作者姓名：姚永刚   
  论文题目：中华民族源流探讨－线粒体DNA的研究   
  作者简介：姚永刚，男， 1975年10月出生，1997年9月师从于中国科学院昆明动物研究所张亚平院士，于2003年3月获博士学位。   
     
  中 文 摘 要   
  　

中华民族源流问题目前已有较多的研究，但争议颇多。对我国民族人群的线粒体DNA（mtDNA）进行系统的研究，可为解决这些争论提供一些遗传学上的证据。近二十年来，mtDNA由于自身比较独特的遗传特性（母系遗传、缺乏重组和进化速率高）而被广泛地应用于人类群体的起源和演化、人群过去动态的估计、单个人群的区域性微分化和人口历史学等研究。本文首先总结了近年来世界各人群mtDNA的研究进展，并对世界各人群中主要mtDNA单倍型类群（haplogroup）作了归纳，重点介绍了目前mtDNA突变筛选方法和思路、mtDNA与核基因标记结果的异同、mtDNA与语言的协同进化关系、古老DNA研究以及关于这种遗传标记本身遗传特性的争论等问题。

在系统分析了我国6个不同省份的汉族地理人群mtDNA控制区和编码区序列信息后，结合报道的全序列信息，建立了我国人群中mtDNA单倍型类群的系统发育关系。在此基础上，通过将各地理群体中检测到的mtDNA世系归入不同的类群，结果显示：不同地区汉族人群中各种单倍型类群的频率分布不同，汉族人群中存在明显的地理分化。南方群体间的遗传差异比北方群体间的差异大。年龄古老的mtDNA单倍型类群在南方群体中出现的频率普遍高于北方群体。以往将汉族地理样本按地理位置合并为“北部汉族”和“南部汉族”的做法，或者选用一个或两个汉族地理群体来代表整个汉族人群，都不能很好的反映其遗传结构的全貌。南方汉族群体中含有较北方群体更多的古老类群和未定类型这个结果可以用中华民族南方起源的假说来解释，但在人群最初扩张迁移之后，群体的动态发展却存在很多推测和想象上的空间。南北汉族人群间基因库的差异始于这种早期由南向北的迁移和群体分化，但这种分化在随后由于战乱、灾荒、寻求发展等原因造成的人群由北向南、由东向西的迁移事件中或多或少地变得模糊。汉族的形成和扩张在很大程度上是汉文化对其他土著和周边人群的征服和同化，是个极其复杂的过程。现今各地汉族人群mtDNA带有其历史遗传和迁移的印记。

这里在汉族人群数据分析过程中建立的mtDNA系统地理发育分析方法，对于如何有效挖掘古老DNA数据中蕴含的信息和甄别报道的mtDNA数据中存在的错误很有借鉴作用。依据这种系统地理分析的思想，我们对于报道的来自山东境内距今2,500和2,000年前的古老DNA进行了重新分析。结果提示，这批古人群中某些mtDNA类型在山东境内的现代人群以及其他省份人群中都有分布，2,000多年前该地人群和我国现代南方人群在母系遗传结构组份上有较大相似性，而与现代欧洲人群没有联系。这种结果不支持早前提出的山东人群基因库在过去3000年内由欧洲型向东亚型转换的解释。

基于同样的策略，我们对史载记录中主要是氐羌部落和百越部落起源的民族群体的母系基因库进行了比较分析，集中探讨历史族源与群体遗传结构的关系。结果表明，氐羌和百越族源的群体在单倍型多样度、核苷酸多样度、特定单倍型类群的分布上存在差异。起源于南方百越部落的群体总体上较西北氐羌部落起源的群体具有更高的遗传多样性；百越起源的群体在单倍型类群的分布上具有明显的南方群体的特征，而西北氐羌起源群体则保留有其祖先类群组份。百越群体的群体扩展时间相对于氐羌群体更为古老，有些氐羌群体，如怒族、拉祜族等，在向西南省份迁移和发展的过程中，可能经受了较强的建群者效应。根据族源进行群体归组的AMOVA分析、基于群体间的净遗传距离、单倍型匹配率的比较、以及依据遗传距离构建的群体间关系树和依据单倍型类群频率数据进行的主成分分析，都支持氐羌和百越起源的群体间存在遗传上差异，族源的印记或多或少的在群体中得到保留；群体间文化上的相对隔离、交流、群体自身经受的漂变，以及遵循的婚配习俗深刻地影响群体的遗传结构。

地理上欧亚大陆交接地带的人群，由于历史上的贸易和在此之前的人群交流，提供了一个研究特定区域内有着不同迁移历史、不同文化婚俗人群间遗传上的相对隔离和基因融合的很好模型。我们选择来自新疆地区的有着不同历史来源、文化习俗和迁移时间的群体，集中讨论同一地区不同人群的母系遗传结构，同时对于历史上的丝绸之路和有史以前该地区东西方交流融合现象进行探讨。研究表明，新疆地区绝大多数个体的线粒体DNA世系都可明确地归属到东亚人群和欧洲人群基因库中，这种遗传分布格局进一步支持中亚地区是东西方人群基因融合之地的推测。研究中还发现，来自同一地区的不同民族群体中的欧亚人群基因融合程度不同，较早的居民如维吾尔族和哈萨克族等，含有较多的欧洲人群特有组份，而较近迁移进入该地区的民族人群，如蒙古族和回族，则含有更多的东亚人群特有组份。这种群体母系遗传结构上的差异和民族群体本身的族源历史比较吻合，蕴涵着丰富的历史迁移信息。族源、婚俗以及政治和经济因素对新疆地区不同民族人群的母系结构有着非常重要的影响。

