Abstract
The Iberian Peninsula has been the focus of attention of numerous studies dealing with mitochondrial DNA (mtDNA) variation, most of them targeting the control region segment. In the present study we sequenced the control region of 3,024 Spanish individuals from areas where available data were still limited. We also compiled mtDNA haplotypes from the literature involving 4,588 sequences and 28 population groups or small regions. We meta-analyzed all these data in order to shed further light on patterns of geographic variation, taking advantage of the large sample size and geographic coverage, in contrast with the atomized sampling strategy of previous work. The results indicate that the main mtDNA haplogroups show primarily clinal geographic patterns across the Iberian geography, roughly along a North-South axis. Haplogroup HV0 (where haplogroup U is nested) is more prevalent in the Franco Cantabrian region, in good agreement with previous findings that identified this area as a climate refuge during the Last Glacial Maximum (LGM), prior to a subsequent demographic re-expansion towards Central Europe and the Mediterranean. Typical sub-Saharan and North African lineages are slightly more prevalent in South Iberia, although at low frequencies; this pattern has been shaped mainly by the transatlantic slave trade and the Arab invasion of the Iberian Peninsula. The results also indicate that summary statistics that aim to measure molecular variation, or AMOVA, have limited sensitivity to detect population substructure, in contrast to patterns revealed by phylogeographic analysis. Overall, the results suggest that mtDNA variation in Iberia is substantially stratified. These patterns might be relevant in biomedical studies given that stratification is a common cause of false positives in case-control mtDNA association studies, and should be also considered when weighting the DNA evidence in forensic casework, which is strongly dependent on haplotype frequencies.
Highlights
Analysis of mitochondrial DNA variation has become a convenient tool for the analysis of population substructure
Data from the literature were supplemented with a significant amount of newly generated control region sequences
It is important to note that the resolution of the control region is limited for the classification of some sequences belonging to certain haplogroups (e.g. H); this strategy allows us to reveal patterns of haplogroup geographic variation that would remain unnoticed using other methods
Summary
Analysis of mitochondrial DNA (mtDNA) variation has become a convenient tool for the analysis of population substructure. Population structure is commonly identified when samples have deviations from Hardy-Weinberg proportions, or deviations from panmixia. Conditions such as behavioral restrictions of mating, inbreeding, selection, or migration are the main causes of population substructure, since they are responsible for potential mates not having an equal chance of being selected as partners. This definition, does not apply to mtDNA variation because this marker is inherited in a matrilineal way. The detection of differences in haplotype frequencies has the main limitation that large sample sizes are usually needed. Phylogeographic analysis can help reveal the existence of particular haplogroups or haplotypes that are more prevalent in specific regions
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