Y-chromosome analysis in Egypt suggests a genetic regional continuity in northeastern Africa

Human Biology, Oct 2002 by Manni, Franz, Leonardi, Pascal, Barakat, Abdelhamid, Rouba, Hassan, Et al

Abstract The geographic location of Egypt, at the interface between North Africa, the Middle East, and southern Europe, prompted us to investigate the genetic diversity of this population and its relationship with neighboring populations. To assess the extent to which the modern Egyptian population reflects this intermediate geographic position, ten Unique Event Polymorphisms (UEPs), mapping to the nonrecombining portion of the Y chromosome, have been typed in 164 Y chromosomes from three North African populations. The analysis of these binary markers, which define 11 Y-chromosome lineages, were used to determine the haplogroup frequencies in Egyptians, Moroccan Arabs, and Moroccan Berbers and thereby define the Y-chromosome background in these regions. Pairwise comparisons with a set of 15 different populations from neighboring European, North African, and Middle Eastern populations and geographic analysis showed the absence of any significant genetic barrier in the eastern part of the Mediterranean area, suggesting that genetic variation and gene flow in this area follow the "isolation-by-distance" model. These results are in sharp contrast with the observation of a strong north-south genetic barrier in the western Mediterranean basin, defined by the Gibraltar Strait. Thus, the Y-chromosome gene pool in the modern Egyptian population reflects a mixture of European, Middle Eastern, and African characteristics, highlighting the importance of ancient and recent migration waves, followed by gene flow, in the region.

KEY WORDS: EGYPT, Y CHROMOSOME, GENETIC BARRIERS, MEDITERRANEAN BASIN, GENE FLOW

The northern part of Africa is separated from the rest of the continent by the Sahara Desert, which continues almost uninterrupted from the Atlantic Ocean to the Red Sea. As a consequence of this important geographic boundary, human movements may have been limited to the Nile Valley and Red Sea to the east, and to the Atlantic coast on the west. These two areas represent potential pivotal regions that may shed light on different migration waves during human history, including those related to the "out-of-Africa" hypothesis on the origin of modern humans and also to populations movements towards Africa, such as Neolithic diffusion of agriculturalists from the Middle East.

Despite the importance associated with the northeastern part of Africa in human history, few studies have defined the genetic background prevailing in modern populations in this key geographic region. When this genetic data is used to determine the genetic affinity of the Egyptian population in a wider regional context, the results are controversial. On the basis of classical genetic polymorphisms, Egypt, together with Libya, is more closely related to European populations than other African populations (Bosch et al. 1997), whereas autosomal short tandem repeat loci (STRs) suggest important genetic differences between the Egyptian and European populations (Klintschar et al. 1999). Both of these studies do show considerable intrapopulation variability. In addition, there is evidence of gene flow from sub-Saharan Africa to North Africa. Mitochondrial DNA (mtDNA) studies in different populations inhabiting the Nile Valley have suggested that this region acted as a corridor for gene flow between sub-Saharan and North Africa (Krings et al. 1999), an observation also suggested by mitochondrial DNA analysis in remains of ancient Nubians (Fox 1997). Indeed, the development of early civilizations along the valley may have increased gene flow along the Nile.

Africa has three contact zones with neighboring continents: the Sinai Desert in the northeast, the Gibraltar Strait in the northwest, and the Bab-el-Mandeb Strait, which separates Ethiopia from the Arabian Peninsula. In the last few years, several studies have attempted to reveal migration routes from Africa both to Europe and to Asia, as well as to identify areas of gene flow between these continents. In this context, populations on either side of the Gibraltar Strait (which separates northwest [NW] Africa from the Iberian Peninsula by only 15 km and is a possible contact zone between African and southern European populations) exhibit strong genetic differences (Bosch et al. 2000a; 2000b, 2001; Comas et al. 2000). Even if a certain degree of gene flow into Southern Iberia may have existed (Flores et al. 2000; 2001), the Gibraltar Strait appears to have represented a strong barrier to genetic exchange between the two continents. In contrast, mtDNA studies indicate considerable gene flow through the Bab-el-Mandeb Strait in East Africa, leading to the proposal of an early exit of modern humans from Africa via East Africa and the Arabian Peninsula (Quintana-Murci et al. 1999). The third contact zone, the Sinai Desert and Sinai Peninsula, is potentially an important migratory route, both terrestrial and coastal, enabling contacts with the Middle East, Europe, and Asia. However, to date there have been no studies to evaluate how this contact zone has or has not influenced gene flow.