Characterizing the history of sub-Saharan African gene flow into southern Europe. P. Moorjani1, N. Patterson2, J. Hirschhorn1,3, D. Reich1,2 1) Department of Genetics, Harvard Medical School, Boston, MA; 2) Broad Institute, Cambridge, MA; 3) Divisions of Endocrinology and Genetics and Program in Genomics, Children's Hospital, Boston, MA.
Recent analyses of whole-genome SNP data sets have suggested a history of sub-Saharan African ancestral contribution into southern Europe but not in northern Europe, consistent with previous analyses based on the Y-chromosome and mitochondrial DNA. However, there has been no characterization of the proportion of African admixture in southern Europe, or of its date. Here we analyze data from ~450,000 autosomal SNPs in the Population Reference Sample, ~650,000 SNPs from the Human Genome Diversity Panel, and ~1.5 million SNPs from the HapMap Phase 3 Project, and studied patterns of correlation in allele frequencies across populations to confirm the evidence of African ancestry in many southern European populations but not in northern Europeans. Using methods that can infer admixture proportions in the absence of accurate ancestral populations, we estimated that the proportion of sub-Saharan African ancestry in Spain is 2.4 +/- 0.3%, in Tuscany 1.5 +/- 0.3%, and in Greece 1.9 +/- 0.7% (1 standard error). We also studied the decay of admixture linkage disequilibrium with genetic distance, which provided a preliminary estimate of the date of African gene flow into Spain of roughly 60 generations ago, or about 1,700 years ago assuming 28 years per generation. This date is consistent with the historically known movement of individuals of North African ancestry into Spain, although it is possible that this estimate also reflects a wider range of mixture times.