Speaker
Description
Mutations are the source of genetic variation upon which natural selection acts and therefore the driving force of evolution. In order to understand the generation of diversity among life forms, from the variety of Galapagos finches to the spread of antibiotic resistant bacterial strains, as well as the diversity between cells in an organism, such as in cancer evolution, we need a quantitative characterization of the dynamics of mutation accumulation as well as their effects on fitness. The distribution of fitness effects (DFE) of spontaneous mutations is an important quantity in evolutionary biology but is difficult to measure experimentally. In previous studies on microorganisms, the quality of DFE estimation was often limited by a small sample of mutations and by a sampling bias due to the effect of natural selection, which purges strongly deleterious mutations. In addition, the dynamics of the mutation accumulation process has never been experimentally revealed, due to the lack of appropriate tools. Using a microfluidic setup we followed the growth of thousands of individual Escherichia coli cells for hundreds of generations as they accumulate mutations [1,2]. Individual cells grow in separate microchannels whose geometry allows blocking natural selection, thus producing an unbiased sample of tens of thousands of mutations. Lethal and strongly deleterious mutations can also be detected as they appear, in contrast to previous studies. This high-throughput data allowed a quantitative characterization of the DFE, showing that it is dominated by neutral mutations, with a surprisingly weak average cost for non-lethal mutations, and 1% of lethal mutations. Using a fluorescent reporter of nascent mutations based on the expression of fluorescent Mismatch Repair protein MutL, allowing detecting nascent mutations as fluorescent foci in the cells, we also follow directly the dynamics of the mutation accumulation process in single cells. 1. Robert L., Ollion J.,Robert J., Song X., Matic I., Elez M., 2018 Science 359(6381):1283-1286 2. Robert L., Ollion J., Elez M., 2019 Nat. Protoc. 14(11):3126-3143
