Evolutionary dynamics of non-GII genotype 4 (GII.4) noroviruses reveal limited and independent diversification of variants.

Noroviruses are extremely diverse, with ≥30 genotypes infecting humans. GII genotype 4 (GII.4) noroviruses, the most prevalent genotype, present a constant accumulation of mutations on the major capsid protein (VP1), resulting in the chronological emergence of new variants every 2-8 years. On the other hand, non-GII.4 noroviruses present a limited number of changes on the capsid protein over time. Despite limited diversification, non-GII.4 viruses can also be associated with large outbreaks. To gain insights into the evolutionary dynamics of non-GII.4 viruses, we performed variant-specific phylogenetic analyses on a comprehensive dataset of 13 genotypes. Although the genotypes with a single variant presented a linear (clock-like) evolution, maximum-likelihood analyses revealed a lack of clock-like signals for the genotypes with ≥3 variants: GI.3, GII.6 and GII.17. Notably, the evolutionary pattern of non-GII.4 viruses showed clock-like signals when each variant was analysed separately. A minimal impact on the long-term clock-like evolution of VP1 was detected due to the exchange (recombination) of the polymerase types. The linear evolution, without replacement among variants, is explained by minimal changes at the protein level due to the higher ratio of synonymous compared to non-synonymous substitutions in their evolution. Taken together, these data indicate that (i) the variants of non-GII.4 noroviruses evolve and persist in the population independently, probably due to strong evolutionary constraints on VP1, and (ii) variant-specific analyses with robust sequence databases that cover long periods of surveillance are needed to limit the potential for misinterpretation of the evolutionary dynamics of non-GII.4 noroviruses.