mtDNAs into haplogroups, and we followed the haplogroup notation system described by Richards et al. (2000), Yao et al. (2002a, 2003), and Kong et al. (2003b). Briefly, by a first round of haplogroup-specific HVS-I motif searching and (near-)matching with the published data set with coding region information available, we tentatively assigned each mtDNA to respective haplogroups. Then, the haplogroup-specific HVS-II and/or coding region motif(s) of the mtDNAs were further tested to confirm the predicted haplogroup status of each mtDNA. If the predicted HVS-II or coding region motifs of the mtDNA were not observed, additional typing efforts were employed to further allocate the mtDNA into the nested haplogroup it belongs to. For example, one mtDNA HVS-I sequence with mutations at sites 16223 and 16362 could be D, G, or another type in M or N. We firstly tested for haplogroup Dspecific polymorphism, 5178A (-5176AluI), to confirm the D status. Site 4833 (+4831HhaI) would be needed to test for G status if 5178A was found to be absent in that sample. If both mutations were absent, we would start a further round of typing in other coding-region fragment(s) to determine the status of the sample. Through this discrimination process, each mtDNA could be easily identified without exhaustively typing for all haplogroups' characteristic mutations. For the reported mtDNAs in Comas et al. (1998, 2004), we tentatively assigned them to respective haplogroups according to information provided by HVS-I and II and/or by (near-)matching with the published data sets with coding region information as well as the mtDNAs analyzed in current study.
Data Analyses
The frequencies of mtDNA haplogroups in each population were estimated. We focused on the comparison of the haplogroup distribution patterns between the samples with different ethno-origins, demographic histories, and religions. Fisher exact tests were performed to quantify the difference between the frequencies of certain haplogroup(s) in the samples. Furthermore, a principal component (PC) map was constructed on the basis of the haplogroup frequency matrix to show the clustering pattern of the samples.
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Results |
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Discussion |
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As shown in tables 1 and 2, nearly all mtDNA lineages identified in samples from Xinjiang, China, consisted of a subset of the haplogroups specific to eastern and western Eurasia. This result was in agreement with the suggestion of an extensively genetic admixture in this region, based on classic genetic data and less resolved mtDNA data (Zhao and Lee 1989; Cavalli-Sforza, Menozzi, and Piazza 1994; Comas et al. 1998, 2004; Yao et al. 2000). The central heartland hypothesis, which is based on an interpretation of Y-chromosome evidence (Wells et al. 2001) and suggests that most of the modern Eurasian lineages are descended from the Central Asian pool, would conflict with the distribution pattern of mtDNA haplogroups in this region and could not account for the absence of some of the eastern or western Eurasian-specific deep-rooting haplogroups in the Central Asian mtDNA pool.
A prominent feature that should be mentioned is the extent to which the admixture varied among these populations, although our comparison of the well resolved matrilineal components of populations from Xinjiang supported the genetic admixture hypothesis. A decreasing tendency of the western Eurasian-specific haplogroup frequency was observed in these populations from the same region, with the highest frequencies present in the earlier inhabitants, such as the Uygur, Uzbek, and Kazak, followed by Mongolian and Hui; the latter two populations have relatively recent migration histories. This distribution pattern was, coincidentally, well in agreement with the historical ethno-origins of these populations (Ge, Wu and Chao 1997; Chen 1999) and thus suggested that the matrilineal genetic structures of the populations bear the imprints of their migratory histories (Yao et al. 2002a, 2002b; Yao and Zhang 2002). As a result, it is not strange that no western Eurasian lineage was found in the Xinjiang Han samplea sample with the most recent migration history (Yao et al. 2002a). Intriguingly, the haplogroups that were mainly found in southern Pakistan, India, the Near East/Caucasus region, the Iranian plateau, and the Arabian Peninsula, such as HV2, R2, and U7 (Quintana-Murci et al. 2004), were only present in Uygur and Uzbek, which harbored an approximately equal amount of western Eurasian types. Haplogroup U2e, which is European-specific (cf. Quintana-Murci et al. 2004), was also restricted to these two samples. It thus seems that the Uygur and Uzbek populations were more open to accept external genes than another earlier inhabitant, the Kazaks, and this also explains why the Kazak populations harbored fewer western Eurasian types compared with Uygurs and Uzbeks.
Insights into the ethno-origins of these populations could be further solidified by focusing on the eastern Eurasian-specific components in these samples. The high frequencies of the north-prevalent haplogroups, such as M8, D, and G (Yao et al. 2002a; Kong et al. 2003a), found in Mongolian (46.9%), Kazak (45.3%), Uygur (36.2%), and Uzbek (32.8%) samples were in accordance with their northern origin as well as their current geographic distributions. As demonstrated in figure 3, the two Mongolian samples from Inner Mongolia (Kong et al. 2003a) and Xinjiang presented a close genetic relationship, notwithstanding the long geographic distance between their sampling locations. It thus seems that the Mongolians in Xinjiang, who heavily migrated to this region during the Yuan Dynasty (A.D.1,2061,368), maintained most of their ancestral matrilineal components while assimilating genes from other ethnic groups.
A similar condition would also apply to other regional samples belonging to the same ethnic group, for instance the Uygur and Uighur. Although these two samples were separated from each other in the PC map (fig. 3a), the percentages of the total eastern Eurasian types (Uygur, 57.4%; Uighur, 56.4%) and western Eurasian types (Uygur, 42.6%; Uighur, 43.6%) were approximately equal. The difference between Uygur and Uighur was significantly smaller than the differences between each of them and the Han or Hui.
Our results constructed a good story about the unique ethnohistory of the Hui. According to historical records (Du and Yip 1993; Ge, Wu, and Chao 1997; Chen 1999), Huis traced their origin to the Persians and Arabs, who migrated to the southeast coast of China (e.g., Fujian and Guangdong Provinces) in the 7th century A.D., and later to the Central Asians, Persians, and Arabs who migrated to China during the Yuan Dynasty for trade and war reasons. At that time, most of the immigrants did not bring their family members and mainly married Han women. The intermarriages between the Hui and Han were further reinforced by the imperial edict in the Ming Dynasty (A.D. 1,3681,644; Du and Yip 1993; Chen 1999). The prevalent presence of the eastern Eurasian-specific mtDNA haplogroups (93.3%) in the Hui compared with other Muslim samples, if not of recent gene flow from the Han people, would be attributed to the historical Han women contribution. The high frequencies of the south-prevalent haplogroups (e.g., B, 17.8%; F, 13.3%; and M7, 11.1%), the presence of an M7b3 type (Hui98), and other matched types across China in the Hui samples added further tallies to this suggestion and mirrored the trace of the earlier inhabitants along the southeast coast of China (Du and Yip 1993; Chen 1999). In addition, neither Hui nor Han samples harbored any H and U types that were highly present in other populations in Central Asia. At this point, the matrilineal gene pool of the Hui was shaped by their historically encouraged intermarriage with the Han people, and the religion did not have much influence on it.
In short, the peopling in Xinjiang is more complex than expected. Although the coexistence of the eastern and western Eurasian lineages in the ethnic groups from the Silk Road region generally supported the genetic admixture hypothesis, the extent of admixture varied and was more prone to be influenced by economic and political episodes (at least, this applied to the Han and Hui populations). The observed matrilineal genetic structure is thus a reservoir that aggregated all kinds of genetic and demographic effects.
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Electronic Database Information |
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Acknowledgements |
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Footnotes |
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Takashi Gojobori, Associate Editor
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References |
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