Detecting and dissecting the fine-scale genetic population structure of Spain. C. Bycroft1, A. Carracedo2, C. Fernandez-Rozadilla1,2, C. Ruiz-Ponte2, I. Quintela-García2, S. Myers1, P. Donnelly1 1) Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK; 2) Galician Public Fundation of Genomic Medicine (FPGMX)-Grupo de Medicina Xenómica-Centro de Investigación Biomédica en Red de Enfermedades Raras (CIBERer)-IDIS, Santiago de Compostela, Spain.

   The Iberian Peninsula has been the subject of diverse population movements, both from Europe and North Africa, over millennia. The patterns of genetic variation within Spain reflect these movements, together with local isolation of populations due to cultural, linguistic, and geographical factors. Here we describe the largest genome-wide study of population structure across Spain, involving over 500,000 markers in ~600 individuals, analysed with powerful, recently developed statistical tools which exploit patterns of linkage disequilibrium. These analyses reveal striking patterns of genetic population structure at very fine geographic scales and shed light on the demographic history of populations within Spain. The genetic differences we identify distinguish individuals from many parts of Spain, including Galicia, the Basque Country, Catalonia, and the Balearic Islands. These differences correlate with both physical barriers to migration, and different languages spoken within Spain. Interestingly, we detect some genetic clusters that span from the north to the south of Spain, while groups differ more strongly east-west, suggesting predominantly north-south movements of people within Spain. Differing amounts of ancestry shared with individuals from Basque-speaking regions in populations from different parts of Spain also suggests a historic population movement out of the Basque region, largely in a southerly direction. Finally, we use a mixture model-based approach and a recently developed method, GLOBETROTTER, to quantify and date the genetic impact of migrations into Spain from groups including the Moors from North Africa and other European populations.

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