Figure 3.

Positive epistasis increases plasmid stability. A schematic representation of the fitness of a bacterial strain carrying zero, one or two plasmids. In this example, the strain carrying two plasmids can present either multiplicative fitness (no epistasis between plasmids, e=0) or positive epistasis between plasmids (e>0). The arrows represent the change in fitness associated with the loss of a plasmid. The strain carrying two plasmids that produce multiplicative costs (e=0) will experience the same fitness advantage associated with the loss of each of the plasmids as in the strain that carries the single plasmids (orange and blue arrows, the increase in fitness is the same). On the other hand, in the case of positive epistasis, the advantage associated with the loss of a plasmid will be lower than in the previous cases (orange dotted arrow); the loss of the plasmid would even produce a decrease in fitness if the fitness of the strain with two plasmids is higher than the fitness of the strain with only one of the plasmids (blue dotted arrow).