Evolution of mating system and inbreeding depression in the Mimulus moschatus (Scrophulariaceae) alliance

Abstract

The transition from cross- to self-fertilization is considered a major pattern in the evolution of angiosperms. Yet, evolutionists continue to struggle to explain the evolutionary processes involved in maintaining both self- and cross-fertilization, which often occur within the same species. The diversity of mating systems suggests that selective pressures are also diverse, sometimes promoting selfing and other times promoting outcrossing. Inbreeding depression is commonly invoked as the primary selective force balancing the advantages of selfing or promoting outcrossing. The interaction between levels of inbreeding depression and mating system evolution has been fertile ground for both theoretical and empirical studies; however, a long-term patterns and processes remain ambiguous. I examined the relationship of inbreeding depression and outbreeding depression to mating system in a group of closely related Mimulus taxa, specifically incorporating information on their evolutionary relationships. I posed the following questions: Do selfing populations have low inbreeding depression and outcrossing populations have high outbreeding depression? Is selfing an evolutionary 'dead-end'? Are morphological traits correlated with molecular estimates of mating system? How evolutionary labile is mating system and inbreeding depression? Is inbreeding depression negatively correlated with outbreeding depression? Results from this study largely supported theoretical expectations. Inbreeding depression was lowest in the most selfing species and highest in the most outcrossing species. Outbreeding depression was not observed. Many populations actually experienced positive fitness consequences of between-population crosses. The question of selfing species being evolutionary dead-ends remained equivocal. Flower morphology was strongly related to molecular estimates of mating system as expected. Contrary to expectations, inbreeding depression appears to evolve much more quickly than does mating system. I conclude that in the Mimulus moschatus alliance, inbreeding depression is not as strong a selective force as often implied in the evolution of mating system. Although generally low, inbreeding depression can be high in some populations of rare taxa. Outbreeding depression was minimal. Last, inbreeding depression was positively correlated with outbreeding depression, suggesting that mediating the negative effects of inbreeding depression cannot occur by the introduction of foreign genes for many populations.