The specific aims of this proposal are to clone, identify and sequence the genes responsible for one genetically complex trait, the ability to grow at 42 degrees. The sequences of the relevant genes will be determined from strains able to grow at 42 degrees and compared with the S288c reference strain (unable to grow at 42 degrees) sequence. The cloned genes will be used to make a series of defined isogenic strains which differ at the high temperature growth loci. These isogenic strains will be tested for high temperature growth to determine the contribution of each allele/locus to phenotype as well as the interactions of the loci (dominant vs. recessive, additive vs. epistatic) i precise genetic and molecular terms. This proposal is health related in three ways. First, the ability to grow at high temperatures is a virulence trait in S. cerevisiae and other fungi. Understanding the ability to grow at high temperatures will aid our understanding of fungal pathogenesis. Second, this study on one virulence trait is a prelude to a dissection of other fungal virulence traits and to the development of S. cerevisiae as a model for the pathogenic fungi. This study will tell us how an opportunistic pathogen has emerged from the gene pool of a harmless saprophyte. A better understanding of fungal pathogenesis will also aid the development of antifungal drugs. Finally, this proposal addresses a major problem in genetics -- complex polygenic traits and genetic diversity. Complex polygenic traits are not well understood. Many human genetic diseases are quantitative in nature. The development of a microbial model for complex polygenic traits will allow these traits to be understood in precise genetic and molecular terms and thereby aid the understanding of quantitative traits in higher organisms. The broad long term objectives of this proposal are: l.) the development of S. cerevisiae as a microbial model for the analysis of complex polygenic traits, 2.) the identification of the genetic mechanisms which allow specific S. cerevisiae strains to grow at very high temperatures, and 3.) the development of S. cerevisiae as genetically manipulable model for the pathogenic fungi and as a host for the interspecific reconstitution of fungal virulence.