Fig. 1

Maximum clade credibility (MCC) tree showing a the phylogenetic relationships between global CC59 sequences and b the relative genetic diversity of global CC59 over time. The MCC tree is summarised from 9000 BEAST posterior phylogeny samples. Branches are coloured according to the inferred ancestral location under a discrete phylogeography analysis. Due to low power at the root of the tree, we were unable to infer the ancestral location with a posterior probability greater than 0.5 (branches coloured black). Purple dots denote the ancestral nodes of two major clades, one which was inferred to have originated in the USA and the other in East Asia. A black triangle denotes a clade of sequences from the UK found within the USA-associated clade and a black square denotes a clade of sequences from the Netherlands found within the East Asia clade. Median and 95 % highest posterior density (HPD) intervals for the relative genetic diversity of CC59 over time are plotted against time, with the lower limit of the timescale corresponding to the lower 95 % HPD value of the root height of the tree. The phylogeny and the plot of the relative genetic diversity are shown on the same timescale. PP posterior probability