Molecular evolution of novel swine-origin A/H1N1 influenza viruses among and before human

We find that the novel A/H1N1 influenza viruses exhibit very low genetic divergence and suffer strong purifying selection among human population and confirm that they originated from the reassortment of previous triple-reassortant swine influenza viruses including genomic segments from both avian and human lineages with North American and Eurasian swine lineages. The longer phylogenetic branch length to their nearest genetic neighbors indicates that the origin of the novel A/H1N1 is unlikely to be a very recent event. Seventy-six new unique mutations are found to be monomorphically fixed in the novel A/H1N1 virus lineages, suggesting a role of selective sweep in the early evolution of this virus. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11262-009-0393-7) contains supplementary material, which is available to authorized users.