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Study Finds Admixture Has Shaped Modern West Eurasian Populations

NEW YORK (GenomeWeb) – Recent admixture is widespread throughout West Eurasian populations, according to a new analysis conducted by a team of Oxford University-led researchers.

Oxford's George Busby and his colleagues used haplotype-based techniques to examine traces of admixture events left behind in the genomes of modern-day West Eurasian populations. As they reported today in Current Biology, they found that admixture, including the flow of genetic material from outside West Eurasia, is a near-universal property across groups in the region. The researchers were further able to gauge the timing of some admixture events and link them to known historical population movements.

"We now have the statistical machinery to uncover which historical events have produced the mosaic genomes of people in Europe today," Busby said in a statement. "The successful reconstruction of the genetic history of a region of the world that has been well investigated both archaeologically and historically suggests that these approaches have the potential to be applied to areas where history has not been so well recorded and where genetics might be the only way of recovering history."

To examine historical events that have left a mark on population genetic patterns in West Eurasians, Busby and his colleagues examined a dataset of more than 1,200 phased genomes from West Eurasian individuals — representing 63 populations —and 957 genomes from individuals from other spots around the world.

Using the fineSTRUCTURE genetic clustering algorithm, the researchers identified groups of individuals within their cohort that were similar genetically and developed a tree reflecting the hierarchical relationships between the clusters. Through this, they redefined their West Eurasian cohort as a monophyletic clade of 82 fineSTRUCTURE clusters of 1,000 individuals, spanning mainland Europe, Sardinia, Sicily, Cyprus, western Russia, the Caucasus, Turkey, Iran, and some individuals from Tajikistan and Turkmenistan.

Generally, the researchers found that most people who were from the same geographic area belonged to the same genetic cluster. Greek, Basque, and Mordovian individuals, for instance, were placed within the same genetic cluster. However, the researchers noted that people from larger geographic populations, such as Spain, were sometimes split into many genetic clusters, which could reflect population substructure.

By turning to GLOBETROTTER, the researchers then delved into the influence of admixture in each of these clusters. They painted each recipient individual's chromosomes to reflect the recipient's mosaic of different ancestries by drawing on a set of donor groups from 18 world regions as well as other clusters from within West Eurasia. They then ascertained whether admixture was likely to have taken place in that recipient group and characterized the composition and portion from each donor group.

The vast majority of clusters — some 78 percent — exhibited evidence of admixture, the researchers reported. Almost all West Eurasian clusters have some ancestry from outside the region, they added.

For instance, the researchers noted that the West Central Asian, Caucasus, and Turkish clusters contained East Asian ancestry, though Northern European clusters did not. In addition, Italians, Sardinians, the French and Spanish, and a few other groups, contained ancestry from the Levant.

"Much as different cultures have often borrowed elements from each other, we are now seeing that the genomes of people alive in Europe today contain ancestry from multiple different places, from within Europe and outside," senior author Cristian Capelli said.

Busby and his colleagues further gauged the timing of admixture events by modeling the decay of linkage disequilibrium between ancestral chunks.

The researchers noted that admixture events involving a Slavic source occurred in Balkan, Germanic, and eastern European populations toward the end of the first millennium of the Common Era, a time known as the European Migration Period or Völkeranderung.

Another event that occurred between 300 CE and 1200 CE across Northern and Western Europe involved minor West Eurasian source groups, which suggested to the researchers that there was a substantial amount of movement during the Völkeranderung.

This approach, the researchers added, could be used to fill in some of the blanks left in history books.

"History is often written by the winners and the elites — we often do not hear about the everyday life of people," Busby said. "By studying the DNA of populations and understanding how different groups are ancestrally related to each other, our analysis tells the story of all people."