Mammalian orthoreovirus reassortment proceeds with little constraint on segment mixing

Abstract

Segmentation of viral genomes gives the potential for genetic exchange within co-infected cells. However, for this potential to be realized, co-infecting genomes must mix during the viral lifecycle. The efficiency of reassortment in turn dictates its potential to drive evolution. The opportunity for mixing within co-infected cells may vary greatly across virus families, such that the evolutionary implications of genome segmentation differ as a result of core features of the viral lifecycle. To investigate the relationship between viral replication compartments and genetic exchange, we quantified reassortment in mammalian orthoreovirus (reovirus). Reoviruses carry a 10-segmented, double-stranded RNA genome, which is replicated within proteinaceous structures termed inclusion bodies. We hypothesized that inclusions impose a barrier to reassortment. We quantified reassortment between wild-type ( wt ) and variant ( var ) reoviruses that differ by one nucleotide per segment. Wt/var systems in both T1L and T3D backgrounds revealed frequent reassortment without bias towards particular genotypes. However, reassortment was more efficient in the T3D serotype. Since T1L and T3D viruses exhibit different inclusion body morphologies, we tested the impact of this phenotype on reassortment. In both serotypes, reassortment levels did not differ by inclusion morphology. Reasoning that the merging of viral inclusions may be critical for genome mixing, we then tested the effect of blocking merging. Reassortment proceeded efficiently even under these conditions. Our findings indicate that reovirus reassortment is highly efficient despite the localization of many viral processes to inclusion bodies, and that the robustness of this genetic exchange is independent of inclusion body structure and fusion. Importance Quantification of reassortment in diverse viral systems is critical to elucidate the implications of genome segmentation for viral evolution. In principle, genome segmentation offers a facile means of genetic exchange between coinfecting viruses. In practice, there may be physical barriers within the cell that limit mixing of viral genomes. Here, we tested the hypothesis that localization of the various stages of the mammalian orthoreovirus lifecycle within cytoplasmic inclusion bodies compartmentalizes viral replication and limits genetic exchange. Contrary to this hypothesis, our data indicate that reovirus reassortment occurs readily within co-infected cells and is not strongly affected by the structure or dynamics of viral inclusion bodies. We conclude that the potential for reassortment to contribute to reovirus evolution is high.