FIG. 1.

Nucleotide phylogenetic tree for the HA gene of Sw/IN/99. The evolutionary relationship of this gene to HA genes from reference viruses was estimated by the method of maximum parsimony (PAUP software, v.4.0b2; David Swofford, Smithsonian Institution) using the tree-bisection-reconnection branch swapping algorithm and with the MULTREES option in effect. The tree shown represents the best (score = 1,385) of 9,698 rearrangements that were generated. Horizontal line distances are proportional to the minimum number of nucleotide changes needed to join nodes and gene sequences. The vertical lines are simply for spacing the branches and labels. The tree is rooted to an H3 HA sequence. The accession numbers for the reference virus sequences used in this analysis are available upon request.