Most of the viral gene products are shared among a viral population in a host cell, which accumulates up to $10^6$ to $10^7$ genomes. High mutation rates in viral genome replication bring genetic variety to the intracellular population, and this makes the situation social: mutant genomes that do not code intact gene products can survive as free riders, by using the gene products from the other genomes in the intracellular population. We have previously shown that two plant RNA viruses, Japanese soil-borne wheat mosaic virus and tomato mosaic virus, infect a host cell by 5-6 and ~4 genomes in average, respectively, and proposed that this cell infection by a limited number of genomes enhance stochastic separation of free riders from others, resulting in exclusion of free riders from the whole population [1,2]. Here we simulated the evolution of average number of viral genomes to infect a cell (multiplicity of infection, or MOI, in virology). The simulation showed that MOI evolves to 4-5 in silico. This result could be explained by a balance between exclusion of free riders by smaller MOI and avoidance of stochastic failure of infection. Consistently, our further experiments showed that a tripartite plant RNA virus, cucumber mosaic virus (CMV), also has MOIs of 3-5 for its multiple genomic segments in multiple hosts.
The MOI of 4-5 does enhance separation of free riders from others, but still allows their co-existence for a certain period of time. This requires “decisions” by viruses. The intracellular populations exclusively consisting of free riders cannot infect new adjacent cells, and those exclusively consisting of intact genomes will infect the adjacent cells at high probability; those consisting of both free riders and intact genomes need to decide whether to infect the adjacent cells, depending on the ratio of free riders in each intracellular population. Our simulations showed that a majority-decision-like system, where the cell-to-cell infection probability sharply changes between 0 and 1 at a certain threshold for the ratio of free riders, can be favoured by plant viruses. The simulation result was supported by a co-inoculation experiment of CMV variants that carry a functional cell-to-cell movement protein gene or a dysfunctional one.
 Miyashita and Kishino (2010) “Estimation of the size of genetic bottlenecks in cell-to-cell movement of soil-borne wheat mosaic virus and the possible role of the bottlenecks in speeding up selection of variations in trans-acting genes or elements.” J Virol 84(4) 1828-1837.
 Miyashita, Ishibashi, Kishino and Ishikawa (2015) “Viruses roll the dice: the stochastic behavior of viral genome molecules accelerates viral adaptation at the cell and tissue levels.” PLOS Biol 13(3) e1002094.