Understanding the strength and nature of reproductive isolation is necessary for insight into the processes of speciation and introgression. Here, we investigate prezygotic and postzygotic reproductive barriers between Mimulus guttatus and Mimulus nasutus, two wildflower species with highly divergent mating systems. These species often grow in sympatry and produce fertile hybrid offspring where they co-occur. Recent work has shown that Dobzhansky - Muller incompatibilities contribute to F1 and F2 hybrid sterility between a single line-cross between M. nasutus and M. guttatus. In this work, we utilize a common garden experiment involving a number of sympatric and allopatric populations of M. nasutus, M. guttatus, F1, and F2 reciprocal hybrids to determine whether or not comparable reproductive isolation exists throughout the species' range. In related field experiments, we demonstrate postzygotic isolation in the form of reduced germination rates of F1 and F2 hybrids. In these field studies, reciprocal F2 crosses and F1 plants with M. guttatus as the maternal parent showed significantly reduced germination success relative to pure M. guttatus or pure M. nasutus. Hybrid F1 and F2 seeds with M. guttatus cytoplasm also germinated at significantly lower rates than those hybrids with M. nasutus cytoplasm, indicating that incompatibles exist between hybrid nuclear genes and M. guttatus cytoplasm, but not with M. nasutus cytoplasm. Albino seedlings were found in all F2 crosses, but were not found in any of the F1 or parental crosses. These lethal achlorotic individuals occurred in a proportion not significantly different from 1/16, which suggests that the segregation of a single two-locus Dobzhansky-Muller incompatibility results in this particular form of hybrid breakdown. These results give us insight into the nature of reproductive isolation. In other studies, we are documenting prezygotic isolating mechanisms, natural hybridization rates, and introgression in sympatric areas.

Key words: Mimulus guttatus , Mimulus nasutus , Dobzhansky-Muller interactions, hybridization, reproductive isolation