| Understanding the genetic and functional basis for natural variation in traits and fitness represents a significant challenge for evolutionary biology. In this thesis, I examine natural genetic, functional, and trait variation within a well-characterized transcriptional regulatory network of the yeast Saccharomyces cerevisiae. Working with yeast allows large sample collection, ease of phenotype measurements and fitness evaluation, and subsequent hypothesis driven testing of variants through a multitude of laboratory techniques. To take advantage of this system, I collected 77 new S. cerevisiae strains from three habitats and three continents, making this the largest natural isolate collection with sufficient depth for population genetic study. In order to understand the functional consequences of genetic variation, I collected three kinds of data for the glucose repression network. First, traditional sequence variation was analyzed throughout the glucose repression network. This included estimating levels of segregating genetic variation from several relevant genomic regions, testing for natural selection, and reconstructing the genealogical history of the strains. Second, growth rates were measured on several media, both as a proxy for fitness and to characterize an ecologically relevant phenotype. Measurements were made on two natural habitat media (grape and fig extracts), and three synthetic media (glucose, raffinose, and galactose) known to affect the glucose repression network. Third, gene expression was measured from functionally relevant genes throughout the galactose metabolism pathway, which is part of the glucose repression network. Measurements were made using a sensitive and precise method from strains growing on the three synthetic media types. Collectively, these studies provide a rich and unique dataset that builds on sequence variation to provide information about its functional and fitness consequences within an ecologically relevant range of defined habitats. This information provides insights into the genetic basis for trait and fitness variation within natural populations, for functional integration across a transcriptional gene regulatory network, and for the canalizing effects of domestication of widely utilized laboratory yeast strains. |
