We compared the ADMIXTURE results to those of SOURCEFIND (
20), a haplotype-based analysis with greater sensitivity. We modeled the genomes of the British and Irish fineSTRUCTURE “target” clusters as a mixture of haplotypes from other British, Norwegian, Danish, or Swedish (i.e., “Scandinavian”) reference clusters (
SI Appendix, Supplementary Data 6). We calculated the mean total ancestry contributions from reference to target (
Fig. 3B) and the proportions of those reference clusters whose haplotypes account for more than 5% of the ancestry of any one target cluster (
Fig. 3C). Mirroring the ADMIXTURE analysis, we observe the highest levels of Norwegian-like haplotypes in Shetland, then Orkney, followed by the Hebrides, the Isle of Man, and Ireland (
Fig. 3B). Our estimates of Norwegian ancestry across northern Britain are in agreement with our ADMIXTURE analysis. We estimate the total of Norwegian-like ancestry in Orkney and Shetland to be about 20 to 25%. The proportion of Norwegian-like ancestry also tends to be higher in individuals with ancestry from the north of Orkney and of Shetland. The largest source of Norwegian-like haplotypes in Britain and Ireland comes from 2 clusters, NOR9 and NOR10, predominantly consisting of individuals sampled from the western Norwegian counties of Hordaland and Sogn & Fjordane (
Fig. 3C), the area whence most Norse Vikings set sail.
[h=3]Ancient Genetic Links.[/h]We reasoned that the isolated regions in the north of Britain and Ireland may act as proxies for the historical populations of these regions—which are as yet unsampled with ancient DNA. Therefore, we set out to identify possible British or Irish source regions for previously reported ancient Gaels from Iceland (
21). We estimated shared genetic drift between these 27 ancient Icelanders and modern British and Irish populations we have described and modern Scandinavians we have used elsewhere in this study.
We first confirmed that our underlying modern British and Irish structure does not dramatically change the ancestry estimates of the ancient Icelanders (
SI Appendix, Supplementary Data 7). Using Human Genome Diversity Project (HGDP) Yorubans as an outgroup, we calculated the
D statistic of 2 groups of ancient individuals to either British or Irish genetic regions, or modern Scandinavia (
Fig. 4). The British or Irish regions included some merged fineSTRUCTURE clusters in Orkney, Shetland, and the Hebrides, Donegal (
Donegal 1 and
2), Ireland (
N Ireland,
C Ireland), and S Ireland (
Munster) to increase sample sizes of those regions. The 2 groups of ancient Icelanders were those of predominantly Gaelic (
n = 7) or Norse (
n = 10) ancestry (see
SI Appendix, Supplementary Data 7 for more information). All but one of the predominantly Norse ancient Icelanders share significantly (Z > 3) more drift with modern Scandinavian individuals than with the British sample (
Fig. 4). The predominantly Gaelic ancient Icelanders show differing affinities across the British and Irish groups. They show the greatest affinity to the “Gaelic” populations of Scotland and Ireland as opposed to English regions (
Fig. 4). The smallest estimates of
D, which correspond to the largest British/Irish affinity compared to Scandinavia, are to Donegal, the Hebrides, and Argyll. This corresponds to the northwestern region of the British Isles and Ireland which is known to have experienced heavy Viking activity (
1). Interestingly we observe some ancient Gaelic Icelanders, in contrast, share affinity with different clusters (
SI Appendix, Fig. S8), notably KNS-A1, who shows greater affinity to the south of Ireland. Our results provide genetic evidence either of Viking-mediated migration of Gaels from the northwest of the British Isles and Ireland or, at least, that these modern regions represent the best proxy of the true ancient Gaelic source populations in the absence of direct ancient DNA sampling.
Fig. 4.
Shared drift between ancient Gaels and modern British and Irish populations. The
D statistics of ancient predominantly Gaelic (
n = 7) and predominantly Norse (
n = 10) Icelanders, comparing affinity to either modern British or Irish genetic regions and modern Scandinavia. A negative
D statistic indicates more shared drift with Britain or Ireland, and a positive value indicates more shared drift with modern Scandinavia. Shown are the estimates with SEs (
Left), with estimates with a |Z| > 3 shown as filled circles and |Z| < 3 shown as hollow circles. We also show the values of the ancient Gaelic Icelander
D-statistic estimates for each modern British or Irish region mapped to the general geographic position of that cluster(s). This was plotted in the statistical computing language R (39) and the packages ggplots2 and rworldxtra.