Widespread coexistence of self-compatible and self-incompatible phenotypes in a diallelic self-incompatibility system in Ligustrum vulgare (Oleaceae)

The breakdown of self-incompatibility (SI) in angiosperms is one of the most commonly observed evolutionary transitions. While multiple examples of SI breakdown have been documented in natural populations, there is strikingly little evidence of stable within-population polymorphism with both inbreeding (self-compatible) and outcrossing (self-incompatible) individuals. This absence of breeding system polymorphism corroborates theoretical expectations that predict that in/outbreeding polymorphism is possible only under very restricted conditions. However, theory also predicts that a diallelic sporophytic SI system should facilitate the maintenance of such polymorphism. We tested this prediction by studying the breeding system of Ligustrum vulgare L., an insect pollinated hermaphroditic species of the Oleaceae family. Using stigma tests with controlled pollination and paternity assignment of open-pollinated progenies, we confirmed the existence of two self-incompatibility groups in this sp...