The genetic basis of species differences provides insight into past evolutionary change and has long been a subject of contention among evolutionary biologists. Most traits that differentiate strains or species do not show simple Mendelian inheritance, but vary continuously in segregating hybrid populations. QTL studies have suggested that genes of large effect play an important role in the phenotypic differences between crop species and their wild relatives, and also underlie divergence in some wild plant systems. In this study, we investigate the genetic architecture of phenotypic differences between two flowering plant species with highly divergent mating systems: Mimulus guttatus (outcrossing) and M. nasutus (selfing). In order to characterize the nature of this floral divergence at the level of individual loci, we constructed a framework linkage map of the hybrid genome based on microsatellites, AFLPs, and gene-based markers. We locate and characterize quantitative trait loci (QTLs) underlying traits associated with the divergent mating systems. We also use this map to investigate the genetic basis of reproductive isolating barriers, including segregation distortion and partial hybrid sterility. These investigations provide an unusually detailed view of the complex genetic changes that occurred during the evolution of self-fertilization.

Key words: mating system, Mimulus, QTL mapping, reproductive isolation, self-fertilization, speciation