Multiplicity of infection does not accelerate infectivity evolution of viral parasites in laboratory microcosms

Coinfection with multiple parasite genotypes (multiplicity of infection) creates within-host competition and opportunities for parasite recombination, and is therefore predicted to be important for both parasite and host evolution. We tested for a difference in the infectivity of viral parasites (lytic phage Φ2) and resistance of their bacterial hosts (Pseudomonas fluorescens SBW25) under both high and low multiplicity of infection (MOI) during coevolution in laboratory microcosms. Results show that MOI has no effect on infectivity and resistance evolution during coevolution over approximately 80 generations of host growth, and this is true when the experiment is initiated with wild-type viruses and hosts, or with viruses and hosts that have already been coevolving for ~330 generations. This suggests that MOI does not have a net effect of accelerating parasite adaptation to hosts through recombination, or retarding adaptation to hosts through between-parasite conflict in this system.