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Hidden Genetic Structure Reflecting Geography, History Uncovered Within Irish Population

NEW YORK (GenomeWeb) – Researchers have unraveled hidden genetic structure within the Irish population that reflects both geography and historical migrations.

Previous studies have indicated that Ireland, which has been continuously inhabited for some 10,000 years, is fairly genetically homogenous. But using haplotype-based methods to examine about 1,000 people from the island, researchers from Trinity College Dublin have uncovered 23 genetic clusters. As they reported today in PLOS Genetics, they also teased out a west-east cline of Celtic-British ancestry and estimated admixture dates that correspond with historical events, such as the Norse-Viking migrations.

"The long and complex history of population dynamics in Ireland has left an indelible mark on the genomes of modern inhabitants of the island," senior author Russell McLaughlin from Trinity College said in a statement. "We have shown that, using only genetic data, we can accurately reconstruct elements of this past and demonstrate a striking correlation between geographical provenance and genetic affinity."

A similar study appearing in Scientific Reports in December from a Royal College of Surgeons-led team clustered the Irish population into 10 geographically distinct groups with varying degrees of Gaelic Irish and Irish-British ancestry.

In this study, McLaughlin used the ChromoPainter tool to uncover similar haplotypes from within a SNP dataset of 1,035 people from the Republic of Ireland and Northern Ireland. The fineSTRUCTURE algorithm then placed the resulting co-ancestry matrix into 23 clusters.

He and his colleagues noted that this approach was recently used as part of the People of the British Isles study to uncover hidden genetic structure within the UK population. Here, too, they reported that the approach uncovered finer-scale local population structure.

Overall, the clusters the researchers uncovered broadly matched with the historical Irish provinces of Ulster, Leinster, Munster, and Connacht, and a principal components analysis of ChromoPainter findings uncovered both north-south and east-west clines.

The highest genetic diversity was found among clusters on the west coast, which the researchers said could be due to residual ancient, possible Celtic structure in the region, which was also less affected by historical migration. Eastern populations, meanwhile, appeared fairly homogenous, possibly due to increased gene flow and migration there from Britain.

By folding in samples from the People of the British Isles study as well as data on 18 ancient British individuals, the researchers estimated the extent of British admixture among their Irish cohort. With the ADMIXTURE algorithm, they found that British ancestry was lowest among people from the west coast of Ireland and higher among people in the east. Increased British ancestry on the east coast could be due to increased English influence there following the Norman invasion or to it being geographically closer to England, the researchers said. The Ulster cluster also harbored a high British component, which the researchers said was likely due to the Ulster Plantations in the 17th century and the remaining ethnically British population.

Using GLOBETROTTER and a further 4,514 European samples, the researchers teased out an ancestral influence of Scandinavians and northern Europeans on Irish genomes. The strongest signal could be traced to south and central Leinster, where a large historical Viking settlement — now Dublin — is.

They likewise timed an influx of British ancestry to the Norman period, though not the Ulster Plantations era, which suggested to the researchers that the admixture events between Irish and British populations are too complex to model with GLOBETROTTER.

Still, this suggested to the researchers that historical migrations have had a greater influence on Irish genetics than appreciated and that this hidden variability could have effects on studies searching for rare genetic variants.

"Understanding this fine‐grained population structure is crucially important for ongoing and future studies of rare genetic variation in health and disease," McLaughlin added.

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