Fig 2. Minimum spanning tree of 38 B. anthracis strains belonging to the A.Br.011/009 polytomy.

Tree drawing is based upon 1934 chromosomal SNPs. The six branches constituting the polytomy are numbered as previously reported [17]. The diameter of each circle varies according to the number of isolates having the same genotype. The length of each branch is proportional (logarithmic scale) to the indicated number of SNPs. Branches with no indication of length value have a length of one SNP. A tentative dating of the initial outbreak represented by the red star is indicated, based on the assumption that (1) the split towards WNA-Senegal-Gambia occurred within 1550–1650 (2) the average expansion rate of lineage 2 which remained in the same ecosystem (within France) did not vary from the founder outbreak event until the end of the nineteenth century. Under these hypotheses, the outbreak most likely occurred during the hundred years war between France and England, 1350–1450.