FIGURE 5.

Numerical results illustrating the effect of a constant selective pressure in the stability of nontransmissible multicopy plasmids. (a–g) Each box illustrates the temporal dynamics of the plasmid‐bearing subpopulation in a pairwise competition experiment inoculated with equal initial fractions of PF and PB. From left to right, α = 0, 0.2, 0.26, 0.28, 0.6, and 1. The dotted line denotes MSα = κ _n  + μ _n (1 − κ _n ) for n = 19 and κ _n  = 0.27. Note that for values of α < MSα, plasmids are unstable and eventually cleared from the population, while for α > MSα the plasmid‐bearing subpopulation increases in frequency until reaching fixation. For α = MSα, the selective pressure in favor of the plasmid compensates its fitness cost and therefore the plasmid fraction remains constant throughout the experiment. (h) Minimum selective pressure required to avoid plasmid loss for a range of PCNs. Different curves represent plasmids with different fitness costs (light purple denotes cost‐free plasmids and dark purple a very costly plasmid). Note that, for costly plasmids, there exists a nonmonotone relationship between MSα and PCN. (i) Time elapsed before plasmid fraction in the population is stabilized, for different copy numbers (5 in magenta, 19 in black, and 30 in cyan). Dotted lines represent plasmid fixation, while dashed lines denote stable coexistence between plasmid‐free and plasmid‐bearing subpopulations, and solid lines plasmid extinction. The vertical line indicates MSα, the minimum selective pressure that stably maintains plasmids in the population. Black letters indicate the parameter values used in the examples shown in (a–g).