Inbreeding depression is high in a self-incompatible perennial herb population but absent in a self-compatible population showing mixed mating.

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self-incompatibility (SI) and outcrossing toward self-compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self-fertilization. In contrast, populations that show intermediate selfing rates (a mixed-mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self-fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed-mating populations that are similar to those expected for outcrossing populations. Using hand-pollination crosses, we compared levels of inbreeding depression and heterosis between populations of (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between-population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in has been recent. SC in thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.