Early divergent strains of Yersinia pestis in Eurasia 5,000 years ago.
Bottom Line: How and when it originated remains contentious.We also identify a temporal sequence of genetic changes that lead to increased virulence and the emergence of the bubonic plague.Our results show that plague infection was endemic in the human populations of Eurasia at least 3,000 years before any historical recordings of pandemics.
Affiliation: Center for Biological Sequence Analysis, Department of Systems Biology, Technical University of Denmark, Kemitorvet, Building 208, 2800 Kongens Lyngby, Denmark.Show MeSH
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Mentions: Besides applying standard precautions for working with ancient DNA (Willerslev and Cooper, 2005), the authenticity of our findings are supported by the following observations: (1) The Y. pestis sequences were identified in significant amounts in shotgun data from eight of 101 samples, showing that this finding is not due to a ubiquitous contaminant in our lab or in the reagents. Indeed, further analysis showed that one of these eight was most likely not Y. pestis. We also sequenced all negative DNA extraction controls and found no signs of Y. pestis DNA in these (Table S3). (2) Consistent with an ancient origin, the Y. pestis reads were highly fragmented, with average read lengths of 43–65 bp (Table S3) and also displayed clear signs of C-T deamination damage at the 5′ termini typical of ancient DNA (Figure 3, Figure S1). Because the plasmids are central for discriminating between Y. pestis and Y. pseudotuberculosis, we tested separately for DNA damage patterns for the chromosome and for each of the plasmids. For the seven samples, we observe similar patterns of DNA damage for chromosome and plasmid sequences (Figure 3, Figure S1). (3) We observe correlated DNA degradation patterns when comparing DNA degradation in the Y. pestis sequences and the human sequences from the host individual. Given that DNA decay can be described as a rate process (Allentoft et al., 2012), this suggests that the DNA molecules of the pathogen and the human host have a similar age (Figure 3, Figure S1, Table S3 and Supplemental Experimental Procedures). (4) Because of the high sequence similarity between Y. pestis and Y. pseudotuberculosis, we mapped all reads both to the Y. pestis CO92 and to the Y. pseudotuberculosis IP32953 reference genomes (Chain et al., 2004). Consistent with being Y. pestis, the seven investigated samples displayed more reads matching perfectly (edit distance = 0) toward Y. pestis (Figure 3, Figure S2). One sample (RISE392) was most likely not Y. pestis based on this criterion. (5) A naive Bayesian classifier trained on known genomes predicts the seven samples to be Y. pestis with 100% posterior probability, while RISE392 is predicted to have 0% probability of being Y. pestis (Figure S2, Table S3). (6) If the DNA was from other organisms than Y. pestis, we would expect the reads to be more frequently associated with either highly conserved or low-complexity regions. However, we find the reads to be distributed across the entire genome (Figure S2), and comparison of actual coverage versus the coverage that would be expected from read length distributions and mappability of the reference sequences are also in agreement for the seven samples (Figure 3). (7) In a maximum likelihood phylogeny, the recovered Y. pestis genomic sequences of RISE505 and RISE509 are clearly within the Y. pestis clade and basal to all contemporary Y. pestis strains (Figure 4) (see below).
Affiliation: Center for Biological Sequence Analysis, Department of Systems Biology, Technical University of Denmark, Kemitorvet, Building 208, 2800 Kongens Lyngby, Denmark.