Hybrid frequencies confirm limit to coinfection in the RNA bacteriophage phi 6

Coinfection of the same host cell by multiple viruses may lead to increased competition for limited cellular resources, thus reducing the fitness of a n individual virus. Selection should favor viruses that can limit or preven t coinfection, and it is not surprising that many viruses have evolved mech anisms to do so. Here we explore whether coinfection is limited in the RNA bacteriophage phi 6 that infects Pseudomonas phaseolicola. We estimated the limit to coinfection in phi 6 by comparing the frequency of hybrids produc ed by two marked phage strains to that predicted by a mathematical model ba sed on differing limits to coinfection. Our results provide an alternative method for estimating the limit to coinfection and confirm a previous estim ate between two to three phages per host cell. In addition, our data reveal that the rate of coinfection at low phage densities may exceed that expect ed through random Poisson sampling. We discuss whether phage phi 6 has evol ved an optimal limit that balances the costly and beneficial fitness effect s associated with multiple infections.