COEVOLUTION IN BACTERIAL-PLASMID POPULATIONS.

Evolutionary changes are described in plasmid-containing strains isolated after approximately 800 generations of growth in glucose-limited chemostat culture. The reproductive fitness increased dramatically over this period. Genetic changes associated with the increases in fitness were localized to both the bacterial and the plasmid chromosomes. In addition, some of the genetic changes on the bacterial and the plasmid chromosomes interact to minimize the deleterious effect of the plasmid. Thus, the changes observed may be considered coevolutionary. Reductions in the deleterious effects of the plasmid were shown to be associated with a decrease in plasmid copy number and an increase in the rate of segregational loss of the plasmid.