Studies of the genus Mimulus have made important contributions to our understanding of the effects of inbreeding on plant fitness. The genetic and phenotypic changes produced by inbreeding may have effects beyond the boundary of the inbreeding population, however, by altering ecological interactions with other species. Genetic studies of ecological interactions have focused primarily on adaptive evolutionary change, but few studies have considered the nonadaptive loss of genetic variation produced by inbreeding. Mimulus guttatus has proven to be an excellent system in which to study the interspecific effects of inbreeding. In greenhouse studies we have demonstrated that inbreeding in M. guttatus can reduce tolerance to attack by spittlebugs (Philaenus spumarius). Spittlebugs have almost no impact on outbred plants but can reduce biomass by as much as 30% in inbred plants. Herbivory greatly increases inbreeding depression and could therefore affect mating-system evolution, but the interaction between inbreeding and herbivory is two-sided. Spittlebugs can take 10% longer to develop on inbred plants and even display a preference for less inbred host plants, suggesting that inbreeding reduces host plant quality. Recently we have produced self and outcross progeny from four M. guttatus populations by controlled hand-pollinations in the greenhouse and transplanted these seedlings back into their home sites in Napa County, California. These ongoing studies are examining the natural colonization of inbred and outbred M. guttatus by a range of herbivores and will document the impact of herbivory in the field.

Key words: herbivory, host plant quality, inbreeding, mating system, Mimulus guttatus, tolerance