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Irish Genetic Population Study Uncovers Influence of Historical Events

NEW YORK (GenomeWeb) – A genetic analysis of the people of the island of Ireland has revealed a fine-scale population structure that largely reflects geographic and political boundaries.

Researchers from the Royal College of Surgeons in Ireland combined data from four cohorts of European ancestry, including the Irish DNA Atlas and Peoples of the British Isles datasets, to examine the genetic ancestry of the Irish population. As they reported today in Scientific Reports, the researchers were able to cluster Irish individuals into 10 groups that were geographically distinct. Of these clusters, seven had Gaelic Irish ancestry and three had combined Irish-British ancestry, which the researchers traced to the Ulster Plantations, when English and Scottish settlers arrived in Ulster.

The researchers also uncovered genetic influences among the Irish clusters from populations in France and Norway.

"Our work informs on Irish history; we have demonstrated that the structure emerging from genetic similarity within Ireland, mirrors historical kingdoms of Ireland, and that Ireland acts as a sink of 'Celtic' ancestry," first author Edmund Gilbert from the Royal College said in a statement.

The researchers amassed SNP genotype dataset of 536 Irish people — from the Irish DNA Atlas, the Trinity Student Study dataset, or the Peoples of the British Isles — as well as 101 Scottish, 131 Welsh, 96 Orcadians, and 1,239 English individuals. The Irish samples included 192 people with four generations of Irish ancestry from specific regions.

Using the FineStructure algorithm, they clustered these samples to uncover seven clusters of predominantly Irish individuals. Three other clusters spanned both Irish and British individuals, suggesting a shared ancestry.

The distribution of the Irish clusters followed geographic and political boundaries within Ireland, especially of the provinces, Gilbert and his colleagues reported. For instance, two clusters followed the boundaries of Munster, while another followed that of Connacht. The two Munster clusters were further confined by the boundaries of the ancient Dál Cais and Eóganacht kingdoms. This suggested to the researchers that, at least until the mid-19th century, there wasn't widespread movement within Ireland.

Still, the researchers noted that the level of differentiation between the Irish clusters was smaller than what has been observed in Britain. When they examined barriers to gene flow within the populations, they noted a number of boundaries in the British samples, but in Ireland they found a general trend of gene flow, though with some low-migration regions.

By adding in wider European genetic data, the researchers modeled their genetic influence on Irish and British populations. For the seven Gaelic Irish population clusters, Gilbert and his colleagues found genetic influences from French, Belgian, Danish, and Norwegian populations. They traced the French genetic influence to a northwestern French population that had previously been linked to other Celtic populations, including in Scotland, Wales, and Cornwall.

The researchers also uncovered a high level of Norwegian-like ancestry among their Irish clusters as well as in some of their British clusters. They attributed this Norwegian-like ancestry to coastal parts of Norway, including regions where Norse Vikings were active. Globetrotter modeling further timed this Norwegian genetic influence to about the time that Vikings were active.

The three Irish-British clusters, meanwhile, mainly included Irish from Northern Ireland, Scottish from southern Scotland, and English from northern England. Globetrotter analysis of these populations traced the influx of British ancestry to the 17th and 18th centuries, which the researchers said largely coincided with the Ulster Plantations period of the 16th and 17th centuries.

In addition to informing genealogical and historical studies, the researchers said their finding could help the study of genetic diseases in Ireland. "Having a genetic map of the Irish Population will be invaluable in future studies of the genetic component of some common diseases in the Irish population, especially those diseases which show a difference in prevalence rates across the Island of Ireland," added author Sean Ennis from University College Dublin.