Molecular phylogeny and evolutionary dynamics of matrix gene of avian influenza viruses in China

In China, several subtype avian influenza viruses consistently circulate in poultry. Numerous studies have focused on the evolution of the hemagglutinin gene; however, studies on the evolution of the matrix (M) gene are limited. In this study, a large-scale phylogenetic analysis of M gene sequences of avian influenza viruses isolated in China revealed that the M gene has evolved into six different lineages denoted as I-VI. The majority of lineages I and IV were isolated in terrestrial birds, while the majority of lineages II, III, V and VI were isolated in aquatic birds. Lineage I included 148 H9N2 subtype viruses (72.2%), lineage II comprised of 63 H6 subtype viruses (100%), and lineage IV included 157 H5 subtype viruses (97.5%). The mean substitution rates of different lineages ranged from 1.32×10(-3) (lineage III) to 3.64×10(-3) (lineage IV) substitutions per site per year. According to the most recent common ancestor of all lineages, lineage III was the oldest lineage, formed in 1981 or even earlier. And lineage V was the most recent, established around the year 2000. Selective pressure on M2 was stronger than that on M1. The strongest selection pressure was observed in lineage IV. In addition, site-by-site analyses of each lineage identified 8 positive selection sites, all in M2. Most of the sites (5 out of 8) were located in the extracellular domain, which is an antigen for vaccine development. The positive selection sites (amino acid positions 66, 82 and 97) are likely associated with virus budding. This study enhanced our knowledge of M gene evolution of avian influenza viruses, and is expected to improve the early detection of new viruses and lead to vaccine development.