FIG. 2.

Phylogenetic tree of influenza B virus gene segments HA1 (a) and NA (b). Nucleotide sequences were analyzed for phylogenetic relationships by the distance method using the PHYLIP software package. Trees were constructed by the neighbor-joining method and are rooted to Lee40. The scale indicates the number of nucleotide substitutions per site. Strain abbreviations are listed in Table 1. Reassortment viruses are boldfaced. Columns adjacent to the phylogenetic tree give the derivations of the HA and NA gene segments for each strain and the lengths of the HA and NA gene segments relative to those of Lee40.