A preliminary panorama of the diversity of N1 subtype influenza viruses.

N1 subtype influenza viruses have caused many epidemics and even a few pandemics in humans, pigs and fowls including 1918 human H1N1 pandemic, which killed 20-50 million people and the current avian H5N1 pandemic in the Eastern Hemisphere, which has caused great economic losses and posed a severe threat to human public health. To elucidate the whole diversity of N1 influenza viruses from a dynamic view, 202 neuraminidase (NA) sequences of N1 subtype influenza isolates were selected and analyzed in this study. Our results showed that N1 influenza isolates could be divided into three distinct lineages (Human, Classic Swine and Avian), which largely circulated in the humans, pigs and fowls respectively, though viruses in the Avian lineage could infect mammals and even there was a sublineage in the Avian lineage wholly isolated from pigs. The Avian lineage and the Human lineage, which have existed at least for decades, possibly began divergence around in 1890 through regression analysis. Both of the Human and Avian lineages could be further divided into some sublineages, and the correlation between these lineages (or sublineages) and their isolation places, isolation time, hemagglutinin (HA) subtypes, host species, virulence, or epidemics were discussed. The panorama of the diversity of N1 influenza viruses presented in this study provided a framework for the studies on the evolution and epidemiology of N1 influenza viruses.