Estimating Scandinavian and Gaelic ancestry in the male settlers of Iceland

Abstract

We present findings based on a study of Y-chromosome diallelic and microsatellite variation in 181 Icelanders, 233 Scandinavians, and 283 Gaels from Ireland and Scotland. All but one of the Icelandic Y chromosomes belong to haplogroup 1 (41.4%), haplogroup 2 (34.2%), or haplogroup 3 (23.8%). We present phylogenetic networks of Icelandic Y-chromosome variation, using haplotypes constructed from seven diallelic markers and eight microsatellite markers, and we propose two new clades. We also report, for the first time, the phylogenetic context of the microsatellite marker DYS385 in Europe. A comparison of haplotypes based on six diallelic loci and five microsatellite loci indicates that some Icelandic haplogroup-1 chromosomes are likely to have a Gaelic origin, whereas for most Icelandic haplogroup-2 and -3 chromosomes, a Scandinavian origin is probable. The data suggest that 20%-25% of Icelandic founding males had Gaelic ancestry, with the remainder having Norse ancestry. The closer relationship with the Scandinavian Y-chromosome pool is supported by the results of analyses of genetic distances and lineage sharing. These findings contrast with results based on mtDNA data, which indicate closer matrilineal links with populations of the British Isles. This supports the model, put forward by some historians, that the majority of females in the Icelandic founding population had Gaelic ancestry, whereas the majority of males had Scandinavian ancestry.

Figure 1

Haplogroups are represented by circles and are identified by numbers, on the basis of the nomenclature of Tyler-Smith (1999) and Jobling et al. (1997). Lines denote mutation events, which are labeled with the associated diallelic marker.

Figure 2

The area of each pie denotes sample size, and the pie slices denote the proportion of chromosomes belonging to each haplogroup. Haplogroups were assigned using the diallelic markers 92R7, M9, SRY-1532, YAP, and TAT. Sources of data are as follows: for pies 1–4, the present study; for pie 5, Hill et al. (2000); for pie 6, T. Zerjal, L. Beckman, and C. Tyler-Smith (personal communication); for pies 7–10, Karafet et al. (1999); and for pie 11, Previdere et al. (1998).

Figure 3

Median-joining network showing the phylogenetic relationships of Icelandic Y-chromosome haplotypes within haplogroup 1. Haplogroup membership was determined by the seven diallelic markers listed in the Material and Methods section. Haplotypes are defined by character states at eight microsatellite loci: DYS19, DYS388, DYS389I, DYS389b, and DYS390–DYS393. The allele sizes of the two DYS385 loci are shown below each haplotype. Circles denote haplotypes, and their sizes denote the frequencies of the haplotypes in the Icelanders. Lines denote mutational changes in the repeat numbers of microsatellite loci, with both the locus and the change in repeat number indicated. Reticulations denote the presence of recurrent mutations that could not be resolved with the available markers. Circles are shaded with respect to haplotype sharing with Scandinavians and Gaels, on the basis of a comparison of a restricted set of five microsatellites: DYS19 and DYS390–DYS393. Broken lines denote mutations at loci that are not included in the restricted set of microsatellites and therefore do not discriminate between haplotypes of Icelanders and those of Scandinavians and Gaels. Gray-shaded and blackened areas denote the relative frequencies of a haplotype in Gaels and Scandinavians, respectively. Unshaded and unblackened haplotypes are found only in Iceland.

Figure 4

Median-joining network showing the phylogenetic relationships of Icelandic Y-chromosome haplotypes within haplogroup 2. For definitions, see the legend for figure 3.

Figure 5

Median-joining network showing the phylogenetic relationships of Icelandic Y-chromosome haplotypes within haplogroup 3. For definitions, see the legend for figure 3.

Figure 6

The scatterplot presents the relationship between p_I − p_G and p_S−p_G, where p_I, p_G, and p_S denote the haplotype frequencies for the Icelanders, Gaels, and Scandinavians, respectively. The slope of the least-squares fitted regression line approximates the m_R estimate of Scandinavian ancestry in Icelandic Y chromosomes.