Figure 4.

Stability of the small plasmid, pNI105, after evolution. Stability of pNI105 (percentage of plasmid-bearing cells,±s.e.) in the populations with the different plasmid combinations after 100 generations of evolution. PAO1/pNI105 is the control strain, carrying pNI105 alone. The plasmids of the PAO1/pNI105 PAMBL-1 strain display multiplicative fitness effects. The remaining plasmid combinations display positive epistasis and are grouped together. In the four populations where pNI105 is both associated with a large plasmid and also displays positive epistasis, there are no significant differences in the stability of pNI05 among them (one-way analysis of variance, P=0.193, F=1.73, df=3, 20). ‘Average' is the average of the results (±s.e.) of the stability of pNI105 in the strains showing positive epistasis between plasmids.