Many bacteria carry extra DNA molecules beyond their chromosome, so-called plasmids. While plasmids are apriori costly to the cell, they can also contain beneficial genes such as antibiotic resistance genes. Many plasmids exist in the cell in multiple copies. This has consequences for the evolution of genes on plasmids. First, the more copies of a gene are present, the higher is the probability that one of them mutates. Second, the spread of such a mutation is affected by the distribution of plasmid variants to the daughter cells at cell division. Third, the fitness of any cell may depend on its plasmid composition. We developed a mathematical framework to study bacterial adaptation driven by beneficial alleles on multicopy plasmids. We find that the probability of successful adaptation may increase or decrease with the plasmid copy number, depending on the type of the adaptive mutation. The fixation process may contain a phase, in which (nearly) all cells contain both alleles.
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