title: Epistatic interactions between ancestral genotype and beneficial mutations shape evolvability in Pseudomonas aeruginosa.
abstract: The idea that interactions between mutations influence adaptation by driving populations to low and high fitness peaks on adaptive landscapes is deeply ingrained in evolutionary theory. Here, we investigate the impact of epistasis on evolvability by challenging populations of two Pseudomonas aeruginosa clones bearing different initial mutations (in rpoB conferring rifampicin resistance, and the type IV pili gene network) to adaptation to a medium containing l-serine as the sole carbon source. Despite being initially indistinguishable in fitness, populations founded by the two ancestral genotypes reached different fitness following 300 generations of evolution. Genome sequencing revealed that the difference could not be explained by acquiring mutations in different targets of selection; the majority of clones from both ancestors converged on one of the following two strategies: (1) acquiring mutations in either PA2449 (gcsR, an l-serine-metabolism RpoN enhancer binding protein) or (2) protease genes. Additionally, populations from both ancestors converged on loss-of-function mutations in the type IV pili gene network, either due to ancestral or acquired mutations. No compensatory or reversion mutations were observed in RNA polymerase (RNAP) genes, in spite of the large fitness costs typically associated with mutations in rpoB. Although current theory points to sign epistasis as the dominant constraint on evolvability, these results suggest that the role of magnitude epistasis in constraining evolvability may be underappreciated. The contribution of magnitude epistasis is likely to be greatest under the biologically relevant mutation supply rates that make back mutations probabilistically unlikely.
authors.affiliation: Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, United Kingdom. danna.gifford@manchester.ac.uk.|Current Address: Faculty of Life Sciences, University of Manchester, Michael Smith Building, Dover St., Manchester, M13 9PL, United Kingdom. danna.gifford@manchester.ac.uk.
authors.forename: Danna R
authors.initials: DR
authors.lastname: Gifford
authors.affiliation: Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, United Kingdom.|Current Address: Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland.
authors.forename: Macarena
authors.initials: M
authors.lastname: Toll-Riera
authors.affiliation: Department of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, United Kingdom.
authors.forename: R Craig
authors.initials: RC
authors.lastname: MacLean
mesh_terms: D005075:Biological Evolution; D002244:Carbon / Q000378:metabolism; D024881:Drug Resistance, Bacterial; D004843:Epistasis, Genetic; D010861:Fimbriae, Bacterial / Q000235:genetics; D005838:Genotype; D009154:Mutation; D011550:Pseudomonas aeruginosa / Q000235:genetics*; D012293:Rifampin / Q000494:pharmacology; D012694:Serine / Q000378:metabolism*
chemical_list: D012694:Serine; D002244:Carbon; D012293:Rifampin
keywords: Antibiotic resistance; epistasis; evolvability; experimental evolution