Segmentation of viral genomes allows exchange of intact genes between related viruses when they coinfect the same cell (Fig 1). This exchange is a type of recombination called reassortment. Classical recombination involves the joining of nucleic acid sequences derived from two different templates into one chimeric product. During reassortment, however, entire gene segments are swapped to give rise to chimeric genomes. In both cases, novel genotypes are formed, giving the potential for viruses to evolve. As with genetic change through mutation, most reassortment events yield progeny viruses that are less fit than either parent (Fig 2). Occasionally, however, reassortment gives rise to a combination of genes particularly well suited to a given set of selection pressures, and increased fitness results. Open in a separate window Fig 1 Reassortment requires viruses to meet on multiple scales. For reassortment to occur between viruses of two distinct genotypes, these viruses must infect the same host (A) and the same tissue within that host (B). Either the inoculating viruses or their progeny must come together within the same cell (C). Finally, the coinfecting viral genomes must mix within the coinfected cell, and replicated segments must be copackaged, processes which may be limited by compartmentalization of viral replication and selectivity of genome incorporation, respectively (D). When all of these criteria are met, progeny viruses of both reassortant and parental viral genotypes will emerge from the cell (E).