Phylogenetic analysis of the N8 neuraminidase gene of influenza A viruses.

Phylogenetic analysis of the N8 neuraminidase (NA) genes from 18 influenza A viruses, representing equine and avian hosts in different geographic locations, revealed three major lineages: (i) currently circulating equine 2 viruses; (ii) avian viruses isolated in the Eurasian region, including A/Equine/Jilin/1/89, a recent avian-like N8 isolate found in horses in China; and (iii) avian viruses isolated in North America. Comparison of mutation rates indicated that avian N8 genes have evolved more slowly than their equine counterparts. That is, in both avian lineages, 72% of the nucleotide changes were silent in the terminal branches of the phylogenetic tree, whereas in equine 2 viruses, 59% of the nucleotide changes were silent. This suggests greater selective pressure on the NA gene from the mammalian immune system, leading to progressive evolution. Alternatively, the slower mutation rate for avian N8 genes could reflect a selective advantage gained from a longer, continuous span of evolution. The shape of the phylogenetic tree, the evolutionary rate, and the calculated date of origin for the N8 equine 2 virus lineage were comparable to findings for the equine 2 virus hemagglutinin (HA) gene (Bean et al., J. Virol. 66, 1129-1138, 1992). This suggests that both viral membrane glycoproteins of equine 2 viruses have evolved together and have been subjected to similar levels of selective pressure. Several amino acid residues were found to differ among the three host-specific lineages, but they may not be involved in host restriction of the NA, as they are shared by EQ/Jilin/1/89 and viruses of avian origin. The present findings complement detailed structural information on the N2 and N9 subtypes and should prove valuable in understanding future X-ray diffraction studies of N8 crystals.