Data from: Costs of selfing prevent the spread of a self-compatibility mutation that causes reproductive assurance

In flowering plants, shifts from outcrossing to partial or complete self-fertilization have occurred independently thousands of times, yet the underlying adaptive processes are difficult to discern. Selfing’s ability to provide reproductive assurance when pollination is uncertain is an oft-cited ecological explanation for its evolution, but this benefit may be outweighed by genetic costs diminishing its selective advantage over outcrossing. We directly studied the fitness effects of a self-compatibility (SC) mutation that was backcrossed into a self-incompatible (SI) population of Leavenworthia alabamica, illuminating the direction and magnitude of selection on the mating-system modifier. In array experiments conducted in two years, SC plants produced 17-26% more seed, but this advantage was counteracted by extensive seed discounting -- the replacement of high-quality outcrossed seeds by selfed seeds. Using a simple model and simulations, we demonstrate that SC mutations with these attributes rarely spread to high frequency in natural populations, unless inbreeding depression falls below a threshold value (0.57 ≤ threshold ≤ 0.70) in SI populations. A combination of heavy seed discounting and moderate inbreeding depression likely explains why outcrossing adaptations such as self-incompatibility are maintained generally, despite persistent input of selfing mutations and frequent limits on outcross seed production in nature.在显花植物中，从异交（outcrossing）转向部分或完全自花受精的演化事件已独立发生数千次，但其背后的适应性演化过程仍难以厘清。当传粉过程存在不确定性时，自交能够提供生殖保障（reproductive assurance），这是学界常引用的自交演化生态学解释，但该益处可能被遗传成本抵消，进而削弱自交相较于异交的选择优势。我们直接研究了经回交转入阿拉巴马列瓦沃西亚草（Leavenworthia alabamica）自交不亲和性（self-incompatible, SI）种群的自交亲和性（self-compatibility, SC）突变的适合度效应，阐明了交配系统修饰基因上选择作用的方向与强度。在两年开展的田间阵列实验中，自交亲和植株的结籽量较对照高出17%至26%，但该优势被广泛存在的种子贴现（seed discounting）抵消——即自交种子替代了高品质的异交种子。通过构建简单模型与模拟实验，我们证明了具备上述特征的自交亲和突变体极少能在自然种群中扩散至高频，除非自交不亲和种群中的近交衰退（inbreeding depression）阈值降至0.57 ≤ δ_threshold ≤ 0.70。鉴于普遍存在的重度种子贴现与中度近交衰退，即便自交突变持续产生、且自然环境中异交结籽量常受限制，为何自交不亲和这类异交适应性状仍能得以维持，这一现象或可由此得到合理解释。