Hitchhiking microbes

108-365 Fresh Bread

Saccharomyces cerevisae is effectively a domesticated species. Photo by Flickr user bcymet.

It is quite clear that humans play a major role in altering ecosystems today. Historic migration of human populations has been shown to have many interesting associated evolutionary consequences1,2. Worldwide travel makes it difficult to stop anything from going anywhere, and there is now even discussion about the impact of microbes catching a ride to Mars on space shuttles!
The interesting associated migration I’d like to discuss here is that of our good friend, the budding yeast Saccharomyces cerevisiae. My previous interests in this delightful organism were limited to its use in fermenting some of my favorite beverages. However, there is so much more to this microscopic morphon than meets the eye. Not until yeast geneticist Duncan Greig came to UBC did it become clear to me how little we know of this species in nature. But the idealistic thoughts of having field sites at vineyards in French wine country were quickly washed away by his description of how difficult it can be to find the right yeast, which is not easy to identify to the species level3.
Yet the point about vineyards is intriguing. It might be argued that S. cerevisiae is now a domesticated species. These microbes are not motile on their own, and human use has been shown to impact the evolution of different strains since the origin of alcohol production in pre-historic human populations4. Populations of yeast exhibit genetic differentiation associated with different alcohol breweries or distilleries and different geographic regions across the globe.

Figure 1 from reference 4: neighbor-joining tree of 651 yeast strains, with color indicating the source each strain was isolated from.

Figure 1 from reference 4: neighbor-joining tree of 651 yeast strains, with color indicating the source each strain was isolated from.

These results correlate with historic data to show the human-initiated origins of each strain4. The closely related species Saccharomyces paradoxus, which is not used by humans but considered a close wild relative of S. cerevisiae, shows a very different pattern of geographic population structure, indicating isolation across the continents and thus barriers to migration that S. cerevisiae does not experience as a results of its association with humans3,5. Therefore, we see how the association of humans using yeast in food products has allowed its migration across continents which may have otherwise been impossible.

Figure 1 from reference 3: Genetic variation in S. paradoxus compared to S. cerevisiae. Neighbor-joining tree based on SNP data.

Legras et al. address the question of whether this merits S. cerevisiae yeast as being considered domesticated. Though some of the ancient strains, such as those used in rice wine for example, could be called domestic, other strains are not so clear. Vineyard yeast strains are interesting in that they are less controlled by winemakers and often introduced to the winemaking process simply by merit of living on the skins of the growing grapes.
Hyma and Fay5 have examined this recently in North America, showing that there is migration from yeast inhabiting nearby oak trees into these vineyard yeast. Interestingly, there is no evidence for migration in the reverse direction past vineyard environments. Oak trees surrounding vineyards were found to have wine strains of yeast, yet away from vineyards, these wine strains have not been found. It may be the case that vineyard strains are not able to establish or persist in such populations, perhaps due to too little migration away from vineyards without the aid of humans.
These strains of yeast isolated from oak trees or other wild sources still show that despite being mainly domesticated by humans and moving across the globe via human migration, wild S. cerevisiae does still exist, and we know so little about it and its life history3! Understanding more about how it behaves, reproduces, and survives in nature will likely greatly inform studies of what is sometimes generically thought of as a lab-based model organism in evolutionary biology.
1. Agostini HT, Yanagihara R, Davis V, Ryschkewitsch CF, Stoner GL (1997) Asian genotypes of JC virus in Native Americans and in a Pacific Island population: Markers of viral evolution and human migration. PNAS, 94: 14542-14546.
2. Hume JCC, Lyons EJ, Day KP (2003) Human migration, mosquitoes and the evolution of Plasmodium falciparumTrends in Parasitology, 19: 144-149.
3. Greig D, Leu J-Y (2009) Natural history of budding yeast. Current Biology, 19: R886-R890.
4. Legras J-L, Merdinoglu D, Cornuet J-M, Karst F (2007) Bread, beer and wine: Saccharomyces cerevisiae diversity reflects human history. Molecular Ecology, 16: 2091-2102.
5. Hyma KE, Fay JC (2013) Mixing of vineyard and oak-tree ecotypes of Saccharomyces cerevisiae in North American vineyards. Molecular Ecology doi: 10.1111/mec.12155

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