The distribution of mutational fitness effects of phage φX174 on different hosts.

Adaptation depends greatly on the distribution of mutation fitness effects (DMFE), but the phenotypic expression of mutations is often environment dependent. The environments faced by multihost pathogens are mostly governed by their hosts and therefore measuring the DMFE on multiple hosts can inform on the likelihood of short-term establishment and longer term adaptation of emerging pathogens. We explored this by measuring the growth rate of 36 mutants of the lytic bacteriophage φX174 on two host backgrounds, Escherichia coli (EcC) and Salmonella typhimurium (StGal). The DMFE showed higher mean and variance on EcC than on StGal. Most mutations were either deleterious or neutral on both hosts, but a greater proportion of mutations were deleterious on StGal. We identified two mutations with beneficial fitness effects on EcC that were neutral on StGal. Host-specific differences in fitness were associated with particular functional classes of genes involved in the initial stages of infection in accordance with previous studies of host specificity. Overall, there was a positive correlation between the effects of mutations on each host, suggesting that most new mutations will have general, rather than host-specific fitness effects. We consider these results in light of simple fitness landscape models of adaptation and discuss the relevance of context-dependent DMFE for multihost pathogens.