Maintenance of the 2μm circle plasmid in populations of Saccharomyces cerevisiae

The 2μm circle plasmid is maintained at high frequencies in populations of yeast cells. To find out how the plasmid is maintained, three forces were measured: the selective advantage or disadvantage conferred by 2μm circles, the rate of generation of [Cir⁰] cells, and the rate of illegitimate transfer of 2μm circles from cell to cell. It was found that under the conditions used, 2μm circles confer a selective disadvantage of about 1%, that [Cir⁰] cells are generated at the rate of 7.6 x 10^-5 per [Cir⁺] cell per generation, and that illegitimate transfer of 2 μm circles occurs at a rate less than 10^-7 per recipient cell per generation. The most likely explanation of 2μm circle maintenance is that the plasmid is sexually transmitted at such a rate that it spreads through populations despite selection against it.