Background: 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 towards 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. Methodology: Using hand-pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. Main Results: 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. Conclusions: 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 towards complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplify a situation in which the mating system is likely not at an equilibrium with inbreeding depression.

Inbreeding depressionThis file contains the raw data (per individuals) used for the inbreeding depression and heterosis calculations. The file contains four pages: cross data, phenotypic data, longevity data and pollen/ovule data.Isolation_by_distanceThis file contains the Fst values and distance (km) between each pair of Linaria cavanillesii populations.