A genetic perspective on the recent demographic history of Ireland and Britain

Background: While subtle yet discrete clusters of genetic identity across Ireland and Britain have been identified, their demographic history is unclear. Methods: Using genotype data from 6,574 individuals with associated regional Irish or British ancestry, we identified Irish-like and British-like genetic communities using network community detection. We segregated Identity-by-Descent (IBD) and Runs-of-Homozygosity (ROH) segments by length and approximated their corresponding time periods. Through this, we inferred the regional Irish and British demographic histories in these time periods by (1) estimating genetic relatedness between communities, (2) estimating changes in effective population sizes, (3) inferring recent migration rates across Ireland and Britain, and (4) estimating changing affinities to regional European populations. For a subset of the Irish communities, we determined the enrichment/depletion of surnames within the genetic communities. Results: Through patterns of IBD-sharing and ROH, we find evidence of recent population bottlenecks in the Orcadian, Manx and Welsh communities. While the structure in Ireland is subtler, the communities share relatively more IBD segments that are shorter in length, and the genetic differences between the Irish communities are more subtle on average, when compared to the British communities. Regional effective population size trajectories indicate a similar demographic history throughout the island of Ireland. Further, we observe a stable migration corridor between north-east Ireland and south-west Scotland while there is a recent migration barrier between South-Eastern Ireland and Western Ireland. We observed an enrichment of Anglo-Norman and English surnames in the Wexford community while within the West Ulster-Argyll community, we saw an enrichment of Gallowglass and Scottish surnames. Conclusions: Using well-annotated Irish and British reference genotypes, we observed temporal changes in genetic affinities within and between genetic communities in Ireland and Britain. In addition, using effective population size estimates and levels of haplotype-sharing, we detected varying degrees of genetic isolation in some Irish and British genetic communities across time. Using these new insights into the regional demographic history of Ireland and Britain across different time periods, we hope to understand the driving forces of rare allele frequencies and disease risk association within these populations. ### Competing Interest Statement OH has received speaking honoraria from Janssen Cilag, Biogen, Sanofi Aventis, Novartis, Merck-Serono and Cytokenetics. She has been a member of advisory panels for Biogen, Novartis, Cytokenetics, Denali and Neurosense. She serves as Editor-in-Chief of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia.