yEvo (“yeast evolution lab”) provides experimental evolution teaching modules to high school classrooms to bring genuine research experiences to students. Our goal is to show students how evolution can be applied to solve biological problems by asking them to carry out their own evolution experiments. Recently, we developed an experiment where students evolve yeast in wood hydrolysate media, which comes from lumber waste and contains plant sugars that yeast can convert to biofuel. Wood hydrolysate is a stressful condition for yeasts, as it contains acetic acid and harsh phenolic compounds, therefore yeast struggle to grow in this media. Mutations that yeast acquire while evolving in this media could be used to engineer a better biofuel-producing yeast. Students at Foster High (Tukwila, WA) evolved yeast in this media for seven weeks with transfers to fresh media every week. We retrieved the student’s evolved yeast and they showed striking new characteristics. We then used whole genome sequencing to identify underlying mutations. Some yeasts developed ring-like growths above the wood hydrolysate media and whole genome sequencing of two of these clones revealed mutations in regulators of cell-cell adhesion. Other yeasts reached very high population densities but whole genome sequencing revealed no novel mutations. I then utilized DNA staining and flow cytometry to check the DNA content of these evolved yeasts to see if changes in ploidy caused their new characteristics. The results showed that these yeast effectively became diploid, which has been shown by other labs to increase fitness in sugar-rich environments. yEvo is currently designing future experiments to further expand upon these interesting new results so that we can further understand mechanisms of adaptation to wood hydrolysate.

Experimental evolution can determine genetic interactions during natural selection in complex systems. Using whole-genome sequencing of 95 parallel populations of haploid Saccharomyces cerevisiae experimentally evolved for 250 generations, we discovered a possible epistatic interaction between two sets of beneficial mutations. The first mutation is a transposable element (TE) insertion into the promoter of FLO1, giving rise to a cellular aggregation phenotype known as flocculation. The second set are putative loss of function mutations in genes encoding members of the SAGA-complex, which is thought to increase expression of genes proximal to TEs. We hypothesize that without the members of the SAGA-complex, the FLO1 gene will be unexpressed, abrogating the flocculation phenotype. We isolated three flocculant clones with TE insertions from different experimental populations and crossed them with three clones with the deletion in the SAGA-complex. Through meiosis, the yeast sporulated into four cells. The ratios of flocculant to wildtype haploid cells are used to determine an epistatic interaction. A 2:2 ratio suggests a non-epistatic interaction while a 1:3 flocculant to wildtype ratio suggests an epistatic interaction. The project is in the early stages but segregation ratios suggest the members of the SAGA-complex with deletion mutations do not hinder the expression of the FLO1 gene. Our alternate hypothesis is members of the SAGA-complex have no effect on the activation of TE insertions that promote the expression of the FLO1 gene. While the initial hypothesis might not hold, this experiment will give us a further understanding of genetic interactions during evolution.