Data from: Frequency-dependent fitness in gynodioecious Lobelia siphilitica

Selection is frequency dependent when an individual's fitness depends on the frequency of its phenotype. Frequency-dependent selection should be common in gynodioecious plants, where individuals are female or hermaphroditic; if the fitness of females is limited by the availability of pollen to fertilize their ovules, then they should have higher fitness when rare than when common. To test whether the fitness of females is frequency dependent, we manipulated the sex ratio in arrays of gynodioecious Lobelia siphilitica. To test whether fitness was frequency dependent because of variation in pollen availability, we compared open-pollinated and supplemental hand-pollinated plants. Open-pollinated females produced more seeds when they were rare than when they were common, as expected if fitness is negatively frequency dependent. However, hand-pollinated females also produced more seeds when they were rare, indicating that variation in pollen availability was not the cause of frequency-dependent fitness. Instead, fitness was frequency dependent because both hand- and open-pollinated females opened more flowers when they were rare than when they were common. This plasticity in the rate of anthesis could cause fitness to be frequency dependent even when reproduction is not pollen limited, and thus expand the conditions under which frequency-dependent selection operates in gynodioecious species.当个体的适合度取决于自身表型的频率时，自然选择便呈现频率依赖性特征。频率依赖选择在雌全异株植物（gynodioecious plants）中应属普遍现象，这类植物的个体可划分为雌性植株与雌雄同株植株；若雌性个体的适合度受限于为其胚珠完成受精的花粉供给量，那么当雌性个体在种群中较为稀有时，其适合度理应高于种群中占比较高时的水平。为检验雌性个体的适合度是否具有频率依赖性，我们在人工构建的雌全异株物种锡利齐山梗菜（Lobelia siphilitica）群落中对性比进行了操控。为探究适合度的频率依赖性是否由花粉可获得性的差异所介导，我们设置了自然开放传粉与人工辅助传粉两组处理进行对比。实验结果显示，自然开放传粉的雌性植株在种群中稀有时所结种子量显著高于其常见时的水平，这契合适合度呈负频率依赖的理论预期。但值得注意的是，人工辅助传粉的雌性植株同样表现出稀有时结籽更多的现象，这表明花粉可获得性的差异并非导致适合度频率依赖性的原因。进一步分析表明，适合度的频率依赖性实则源于两类传粉处理下的雌性植株，在种群稀有时开放的花朵数量均显著多于常见时。这种开花速率（anthesis）的表型可塑性，即便在繁殖过程不受花粉限制的情况下，也可引发适合度的频率依赖性，进而拓展了频率依赖选择在雌全异株物种中发挥作用的情境范围。