Voillemot Marie, Pannell John R
Department of Ecology and Evolution University of Lausanne Lausanne Switzerland.
Ecol Evol. 2017 Sep 12;7(20):8535-8544. doi: 10.1002/ece3.3354. eCollection 2017 Oct.
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.
高度的近亲繁殖衰退被认为是阻止雌雄同体植物从自交不亲和（SI）和异交向自交亲和（SC）和自交进化转变的主要因素之一。然而，当自交确实进化时，近亲繁殖衰退可以迅速消除，从而允许完全自花受精的进化。相比之下，表现出中等自交率（混合交配系统）的种群通常表现出与异交物种相似的近亲繁殖衰退水平，这表明对近亲繁殖的选择可能是阻止向完全自花受精转变的原因。这意味着，种群间杂交应该揭示出混合交配种群的杂种优势模式，类似于异交种群预期的模式。我们通过人工授粉杂交，比较了车前科一种多年生草本植物不同种群间的近亲繁殖衰退水平和杂种优势，该植物表现出不同的交配系统。自交不亲和种群表现出高度的近亲繁殖衰退，而表现出混合交配的自交亲和种群则没有近亲繁殖衰退。相比之下，我们发现基于种群间杂交的杂种优势在自交不亲和种群和自交亲和种群中相似。我们的结果与衍生的自交亲和种群中近亲繁殖衰退的快速消除一致，尽管混合交配持续存在。然而，向自交亲和转变后异交的维持与这样的预测不一致，即已经消除近亲繁殖衰退的种群应该向完全自交进化，这表明该植物向自交亲和的转变是最近发生的。因此，该植物的自交亲和体现了一种交配系统可能与近亲繁殖衰退不平衡的情况。
