EF: Ecological Genetics of Host-Specific Species Formation

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Ecological Genetics of Host-Specific Species FormationWilliam T. Starmer and Scott E. ErdmanSyracuse UniversityThe central goal of the research is to integrate knowledge of the ecology and genetics of microorganisms in a single system to understand the mechanisms responsible for formation of new species. The project will focus on fungal speciation mechanisms and target two closely related yeast species that live in different types of columnar cacti in the Sonoran Desert. The two yeasts, within the Pichia amethionina complex, only live in rotting stems of cacti and have close ecological associations with other cactus-specific yeasts and drosophila. The two cactus types (Stenocereus and Pachycereus) differ from one another in their stem chemistry. Earlier investigations have demonstrated that an abundant natural product in the form of a triterpene glycoside is found only in Stenocereus species and inhibits growth of one of the yeast species, thus preventing it from growing in that cactus. Genetic studies have shown the inhibition to be due to at least one gene difference between the two cactus dwelling yeast species. A genome-wide knockout library of the common baker's yeast Saccharomyces cerevisae is being employed by this project to identify and study candidate genes that may be identical to the cactus-yeast genes responsible for the species diversification in the Pichia amethionina complex. In general, identification of candidate genes, in the whole genome, provides information on potential physiological function when challenged with the host chemicals and thus an overview of potential cellular effects experienced by the yeast in the natural environment.There are four phases of the research: 1) Catalog all genes that can provide resistance to the triterpene glycosides. This work will be done using modern yeast genetic tools and knowledge of the genomics of S. cerevisae. 2) Determine which genes in the catalog can confer resistance in the cactus-yeast. This work will utilize insertional mutagenesis and expression vectors as tools. 3) Determine which genes that can confer resistance actually do confer resistance in nature. This work will use field surveys and standard laboratory genetic crosses to find the actual genes responsible. 4) Discover the ecological reasons (limits and constraints) that restrict the possible solutions to the actual solutions found in nature. Ecological experimentation in the laboratory using cactus tissue, drosophila, and associated yeasts will be used to find the connection between the ecology and genetics responsible for the actual solution and diversification.The project will be significant to our understanding of fundamental processes involved in the formation of new species. Detailed molecular mechanisms in relation to ecological constraints will provide a unique view of how microorganisms adapt to novel and exotic chemicals in their natural environment and why some genetic solutions are employed more frequently or with preference relative to others.