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

In the present study we have analyzed 44 Y-chromosome biallelic polymorphis ms in population samples from northwestern (NW) Africa and the Iberian Peni nsula, which allowed us to place each chromosome unequivocally in a phyloge netic tree based on >150 polymorphisms. The most striking results are that contemporary NW African and Iberian populations were found to have originat ed from distinctly different patrilineages and that the Strait of Gibraltar seems to have acted as a strong (although not complete) barrier to gene fl ow. In NW African populations, an Upper Paleolithic colonization that proba bly had its origin in eastern Africa contributed 75% of the current gene po ol. In comparison, similar to 78% of contemporary Iberian Y chromosomes ori ginated in an Upper Paleolithic expansion from western Asia, along the nort hern rim of the Mediterranean basin. Smaller contributions to these gene po ols (constituting 13% of Y chromosomes in NW Africa and 10% of Y chromosome s in Iberia) came from the Middle East during the Neolithic and, during sub sequent gene flow, from Sub-Saharan to NW Africa. Finally, bidirectional ge ne flow across the Strait of Gibraltar has been detected: the genetic contr ibution 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 chro mosomes. The Islamic rule of Spain, which began in A.D. 711 and lasted almo st 8 centuries, left only a minor contribution to the current Iberian Y-chr omosome pool. The high-resolution analysis of the Y chromosome allows us to separate successive migratory components and to precisely quantify each hi storical layer.