Genetics and Population History of Caucasus Populations

Human Biology, Dec 2003 by Bulayeva, Kazima, Jorde, Lynn B, Ostler, Christopher, Watkins, Scott, Et al

Abstract We describe aspects of genetic diversity in several ethnic populations of the Caucasus Mountains of Daghestan using mitochondrial DNA sequences and a sample of 100 polymorphic AIu insertion loci. The mitochondrial DNA (mtDNA) sequences are like those of Europe. Principal coordinates and nearest neighbor statistics show that there is little detectable structure in the distances among populations computed from mtDNA. The AIu frequencies of the Caucasus populations suggest that they have undergone more genetic drift than most other groups since the dispersal of modern humans. Genetic differences among these populations are not large; instead, they are of the same order as distances among populations of Europe. We compare two methods of inference about the demography of ancient colonizing populations from Africa, one based on conventional FST statistics and one based on mean Alu insertion frequencies. The two approaches agree reasonably well if we assume that there was demographic growth in Africa before the diaspora of ancestors of contemporary regional human groups outside Africa.

KEYWORDS: DAGHESTAN, ALU INSERTION POLYMORPHISM, MTDNA, POPULATION STRUCTURE

In this paper we describe patterns of genetic differentiation among several populations of the Caucasus Mountains of Daghestan and compare them with a larger sample of human groups. The Caucasus Mountains, between the Black and Caspian seas, are astride what must have been a major corridor of movement since the expansion of modern humans. The inaccessible mountains may have functioned as a refuge and cul-de-sac off these migration streams. Today, ethnic groups in the Caucasus are characterized by extreme cultural and linguistic differentiation in a small geographic area. The groups are thought to be of great antiquity.

It is known from previous work (Barbujani et al. 1994) that Caucasus populations are not part of the system of gene frequency clines extending from Anatolia across Europe to the northwest. The inference is that they are not descendants of the Neolithic farmers whose expansion across Europe is responsible for the gene frequency clines. They may be, instead, descendants of the earlier "layer" of the population of Europe. In this they are like Basques, and indeed some linguists see a genetic relationship between Basque and Caucasian languages. The most extreme lumping (Ruhlen 1994) places Caucasian languages with Basque, a Siberian language with few speakers, Chinese and related languages, and the Athapascan languages of North America.

Daghestan is a southern Russian republic between the Black and Caspian seas. The southern two-thirds of Daghestan is in the Caucasus Mountains, reaching 2000-4000 meters above sea level. The northern third is a flat plain that extends along the western shores of the Caspian Sea (Figure 1). The republic of roughly 50,000 square kilometers has a population of about two million people. While many of them are urban, there remain many isolated ethnic groups that rely on subsistence agriculture, herding, and craft production, especially in the difficult Caucasus Mountains.

Many rural people live in remote mountain villages, known as auls, which have been geographically and reputed to be genetically isolated for thousands of years (Bulayeva 1991; Gammer 1994). These auls often exhibit unique customs, languages and dialects, and architectural styles. They are characterized by elevated rates of inbreeding, encouraged by Muslim traditions of marriages within families. Migration from highland to lowland regions has occurred for some of the groups, leading to outbred populations residing either in large lowland agricultural villages or cities. Within the auls valuable properties (e.g., farming terraces and sheep) usually were kept in the same family from one generation to the next by arrangement of marriages within the family.

The region has been predominantly Muslim since the 12th to 14th centuries. Before the introduction of Islam many groups were Christian (Aglarov 1988; Gadjiev 1971), but there was little Byzantine presence in the region. In the latter part of this millennium Daghestan was a locus of conflict among the Ottoman, Persian, and Russian empires.

The mountain auls have undergone remarkable linguistic and ethnic differentiation. There are auls of goldsmiths, woodcarvers, tinsmiths, boot-makers, dancers, singers, and many more. But the main occupations of highlanders are growing crops, primarily on hillside terraces, and stock raising, primarily sheep. Despite the harsh environment highlander groups have persisted for many centuries. In fact, some of them may have contributed to the initial exploitation of some important world crops on the hillside terraces (Vavilov 1936).

Materials and Methods

Populations. We describe HVS-I mtDNA sequences from five Daghestan populations: Kubachi, Novo-Kurush, Novo-Mehelta, Urkarah, and Stalskoe, as well as AIu insertion frequencies in a partially overlapping sample of populations: Kubachi, Urkarah, Stalskoe, Nogais, and Makhachkala. We compare both sets of data with comparable data from other populations.