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Abstract Features of the natural life cycle of the budding yeast Saccharomyces cerevisiae were crucial to its domestication as a laboratory experimental model, especially the ability to maintain stable haploid clones and cross them at will to combine alleles via meiosis. Stable haploidy results from mutations in HO, which encodes an endonuclease required for haploid-specific mating-type switching. Previous studies found an unexpected diversity of HO alleles among natural isolates within a small geographic area. We developed a hands-on field and laboratory activity for middle-school students in Denver, CO, USA, to isolate wild yeast from oak bark, identify species via DNA sequencing, and sequence HO from S. cerevisiae isolates. We find limited HO diversity in North American oak isolates, pointing to efficient, continuous dispersal across the continent. In contrast, we isolated the “dairy yeast,” Kluyveromyces lactis, from a tree <10 m away and found that it represents a new population distinct from an oak population in an adjacent state. The outreach activity partnered middle-school, high-school, and university students in making scientific discoveries and can be adapted to other locations and natural yeast habitats. Indeed, a pilot sampling activity in southeast Texas yielded S. cerevisiae oak isolates with a new allele of HO and, from a nearby prickly pear cactus, a heat-tolerant isolate of Saccharomyces paradoxus. 

Gametogenesis in diploid cells of the budding yeast Saccharomyces cerevisiae produces four haploid meiotic products called spores. Spores are dormant until nutrients trigger germination, when they bud asexually or mate to return to the diploid state. Each sporulating diploid produces a mix of spores of two haploid mating types, a and α. In asexually dividing haploids, the mating types result from distinct, mutually exclusive gene expression programs responsible for production of mating pheromones and the receptors to sense them, all of which are silent in diploids. It was assumed that spores only transcribe haploid- and mating-type-specific genes upon germination. We find that dormant spores of each mating type harbor transcripts representing all these genes, with the exception of Mata1, which we found to be enriched in a spores. Mata1 transcripts, from a rare yeast gene with two introns, were mostly unspliced. If the retained introns reflect tethering to the MATa locus, this could provide a mechanism for biased inheritance. Translation of pheromones and receptors were repressed at least until germination. We find antisense transcripts to many mating genes that may be responsible. These findings add to the growing number of examples of post-transcriptional regulation of gene expression during gametogenesis.