Frequency-dependent ecological interactions increase the prevalence and shape the distribution of pre-existing drug resistance.

The evolution of resistance remains one of the primary challenges for modern medicine from infectious diseases to cancers. Many of these resistance-conferring mutations often carry a substantial fitness cost in the absence of treatment. As a result, we would expect these mutants to undergo purifying selection and be rapidly driven to extinction. Nevertheless, pre-existing resistance is frequently observed from drug-resistant malaria to targeted cancer therapies in non-small cell lung cancer (NSCLC) and melanoma. Solutions to this apparent paradox have taken several forms from spatial rescue to simple mutation supply arguments. Recently, in an evolved resistant NSCLC cell line, we found that frequency-dependent ecological interactions between ancestor and mutant ameliorate the cost of resistance in the absence of treatment. Here, we hypothesize that frequency-dependent ecological interactions in general may play a major role in the prevalence of pre-existing resistance. We combine numerical simulations with robust analytical approximations to provide a rigorous mathematical framework for studying the effects of frequency-dependent ecological interactions on the evolutionary dynamics of pre-existing resistance. First, we find that ecological interactions significantly expand the parameter regime under which we expect to observe pre-existing resistance. Even when positive ecological interactions between mutants and ancestors are rare, these clones provide the primary mode of evolved resistance because their positive interaction leads to significantly longer extinction times. Next, we find that even in the case where mutation supply is sufficient to predict pre-existing resistance, frequency-dependent ecological forces still contribute a strong evolutionary pressure that selects for increasingly positive ecological effects. Finally, we genetically engineer several of the most common clinically observed resistance mechanisms to targeted therapies in NSCLC, a treatment notorious for pre-existing resistance, and where our theory predicts positive ecological interactions to be common. We find that all three engineered mutants display a positive ecological interaction with their ancestor, as predicted. Strikingly, as with our originally evolved resistant mutant, two of the three engineered mutants have ecological interactions that fully compensate for their substantial fitness costs. As a whole, these results suggest that frequency-dependent ecological effects may provide the primary mode by which pre-existing resistance emerges.