Frank, S. A. 2001. Multiplicity of infection and the evolution of hybrid incompatibility in segmented viruses. Heredity 87:522-529.

Some viral genomes are divided into segments. When multiple viruses infect a single cell, progeny form by reassorted mixtures of genomic segments. Hybrid incompatibilities arise when a progeny virus has incompatible segments from different parental viruses. Hybrid incompatibility has been observed in influenza and in the multiparticle plant virus Dianthovirus. Hybrid incompatibility provides an opportunity to study rates of viral evolution, divergence and speciation, and the extent of epistatic interactions among components of the viral genome. I used mathematical and computer simulation models to study hybrid incompatibility between diverging strains. The models identify multiplicity of infection as a key factor. When many viral particles infect each host cell, the effective ploidy of the genetic system is high. High ploidy dilutes the contribution of each locus to the phenotype, weakening the selective intensity on each locus. Weaker selection on variant alleles allows the population to maintain greater genetic diversity and to be more easily perturbed by stochastic fluctuations. Greater diversity and stochastic fluctuations explore more widely the space of epistatic interactions, causing more frequent shifts among favoured combinations of alleles. Variable ploidy of viral genetics differs from standard Mendelian genetics.