Suffering makes you weaker: limited evolutionary adaptation in competitively inferior populations

Data collected from an experimental evolution project.

54 evolution lines, including 18 E. coli monoculture lines, 18 P. fluorescens monoculture lines and 18 coculture lines were studied. Each line were propagated at three temperatures. Nine incubators were used in this study, and randomly divided into three blocks. The three incubators in each block were set 19, 26 and 31°C. There were two within-incubator replicate microcosms for each species composition. Therefore, a total of 36 evolution lines were established for each species (2 culture types × 3 blocks × 3 temperatures × 2 within-incubator replicates).

Population density was measured as colony formation units per ml.

Fitness of each evolved population against it ancestral strain at each of the three temperatures (19, 26 or 31°C) was measured by competition experiment. Fitness of evolved E. coli populations and their ancestor, all of the strain REL606, relative to a reference strain, REL606Ara+, was measured by head-to-head competition assays. Fitness of evolved and ancestral P. fluorescens populations, all of the strain SBW25EeZY6KX, was measured against a reference strain SBW25. Relative fitness of each tested population against the reference strain was estimated from the Malthusian parameters. Fitness of each evolved population relative to its ancestor (change in fitness during the evolution experiment) was calculated as difference between the two, analogous to the selection coefficient index.

Population-level competitive effect of an effect species on a response species could be estimated by the relative yield (RY) of the response species, that is, the ratio of its biomass in cocultures to that in monocultures. Interspecific effects of the ancestral and evolved E. coli (B REL606) populations on the ancestral P. fluorescens (SBW25EeZY6KX) strain were measured. And RY of the response population was calculated. The difference in competitive effects between evolved E. coli and the ancestral strain was then estimated as RY of ancestral P. fluorescence in the presence of evolved E. coli minus that in the presence of ancestral E. coli. This was termed as “change in interspecific effect”. Changes in interspecific effects of P. fluorescens populations were measured using the same method.