Experimental evolution of plasmid genomes
The ubiquity of plasmids in all prokaryotic phyla and habitats and their ability to transfer between cells marks them as prominent constituents of prokaryotic genomes. Many plasmids are found in their host cell in multiple copies. The multi-copy state of plasmids leads to an increased mutational supply of plasmid-encoded genes and genetically heterogeneous plasmid genomes. Nonetheless, the segregation of plasmid copies into daughter cells during cell division is considered to occur in the absence of selection on the plasmid alleles. Consequently, I hypothesize that genetic drift of plasmid alleles during cell division has implication for the evolutionary rates of plasmids by lowering the number of accumulating mutations that are expected from the mutational supply. To test this hypothesis, I performed an experimental evolution experiment of low- and high-copy non-mobile plasmids in an Escherichia coli host. The results of the experiment revealed that the evolutionary rate of multicopy plasmids does not reflect the increased mutational supply expected according to their copy number. The results further suggest that many plasmid mutations are quickly lost due to genetic drift during cell division. Here I term this special type of allele dynamics in the population as ‘segregational drift’. Thus, segregational drift of multicopy plasmids interferes with the retention and fixation of novel plasmid variants. Furthermore, an examination of the experimentally evolved hosts reveals a significant impact of the plasmid type on the host chromosome evolution. In conclusion, depending on the selection pressure on newly emerging variants, plasmid genomes may evolve slower than haploid chromosomes, regardless of their higher mutational supply. Plasmid copy number is thus an important determinant of plasmid evolvability due to the manifestation of segregational drift.
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