Antibiotic treatment affects the effect sizes of spontaneous mutations on bacterial population-growth characters
Authors: Wei-Chin Ho; Jadon Gonzales; Michael Lynch
Affiliation: Arizona State University
Keywords: Complex traits
The nature of spontaneous mutations, including rates and effect sizes, largely determines genotypic and phenotypic evolution dynamics. Therefore, studying how environmental factors impact the mutations is critical for understanding the organismal evolution in different environments. While the mutation rates have been found to be plastically different in various environments, whether the effect sizes of mutations are so remains unknown. To answer this question, for ~80 lines of Escherichia coli that had accumulated spontaneous mutations in rich medium with or without the antibiotic norfloxacin, we measured their growth curves in their home environment using the microplate spectrophotometer. By analyzing the phenotypic effects and the numbers of mutations among these mutation-accumulated lines, we quantified mutational effect sizes for two traits: maximum growth rate and carrying capacity. The results show that, while the mutational effects of both traits are positively correlated in each home environment, norfloxacin significantly aggravates the mutational effects of maximum growth rates but not the carrying capacities. To further dissect the source of the observed effect-size changes, we measured and analyzed the same traits in the same mutation-accumulated lines in their non-home environment. Besides the direct environmental impact, the results show that the altered spectra of spontaneous mutations in different environments also influence the observed effect sizes. In addition, the mutational correlation of two traits becomes insignificant in the non-home environment. To sum up, our data demonstrate complex context-dependent behaviors of mutational effects and mutaional correlations. These results will enhance our knowledge of genotype-environment interaction, quantitative trait evolution, and microbial responses to antibiotics.