Genome segmentation, the division of the hereditary material into multiple physical units, is widespread across all domains of life. Although many viruses also have segmented genomes, some plant viruses go a step further and package each segment into a different virus particle. For plant RNA viruses, separate packaging of each genome segment imposes a massive cost, however, since all virus genome segments must enter the same cell to cause infection. The benefits of multipartition are poorly understood, and it is therefore a mystery how multipartite viruses can thrive. Recent work has shown that frequency of the different types of viral particles can change over time, and that it converges on an equilibrium. The virus particles containing different genome segments are then present at different frequencies, and there is evidence these changes might be adaptive. Moreover, this equilibrium is different for alternative host species, suggesting that multipartition might also have evolved to regulate gene expression in different environments.<br/><br/>Here, I will test the hypothesis that multipartition has evolved to regulate gene expression in a robust yet flexible manner, using a multidisciplinary approach including mathematical modeling, laboratory experiments and ecological fieldwork. Models will be used to predict the robustness and the existence of multiple genome segment frequency equilibria. Experimental evolution will be used to test whether the genome segment frequency equilibrium changes under different environmental conditions. Experiments will also be performed to test how changes in genome segment frequency affect the expression of viral proteins. Finally, fieldwork will be performed to measure genome segment frequencies in the natural virus populations. Knowledge utilization will focus on a two-way interaction with plant inspection services, seed producers and biotechnology companies, to develop more effectively ways to monitor multipartite viruses and better approaches for engineering resistance against viruses in plants.<br/>