The Impact of Genetic Background During Laboratory Evolution of Pseudomonas aeruginosa in a Cystic Fibrosis-like Environment

Abstract

Abstract Genetic background has the potential to influence both the tempo and trajectory of adaptive change: different genotypes of a given species may adopt varied solutions to the same environmental challenge, or they may approach the same solution at different rates. Laboratory selection has been used widely to experimentally examine the evolutionary consequences of variation in genetic background, although largely using genotypes differing by only a few mutations. Here, we leverage natural variation in the bacterium Pseudomonas aeruginosa to investigate whether different adaptive solutions are accessible from distant points of departure on an adaptive landscape. We evolved seventeen diverse genotypes in a laboratory medium that partially mimics the lung sputum of cystic fibrosis patients, and we measured changes in ten phenotypes as well as in fitness. Using phylogenetically-informed analyses, we found that genetic background impacted the tempo, but not the trajectory, of phenotypic evolution: different starting genotypes converged towards similar phenotypes, but at varying rates. Our findings add to a growing body of evidence supporting widespread diminishing returns epistasis during adaptation. The importance of genetic background towards the trajectory of adaptation remains inconsistent across experimental systems and conditions.