Influenza virus A(H7N9) has now caused five epidemic waves of human infections in China since 2013. Avian influenza A viruses are thought to face strong selection to adapt to novel conditions when establishing themselves in humans. The authors were interested in whether adaptive evolution has occurred in human-isolated A(H7N9) viruses. They examined all of the available genomes and generated maximum likelihood trees for all eight genes. The authors identified three genes that had significant signals of positive selection. In addition, they identified a further 34 sites that had significant signals for parallel evolution in all eight genes, including seven sites (Q591K, E627K and D701N in the PB2 gene; R156K, V202A and L244Q in HA; and R289K in NA) well known to play roles in crossing species barriers for avian influenza A viruses. Therefore it does appear that A(H7N9) viruses have undergone adaptive evolution to humans and the sites where parallel evolution have occurred may play roles in crossing species barriers.
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