Abstract
Many studies have tackled the existence of a genetic barrier in the Strait of Gibraltar between Iberian and North African populations, often with controversial conclusions. Here, we address this issue using a collection of Western Mediterranean populations and two dimensionality reduction methods: principal component analysis (PCA) and spatial PCA (sPCA). Our four different data sets consisted of (i) 16 polymorphic Alu insertions in 12 populations; (ii) 35 single-nucleotide polymorphisms in 13 populations; (iii) 13 short tandem repeats in 11 populations; and (iv) all 64 markers in 9 populations. In all PCA plots, South European and North African samples were visually distinguishable along the first PC. Several smaller clusters were also identifiable, especially on the African side of our geographical setting. sPCA indicated a single global structure for each of the marker sets and no local structures. These results are more compatible with a clinal distribution of allele frequencies rather than with abrupt changes, suggesting that isolation-by-distance, rather than a barrier to gene flow, is a more likely mechanism of genetic differentiation in the Western Mediterranean. An alternative/complementary explanation is progressive introgression from North African to Southwestern European populations.
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Acknowledgements
The authors would like to thank two anonymous reviewers for their useful and constructive comments on the manuscript. This study has been financially supported by the CGL2008-03955 and CGL2011-27866 projects of the Spanish Ministerio de Educación y Ciencia, as well as the 2005SGR00252 project of the Generalitat de Catalunya. The work of GA has been financed by an FPU grant from the Ministerio de Educación y Ciencia (reference: AP2005-4425). The authors would also like to thank all the donors for providing blood samples and the collaborators for collecting them: F Luna, MS Mesa and C Rodríguez (Spanish samples); N Harich and M Kandil (Moroccan samples); H Chaabani (Tunisian sample), JM Dugoujon (North African and French samples); and, two colleagues: M Gayà-Vidal for helping in the SNP genotyping and E González-Pérez for providing the PAI dataset. Also thanks to Professor M Stoneking for his advice, technical and material support during GA’s visit at the Max Planck Institute for Evolutionary Anthropology in Leipzig (Germany), where most of the STR genotyping was carried out. Finally, the authors would like to thank Professor WH Stone and ME Weale for reviewing and discussing the material presented here.
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Athanasiadis, G., Moral, P. Spatial principal component analysis points at global genetic structure in the Western Mediterranean. J Hum Genet 58, 762–765 (2013). https://doi.org/10.1038/jhg.2013.94
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DOI: https://doi.org/10.1038/jhg.2013.94
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