Abstract
All genes critical for plasmid replication regulation are located on the plasmid rather than on the host chromosome. It is possible therefore that there can be copy-up “cheater” mutants. In spite of this possibility, low copy number plasmids appear to exist stably in host populations. We examined this paradox using a multilevel selection model. Simulations showed that, a slightly higher copy number mutant could out-compete the wild type. Consequently, another mutant with still higher copy number could invade the first invader. However, the realized benefit of increasing intra-host fitness was saturating whereas that of inter-host fitness was exponential. As a result, above a threshold, intra-host selection was overcompensated by inter-host selection and the low copy number wild type plasmid could back invade a very high copy number plasmid. This led to a rock-paper-scissor (RPS) like situation that allowed the coexistence of plasmids with varied copy numbers. Furthermore, another type of cheater that had lost the genes required for conjugation but could hitchhike on a conjugal plasmid, could further reduce the advantage of copy-up mutants. These sociobiological interactions may compliment molecular mechanisms of replication regulation in stabilizing the copy numbers.
Highlights
Plasmids are extra chromosomal elements of circular DNA in bacteria, which replicate independent of the host genome
The presence of useful genes does not seem to be central to the evolution and stability of plasmids because a useful gene could be incorporated in the bacterial chromosome [11] saving the cost of carrying the plasmid
In the case of genes involved in extracellular products such as virulence factors, a ‘cheater’ strain that does not produce the virulence factor can invade the virulent population. Having such genes on plasmids make horizontal gene transfer possible coercing the cheaters to make the extracellular products [12]. Such systems can be robust to bacterial cheaters, they may not be robust to plasmid cheaters with the gene for the extracellular products deleted
Summary
Plasmids are extra chromosomal elements of circular DNA in bacteria, which replicate independent of the host genome. Plasmids exploit the machinery of host cell for their replication, but many of them carry useful or conditionally advantageous genes and cannot be generalized as parasites. In the case of genes involved in extracellular products such as virulence factors, a ‘cheater’ strain that does not produce the virulence factor can invade the virulent population Having such genes on plasmids make horizontal gene transfer possible coercing the cheaters to make the extracellular products [12]. Such systems can be robust to bacterial cheaters, they may not be robust to plasmid cheaters with the gene for the extracellular products deleted. Many plasmids carry useful genes, that does not appear to be necessary and sufficient cause of plasmid stability
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