UC Berkeley

Unbiased genetic dissection of widely observable phenotypic traits in the wild has

long been the goal of evolutionary geneticists. Mice, bacteria, rice and stickleback fishes

have historically been among the prime model organisms in the field. Here, we leverage

the recent surge of Saccharomyces yeast as a model genus in ecology and evolution to

begin answering questions about the genetic basis of ancient trait differences that have

evolved between species, over long evolutionary time scales. Specifically, we want to

know what genetic mechanisms evolution has used to create new traits in the distant

past, and what biological functions have been the focus of adaptation in the past. I

introduce the field and our questions of interest in the introductory Chapter 1. Next,

Chapter 2 delves deeper into the methods that have been used in the past to dissect

interspecies genetics, reviewing the literature and drawing general conclusions from

what we have learned so far. In Chapter 3, we develop a new application of the

reciprocal hemizygote test on a genome-wide scale to drill down to the single gene level

and dissect the ability of S. cerevisiae to grow at high temperatures relative to other

Saccharomyces species. We uncover a suite of housekeeping genes genetically

responsible for this derived phenotype and reveal a likely defect in cell division in S.

paradoxus as the culprit for cell death at high temperatures. Finally, in Chapter 4, we

investigate two more species-specific differences: 1) resistance to the drug benomyl, a

microtubule poison and 2) cold tolerance. Unexpectedly, while microtubules are the

molecular target of benomyl, it is the genes encoding water channels in the cell

membrane tha seem to be at the genetic root of these phenotypes. Our data begin to

connect the dots between the growth advantage in S. paradoxus in benomyl and at low

temperatures, relative to S. cerevisiae.