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Genetics, Vol. 168, 9-19, September 2004, Copyright © 2004
doi:10.1534/genetics.104.030205
Co-infection Weakens Selection Against Epistatic Mutations in RNA Viruses
Rémy Froissart*,1,
Claus O. Wilke
,
,
Rebecca Montville*,
Susanna K. Remold*,
Lin Chao
and
Paul E. Turner*,2
* Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut 06520
Digital Life Laboratory 136-93, California Institute of Technology, Pasadena, California 91125
Keck Graduate Institute, Claremont, California 91711
Section of Ecology, Behavior and Evolution, Division of Biology, University of California, San Diego, California 92093
2 Corresponding author: Dept. of EEB, Science Area Receiving, 266 Whitney Ave., Yale University, New Haven, CT 06520.
E-mail: paul.turner{at}yale.edu
Co-infection may be beneficial in large populations of viruses because it permits sexual exchange between viruses that is useful in combating the mutational load. This advantage of sex should be especially substantial when mutations interact through negative epistasis. In contrast, co-infection may be detrimental because it allows virus complementation, where inferior genotypes profit from superior virus products available within the cell. The RNA bacteriophage 
6 features a genome divided into three segments. Co-infection by multiple 6 genotypes produces hybrids containing reassorted mixtures of the parental segments. We imposed a mutational load on 6 populations by mixing the wild-type virus with three single mutants, each harboring a deleterious mutation on a different one of the three virus segments. We then contrasted the speed at which these epistatic mutations were removed from virus populations in the presence and absence of co-infection. If sex is a stronger force, we predicted that the load should be purged faster in the presence of co-infection. In contrast, if complementation is more important we hypothesized that mutations would be eliminated faster in the absence of co-infection. We found that the load was purged faster in the absence of co-infection, which suggests that the disadvantages of complementation can outweigh the benefits of sex, even in the presence of negative epistasis. We discuss our results in light of virus disease management and the evolutionary advantage of haploidy in biological populations.
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