Biogeography, population genetics and rapid centromere evolution in Saccharomyces cerevisiae

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Saccharomyces cerevisiae is the species of yeast that puts the alcohol into sake, wine and beer and is used to make bread. Some S. cerevisiae strains also live independently of humans in the bark of oak trees and the soil around them. Partly because sake, wine, beer and bread are so interesting and partly because this single-celled fungus is so simple yet has much in common with animal and plant cells, researchers have studied it in the laboratory for decades. Now S. cerevisiae is probably better understood at the cellular and molecular level than any animal, plant or other fungus, yet little is known about its ecology. This study will lead to a better understanding of S. cerevisiae in Europe and how far they migrate and mix with their relatives in different habitats and geographic locations. S. cerevisiae has been discovered in the bark of oak trees in North America, South East Asia and various other parts of the world, they have also been found in soils from Holland and Finland, but they have not yet been discovered in the oaks of Europe. In a recent study, scientists in North America discovered that pretreating bark and soil with alcohol and sugar led to the isolation of S. cerevisiae, even though their occurrence in bark and soil would otherwise rarely be noticed. The use of this technique is likely to lead to the discovery of S. cerevisiae in European oaks. In this study, small amounts of soil, bark or grapes will be taken back to the lab and analysed for the presence of yeasts. The DNA sequences of the yeasts that are discovered in this way are studied for signs of genetic subdivision. If there are signs that different types of DNA sequence prevail among the yeasts from different habitats or different geographic regions, then that suggests that there has been little or no admixture between these sites now or even in the last few thousand years. The level of difference among the DNA sequences from different sites can lead to an estimate of approximately how long it has been since there was reasonable mixture between the yeasts of different habitats or regions. This study will use what is probably the most rapidly evolving type of DNA sequence in yeasts - the centromere - something that changes fast is most likely to pick up the differences among even very similar yeasts, and so will minimise the amount of DNA sequence necessary to spot differences. Though useful for the purposes of this study, the rapid evolution of centromeres is also very curious. Centromeres play a crucial role in any kind of cell division. They form the points at which the cell's machinery attaches to all the genetic material of a cell and organises the equal division of DNA into the next generation of cells. Laboratory experiments in the 1980s showed that certain types of change to the DNA sequence of a centromere resulted in various types of failure, from the most devastating failures in growth and reproduction to reduced fertility among a yeast's offspring. Why would something so important be evolving so fast? Might this rapid evolution have consequences for growth, fertility or the reproductive isolation between species? I will also use the data from the investigation into yeast ecology to address this question. These data from natural populations together with a couple of simple laboratory experiments should reveal the causes and consequences of rapid centromere evolution.