ORIGIN AND EVOLUTIONARY PATHWAYS OF THE H1 HEMAGGLUTININ GENE OF AVIAN, SWINE AND HUMAN INFLUENZA-VIRUSES - COCIRCULATION OF 2 DISTINCT LINEAGES OF SWINE VIRUS

The nucleotide sequences of the HA1 domain of the H1 hemagglutinin gen es of A/duck/Hong Kong/36/76, A/duck/Hong Kong/196/77, A/sw/ North Ire land/38, A/sw/Cambridge/39 and A/Yamagata/120/86 viruses were determin ed, and their evolutionary relationships were compared with those of p reviously sequenced hemagglutinin (H1) genes from avian, swine and hum an influenza viruses. A pairwise comparison of the nucleotide sequence s revealed that the genes can be segregated into three groups, the avi an, swine and human virus groups. With the exception of two swine stra ins isolated in the 1930s, a high degree of nucleotide sequence homolo gy exists within the group. Two phylogenetic trees constructed from th e substitutions at the synonymous site and the third codon position sh owed that the H1 hemagglutinin genes can be divided into three host-sp ecific lineages. Examination of 21 hemagglutinin genes from the human and swine viruses revealed that two distinct lineages are present in t he swine population. The swine strains, sw/North IreIand/38 and sw/Cam bridge/39, are clearly on the human lineage, suggesting that they orig inate from a human A/WSN/33-like variant. However, the classic swine s train, sw/Iowa/15/30, and the contemporary human viruses are not direc t descendants of the 1918 human pandemic strain, but did diverge from a common ancestral virus around 1905. Furthermore, previous to this th e above mammalian viruses diverged from the lineage containing the avi an viruses at about 1880.