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Fine-scale Genetic Map of Britain Gives Clues to Ancient Population Movements

NEW YORK (GenomeWeb) – An international team of researchers has developed a fine-scale genetic map of the British population that largely reflects regional geography and known historical events, but also provides insight into historical questions.

By genotyping more than 2,000 people living in rural parts of the UK whose grandparents all hailed from within 80 kilometers of one another, the team led by the University of Oxford's Peter Donnelly got a glimpse into the past and how population structure in the UK has changed over time and has been influenced by influxes from continental Europe, as they reported in Nature today.

"It's really the first time that scientists have looked in great detail within a country at patterns of genetic variation, certainly the first time within the UK," Donnelly said during a press briefing. "And what was striking was the pattern of variation that we saw and the way it aligned with geography."

Drawing on the People of the British Isles collection, Donnelly and his colleagues assembled a cohort of 2,039 people from rural regions of the UK who could trace their ancestors back to the area. That way, the researchers said they could, in effect, analyze the genomes of those individuals' grandparents to get a picture of what the genetic makeup of the UK in the late 19th century was, prior to the population movements of the 20th century.

The researchers genotyped their cohort and used the variation they observed at about half a million SNPs to cluster their samples by genetic similarity using the fineSTRUCTURE algorithm.

Overall, Donnelly noted that the Caucasians from the UK population they studied are more similar to each other than to people from other countries, and that he and his colleagues sifted through rather subtle differences between them.

The researchers overlaid the genetic clusters they found on a map of the UK, and found that they reflected different regions of the UK.

The first group to split off in this clustering analysis was the Orkney population off of the north of Scotland, followed by the Welsh splitting off from the non-Orkney UK population. These splits then continued to place the UK individuals into increasingly finer groups, eventually totaling 17 clusters.

The finding that Orkney differed genetically from the rest of the UK is in line with the historical record, the researchers noted. The islands were settled and controlled by Norse Vikings in the 9th century and were part of Norway from 875 through 1472.

This clustering was able to separate groups living in close proximity such as those living in Cornwall and Devon. Intriguingly, the clusters there, Donnelly added, reflect the modern county boundary.

Many of these divisions, the researchers said, reflect geography and relative isolation from each other.

One large cluster — which represents about half the study population — covered southern, central, and the east coast of England. This region, the researchers noted, largely lacks geographical or political boundaries that would've prevented mixture.

Additionally, in the Celtic areas of the UK, the researchers saw no evidence of a general Celtic population, but rather a number of genetically diverse groups. The Cornish in Southwest England, for example, are more similar genetically to other English clusters than to Welsh or Scottish clusters.

To see how populations from continental Europe contributed to the genetic makeup of these UK clusters, Donnelly and his colleagues applied their fineSTRUCTURE analyses to a set of more than 6,000 samples from continental Europeans. Then they characterized the ancestry profile of each UK cluster as it related to the 51 European groups they found.

Some European groups — such as those from western Germany, northern Belgium, northwest France, Denmark, and southern France and Spain — figured largely in the ancestry profiles of all the UK clusters. Others, however, were only present in a few UK populations.

For instance, Norwegian groups figure prominently in the Orkney clusters' ancestry profiles, but are mostly absent from other UK clusters. "[This] confirms what we already knew about history, but also confirms that our approach is doing something sensible," first author Stephen Leslie from the University of Melbourne said.

Donnelly, Leslie, and their colleagues could also broadly describe the relative order of the peopling of Britain.

The earliest migrants who made a substantial genetic contribution to UK ancestry after the last Ice Age, they said, arrived from western Germany, Belgium, and northwestern France as their DNA has had a chance to spread throughout Britain.

Other groups that gave smaller contributions came from southern France and Spain, part of Denmark, and possibly parts of Norway and Sweden. Another migration from France contributed to UK ancestry, though not to the Welsh.

The Welsh cluster, the researchers added, seems to be more similar to the early post-Ice Age settlers of Britain than other UK populations.

One mystery that has vexed historians and archaeologists is about what happened in Britain following the decline of the Roman Empire in 410 AD and the arrival of the Anglo-Saxons in southern England.

The Anglo-Saxons, Mark Robinson, an archaeologist at Oxford, noted, came to dominate Roman Britain linguistically and culturally, but there has been debate regarding whether the Anglo-Saxons replaced the Romano-British population or whether there was a small-scale settlement of Anglo-Saxon migrants and the Romano-British population chose to adopt the Anglo-Saxon culture.

Through their genetic analyses, the researchers estimated that the portion of Saxon ancestry in central, eastern, and southern England was less than 50 percent and likely closer to between 10 percent and 40 percent.

"What this is implying is that once the Saxons arrived, they mixed and intermarried with the Britons, although it was the Saxon culture which prevailed, possibly because the Romano-British culture had become associated with the failure and collapse which occurred after the Roman withdrawal," Robinson said.