Rivkin L Ruth, Case Andrea L, Caruso Christina M
Department of Integrative Biology, University of Guelph, Guelph, Ontario, N1G 2W1, Canada.
Department of Biological Sciences, Kent State University, Kent, Ohio, 44242.
Evolution. 2015 May;69(5):1232-43. doi: 10.1111/evo.12654. Epub 2015 Apr 29.
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其表型的频率时，选择就是频率依赖的。频率依赖型选择在雌雄异株植物中应该很常见，在这类植物中个体为雌性或雌雄同体；如果雌性的适合度受到用于使胚珠受精的花粉可用性的限制，那么它们在稀有时应该比在常见时有更高的适合度。为了测试雌性的适合度是否是频率依赖的，我们在雌雄异株的北美半边莲阵列中操纵了性别比例。为了测试适合度是否因花粉可用性的变化而具有频率依赖性，我们比较了开放授粉和人工辅助授粉的植株。正如如果适合度是负频率依赖时所预期的那样，开放授粉的雌性在稀有时比在常见时产生更多的种子。然而，人工辅助授粉的雌性在稀有时也产生更多的种子，这表明花粉可用性的变化不是适合度频率依赖的原因。相反，适合度是频率依赖的，因为人工辅助授粉和开放授粉的雌性在稀有时比在常见时开放更多的花。即使在繁殖不受花粉限制时，这种开花速率的可塑性也可能导致适合度具有频率依赖性，从而扩展了频率依赖型选择在雌雄异株物种中起作用的条件。
