ELSEVIER Virus Research 55 (1998) 143-156 Virus Research Influence of host species on the evolution of the non structural (NS) gene of influenza A viruses Yoshihiro Kawaoka Owen T. Gorman c, Toshihiro Ito d, Krisna Wells a, Ruben O. Donis e, Maria R. Castrucci f, Isabella Donatelli f, Robert G. Webster a,b a Department of Virology/Molecular Biology, St. Jude Children's Research Hospital, 332 North Lauderdale, Memphis, TN 38105, USA b Department of Pathology, University of Tennessee, Memphis, 800 N. Madison Avenue, Memphis, TN 38163, USA C US Fish and Wildlife Service, Flagstaff, AZ 86001, USA d Department of Veterinary Public Health, Faculty of Agriculture, Tottori University, Tottori 680, Japan e Department of Veterinary and Biomedical Sciences, University of Nebraska-Lincoln, Lincoln, NE 68583-0905, USA f Department of Virology, Instituto Superiore di Sanita, 299 v.le Regina Elena, 00161 Rome, Italy Received 12 December 1997; received in revised form 20 March 1998; accepted 20 March 1998 Abstract The matrix (M) and nonstructural (NS) genes of influenza A viruses each encode two overlapping proteins. In the M gene, evolution of one protein affects that of the other. To determine whether or not this evolutionary influence operating between the two M proteins also occurs in the NS gene, we sequenced the NS genes of 36 influenza A viruses isolated from a broad spectrum of animal species (wild and domestic birds, horses, pigs, humans, and sea mammals) and analyzed them phylogenetically, together with other previously published sequences. These analyses enabled us to conclude the following host species-related points that are not found in the other influenza A virus genes and their gene products. (I) The evolution of the two overlapping proteins encoded by the NS gene are lineage-dependent, unlike the M gene where evolutionary constraints on the MI protein affect the evolution of the M2 protein (Ito et aI., J. Virol. 65 (1991) 5491-5498). (2) The gull-specific lineage contained nonH13 gull viruses and the non-gull avian lineage contained H13 gull viruses, indicating that the gull-specific lineage does not link to the H13 HA subtype in the NS gene unlike findings with other genes. (3) The branching topology of the recent equine lineage (H7N7 viruses isolated after 1973 and H3N8) indicates recent introduction of the NS, M, and PB2 genes into horses from avian sources by genetic reassortment. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Influenza virus; NS gene; Evolution * Corresponding author. Present address: Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive West, Madison, WI 53706, USA. Tel.: + I 608 2654925; fax: + I 608 2655622; e-mail: kawaokay@svm.vetmed.wisc.edu 0168-1702/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PI! SO 168-1702(98)00038-0 .