- Osmotic stress is a common challenge faced by organisms in environments with variable salinity or aridity. Understanding the evolutionary mechanisms underlying adaptation to osmotic stress may have wide applications, including management of populations experiencing climate change, or engineering industrial organisms involved in processes such as fermentation. While Saccharomyces cerevisiae is a model organism in genetics research, we investigated a Japanese sake strain (Y12) that is genetically distinct from common laboratory strains. This initially isogenic ancestral strain was used to found three replicate populations that were evolved previously under hyper-osmotic stress conditions for 750 generations. The ancestral and evolved strains were subsequently subjected to genotypic and phenotypic assays to characterize the extent of evolution that occurred over this long-term experiment. Viability under osmotic stress conditions appears increased in one of the evolved populations after preculture in sorbitol, relative to the Y12 ancestor, but not in the other two. Whole-genome sequencing identified 7 candidate mutations in the evolved strains, two of which were successfully validated with Polymerase Chain Reaction (PCR) and Sanger sequencing, and were found to result in nonsynonymous changes to genes. This work verifies that genetic changes arose over the course of a previous long-term evolution experiment, and suggests that the evolved Y12 strains adapted to sorbitol stress by an induced quiescence mechanism rather than a growth or viability phenotype.
Key Words: Experimental Evolution, Genetics, Osmotic Stress, Saccharomyces cerevisiae