Distinct lineages of influenza virus H4 hemagglutinin genes in different regions of the world.

To understand the determinants of influenza virus evolution, phylogenetic relationships were determined for nine hemagglutinin (HA) genes of the H4 subtype. These genes belong to a set of viruses isolated from several avian and mammalian species from various geographic locations around the world between 1956 and 1985. We found that the HA gene of the H4 subtype is 1738 nucleotides in length and is predicted to encode a polypeptide of 564 amino acids. The connecting peptide, which is removed from the precursor polypeptide by peptidases to yield the mature HA1 and HA2 polypeptides, contains only one basic amino acid. This type of connecting peptide is a feature of all avian avirulent HAs. On the basis of pairwise nucleotide sequence homology comparisons the genes can be segregated into two groups: influenza virus genes isolated in North America and those isolated from other parts of the world. A high degree of homology exists between pairs of genes from viruses of similar geographic origin. The nucleotide sequences within a group differ by 1.5 to 10.6%; in contrast, between groups the differences range from 15.8 to 19.4%. An evolutionary tree for the nine sequences suggests that North American isolates have diverged extensively from those circulating in other parts of the world. Geographic barriers which determine flyway outlay may prevent the gene pools from extensive mixing. The lack of correlation between date of isolation and evolutionary distance suggests that different H4 HA genes cocirculate in a fashion similar to avian H3 HA genes (H. Kida et al., 1987, Virology 159, 109-119) and influenza C genes (D. Buonagurio et al., 1985, Virology 146, 221-232) implying the absence of selective pressure by antibody that would give a significant advantage to antigenic variants. In contrast to avian influenza virus genes, human influenza virus genes evolve rapidly under the selective pressure of antibody.