DISTRIBUTION OF FITNESS EFFECTS CAUSED BY RANDOM INSERTION MUTATIONS IN ESCHERICHIA-COLI

Very little is known about the distribution of mutational effects on o rganismal fitness, despite the fundamental importance of this informat ion for the study of evolution. This lack of information reflects the fact that it is generally difficult to quantify the dynamic effects of mutation and natural selection using only static distributions of all ele frequencies. In this study, we took a direct approach to measuring the effects of mutations on fitness. We used transposon-mutagenesis t o create 226 mutant clones of Escherichia coli. Each mutant clone carr ied a single random insertion of a derivative of Tn10. All 226 mutants were independently derived from the same progenitor clone, which was obtained from a population that had evolved in a constant laboratory e nvironment for 10,000 generations. We then performed competition exper iments to measure the effect of each mutation on fitness relative to a common competitor. At least 80% of the mutations had a significant ne gative effect on fitness, whereas none of the mutations had a signific ant positive effect. The mutations reduced fitness by about 3%, on ave rage, but the distribution of fitness effects was highly skewed and ha d a long, flat tail. A compound distribution, which includes both gamm a and uniform components, provided an excellent fit to the observed fi tness values.