Independence of evolutionary and mutational rates after transmission of avian influenza viruses to swine

In 1979, an H1N1 avian influenza virus crossed the species barrier, establi shing a new lineage in European swine. Because there is no direct or serolo gic evidence of previous H1N1 strains in these pigs, these isolates provide a model for studying early evolution of influenza viruses. The evolutionar y rates of both the coding and noncoding changes of the H1N1 swine strains are higher than those of human and classic swine influenza A viruses. In ad dition, early H1N1 swine isolates show a marked plaque heterogeneity that c onsistently appears after a few passages. The presence of a mutator mutatio n was postulated (C. Scholtissek, S. Ludwig, and W. M. Fitch, Arch. Virol, 131:237-250, 1993) to account for these observations and the successful est ablishment of an avian H1N1 strain in swine, To address this question, we c alculated the mutation rates of A/Mallard/New York/ 6750/78 (H2N2) and A/Sw ine/Germany/2/81 (H1N1) by using the frequency of amantadine-resistant muta nts. To account for the inherent variability of estimated mutation rates, w e used a probabilistic model for the statistical analysis. The resulting es timated mutation rates of the two strains were not significantly different. Therefore, an increased mutation rate due to the presence of a mutator mut ation is unlikely to have led to the successful introduction of avian H1N1 viruses in European swine.