mtDna and the islands of the North Atlantic: estimating the proportions of Norse and Gaelic ancestry

Abstract

A total of 1,664 new mtDNA control-region sequences were analyzed in order to estimate Gaelic and Scandinavian matrilineal ancestry in the populations of Iceland, Orkney, the Western Isles, and the Isle of Skye and to investigate other aspects of their genetic history. A relative excess of private lineages in the Icelanders is indicative of isolation, whereas the scarcity of private lineages in Scottish island populations may be explained by recent gene flow and population decline. Differences in the frequencies of lineage clusters are observed between the Scandinavian and the Gaelic source mtDNA pools, and, on a continent-wide basis, such differences between populations seem to be associated with geography. A multidimensional scaling analysis of genetic distances, based on mtDNA lineage-cluster frequencies, groups the North Atlantic islanders with the Gaelic and the Scandinavian populations, whereas populations from the central, southern, and Baltic regions of Europe are arranged in clusters in broad agreement with their geographic locations. This pattern is highly significant, according to a Mantel correlation between genetic and geographic distances (r=.716). Admixture analyses indicate that the ancestral contributions of mtDNA lineages from Scandinavia to the populations of Iceland, Orkney, the Western Isles, and the Isle of Skye are 37.5%, 35.5%, 11.5%, and 12.5%, respectively.

Figure 1

Map of the North Atlantic region. The blackened area represents regions where Norse cultural and linguistic dominance was complete during the Viking period. The dark speckled areas in the British Isles provide an indication of the core areas of Viking exploits, on the basis of archaeological sites, place names of Norse origin, and raided monasteries and towns (Bjarnason et al. ; Graham-Campbell and Batey ; Corráin ; Keynes 1999). The arrows show some of the main sailing routes of the Vikings in the North Atlantic.

Figure 2

Schematic phylogenetic representation of mtDNA lineage clusters found in European populations, reconstructed on the basis of information obtained from Torroni et al. (1996), Richards et al. (1998), Macaulay et al. (1999), Quintana-Murci et al. (1999), and Helgason et al. (2000b). Lineage clusters are shown as circles, and the connecting lines represent diagnostic substitutions.

Figure 3

Scatterplot of θ_k values and the percentage of private lineages. The least-squares regression line, where r=0.77, is shown. The curved lines show the 95% confidence region around the regression line.

Figure 4

ρ distances between the mtDNA pools of five North Atlantic populations and those of potential European source populations. ρ distances to the Saami were excluded to maximize clarity in the representation of distances to the other populations. In all cases, the ρ distance to the Saami was ∼1.5.

Figure 5

Multidimensional scaling plot of genetic distances on the basis of haplogroup frequencies. The fourteen dimensions of the genetic distance matrix were reduced to two dimensions, which account for 85% of the genetic variation defined by the original distance matrix.