Maximum clade credibility (MCC) tree from Bayesian coalescent analysis based on a subset of the P gene sequences in this study. The estimated TMRCA for this dataset and its 95% HPD values are indicated. Isolate names are given according to Table 1. Horizontal branches are drawn to a scale of estimated year of divergence, with tip times reflecting sampling date (year). Posterior probability values for all of the major nodes are shown. See materials and methods for details.