FIG. 2.

Phylogenetic trees for the PB1 (a), PA (b), and NS (c) genes of influenza A viruses. Evolutionary analyses of nucleotide sequences of PB1, PA, and NS genes were performed as described in the legend to Fig. 1. The sequence ranges were listed in Materials and Methods. The PB1 and PA trees were rooted to B/Lee/40 and B/Singapore/222/79, respectively. The NS gene tree was rooted to A/Equine/Prague/1/56 (H7N7). Virus names and abbreviations can be found in the legend of Fig. 1.