High-resolution analysis of human Y-chromosome variation shows a sharp discontinuity and limited gene flow between northwestern Africa and the Iberian Peninsula

Abstract

In the present study we have analyzed 44 Y-chromosome biallelic polymorphisms in population samples from northwestern (NW) Africa and the Iberian Peninsula, which allowed us to place each chromosome unequivocally in a phylogenetic tree based on >150 polymorphisms. The most striking results are that contemporary NW African and Iberian populations were found to have originated from distinctly different patrilineages and that the Strait of Gibraltar seems to have acted as a strong (although not complete) barrier to gene flow. In NW African populations, an Upper Paleolithic colonization that probably had its origin in eastern Africa contributed 75% of the current gene pool. In comparison, approximately 78% of contemporary Iberian Y chromosomes originated in an Upper Paleolithic expansion from western Asia, along the northern rim of the Mediterranean basin. Smaller contributions to these gene pools (constituting 13% of Y chromosomes in NW Africa and 10% of Y chromosomes in Iberia) came from the Middle East during the Neolithic and, during subsequent gene flow, from Sub-Saharan to NW Africa. Finally, bidirectional gene flow across the Strait of Gibraltar has been detected: the genetic contribution of European Y chromosomes to the NW African gene pool is estimated at 4%, and NW African populations may have contributed 7% of Iberian Y chromosomes. The Islamic rule of Spain, which began in a.d. 711 and lasted almost 8 centuries, left only a minor contribution to the current Iberian Y-chromosome pool. The high-resolution analysis of the Y chromosome allows us to separate successive migratory components and to precisely quantify each historical layer.

Figure 1

Phylogenetic tree of Y-chromosome haplotypes and their absolute frequencies in NW Africa and the Iberian Peninsula. The identifiers of the genotyped markers, as well as the haplogroup and haplotype designations, are according to Underhill et al. (2000). Italicized markers were not typed, but their allelic states in the chromosomes analyzed can be deduced unequivocally from the phylogenetic tree.

Figure 2

Frequency of Y-chromosome haplotypes H22 (a); H35 (b); H36 (c); H38 (d); H58 (e); H71 (f); H102 (g); H103 (h); and H104 (i), in NW Africa and the Iberian Peninsula. The phylogenetic position of each haplotype is found in the upper right of each panel. The worldwide distribution, according to Underhill et al. (2000), is found in the lower right of each panel. Areas of circles are proportional to sample size. In panel a, 1 = Basques; 2 = Catalans; 3 = Andalusians; 4 = north-central Moroccan Berbers; 5 = Moroccan Arabs; 6 = southern Moroccan Berbers; and 7 = Saharawi.

Figure 3

Median-joining networks relating eight-locus STR haplotypes H35 (a); H38 (b); and H104 (c), within biallelic lineages. Areas of circles are proportional to absolute frequencies. White circles denote NW African individuals, black circles denote Iberians. For the sake of clarity, parallel lines do not always represent parallel mutations.

Figure 4

Median-joining network of STR haplotypes in H58 and H71. Chromosomes to the left of line a carry allele 12f2*10Kb; chromosomes to the right of line a carry allele 12f2*8Kb. Those under line b or within circle c belong to H58, whereas all others belong to H71. White circles denote NW African individuals; black circles denote Iberians. For the sake of clarity, parallel lines do not always represent parallel mutations.