Weakly Deleterious Mutations and Low Rates of Recombination Limit the Impact of Natural Selection on Bacterial Genomes [electronic resource].
- Washington, D.C. : United States. Dept. of Energy. Office of Science, 2015.
Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy
- Physical Description:
- Article numbers e01,302-15 : digital, PDF file
- Additional Creators:
- Lawrence Berkeley National Laboratory
United States. Department of Energy. Office of Science
United States. Department of Energy. Office of Scientific and Technical Information
- Free-living bacteria are usually thought to have large effective population sizes, and so tiny selective differences can drive their evolution. However, because recombination is infrequent, “background selection” against slightly deleterious alleles should reduce the effective population size (<italic>N<sub>e</sub></italic>) by orders of magnitude. For example, for a well-mixed population with 10<sup>12</sup> individuals and a typical level of homologous recombination (<italic>r</italic>/<italic>m</italic>= 3, i.e., nucleotide changes due to recombination [<italic>r</italic>] occur at 3 times the mutation rate [<italic>m</italic>]), we predict that <italic>N<sub>e</sub> </italic>is<italic><</italic>10<sup>7</sup>. An argument for high <italic>N<sub>e</sub></italic> values for bacteria has been the high genetic diversity within many bacterial “species,” but this diversity may be due to population structure: diversity across subpopulations can be far higher than diversity within a subpopulation, which makes it difficult to estimate <italic>N<sub>e</sub></italic> correctly. Given an estimate of<italic>N<sub>e</sub></italic>, standard population genetics models imply that selection should be sufficient to drive evolution if <italic>N<sub>e</sub> </italic>×<italic>s</italic> is >1, where <italic>s</italic> is the selection coefficient. We found that this remains approximately correct if background selection is occurring or when population structure is present. Overall, we predict that even for free-living bacteria with enormous populations, natural selection is only a significant force if<italic>s</italic> is above 10<sup>-7</sup> or so. Because bacteria form huge populations with trillions of individuals, the simplest theoretical prediction is that the better allele at a site would predominate even if its advantage was just 10<sup>-9</sup> per generation. In other words, virtually every nucleotide would be at the local optimum in most individuals. A more sophisticated theory considers that bacterial genomes have millions of sites each and selection events on these many sites could interfere with each other, so that only larger effects would be important. However, bacteria can exchange genetic material, and in principle, this exchange could eliminate the interference between the evolution of the sites. In conclusion, we used simulations to confirm that during multisite evolution with realistic levels of recombination, only larger effects are important. We propose that advantages of less than 10<sup>-7</sup>are effectively neutral.
- Published through SciTech Connect.
mBio (Online) 6 6 ISSN 2150-7511 AM
Morgan N. Price; Adam P. Arkin.
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