EVOLUTIONARY ANALYSIS OF INFLUENZA-C VIRUS M-GENE

The previous study of the 25 hemagglutinin-esterase (HE) glycoprotein genes of influenza C viruses identified four discrete lineages represe nted by C/Yamagata/26/81, C/Aichi/1/81, C/Aomori/74 and C/Mississippi/ 80, respectively. Here we compared the M gene sequence among the 24 vi ruses isolated between 1964 and 1991. A phylogenetic analysis showed t hat these genes have evolved into three distinct lineages. Lineage I i ncluded most of viruses with the HE genes of C/Y amagata/26/81-related lineage. The predominant members of lineage II were viruses having th e HE genes of either C/Aichi/1/81- or C/Mississippi/80-related lineage . Lineage III contained only C/Aomori/74. Phylogenetic positions of se veral strains (C/Yamagata/64, C/Kanagawa/1/76, C/Miyagi/77 and C/Nara/ 1/85) were different between the M and HE gene trees, suggesting that they are reassortants. Furthermore, phylogenetic relationships between C/Mississippi/80-like and C/Aichi/1/81-like viruses were much closer for the M gene than the HE gene, raising the possibility that these tw o virus groups are genetically related by a reassortment event. Nucleo tide changes in the M genes occurred at about 7% positions with a unif orm distribution throughout the molecules. However, the predicted amin o acid sequence of the matrix protein (M1) was conserved almost comple tely among the isolates analyzed. The amino acid sequence of the secon d protein (CM2) encoded by M gene was also highly conserved, but was m ore divergent than the M1 protein sequence, suggesting that the two M gene products are evolving differently in response to selective pressu res or structural and functional constraints.