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

Abstract

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 current practice of assigning evolutionary relationships by antigenic analysis of the hemagglutinin of influenza B viruses would fail to detect reassortants. 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 phylogenetic trees to assess evolutionary relationships. A group of viruses represented by B/Houston/1/92 possess a hemagglutinin derived from a B/Yamagata/16/88-like strain and a neuraminidase derived from a B/Victoria/2/87-like strain. 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 neuraminidase 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 closely related to that of B/Victoria/2/87-like viruses currently circulating in Nanchang, China. A pattern of insertions and deletions in the hemagglutinin and the neuraminidase of different strains of influenza B viruses is observed. 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 influenza viruses.

FIG. 1

Evidence for reassortment of influenza B viruses Hou92 and Mem93 by comparison of the amino acid sequences of their HAs and NAs with sequences from Pan90 (Yam88-like), Mem1297 (Yam88-like), and Nan97 (Vic87-like). Numbering is relative to that of Lee40. Amino acids contained in or identical to those in Hou92 are shaded to indicate conservation of the sequence. ^a, Vic87-like viruses isolated prior to 1990 have a deletion at this position.

FIG. 2

Phylogenetic tree of influenza B virus gene segments HA1 (a) and NA (b). Nucleotide sequences were analyzed for phylogenetic relationships by the distance method using the PHYLIP software package. Trees were constructed by the neighbor-joining method and are rooted to Lee40. The scale indicates the number of nucleotide substitutions per site. Strain abbreviations are listed in Table 1. Reassortment viruses are boldfaced. Columns adjacent to the phylogenetic tree give the derivations of the HA and NA gene segments for each strain and the lengths of the HA and NA gene segments relative to those of Lee40.