Influenza B virus evolution: co-circulating lineages and comparison of evolutionary pattern with those of influenza A...

Sequence analyses and comparison of the genes coding for the nonstructural (NS) and hemagglutinin (HA) proteins of different influenza B viruses isolated between 1940 and 1987 reveal that the number of substitutions is not always proportional to the time between isolates. Examination of 14 influenza B virus NS gene and 10 HA gene sequences by the maximum parsimony method suggested that--as with influenza C viruses--there are multiple evolutionary lineages which can coexist for considerable periods of time. Comparison of the sequence divergence among genes of viruses belonging to type A, B, and C virus suggests that, in man, influenza B viruses evolve slower than A viruses and faster than C viruses. We propose an evolutionary model for influenza B viruses that is intermediate between the pattern for human influenza A viruses and that for influenza C viruses.