Species Interactions Alter the Impact of Mutations in Predictable Ways Across Communities.*

Predicting evolution in microbial communities is critical for problems from human health to global nutrient cycling. Understanding how species interactions impact the effect of mutations in a focal population would enhance our ability to predict evolution. We created a transposon mutant library in Salmonella enterica and measured the fitness of loss of function mutations in 3,550 genes when grown alone versus in competitive co-culture or mutualistic co-culture with Escherichia coli and Methylorubrum extorquens. We found that mutualism reduces the average impact of mutations, while competition had no effect. Additionally, mutant fitness in the 3-species communities can be predicted by averaging the fitness in each 2-species community. These results complement our findings that species interactions alter the impact of diverse perturbations in predictable ways. Our work suggest that species interactions can predictably impact the effect of mutations, in turn suggesting that evolution may ultimately be predictable in multi-species communities.