UC Berkeley

A key goal of evolutionary biology is understanding how organisms build new traits. Many adaptations that distinguish taxa evolved so long ago that they are separated across species barriers, and many of these traits also have a complex genetic basis. In this thesis, I have leveraged advances in tools for interspecies genetics to illuminate the genetic underpinnings of thermotolerance evolution in Saccharomyces yeast, as a case study. We sought to improve the statistical power of screens to detect genetic determinants of this interspecific trait, both through computational advances in data processing and further development of the screening methodology. We wanted to know what traces of the putative ancient selective sweep(s) on thermotolerance factors might linger in the genomes of modern yeasts. We were also curious how initial conditions the evolutionary landscape might have differed from those further along during the evolutionary acquisition of thermotolerance. In Chapter 1, we introduce the field, and the questions within the field that we studied specifically. In Chapter 2, we investigate the population and comparative genetics of thermotolerance loci, as well as the mechanisms by which one key allele contributes to the phenotype. In Chapter 3, we use a barcoded version of the reciprocal hemizygosity analysis via sequencing screening technique to identify dozens of candidate genes involved in the thermotolerance phenotype. In Chapter 4, we delve into thermotolerance across a range of temperatures and discuss models for the early evolutionary landscape of the thermotolerance divergence. Lastly, in Chapter 5, we use directed evolution to gain insight into adaptations that allow a thermosensitive Saccharomyces yeast to acquire thermotolerance.