Abstract
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.
Highlights
Torulaspora microellipsoides is an under-characterized budding yeast of the Saccharomycetaceae family that is primarily associated with viticulture
In working with the yeast T. microellipsoides, we serendipitously discovered that the species undergoes a morphology switch on agar plates that is reminiscent of the white-toopaque switch observed in C. albicans
T. microellipsoides opaque cells appear as larger, slightly ellipsoid budding yeast (Fig. 1b), which is distinct from C. albicans opaque cells, which are very elongated compared to white cells
Summary
Torulaspora microellipsoides is an under-characterized budding yeast of the Saccharomycetaceae family that is primarily associated with viticulture. C. albicans exists primarily as a budding yeast that forms shiny white colonies on agar plates; it can spontaneously and reversibly switch to the opaque colony morphology, wherein colonies are flatter, have a darker, dull colour, and individual cells are larger and elongated[5] This switch is thought to have evolved as an adaptation to life in mammalian hosts and, white and opaque cells differ dramatically in their gene expression profiles, mating competency, virulence, and metabolic properties, with each morphology thought to have specific advantages in different host niches due to the differing properties of each cell type[6,8]. Little if any, research has been reported on the characterization of phenotypic switching in non-canonical, non-pathogenic yeast species, including the Zygosaccharomyces/Torulaspora (ZT) clade of Saccharomyceteae Yeasts in this clade are primarily environmental organisms isolated from niches overlapping those of S. cerevisiae. The most important aspect of our observations is that phenotypic switching between highly divergent yeast-cell types exists in at least one species in the ZT clade, suggesting that more yeasts than previously thought may undergo dramatic morphological transitions resembling the white-to-opaque switch
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