Host origin is a determinant of parallel evolution between influenza virus gene segments.

Several emerging influenza viruses, including H7N9 and H5N6 viruses, trace their origins to reassortment with H9N2 viruses that contributed internal gene segments. However, the evolutionary constraints governing reassortment of H9N2 viruses remain unknown. In seasonal human influenza A viruses, gene segments evolve in parallel at both the gene and protein levels. Here, we demonstrate that parallel evolution in human H3N2 viruses differs from avian H9 viruses, with both genes and proteins of avian H9 viruses characterized by high phylogenetic divergence. Strikingly, protein trees corresponding to avian H9 polymerase subunits diverge despite known functional constraints on polymerase evolution. Gene divergence was consistent across avian H9 isolates from different continents, suggesting that parallel evolution between H9 gene segments is not dependent on regionally defined lineages. Instead, parallel evolution in H9 viruses was dependent upon host origin. Our study reveals the role of the host in parallel evolution of influenza gene segments and suggests that high reassortment potential in avian species may be a consequence of evolutionary flexibility between gene segments. ### Competing Interest Statement The authors have declared no competing interest.