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Ancient Genomes Fill In Missing Parts of British Population History, Influence of Migrants

NEW YORK (GenomeWeb) – By sifting through the genomes of nearly 20 individuals who lived in ancient Britain, two teams of researchers have caught a glimpse into the population history of Britain, as they reported today in a pair of Nature Communications papers. Both teams highlighted the influence of migrations on the genetic makeup of the people of Britain.

The Wellcome Trust Sanger Institute's Richard Durbin and his team sequenced the genomes of 10 individuals from the east of England — namely from Hinxton, Oakington, and Linton — who lived during the late Iron Age, early Anglo-Saxon period, or middle Anglo-Saxon period. A principal components analysis placed these ancient samples within the range of modern English and Scottish samples.

By examining the rare variants present in these ancient genomes versus what's found in modern European populations, the researchers estimated that modern East English populations derive an average 38 percent of their ancestry from Anglo Saxons, while Welsh and Scottish population derive a bit less, about 30 percent on average, of their ancestry from Anglo Saxons.

In addition, the researchers reported that the middle Anglo-Saxon samples from Hinxton shared more rare variants with modern Dutch populations than did the Iron Age samples from Hinxton and Linton. Further, two of the early Anglo-Saxon samples from Oakington were similar to the middle Anglo-Saxon samples while another was similar to the Iron Age samples. A fourth exhibited an intermediate level of allele sharing, suggestive of mixed ancestry.

Using a new method they developed called rarecoal, which fits a demographic model to the distribution of rare alleles found within a large set of samples, Durbin and his colleagues built a population phylogeny of the relationships of European populations. They then placed the ancient samples within this framework.

From this, they found that the Anglo-Saxon individuals are closely related to modern Danish and Dutch populations, while the Iron Age individuals shared ancestors with multiple Northern European populations.

“We wanted to determine where ancient DNA samples would fit with respect to a modern population model and to map individuals into that model," Durbin said in a statement.  "This study, using whole-genome sequencing, allowed us to assign DNA ancestry at extremely high resolution and accurately estimate the Anglo-Saxon mixture fraction for each individual.”

At the same time, a team led by Trinity College Dublin's Daniel Bradley sequenced nine individuals who lived in northern England during the Iron Age, Roman era, or Anglo-Saxon period, finding evidence of both migration and genetic continuity in the region. A number of the Roman-era samples had been decapitated around the time of their deaths, leading to speculation that they'd been gladiators or criminals.

After calling between 210,000 SNPs and 400,000 SNPs in these samples — which had also been analyzed in a cohort of modern Europeans, West Asians, North Africans, and Middle Easterners — the researchers conducted a principal components analysis. This, they reported, placed eight of the nine ancient genomes among a cluster of northwest European samples.

The Roman-era individuals were generally similar to modern Welsh populations, according to a similar analysis drawing on genotype data from more than 3,000 British, Irish, and southern Dutch individuals. The Iron Age sample was similarly placed near modern Welsh individuals.

Through allele-sharing and identical-by-state analysis, the researchers found that the Roman-era individuals showed genetic continuity with the earlier Iron Age samples. However, they differed from the later Anglo-Saxon sample as well as from modern-day inhabitants of Yorkshire and Humberside, suggesting genetic change in the region, possibly during the following Anglo-Saxon period.

The Anglo-Saxon sample, meanwhile, was closest to modern East Anglian, and between the English and Dutch medians, likely reflecting the contribution of Germanic immigrants to eastern England.

Intriguingly, the ninth sample from the Roman era didn't cluster among northwest European, and instead was most similar to the modern Middle Eastern populations of Palestine, Jordan, and Syria, the researchers reported. Isotopic analysis of the bones further indicated that the man had a different, likely arid, origin from the other samples. This finding, Bradley and his team said, reflects the cosmopolitan nature of the Roman Empire, even at its most far-flung reaches.

"This is the first refined genomic evidence for far-reaching ancient mobility and also the first snapshot of British genomes in the early centuries AD, indicating continuity with an Iron Age sample before the migrations of the Anglo-Saxon period,” co-author Matthew Collins from the BioArCh research facility said in a statement.