[Evolutionary analysis of the hemagglutinin-esterase (HE) gene of influenza C virus]

The phylogenetic tree constructed with the nucleotide sequences of the hemagglutinin-esterase (HE) genes of 25 influenza C strains isolated during the period from 1964 to 1988 revealed the existence of four discrete lineages. The evolutionary rate of HE gene was estimated to be 0.49 x 10(-3) substitutions per site per year. In the immunodominant region on HE protein, there was little or no amino acid sequence divergence among viruses on the same lineage, raising the possibility that immune selection may not have played a significant role in the evolution of HE after separation into lineages has occurred. The potential role of pigs in influenza C virus ecology remains to be clarified. Evidence was obtained, however, which suggests, that interspecies transmission of the virus between humans and pigs has occurred in nature.