Graduate Thesis Or Dissertation
 

Experimental Evolution of Roundup Resistance in Outcrossing Populations of Saccharomyces cerevisiae

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rx913x96m

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  • Evolve & Resequence (E&R) experiments subject laboratory populations to environments controlled by investigators, who then document the phenotypic and genomic changes that take place over many generations. These experiments provide powerful tools for testing of a wide variety of evolutionary questions, especially questions about the nature of adaptive traits. While any organism could be studied in this context, microbes are most practical, due to their quick reproductive cycles. One drawback to using microbes in E&R studies is that they reproduce asexually, which generally leads to slower adaptation that is mutation-limited. By comparison, E&R experiments with sexually reproducing populations that maintain high levels of standing genetic variation have an improved ability to dissect the genetics of complex adaptive traits. My thesis takes advantage of this powerful experimental technique by subjecting highly diverse, sexually-reproducing populations of the yeast Saccharomyces cerevisiae to a ten week E&R experiment to investigate the genetics of resistance to the herbicide RoundupTM, a trait with significant ecological relevance for a variety of species. The first major objective of my thesis was to identify specific genomic regions that might underlie RoundupTM resistance by tracking allele frequencies over time in evolving populations. A second objective was to determine whether two different selection treatments – a typical treatment involving strong constant selection pressure versus an increasing selection treatment starting with a low dose of RoundupTM that increased over time – led to different evolutionary outcomes. Our results uncovered several potential candidate regions that may underlie RoundupTM resistance and warrant further investigation. While the two selection treatments led to similar evolved phenotypes, and similar ability to localize potentially causative genes, we did observe evidence for a unique trait architecture that suggests that different regions may be selected for depending on the strength of the RoundupTM dose used. Our results provide insight into the consequences of chronic RoundupTM exposure on non-target organisms, and they also inform the general design strategies of future E&R experiments.
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