Remodeling the Plague Phylogenetic Tree

Understanding the molecular history of any organism requires fitting together ancient DNA with the phylogenetic tree constructed with living exemplars. Constructing a bacterial phylogenetic tree is a snapshot of a moving target because its impossible to sample all of the strains.  A recent study by the East Smithfield group ( Bos et al, 2012 [2]) seeks to fit the recent near complete genomic sequence of Yersinia pestis from the Black Death cemetery at East Smithfield into the current phylogenetic tree.

They pooled their SNP database with those used by Morelli et al [3] for a total of 311 strains, plus the parental species Yersinia pseudotuberculosis as its foundation.  The East Smithfield group expect that the SNP comparison “could provide a qualitative indication of phylogenetic signals that were lost via our original, more conservative analytical approach based strictly on complete genomes.” [2]

New phylogeny of Yersinia pestis (Bos et al, 2012)

Their analysis confirmed that the Black Death strain settles into the base of split between branch 1 & 2. This matches what Haensch et al [4] found in 14th century sites at Hereford and Saint-Laurent-de-Cabrerisse. This indicates that the split occurred after the Black Death, probably due to microevolution in geographically distinct regions. Branch 2 is localized primarily along the Silk Road route in Central Asia, while branch 1 is far more widely distributed  and produced the third pandemic strain [3].  Bos et al further identified two living strains, designated 3.ANT, with SNP profiles that match their East Smithfield Black Death SNP profile [2]. These strains have not been completely sequenced and the plasmid profiles of these strains and the Black Death strain have not been characterized, so we can not yet say that these strains are genetically identical in sequence or genomic architecture to the Black Death strain [2]. Note that genomic architecture (placement of genes in chromosome) will mostly likely effect gene expression and therefore function of the microbe.

The East Smithfield group  observed that a small group of three strains diverged from the main descent line immediately before the Black Death, designated here as 0.ANT3, were all isolated from China [2]. They suggest that these strains may have been produced during a diversifying event that produced the main Black Death strain, possibly in Asia before it reached Europe.

They also observed 11 strains of Yersinia pestis clumped at the 0.ANT1 branch point [2]. By their calculations this split would have occurred between the 8th and 10th century (732-980 AD) overlapping with the documented period of the Plague of Justinian. They suggest that these strains represent genetic radiation that occurred during the Justinian expansion. This is a change from their observations based solely on comparisons of complete genomes [1].

The East Smithfield genomic group still have not incorporated ancient DNA data from any other group in their analysis.

References:

[1] Bos KI, Schuenemann VS, Golding GB, Burbano HA, Waglechner N, et al. (2011) A draft genome of Yersinia pestis from victims of the Black Death. Nature 478: 506.510.

[2] Bos KI, Stevens P, Nieselt K, Poinar HN, DeWitte SN, et al. (2012) Yersinia pestis: New Evidence for an Old Infection. PLoS ONE 7(11): e49803. doi:10.1371/journal.pone.0049803

[3] Morelli G, Song Y, Mazzoni CJ, Eppinger M, Roumagnac P, et al. (2010) Yersinia pestis genome sequencing identifies patterns of global phylogenetic diversity. Nat Genet 42: 1140.1143.

[4] Haensch S, Bianucci R, Signoli M, Rajerison M, Schultz M, et al. (2010) Distinct Clones of Yersinia pestis Caused the Black Death. PLoS Pathog 6(10): e1001134. doi:10.1371/journal.ppat.1001134

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