Genetic characterizations and molecular evolution of human norovirus GII.6 genotype during the past five decades.

The norovirus genotype GII.6 is circulating in the population with a relatively high prevalence, but in-depth molecular characterization studies of GII.6 are needed. In this study, norovirus GII.6 sequences were retrieved and analyzed to demonstrate the molecular characterizations of norovirus GII.6. The results show that GII.6 VP1 gene can be divided into three variants and all variants cocirculated in humans in the past decades. The intragenotypic showed no growth trend over time. Since the overall evolutionary rate was 3.432 × 10-3 substitutions/site/year, the infer most recent common ancestor was estimated as 1913. Only a few amino acid sites were recognized to be under positive selection pressure. The mean effective population size was stable in the recent years. The variant C (especially 87 GII.P7-GII.6 strains) had a higher evolutionary rate and more sites under positive selection pressure compared to other variants. NS4 protein had the higher diversity than other nonstructural proteins, and VP1 and VP2 genes had the same phylogenetic relationships. This study provides a systematic description of genetic characterizations and molecular evolution of GII.6. Research on norovirus molecular epidemiology should be pursued to enrich the genomic data of the diverse genotypes and improve their analysis.