总之，通过对同一民族群体（汉族）不同地理群体的母系遗传结构、历史族源和群体遗传结构的关系以及同一地区有着不同历史来源、文化习俗和迁移历史的民族人群的系统研究，我们有理由认为民族的形成和发展是个复杂的动态过程。单一的历史事件虽然可能会在群体的遗传结构中留下痕迹，但目前观察到的群体的遗传结构反映的是一个很长的时间段内各种遗传效应和历史事件影响的综合。

关键词： 线粒体DNA； 中华民族；起源；遗传结构；族源；丝绸之路；基因融合

  
  Demographic History of the Chinese Ethnic Groups: a Mitochondrial DNA Perspective
Yong-Gang Yao

ABSTRACT   
        The origin and migration of Chinese ethnic groups are still on hot debates, although efforts from multilayered, multidirectional and multidisciplinary frameworks have been carried out to reconstruct the (pre)history. In the past two decades, mitochondrial DNA (mtDNA) variation analyses have been extensively used on tracing the origin and migration pattern of human populations, estimation of past population size and date of expansion event, as well as on the microdifferentiation and demographic history of single population. Hitherto, mtDNA from Chinese ethnic groups has been poorly sampled and understood in its variation, with only limited data available.

In this thesis, we reviewed the recent advances of mtDNA studies of the world populations (with special emphasis on the strategy on choosing which method to detect mutations), the congruity and discrepancy between the genetic profiles revealed by mtDNA and nuclear data, the co-evolution between mtDNA and language, ancient DNA, as well as the arguments about the special genetic characteristics of this genetic marker.

In order to take full advantage of a uniparental marker system, such as mtDNA, we constructed a sufficiently resolved mtDNA phylogeny based on the information provided by the control region and coding region sequences of 6 Han geographic populations, as well as information from published sources. This allows us to define new haplogroups (clades of the mtDNA phylogeny) and to dissect the Han mtDNA pool on a phylogenetic basis. Some of the haplogroups under study differ considerably in frequencies across different provinces. The southern-most provinces show more pronounced contrasts in their regional Han mtDNA pools than the central and northern provinces. These and other features of the geographical distribution of the mtDNA haplogroups observed in the Han Chinese make an initial Paleolithic colonization from south to north plausible, but would suggest subsequent migration events in China that mainly proceeded from north to south and east to west. Lumping together all regional Han mtDNA pools into one fictive general mtDNA pool or choosing one or two regional Han populations to represent all Han Chinese is inappropriate for prehistoric considerations. The formation and development of Han, starting from central China, constituted mainly a political expansion process that led to the cultural assimilation of numerous ethnic groups under the dominant Han culture.

The established mtDNA phylogeny and the phylogeographic method in Han samples are very useful for us to decipher the meager information in short fragments of ancient mtDNAs as well as to pinpoint possible errors in published data sets. The reanalysis of two previously published ancient mtDNA population data sets from Linzi (Shandong Province) then indicates that the ancient populations had features in common with the modern populations from south China rather than any specific affinity to the European mtDNA pool.

Following the same strategy, we examined the maternal genetic structures of 13 populations mainly with ancient Pai-Yuei and Di-Qiang tribe origins. A relatively high percentage of individuals were found to share sequences with those from other populations of the same ethnogenesis. Populations of Pai-Yuei origin showed higher diversity compared with populations of Di-Qiang origin. Moreover, populations of Pai-Yuei origin contained higher frequencies of south prevalent haplogroups, while populations of Di-Qiang origin harbored high amount of north prevalent haplogroups. Mismatch distributions from these populations showed concordant features. Populations of Di-Qiang origin showed younger expansion time compared with Pai-Yuei origin populations. Episodes of extreme size reduction in the past, small sample sizes, samples from isolated subpopulations, as well as consanguineous marriage custom are likely explanations for the absence of evidence of expansion in Nu, Lahu, etc. Phylogenetic analysis and PC analysis of the populations supported the suggestion that current mtDNA variation in these ethnic populations could reveal their ethnohistory to some extent. The relative isolation by culture (IBC), restricted gene flow among the ethnic populations, genetic drift, as well as marriage custom have deep effect on the genetic structure of these populations.

Previous studies have shown that there was extensive genetic admixture in the Silk Road region. 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 can serve as a good model for understanding whether the matrilineal genetic structure of the populations reflect the recent demographic episodes occurred and/or the different cultures they practiced. We focused on the matrilineal genetic structures of 5 ethnic populations from Xinjiang Province, China, through which the ancient Silk Road once ran. In a simple way, we just classified the haplotypes identified into the haplogroups that have been described elsewhere and computed their frequencies, to show whether there are any differences in the genetic structure of these populations that had different migration history and performed various marriage cultures. Nearly all the mtDNA types identified here belonged to defined subhaplogroups of haplogroups M and N (including R) and consisted of subsets of both the eastern and western Eurasian pools, thus providing direct evidence supporting the suggestion that Central Asia is the location of the genetic admixture of the East and the West. Although our samples were from the same geographic location, a decreasing tendency of the western Eurasian-specific haplogroup frequency was observed, with the highest frequency present in the earlier inhabitants (such as Uygur and Kazak), whereas populations with relatively recent migration histories contained fewer western Eurasian types. Moreover, a clear difference was observed in the distribution of the haplogroups in these populations. The frequencies of the eastern Eurasian-specific haplogroups also varied in these samples. Combined with the historical records, ethno-origin, migratory history, and marriage custom might play different roles in shaping the matrilineal genetic structure of different ethnic populations resided in this region.