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Abstract

A population’s ability to escape extinction and persist following catastrophic environmental change depends on the fixation of rare alleles that improve fitness in the new environment. The rare individuals bearing these alleles survive, reproduce, and rescue the population from extinction. What types of mutations underlie this phenomenon, "evolutionary rescue", and how do they affect resistance to other stressors? Evolutionary rescue becomes more complicated when multiple stressors interact and modify the harshness of the environment in unexpected ways. Combined stressors can be more deadly (synergistic), less deadly (antagonistic), or equal to the sum of the isolated stressors’ effects. When the effect of a stressor combination deviates from concentration-addition models (i.e., when interactions occur), how are the genetic mechanisms and fitness outcomes of evolutionary rescue affected? I investigate these questions by exposing Saccharomyces cerevisiae to various concentrations of six metals (Cd, Co, Cu, Mn, Ni, and Zn) and their 15 binary combinations. First, I performed evolutionary rescue experiments in each single metal and measured cross-tolerance of evolved lines to the other metals. Whole-genome sequencing revealed that lines with broader cross-tolerance often carried mutations impacting phosphate metabolism. Next, I quantified the combined effects of the metal pairs using two metrics: a previously established interaction value, a, describing deviation from a null model of additivity, and a new metric, δ, describing how harmful a combination is relative to the average of its components. I observed a wide range of interactions, with manganese involved in antagonistic combinations and copper involved in synergistic ones. Finally, I performed evolutionary rescue experiments in the metal pairs, generating evolved lines in eight of the 15 combinations. Evolutionary rescue in antagonistic metal combinations tended to confer fitness benefits in each individual metal, while synergistic combinations favoured mutations with more specific benefits to the pair’s combined effect. By linking evolved fitness patterns to molecular mechanisms, this work advances understanding of evolutionary rescue under multiple metal stresses, an important step toward predicting and managing populations at risk of extinction in an increasingly human-altered world.