Pairing of Parental Noroviruses with Unequal Competitiveness Provides a Clear Advantage for Emergence of Progeny Recombinants.

Genetic recombination plays a pivotal role in the appearance of human norovirus recombinants that cause global epidemics. However, the factors responsible for the appearance of these recombinants remains largely unknown. In this study, we revealed a selective pressure that restricts parental combinations leading to the emergence of norovirus recombinants. To investigate traces of emerging novel recombinants and their parents in the human population, we isolated mass nucleotide sequence clones of human norovirus genogroups I and II in sewage-affected waters over a 4-year sampling period. Fourteen different phylogenetic combinations of recombinants and their parents were defined from the dozens of phylogenetic lineages circulating in the human population. To evaluate the probability of these combinations, parental lineages of each recombinant were categorized into two groups as HP (relatively higher-competitiveness parents) and LP (relatively lower-competitiveness parents), according to their relative detection frequency. Strong categorization of HP and LP was confirmed by tests with modified data and additional variables. An algorithm that was developed in this study to visualize the chance of mixed infection between parents revealed that HP lineages have a higher chance of mixed infection than LP lineages in the human population. Three parental pairing types in recombinants were defined: HP-HP, HP-LP, and LP-LP. Among these, most recombinants were identified as HP-LP, despite the prediction of dominant emergence of HP-HP-type recombinants. These results suggest that nature favors recombinants of human norovirus that originate from parental pairing of heterogeneous competitiveness. Novel recombinants, generated from inter- and intraspecies recombination of norovirus lineages, often emerge and pose a threat to public health. However, the factors determining emergence of these particular recombinants from all possible combinations of parental lineages remain largely unknown. Therefore, current investigations on these recombinants are inevitably limited to postepidemic analyses, which merely identify genetic or phenotypic changes in the newly emerged recombinants compared to their parents. Here, we provide a new theoretical concept that emergence of novel recombinants could be explained by a combination of parental noroviruses thriving in the human population and those circulating at lower levels. This study could provide an additional and important rationale for the proactive environmental monitoring of potential future epidemics due to viral recombinants.