PHYLOGENETIC ANALYSIS OF THE ENTIRE GENOME OF INFLUENZA-A (H3N2) VIRUSES FROM JAPAN - EVIDENCE FOR GENETIC REASSORTMENT OF THE 6 INTERNAL GENES

Nucleotide sequences of all eight RNA segments of 10 human H3N2 influe nza viruses isolated during a 5-year period from 1993 to 1997 were det ermined and analyzed phylogenetically in order to define the evolution ary pathways of all genes in a parallel fashion. It was evident that t he hemagglutinin and neuraminidase genes of these viruses evolved esse ntially in a single lineage and that amino acid changes accumulated se quentially with respect to time. In contrast, amino acid differences i n the internal proteins were erratic and did not accumulate over time. Parallel analysis of the phylogenetic patterns of all genes revealed that the evolutionary pathways of the six internal genes were not link ed to the surface glycoproteins. Genes coding for the basic polymerase -l, nucleoprotein, and matrix proteins of 1997 isolates were closest p hylogenetically to those of earlier isolates of 1993 and 1994. Further more, all six internal genes of four viruses isolated in the 1995 epid emic season consistently divided into two distinct branch clusters, an d two 1995 isolates contained PB2 genes apparently originating from th ose of viruses before 1993. It was apparent that the lack of correlati on between the topologies of the phylogenetic trees of the genes codin g for the surface glycoproteins and internal proteins was a reflection of genetic reassortment among human H3N2 viruses. This is the first e vidence demonstrating the occurrence of genetic reassortment involving the internal genes of human H3N2 viruses. Furthermore, internal prote in variability coincided with marked increases in the activity of H3N2 viruses in 1995 and 1997.