The yeast 2 micron plasmid: strategies for the survival of a selfish DNA

Area of Science:

• Molecular and Cellular Biology
• Yeast Genetics
• Epigenetics

Background:

• The yeast 2-micron (2 mu) plasmid is considered a "selfish" DNA molecule.
• Selfish genetic elements often face challenges in maintaining their presence within host populations.

Purpose of the Study:

• To investigate the stability and survival mechanisms of the yeast 2 mu plasmid in different yeast life cycle phases.
• To understand the evolutionary strategies employed by selfish DNA to persist in host populations.

Main Methods:

• Continuous culture experiments with haploid yeast populations to measure plasmid loss rates.
• Growth rate comparisons between plasmid-free and plasmid-containing yeast cells.
• Competition experiments in diploid yeast strains and under periodic stationary phase conditions.

Main Results:

• The 2 mu plasmid is lost exponentially from haploid yeast in continuous culture, with plasmid-free cells exhibiting a 1.5%-3% growth advantage.
• Periodic entry into stationary phase and the diploid life cycle stage significantly reduced plasmid loss rates.
• Plasmid-free segregants could not be isolated from diploid yeast, indicating stability at the segregation level.

Conclusions:

• The yeast 2 mu plasmid employs a survival strategy that leverages the host's natural life cycle, specifically the diploid and stationary phases.
• These phases counteract the selective pressure against the plasmid in haploid populations, ensuring its ubiquity in natural yeast strains.