Description
Many viruses have segmented genomes, although the different genome segments are usually packaged into a single virus particle. Some viruses take genome segmentation a step further, and package each individual segment into a different virus particle. These different and physically separated virus particles must then reach a new cell for the virus to be successfully transmitted. We call these viruses multipartite (or multi-component) viruses, and this genome and virus particle organization comes at a huge cost: a predicted inefficient transmission of the virus, both within and between hosts. Why has strange genome organization arisen? Here, we explore this issue for a tripartite plant RNA virus, Alfalfa mosaic virus. First, we revisit classical virological work and consider infection kinetics in detail, documenting that multipartition does indeed impose a huge cost on virus infectivity. Second, we find that the different genome segments are not present at the same frequency, similar to observations with nanoviruses. When perturbed the frequency of these segments converges on a stable equilibrium, which is host-species dependent. These observations suggest that changes in the frequency of genome segments might be a strategy for adapting to different environments, for example because they lead to differences in expression of viral genes. Third, models of virus evolution show the importance of genetic bottlenecks for adaptation by changes in the frequency of genome segments. These bottlenecks generate the variation in segment frequencies on which selection can then act, which allows for rapid, “mutation-free” adaptation to different environments. Finally, we use these models of virus evolution to consider how heterogeneous environments must be to favour a multipartitie over a monopartite genome organization. Our results suggest multipartition might be a novel mechanism by which viruses adapt to changing environments, with parallels to the role of true copy number variation in other organisms.| Period | 23 Aug 2018 |
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| Degree of Recognition | International |