Evolution of influenza virus genes

The nucleotide sequences of the eight different influenza A virus segments(genes) were compared among 14 different subtypes. These comparisonsdemonstrate the presence of molecular clocks in the viral genes; theyaccumulated both silent and amino acid-changing substitutions atapproximately constant rates with respect to time during evolution. Inaddition, comparison of the rates of evolution among the eight viral genes,excluding the P2 gene, revealed a rapid and roughly equal rate of silentsubstitution for different genes. The P2 gene exception is explained as theresult of recombination (reassortment) between distantly related strains.The rate of amino acid-changing substitution differs greatly from gene togene. The rate of silent substitution was estimated to be 1.1 X10(-2)/site/year on the average--that is, about 2 X 10(6) times higher thaneukaryotic gene equivalents, which is remarkable. Strain A/USSR/90/77 wasshown to evolve with a rate that is similar to those of other strains butto behave as if replication was frozen during a certain period (Nakajima etal. 1978). The frozen period was estimated to be 25 yr on the basis of themolecular clock. A similar analysis revealed another example of frozenreplication--in this case, apparently for a period of about 9 yr- -in aduck strain, A/duck/Ontario/77.