Complex traits represent one of the greatest challenges in understanding genetics and
inheritance today, as these traits are dictated by more than a single gene—unlike Mendelian traits. Thermotolerance in the model organism Saccharomyces cerevisiae is one such trait that can also be quantified. Combining the measurement of growth rates at elevated temperatures with genetic analyses, such as QTL analysis, could help in understanding the inheritance of complex traits. In this master's thesis, we undertook the preparation of a thermotolerant strain through iterative crossings of strains isolated from nature. The purpose of the research was to obtain a strain that could be used in the future for QTL analysis to help us understand the genetic architecture of thermotolerance in S. cerevisiae. To quantify growth, we utilized the Pyphe software package, which was developed for the growth rate quantification of individual microbial colonies. We successfully obtained the thermotolerant TopT42 strain, which grew successfully at higher temperatures in liquid media than any of the parental strains isolated from nature. When TopT42 strain was grown at higher temperatures, we observed the emergence of small colonies, which we confirmed were caused by the loss of mitochondrial DNA. This results in one part of the cells in the culture obtaining energy through respiration, while the other part does so through fermentation when grown at elevated temperatures. We conclude that the new TopT42 strain could be further utilized in QTL analysis and contribute to a better understanding of thermotolerance in particular and complex traits in general in S. cerevisiae.
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