Virion aggregation shapes infection dynamics and evolutionary potential.

Viral spread is classically thought to be mediated by single viral particles. However, viruses can also disseminate as aggregates, inside membranous vesicles, and as clusters bound to bacterial or complex surfaces. The implications of collective dispersal for viral infectivity and evolution remain incompletely defined. Here, we used mammalian orthoreovirus to evaluate the impact of aggregation on the propagation of infection and the generation of viral diversity through reassortment. Aggregation of free virions was induced by manipulating pH and ionic conditions. This treatment promoted coordinated delivery of viruses to cells, increasing the number of virions per infected cell and the number of virions per occupied endosome at early times of infection. Likely due to a consolidation of infectious units, aggregation concomitantly reduced the overall infectivity of the viral population and progeny virus yields. When viral populations comprised two genetically distinct viruses, aggregation increased the frequency of mixed infection and genetic exchange through reassortment. Thus, the formation of collective infectious units lowers the replicative potential of mammalian orthoreovirus populations but increases viral evolutionary potential by promoting genetic diversification.IMPORTANCEA deeper understanding of the processes shaping viral evolution will advance our ability to anticipate viral emergence, escape from immune responses, and resistance to therapeutics. Although much is known about how genetic variation fuels viral evolution, how modes of viral spread influence the generation and structure of genetic variation remains poorly characterized. Here, we examine how the collective dissemination of viruses modulates early infection dynamics and viral diversity. We find that, although infection in groups reduces the number of independently infected cells, it results in a more genetically diverse progeny population, an outcome that may enhance evolutionary potential.