Genetics of Differences Between the Mating Systems of Mimulus guttatus and M. nasutus

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9727578 Willis The identification and analysis of quantitative trait loci (QTLs) offers a powerful tool with which to assess directly the genetic basis of evolved differences between interfertile organisms. One evolutionary transition in plants that offers a unique opportunity to address the genetic basis of adaptations is the evolution of self-fertilization from outcrossing, which is perhaps the most frequent evolutionary transition in angiosperms. The genus Mimulus (Scrophulariaceae), within which are many closely related and interfertile species or populations, provides an excellent model system for studying the evolution of self-fertilization and speciation. The common yellow monkeyflowers in the Mimulus guttatus species complex are arguably the most polytypic and most rapidly evolving members of the genus, and populations vary tremendously in terms of the degree of self-fertilization. There are two specific goals of the proposed study. First, we propose to conduct a thorough genetic analysis of the floral differences between a primarily outcrossing population of Mimulus guttatus and a highly selfing population of Mimulus nasutus in order to determine whether genes of large effect contribute to the extreme floral differences between these species. This goal will be accomplished through the mapping of quantitative trait loci affecting floral morphology using a high-resolution linkage map of the Mimulus genome based on microsatellite, AFLP, and RAPD markers. Second, we propose to test whether quantitative trait loci affecting floral traits that are correlated with selfing actually have direct effects on the rate of self-fertilization in a natural population. This goal will be accomplished by analyzing in the field the selfing rates of introgression lines containing a M. nasutus allele for floral QTLs in a M. guttatus background. These experiments will address long-standing and fundamental questions about the role of mutations of large effect on the evolution of adaptations. For example, a fundamental claim of neo-Darwinian evolution is that natural selection acts on very slight phenotypic differences among individuals in a population. If this subtle individual variation has a genetic component, then adaptations will result from the steady fixation of genes of small effect. Conversely, others have argued that adaptation frequently results from drastic changes at a very few genetic loci. These experiments will help to answer basic questions such as this. They will also contribute to our basic understanding of how and why self-fertilization and other forms of inbreeding evolve in natural and domesticated populations of plants and animals.