Role of social wasps in Saccharomyces cerevisiae ecology and evolution

Abstract

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.

Fig. 1.

Yeast flora in the collected wasps. (A) Distribution of yeast isolates (n = 393) from Vespa crabro, Polistes spp,. and Apis mellifera insects (n = 61); white bars indicate the number of isolates per species before grape maturation; black bars indicate the number of isolates per species after grape maturation; italic numbers: percentage of insects bearing at least one isolate per yeast species. (B) Duality diagram for the first two components obtained by correspondence analysis of seasonal profile of the yeast flora of V. crabro guts (n = 57). Cases for the analysis are the individual insects indicated by boxes numbered with their ID (Dataset S1). Variables are the period of collection (A and B) and the occurrence of the specific yeast species; rectangles bearing yeast species names represent the occurrence of yeast species. Black numbered boxes indicate insects bearing S. cerevisiae strains. The third axis having an eigenvalue > 1 is illustrated in SI Appendix, Fig. S2.

Fig. 2.

Microsatellite analysis results. (A) Neighbor-joining tree showing the clustering of 17 S. cerevisiae wasp isolates among 256 yeast strains obtained from different sources. The tree was constructed from the Chord distance between strains based on the polymorphism at 12 loci and was rooted according to the midpoint method. Branches are colored according to the substrate from which strains have been isolated. Color code: red, insect isolates; purple, wine; green, grapes; orange, baked products and beer; pink, clinical; light blue, other fermentations; light brown, other natural sources. The position of wasp isolates within the tree is indicated by a red bar. I, II, III, IV, V, VI, and VII: detail of the subclusters encompassing wasp isolates. (B) Ancestry of the 256 S. cerevisiae strains analyzed by microsatellite analysis. The figure shows the proportion of each strain’s ancestry in each cluster. This set of strains as a whole was inferred to fall into 13 clusters. Vertical lines are partitioned into 13 colored components (each representing one of the most probable inferred ancestors, or K clusters), which represent the individual’s estimated membership coefficients in the K clusters. Ancestry was inferred by Instruct analysis (53) and drawn with Distruct (54).

Fig. 3.

Yeast strain cluster based on the SNPs differences of the genome-mimicking genes. Neighbor-joining tree based on SNP differences of the EXO5, IRC8, and URN1 sequences of yeast strains. The strain membership of a specific cluster was assessed by inferring their most probable ancestor with the Bayesian algorithm implemented in Structure (48).

Fig. 4.

Polistes spp. insects fed with S. cerevisiae cells can maintain the yeast in their gut during the winter and pass them to their progeny after nest foundation in the spring. (A) Five out of six overwintering wasps fed with the labeled yeast strain BY4742-GFP/FOX3 preserved the yeast throughout the winter. Four of four foundresses allowed to form a colony were able to spread yeast cells to their offspring both at the larval stage and after their emergence. F = foundress; in vitro W = worker emerged in vitro; L = larva; owF = overwintering foundress; W = worker, (B) Visualization of BY4742-GFP/FOX3 cells in several conditions (original magnification, 100×), yeast cell wall was visualized with Calcofluor white (blue). Green fluorescence is only produced by the tagged strain when grown in YPO medium (Yeast Peptone 0.2% Oleate medium). Red auto-fluorescence is due to intracellular NADH accumulation in necrotic cells (55).