A white-to-opaque-like phenotypic switch in the yeast Torulaspora microellipsoides

Torulaspora microellipsoides is an under-characterized budding yeast of the Saccharomycetaceae family that is primarily associated with viticulture. Here we report for the first time to our knowledge that T. microellipsoides undergoes a low-frequency morphological switch from small budding haploid (white) yeast to larger, higher ploidy (opaque) yeast. Comparison of transcriptomes by mRNA-seq revealed 511 differentially regulated genes, with white cells having greater expression of genes involved in stress resistance and complex carbohydrate utilization, and opaque cells up-regulating genes involved in ribosome biogenesis. Growth assays showed that white cells are physiologically more resistant to stationary-phase conditions and oxidative stress, whereas opaque cells exhibited greater cold tolerance. We propose that phenotypic switching in T. microellipsoides is an ecological adaptation, as has been suggested for similar morphological switching in distantly related species like Candida albicans, and we propose that this switching is a more broadly utilized biological strategy among yeasts than previously thought.