Reassortment and insertion-deletion are strategies for the evolution of influenza B viruses in nature

The evolution of influenza B viruses is poorly understood. Reassortment of influenza B viruses in nature as a means of genetic variation has not been considered to be a major contributor to their evolution. However, the curre nt practice of assigning evolutionary relationships by antigenic analysis o f the hemagglutinin of influenza B viruses would fail to detect reassortant . In this study, influenza B viruses isolated within the past 10 years from sites in the United States and China were studied by nucleotide sequencing of the hemagglutinin and neuraminidase genes and construction of phylogene tic trees to assess evolutionary relationships. A group of viruses represen ted by B/Housto/1/92 possess a hemagglutinin derived from a B/Yamagata/16/8 8-like strain and a neuraminidase derived from a B/Victoria/2/87-like strai n. A second reassortment event between the hemagglutinin of a B/Yamagata/16 /88-like virus closely related to the B/Beijing/184/93 strain and the neura minidase of a B/Victoria/2/87-like strain is represented by a single virus, B/Memphis/3/93. The neuraminidase of the reassortant viruses is most close ly related to that of B/Victoria/2/87-like viruses currently circulating in Nanchang, China. A pattern of insertions and deletions in the hemagglutini n and the neuraminidase of different strains of influenza B viruses is obse rved. Reassortment plays a role in the evolution of influenza B viruses and may necessitate a change in the methods used to assess and identify new in fluenza viruses.