Diversity at eight polymorphic Alu insertion loci in Chinese populations shows evidence for European admixture in an ethnic minority population from Northwest China

Human Biology, Aug 2002 by Xiao, Feng-Xia, Yang, Jun-Fang, Cassiman, Jean-Jacques, Decorte, Ronny

Abstract We have analyzed eight human-specific Alu insertion polymorphisms in four Chinese populations belonging to three ethnic groups (98 Hans from Shanghai, 80 Hans from Guangzhou, 85 Uyghurs, and 60 Sibos). All populations exhibited high levels of average heterozygosity, and those in Uyghur and Sibo were higher than predicted by the island model of population structure. The degree of genetic differentiation among these populations is statistically significant, and lower than those observed in most parts of the world except for Europe and Sahul (Australia and New Guinea). Phylogenetic analysis of these data with published data from 29 worldwide populations shows that there is a close genetic affinity among all the East Asian populations except for the Uyghur, and that the Uyghur population was found to lie between the East Asian and the West Asian populations on the population tree. The greater heterozygosity and the significant genotype associations between unlinked loci observed for the Uyghurs support the scenario that the Uyghurs might have originated from an admixture between Europeans and East Asians. This study also provides further support for the "out-of-Africa" hypothesis of modern human evolution in East Asia.

KEY WORDS: DNA POLYMORPHISM; ALU INSERTION; HUMAN EVOLUTION

Despite China's being one of the few regions of the world with abundant human fossils from Homo erectus to early Homo sapiens and to modern specimens (Brooks and Wood 1990; Li and Etler 1992), modern humans in this region are now believed to have originated in Africa (Vigilant et al. 1991; Chu et al. 1998; Jorde et al. 2000). However, the time and the actual migration routes by which modern humans spread from Africa to East Asia are still controversial (CavalliSforza et al. 1994). At least three different models for the prehistoric migrations of modem humans in this region have been proposed. The linguistic map in China suggested that the initial peopling took place in northeast Asia, followed by migration of populations to the south, where they mixed with the 'Australoid' setfled in Southeast Asia (Piazza 1998). The archaeological records supported the idea that northern and southern East Asian populations have originated independently from the two distinct populations since late Paleolithic times (Piazza 1998). Recent studies of East Asian populations, using nuclear microsatellite and Y-chromosome polymorphisms, have addressed such an issue and suggested that the first settlement of modern humans occurred in Southeast Asia, followed by a northward migration into northern China (Chu et al. 1998; Su et al. 1999). These studies also revealed substantial genetic differences between northern and southern Chinese people, consistent with previous observations based on protein polymorphisms (Zhao and Lee 1989; Etler 1992). These different models on the peopling of East Asia probably reflect different time spans of evolutionary history of modern humans in this region (Piazza 1998).

Alu sequences represent the largest family of short interspersed elements (SINEs) with more than 500,000 copies present in the human genome (Batzer et al. 1994). These mobile Alu elements are ancestrally derived from the 7SL RNA gene and mobilized throughout the primate genomes by retroposition in the last 65 million years (Rogers 1983; Ullu and Tschudi 1984). Approximately 500 Alu family members are present as a human-specific (HS) subfamily, which have been inserted in the human genome within the past 6 million years (Batzer and Deininger 1991). Some of them have retroposed so recently that they are polymorphic for presence or absence at a specific location within the human genome. Recently, Alu insertion polymorphisms have become a novel set of nuclear DNA markers for the study of human evolution. These repetitive Alu elements represent stable polymorphisms that are identical by descent from a common ancestor (Batzer et al. 1996; Stoneking et al. 1997). Particularly, the ancestral state of each Alu insertion is known with certainty to be the absence of an Alu element, facilitating accurate rooting of population trees (Batzer et al. 1996; Stoneking et al. 1997). Recent studies of worldwide populations using Alu polymorphic insertions have shown evidence for the support of "out-of-Africa" hypotheses for modern humans (Batzer et al. 1996; Stoneking et al. 1997). Other studies have dealt with more recent events in human evolution such as the settlements of the Americas and India (Novick et al. 1998; Majumder et al. 1999).

It is obvious that an extensive study of Chinese populations using Alu insertion polymorphisms might be helpful to clarify the controversial issues of the peopling of East Asia. However, genetic data on Alu insertion polymorphisms is still insufficient for populations from mainland China. In the present study, we have analyzed genetic variation at eight Alu insertion polymorphic loci in four populations of China from three ethnic groups. The aim of this study is to better understand the relationships of these populations as well as their genetic affinities with other populations in East Asia.