H7 HA1 phylogeny. The tree was inferred using the PhyML software under the GTR + Γ model of DNA substitution, with 6 rate categories. 1000 bootstrap replicates were performed. Major geographical lineages are labelled in red and bootstrap support values (proportion of bootstrap replicates) for major clades are labelled in blue. An H15 sequence was used as an outgroup, but was removed in this figure for the purpose of presentation. Lineages are coloured by the background NA subtype of the virus at the tips of the tree, and clades of sequences of the same subtype have been collapsed for the purpose of presentation (numbers of sequences in collapsed clades are given in brackets). Note: FPV = 'fowl plague virus’, a term used to describe H7 avian influenza viruses isolated in the 1920s-1940s.