Background
Mutation is one of the fundamental forces of evolution and the primary source of variation subject to natural selection. Mutation rates have diversified greatly across species and tend to be much higher in multicellular plants and animals than in unicellular organisms. Whether microbial forms of multicellularity impact how mutation rates evolve has been unclear.
MyxoEE-8 was designed to provide a highly accurate estimate of the M. xanthus mutation rate using a classic mutation-accumulation experiment in which evolving populations were reduced to a single cell between growth cycles to minimize selection.
Design summary: A non-motile strain of M. xanthus was used to set up dozens of independent experimental Mutation Accumulation (MA) lines. Most lines went through 75 single-colony bottlenecks of picking and restreaking on nutrient-rich medium, which greatly reduces natural selection and thus allows assessment of random mutation accumulation over time. At regular intervals throughout MyxoEE-8, freezer stocks of each line were prepared and stored at -80 °C. End-point freezer stocks of 46 MA lines were revived by streaking, picking of a single colony and regrowth in liquid culture prior to DNA extraction and whole genome sequencing.
Highlights
~ 85’000 generations of cumulative evolution - 400 days of experimental mutation accumulation with 46 lineages yielded 85,323 generations of cumulative evolution in the absence of selection.
High mutation rate µ - The average rate of new mutation in M. xanthus was estimated at ~ 5.5 × 10 -10 per site per generation - one of the highest rates among free-living eubacteria.
Low effective population size Ne - The effective population size was derived from neutral diversity at four-fold degenerate sites across two dozen M. xanthus natural isolates at Ne ~ 107.
Negative correlation between µ and Ne - These estimates strengthen an already clear negative correlation between mutation rate and effective population size in prokaryotes.
Publication
Wielgoss, S., J. D. Van Dyken and G. J. Velicer. 2024. Mutation Rate and Effective Population Size of the Model Cooperative Bacterium Myxococcus xanthus. Genome Biology and Evolution. READ IT HERE