Host determinants of 2-micron plasmid stability in Saccharomyces cerevisiae

Abstract

Selfish genetic elements are DNA parasites that exploit their host cells for their own reproduction, thereby reducing host fitness. How can host cells evolve to defend themselves against these genetic parasites? I seek to address this question using the 2-micron plasmid, a selfish element naturally found across budding yeasts. This plasmid hijacks host cellular machinery to replicate and segregate itself, resulting in a 1-3% fitness cost to the host. Despite this cost, most Saccharomyces cerevisiae isolates carry the plasmid, indicating that it is a remarkably successful, co-evolved genetic parasite of yeasts. I hypothesized that some S. cerevisiae strains may have evolved the ability to restrict the plasmid and thereby evade parasitism. By screening a panel of natural isolates, I identified three strains that naturally do not harbor the 2-micron. I find that when the plasmid is reintroduced in the laboratory, these strains reproducibly lose the 2-micron, indicating that plasmid loss is a heritable trait. Furthermore, this plasmid loss phenotype is a genetically dominant trait, supporting the hypothesis that these strains have evolved a restriction factor targeting the 2-micron plasmid. I took a QTL mapping strategy to identify a genomic locus underlying plasmid loss. Additionally, I have developed a rapid plasmid loss assay that facilitates monitoring of plasmid occupancy in live cells across a population at single cell resolution. This assay is higher throughput and can better explore population heterogeneity compared to traditional plasmid loss methods. This work will allow us to explore the genetics, molecular mechanism, and possible fitness tradeoffs underlying a naturally evolved parasite resistance.