PT - JOURNAL ARTICLE AU - Marcus M. Dillon AU - Nicholas P. Rouillard AU - Brian Van Dam AU - Romain Gallet AU - Vaughn S. Cooper TI - Diverse phenotypic and genetic responses to short-term selection in evolving <em>Escherichia coli</em> populations AID - 10.1101/027086 DP - 2015 Jan 01 TA - bioRxiv PG - 027086 4099 - http://biorxiv.org/content/early/2015/09/17/027086.short 4100 - http://biorxiv.org/content/early/2015/09/17/027086.full AB - Beneficial mutations fuel adaptation by altering phenotypes that enhance the fit of organisms to their environment. However, the phenotypic effects of mutations often depend on ecological context, making the distribution of effects across multiple environments essential to understanding the true nature of beneficial mutations. Studies that address both the genetic basis and ecological consequences of adaptive mutations remain rare. Here, we characterize the direct and pleiotropic fitness effects of a collection of 21 first-step beneficial mutants derived from naïve and adapted genotypes used in a long-term experimental evolution of Escherichia coli. Whole-genome sequencing was used to identify most beneficial mutations. In contrast to previous studies, we find diverse fitness effects of mutations selected in a simple environment and few cases of genetic parallelism. The pleiotropic effects of these mutations were predominantly positive but some mutants were highly antagonistic in alternative environments. Further, the fitness effects of mutations derived from the adapted genotypes were dramatically reduced in nearly all environments. These findings suggest that many beneficial variants are accessible from a single point on the fitness landscape, and the fixation of alternative beneficial mutations may have dramatic consequences for niche breadth reduction via metabolic erosion.