Figure 1: Temporally structured maximum clade credibility phylogenetic tree of the NA gene of seasonal H1N1 viruses, showing the timeline of amino acid substitutions.

Ancestral state reconstruction of tested amino acids is annotated on the tree backbone to indicate when specific mutations were fixed in the population. The trunk colour change indicates a >95% posterior probability of amino acid fixation. Purple bars on tree nodes indicate the Bayesian credible intervals of ancestral divergence time estimates (>50% posterior probability). Isolates with H275Y mutation before H275Y fixation into the population are indicated with red branches.