FIG. 2.

Nucleotide phylogenetic trees for the NA (A), M (B), and PA (C) genes of Sw/IN/99. The evolutionary relationships were estimated by the method of maximum parsimony as described in the legend to Fig. 1. The score and number of rearrangements for each tree are as follows: NA, score = 2,011 of 475,664 rearrangements; M, score = 1,020 of 10,892,448 rearrangements; and PA, score = 3,850 of 2,345,030 rearrangements. The reassortant H3N2 viruses isolated from pigs in the Midwestern United States since 1998 (16a, 43) are marked by an asterisk. The NA tree is rooted to an influenza A virus of a different subtype (N1), whereas the M and PA trees are rooted to influenza B viruses.