The Justinianic Plague began in 541 in the Eastern Roman Empire. Recurrent outbreaks ravaged Europe and the Mediterranean basin for approximately 200 years, wiping out 25% of the population of the Roman world. Recent genetic studies revealed that the bacterium Yersinia pestis was the cause of the disease, but how it spread and the relationship between strains has been unknown.
In the study, published yesterday in PNAS, researchers analyzed human remains from twenty-one sites in Austria, Britain, Germany, France, and Spain. They reconstructed eight Y. pestis genomes, allowing them to compare these strains to previously published ancient and modern genomes.
“This study shows the potential of palaeogenomic research for understanding historical and modern pandemics by comparing genomes across millennia,” explains senior author Johannes Krause of the Max Planck Institute for the Science of Human History.
The team also found the earliest genetic evidence of plague in Britain from the Anglo-Saxon site of Edix Hill. Using a combination of archaeological dating and the position of this strain in the evolutionary tree, they concluded that the genome is likely related to an ambiguously described pestilence in the British Isles in 544 AD.
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“The retrieval of genomes that span a wide geographic and temporal scope gives us the opportunity to assess Y. pestis’ microdiversity present in Europe during the First Pandemic,” says co-first author Marcel Keller.
The newly discovered genomes revealed that there were multiple closely related strains of Y. pestis circulating during the 200 years of the First Pandemic, some possibly at the same times and in the same regions. “The fact that all genomes belong to the same lineage is indicative of a persistence of plague in Europe or the Mediterranean basin over this time period, instead of multiple reintroductions,” Keller adds.
Additionally, plague genomes appearing towards the end of the First Pandemic showed a big deletion in their genetic code that included two virulence factors. Plague genomes from the late stages of the Second Pandemic some 800–1000 years later show a similar deletion covering the same region of the genomes.
“This is a possible example of convergent evolution, meaning that these Y. pestis strains independently evolved similar characteristics,” says co-first author Maria Spyrou. “Such changes may reflect an adaptation to a distinct ecological niche in Western Eurasia where the plague was circulating during both pandemics.”
Image: Lunel-Viel (Languedoc-Southern France). Victim of the plague thrown into a demolition trench of a Gallo-Roman house; end of the 6th-early 7th century. Credit: 1990; CNRS - Claude Raynaud.