Persistence of Resident and Transplanted Genotypes of the Undomesticated Yeast, Saccharomyces paradoxus in Forest Soil
One might expect yeasts in soil to be highly dispersed via water or insects, forming ephemeral, genetically heterogeneous populations subject to competition and environmental stochasticity. Here, we report years-long persistence of genotypes of the yeast Sacchaormyces paradoxus in space and time. Within 1 km2 in a mixed hardwood forest on scales from centimeters to tens of meters, we detect persistence over 3 years of native genotypes, identified by SNPs genome-wide, of the wild yeast, Saccharomyces paradoxus around Quercus rubra and Q. alba. Yeasts were recovered by enrichment in ethanol-containing medium, which measures only presence or absence, not abundance. Additional transplantation experiments employed strains marked with spontaneous defects in the URA3 gene, which also confer resistance to 5-Fluoroorotic acid (5FOA). Plating soil suspensions from transplant sites on 5FOA medium permitted one-step quantification of yeast colony-forming units, with no interference from other unmarked yeasts or microorganisms. After an initial steep decrease in abundance, the yeast densities fluctuated over time, increasing in association with rainfall and decreasing in association with drought. After 18 months, the transplanted yeasts remain in place on the nine sites. In vitro transplantation experiments into non-sterile soil in petri dishes showed similar patterns of persistence and response to moisture and drought. To determine whether S. cerevisiae, not previously recovered from soils regionally, can persist in our cold-climate sites, we transplanted marked S. cerevisiae alone and in mixture with S. paradoxus in fall, 2017. Five months on, S. cerevisiae persist to the same extent as S. paradoxus.