INFLUENCE OF HOST SPECIES ON THE EVOLUTION OF THE NONSTRUCTURAL (NS) GENE OF INFLUENZA-A VIRUSES

The matrix (M) and nonstructural (NS) genes of influenza A viruses eac h encode two overlapping proteins. In the M gene, evolution of one pro tein affects that of the other, To determine whether or not this evolu tionary influence operating between the two M proteins also occurs in the NS gene, we sequenced the NS genes of 36 influenza A viruses isola ted from a broad spectrum of animal species (wild and domestic birds, horses, pigs, humans, and sea mammals) and analyzed them phylogenetica lly, together with other previously published sequences. These analyse s enabled us to conclude the following host species-related points tha t are nor found in the other influenza A virus genes and their gene pr oducts. (1) The evolution of the two overlapping proteins encoded by t he NS gene are lineage-dependent, unlike the M gene where evolutionary constraints on the M1 protein affect the evolution of the M2 protein (Ito et al,, J. Virol. 65 (1991) 5491-5498). (2) The gull-specific lin eage contained nonH13 gull viruses and the non-gull avian lineage cont ained H13 gull viruses, indicating that the gull-specific lineage does not link to the H13 HA subtype in the NS gene unlike findings with ot her genes. (3) The branching topology of the recent equine lineage (H7 N7 viruses isolated after 1973 and H3N8) indicates recent introduction of the NS, M, and PB2 genes into horses from avian sources by genetic reassortment. (C) 1998 Elsevier Science B.V. All rights reserved.