Replication at periodically changing multiplicity of infection promotes   stable coexistence of competing viral populations
  RNA viruses are a widely used tool to study evolution experimentally. Many standard protocols of virus propagation and competition are done at nominally low multiplicity of infection (m.o.i.), but lead during one passage to two or more rounds of infection, of which the later ones are at high m.o.i. Here, we develop a model of the competition between wild type (wt) and a mutant under a regime of alternating m.o.i. We assume that the mutant is deleterious when it infects cells on its own, but derives a selective advantage when rare and coinfecting with wt, because it can profit from superior protein products created by the wt. We find that, under these assumptions, replication at alternating low and high m.o.i. may lead to the stable coexistence of wt and mutant for a wide range of parameter settings. The predictions of our model are consistent with earlier observations of frequency-dependent selection in VSV and HIV-1. Our results suggest that frequency-dependent selection may be common in typical evolution experiments with viruses. 
Claus O. Wilke (Caltech), Daniel D. Reissig (Medical College Ohio),   and Isabel S. Novella (Medical College Ohio)