Genetic exchanges are one of the main mechanisms generating within-population diversity that selection can act on. Even though some extremely successful genotypes may be thus produced, many deleterious genotypes are also produced; the net balance that could result from systematic experimental gene exchange between two distinct genotypes is unknown, and this is true for any organisms. This project aims at filling this gap by using as model system multipartite viruses, whose genome is divided in several segments packaged independently, each viral particle carrying only one segment and each segment encoding only one gene. Multipartite viruses can exchange entire genomic segments, an additional mechanism of genomic shuffling termed reassortment. Using the octopartite nanovirus faba bean necrotic stunt virus (FBNSV), a parasite of legume plants, we will measure several viral fitness components for intra- and inter-specific single segment (and thus single gene) reassortants. This will allow us to estimate whether reassorting is on average beneficial/detrimental and whether specific viral genes/functions are more likely to produce beneficial reassortants. Competition experiments between reassorting and parental-genotypes will show whether mechanisms favoring species/genotype genomic integrity are at play, and experimental evolution will show whether deleterious effects of reassortment may be alleviated during coevolution of reassorting segments with the rest of the genome. Finally, transmission experiments will investigate whether reassorting may facilitate the transmission of multipartite viruses by allowing for the non-concomitant transmission of different genomic segments. This project may thus provide (i) the first systematic characterization of the fitness and biological trait effects of reassortment; (ii) a potential mechanism through which the between-host transmission cost of multipartitism may be alleviated.