Social wasps are a Saccharomyces mating nest.

The reproductive ecology of Saccharomyces cerevisiae is still largely unknown. Recent evidence of interspecific hybridization, high levels of strain heterozygosity, and prion transmission suggest that outbreeding occurs frequently in yeasts. Nevertheless, the place where yeasts mate and recombine in the wild has not been identified. We found that the intestine of social wasps hosts highly outbred S. cerevisiae strains as well as a rare S. cerevisiae×S. paradoxus hybrid. We show that the intestine of Polistes dominula social wasps favors the mating of S. cerevisiae strains among themselves and with S. paradoxus cells by providing a succession of environmental conditions prompting cell sporulation and spores germination. In addition, we prove that heterospecific mating is the only option for European S. paradoxus strains to survive in the gut. Taken together, these findings unveil the best hidden secret of yeast ecology, introducing the insect gut as an environmental alcove in which crosses occur, maintaining and generating the diversity of the ascomycetes.

Social wasp intestines host the local phenotypic variability of Saccharomyces cerevisiae strains.

Nowadays, the presence of Saccharomyces cerevisiae has been assessed in both wild and human-related environments. Social wasps have been shown to maintain and vector S. cerevisiae among different environments. The availability of strains isolated from wasp intestines represents a striking opportunity to assess whether the strains found in wasp intestines are characterized by peculiar traits. We analysed strains isolated from the intestines of social wasps and compared them with strains isolated from other sources, all collected in a restricted geographic area. We evaluated the production of volatile metabolites during grape must fermentation, the resistance to different stresses and the ability to exploit various carbon sources. Wasp strains, in addition to representing a wide range of S. cerevisiae genotypes, also represent large part of the phenotypes characterizing the sympatric set of yeast strains; their higher production of acetic acid and ethyl acetate could reflect improved ability to attract insects. Our findings suggest that the relationship between yeasts and wasps should be preserved, to safeguard not only the natural variance of this microorganism but also the interests of wine-makers, who could take advantage from the exploitation of their phenotypic variability. Copyright © 2016 John Wiley & Sons, Ltd.

Role of social wasps in Saccharomyces cerevisiae ecology and evolution.

Saccharomyces cerevisiae is one of the most important model organisms and has been a valuable asset to human civilization. However, despite its extensive use in the last 9,000 y, the existence of a seasonal cycle outside human-made environments has not yet been described. We demonstrate the role of social wasps as vector and natural reservoir of S. cerevisiae during all seasons. We provide experimental evidence that queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny. This result is mirrored by field surveys of the genetic variability of natural strains of yeast. Microsatellites and sequences of a selected set of loci able to recapitulate the yeast strain's evolutionary history were used to compare 17 environmental wasp isolates with a collection of strains from grapes from the same region and more than 230 strains representing worldwide yeast variation. The wasp isolates fall into subclusters representing the overall ecological and industrial yeast diversity of their geographic origin. Our findings indicate that wasps are a key environmental niche for the evolution of natural S. cerevisiae populations, the dispersion of yeast cells in the environment, and the maintenance of their diversity. The close relatedness of several wasp isolates with grape and wine isolates reflects the crucial role of human activities on yeast population structure, through clonal expansion and selection of specific strains during the biotransformation of fermented foods, followed by dispersal mediated by insects and other animals.

Characterization of Saccharomyces cerevisiae natural populations for pseudohyphal growth and colony morphology.

In this work we have analyzed the colony and cellular morphologies of natural populations of Saccharomyces cerevisiae strains in response to different environmental stimuli. Among one thousand strains grown on YPD medium, 2.5% exhibited a rough (R) colony phenotype versus a smooth (S) phenotype. When grown on the ammonium-deficient medium SLAD, 56% of the strains showed a filamentous phenotype, often associated (43.8%) with an invasive phenotype, while 4.7% of the strains exhibited only an invasive phenotype. The rough phenotype on YPD was always associated with the filamentous phenotype on SLAD. A subset of 52 strains was further characterized for the growth phenotype under different stimuli (nitrogen deprivation, addition of alcohols, growth on proline as sole nitrogen source). On 27 strains, genetic analysis of the spore products was also performed. The entire set of data showed a wide distribution of dimorphism in the yeast population and great variability with respect to the dimorphic switch capability. Some strains grew with peculiar colony morphologies under different environmental stimuli and some showed colony morphology variations. Ecological implications of the wide spreading of dimorphic behavior and the occurrence of peculiar colony morphologies in natural yeasts are discussed.

Crosses between Saccharomyces cerevisiae and Saccharomyces bayanus generate fertile hybrids.

Crossings between strains of Saccharomyces cerevisiae and Saccharomyces bayanus were carried out. Genetic, molecular and electrophoretic karyotyping data indicated that interspecific hybrids were obtained. The hybrid cells segregated "grande" and "petite" colonies, and the latter ranged between 20 and 50%; unlike "grande" colonies, "petite" colonies did not sporulate and did not ferment maltose. In the hybrids, the extent of sporulation varied between 10 and 20%; only very rare asci (around 10(-4)) held viable ascospores. Clones from the viable ascospores sporulated and produced asci with viable ascospores able to give mating with spores from both hybrid derivatives and parental species. Fertile asci could derive from allotetraploid cells generated by endomitotic events in allodiploid cells, a mechanism that enables overcoming the species barrier between S. cerevisiae and S. bayanus.

Induction and characterization of morphologic mutants in a natural Saccharomyces cerevisiae strain.

Saccharomyces cerevisiae is a good model with which to study the effects of morphologic differentiation on the ecological behaviour of fungi. In this work, 33 morphologic mutants of a natural strain of S. cerevisiae, obtained with UV mutagenesis, were selected for their streak shape and cell shape on rich medium. Two of them, showing both high sporulation proficiency and constitutive pseudohyphal growth, were analysed from a genetic and physiologic point of view. Each mutant carries a recessive monogenic mutation, and the two mutations reside in unlinked genes. Flocculation ability and responsiveness to different stimuli distinguished the two mutants. Growth at 37 degrees C affected the cell but not the colony morphology, suggesting that these two phenotypes are regulated differently. The effect of ethidium bromide, which affects mitochondrial DNA replication, suggested a possible "retrograde action" of mitochondria in pseudohyphal growth.

Evidence for S. cerevisiae fermentation in ancient wine.

Saccharomyces cerevisiae is the principal yeast used in modern fermentation processes, including winemaking, breadmaking, and brewing. From residue present inside one of the earliest known wine jars from Egypt, we have extracted, amplified, and sequenced ribosomal DNA from S. cerevisiae. These results indicate that this organism was probably responsible for wine fermentation by at least 3150 B.C. This inference has major implications for the evolution of bread and beer yeasts, since it suggests that S. cerevisiae yeast, which occurs naturally on the surface bloom of grapes, was also used as an inoculum to ferment cereal products.