Phylogenetic analysis of the three polymerase genes (PB1, PB2 and PA) of influenza B virus

Phylogenetic patterns of the three polymerase (PB2, PB1 and PA) genes of a total of 20 influenza B viruses isolated during a 58 year period, 1940-1998 , were analysed in detail in a parallel manner. All three polymerase genes consistently showed evolutionary divergence into two major distinct lineage s and their amino acid profiles demonstrated conserved lineage-specific sub stitutions. Dendrogram topologies of the PB2 and PB1 genes were very simila r and contrasted with that of the PA gene. It was of particular interest to reveal that even though the PA gene evolved into two major lineages, that of three recent Asian Victoria/1/87-like strains formed a branch cluster lo cated in the same lineage as that of recent Yamagata/16/88-like isolates. D ifferences in the phylogenetic pathways of three polymerase genes were not only a reflection of genetic reassortment between co-circulating influenza B viruses, but also an indication that the polymerase genes were not co-evo lving as a unit. As a result, comparison of the phylogenetic patterns of th e three polymerase genes with previously determined patterns of the HA, NP, M and NS genes of 18 viruses defined the existence of eight distinct genom e constellations, Also, similar phylogenetic profiles among the PA, NP and M genes, as well as between the PB2 and PB1 genes, were observed, suggestin g possible functional interactions among these proteins. Completion of evol utionary analysis of the six internal genes and the HA gene of influenza B viruses revealed frequent genetic reassortment among co-circulating variabl e strains and suggested co-dependent evolution of genes.