Fine scale population structure of Spain and the genetic impact of historical invasions and migrations.
C. Bycroft1 ; C. Fernandez-Rozadilla1,2 ; A. Carracedo2 ; C. Ruiz-Ponte2 ; I. Quintela-García3 ; P. Donnelly1,4 ; S. Myers1,4
As well as being linguistically and culturally diverse, the Iberian Peninsula is unusual among European regions in that its demographic history includes a prolonged and large-scale occupation by people of predominately north-west African origin. Therefore, the Iberian Peninsula provides a unique opportunity for studying fine-scale population structure and admixture, and to test cutting-edge methods of detecting complex or subtle population genetic patterns.Previous studies using Y-chromosome, mtDNA as well as autosomal data have detected limited genetic structure in Iberia. However, powerful new methods and larger datasets mean it has recently become possible to detect and characterise genetic differentiation at a sub-national level. We performed the largest and most comprehensive study of Spanish population structure to date by analysing a dataset of ~1,400 Spanish individuals typed at ~700,000 SNPs. Using the fineSTRUCTURE method we detected striking and rich patterns of population differentiation within Spain, at scales down to tens of kilometres. Strikingly, the major axis of genetic differentiation in Spain runs from west to east, while conversely there is remarkable genetic similarity in the north-south direction.To infer details of historical population movements into Spain, we analysed Spain alongside a sample of ~6,000 individuals from Europe, North Africa, and sub-Saharan Africa. Across Spanish groups, we identify varying genetic contributions from north-west African ancestral populations, at times that all fall within the period of Islamic occupation. We also identify Basque-like admixture within Spanish groups to the south of the Basque-speaking region, implying southerly gene flow from this region. This analysis has revealed details of the strengths and weaknesses of different approaches to investigating population genetic history, as well as providing important new insights into the complex genetic history of Spain.