EPIDEMIOLOGICAL SIGNIFICANCE OF REASSORTMENT BETWEEN INFLUENZA C VIRUSES

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It is well known that reassortment characterized by the exchange of RNA segment between two different influenza C strains occurs in vitro at a high frequency. This study was perfomed to obtain information on how often reassortment of influenza C virus genes occurs in nature. The results obtained in these two years are summarized as follows.1.Antigenic and oligonucleotide fingerprinting analyzes of the previously isolated virus strains raised the possibility that several isolates might have arizen by reassortment from two viruses belonging to different lineages. Comparison of the partial nucleotide sequences of their seven RNA segments with those of the reference strains revealed that Nara/1/85 is a reassortant which inherited the HE and NP genes from a Mississippi/80 (MS/80) -related virus and the other five genes from a Yamagata/81-related virus, and that three strains (Yamagata/5/92, Miyagi/3/93, Miyagi/4/93) isolated during 1992/93 are all reassortants which received HE,P3, NP and M genes from an MS/80-like virus and PB2, PB1, and NS genes from a Yamagata/81-like virus.2.Phylogenetic trees for the HE and M genes were constructed by using the nucleotide sequences of 25 influenza C strains. Comparison between these two trees showed tha strains Yamagata/64, Kanagawa/1/76, and Miyagi/77 are all reassortants which received HE genes from an Aomori/74-like virus and M genes from a Yamagata/81-like virus. It also became evident that the evolutionry relationship between MS/80-like and Aichi/81-like viruses was much closer for the M gene than the HE gene, raising the possibility that these two virus groups are genetically related by a reassortment event.3.Based on these results, we suggest that reassortment of the genome between influenza C viruses occurs in nature frequently and thus contributes to genetic variation of the viruses.