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 species. We also demonstrated the occurrence of self-compatible individuals in different populations of Western Europe arising from a mutation affecting the functioning of the pollen component of SI. Our results show that the observed low frequency of self-compatible individuals in natural populations is compatible with theoretical predictions only if inbreeding depression is very high.

被子植物中自交不亲和性（SI）的破坏是最常见的进化转变之一。虽然已经在自然种群中记录了多个 SI 分解的例子，但几乎没有证据表明近交（自我相容）和异交（自我不相容）个体具有稳定的种群内多态性。这种育种系统多态性的缺失证实了理论预期，即只有在非常有限的条件下才可能发生内/近交多态性。然而，理论还预测二等位孢子体 SI 系统应该有助于维持这种多态性。我们通过研究女贞子的繁殖系统来验证这一预测L.，木犀科的昆虫授粉雌雄同体物种。使用带有受控授粉和开放授粉后代的亲子关系分配的柱头测试，我们证实了该物种中存在两个自交不亲和群体。我们还证明了由于影响 SI 花粉成分功能的突变而在西欧的不同人群中出现了自我相容的个体。我们的研究结果表明，只有在近交衰退非常高的情况下，观察到的自然种群中自我相容个体的低频率才与理论预测相符。