Abstract
Summary Pseudomonas aeruginosa exploits several types of motility behaviours to colonize diverse environments. One of these is swarming motility, a coordinated group movement on a semi‐solid surface. This bacterium needs to express a functional flagellum and produce rhamnolipids to display this type of social motility. A Δ hptB mutant, a gene part of the Gac/Rsm signalling pathway, produces rhamnolipids and expresses a functional flagellum but has an important swarming defect. Experimental‐directed evolution was performed on this mutant under swarming conditions to obtain compensatory mutations and thus identify genes responsible for its deficient swarming phenotype. Unexpectedly, a gain‐of‐function subpopulation emerged from this evolution with mutations in lasR , which codes for a key quorum‐sensing transcriptional regulator. Furthermore, we found that lasR − mutants even emerge at high frequencies in the wild‐type strain when using the same experimental evolution strategy. The resulting evolved population, largely composed of LasR‐defective mutants, is fitter than the original strain in swarming motility. We also established that lasR − mutants have a growth advantage under swarming conditions when compared with wild‐type. Our results demonstrate that a social phenotype, that is, swarming motility, favours the emergence of mutants deficient in a quorum‐sensing regulatory pathway to the benefit of the whole population.