Figure 2.

The phylogenetic diversity of avian influenza A viruses. There are 16 different hemagglutinin (HA) subtypes of influenza A virus circulating in wild birds, falling into two main HA groups and further subdivided into a series of clades (Latorre-Margalef et al. 2013). Within each subtype, there is clear genetic structure based upon avian geography, such that sequences sampled from birds of the Americas or Eurasia fall into two distinct and independently evolving clades regardless of subtype. This phylogeographic division is demonstrated in a phylogeny containing all currently available H4 sequences (right panel), and is also observed in all nine neuraminidases (NAs) and the internal gene segments. Occasionally, there is spillover across these main lineages, as demonstrated by the N8 tree (right panel) in which virus sequences originating in Asia cluster in a clade dominated by North American sequences.