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.

Epistatic interactions between resistance mutations in antibiotic-free environments potentially play a crucial role in the spread of resistance in pathogen populations by determining the fitness cost associated with resistance. We used an experimental evolution approach to test for epistatic interactions between 14 different pairs of rifampicin mutations in the pathogenic bacterium Pseudomonas aeruginosa in 42 different rifampicin-free environments. First, we show that epistasis between rifampicin-resistance mutations tends to be antagonistic: the fitness effect of having two mutations is generally smaller than that predicted from the effects of individual mutations on the wild type. Second, we show that sign epistasis between resistance mutations is both common and strong; most notably, pairs of deleterious resistance mutations often partially or completely compensate for each others' costs, revealing a novel mechanism for compensatory adaptation. These results suggest that antagonistic epistasis between intragenic resistance mutations may be a key determinant of the cost of antibiotic resistance and compensatory adaptation in pathogen populations.