Lydia Robert (Laboratoire Jean Perrin, UPMC)

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4 septembre 11:30 » 12:30 — Bibliothèque PCT - F3.04

Dynamics of mutation accumulation and mutation fitness effects studied at the single cell level

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 mutations accumulation as well as their effects on fitness. Although commonly divided, according to their fitness effects, into three categories : good, bad and neutral, in reality mutations show a distribution of fitness effects (DFE), from strongly deleterious to highly beneficial. This distribution is an important parameter in evolutionary biology but is difficult to measure experimentally. In previous studies on microorganisms, the quality of DFE estimation was often limited either by a small sample of mutations and/or by a sampling bias due to the effect of natural selection, leading to the underrepresentation/overrepresentation of deleterious/beneficial mutations. In studies involving higher organisms, the low number of individuals that can usually be monitored limits the precision of DFE estimation. In addition, the dynamics of the mutation accumulation process has never been experimentally revealed, due to the lack of relevant techniques.

Using a microfluidic setup we followed the growth of thousands of individual Escherichia coli cells for hundreds of generation as they accumulate mutations. Individual cells grow in separate microchannels, thus avoiding any selection bias and therefore 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. 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.

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