EVOLUTION OF THE HEMAGGLUTININ-ESTERASE GENE OF INFLUENZA-C VIRUS

The nucleotide sequences of the haemagglutinin esterase (HE) genes of 18 influenza C virus strains isolated in Japan during the period from 1964 to 1988 (11 published and 7 new sequences) were analysed to exami ne their evolutionary relationships. The phylogenetic tree constructed by the maximum parsimony method revealed the existence of four discre te lineages (I to IV), one of which (lineage III) may have died out in the late 1970s. Sequential evolution was demonstrated within seven st rains of lineage I, which allowed estimation of an evolutionary rate o f 0.49 x 10(-3) nucleotide substitutions per site per year, a value co rresponding to about one-ninth of the rates of human influenza A virus haemagglutinin genes. In the previously proposed immunodominant regio n on HE protein (positions 178 to 217), there was little or no amino a cid sequence divergence among viruses on the same lineage although con siderable divergence was seen among those on different lineages, raisi ng the possibility that immune selection may not have played a signifi cant role in the evolution of the glycoprotein, at least not after sep aration into lineages occurred. It was also found that the HE genes of the seven isolates obtained outside Japan during 1966-1983 could be e ach assigned to one of lineages I, II and IV, which suggests that infl uenza C virus is capable of spreading worldwide.