Little evidence for synergism among deleterious mutations in a nonsegmented RNA virus

Several models have been proposed to account for the segmentation of RNA vi ruses. One of the best known models suggests that segmentation, and mixing of segments during coinfections, is a way to eliminate deleterious mutation s from the genome. However, for validity, this model requires that deleteri ous mutations interact in a synergistic way. That is, two mutations togethe r should have a more deleterious effect than the result of adding their ind ividual effects. Here I present evidence that deleterious mutations in foot -and-mouth disease virus produce a decline in fitness but that the relation ship between the number of mutations fixed and the magnitude of fitness dec line is compatible mainly with a nonsynergistic model. However, the statist ical uncertainties associated with the data still give some room for the ex istence of very weak synergistic epistasis.