Graff Alison
Biology Department, Evolutionary and Population Biology Program, Washington University, 1 Brookings Drive, Campus Box 1137, St. Louis, Missouri, 63130.
Missouri Botanical Garden, P.O. Box 299, St. Louis, Missouri, 63166.
Evolution. 1999 Dec;53(6):1714-1722. doi: 10.1111/j.1558-5646.1999.tb04556.x.
The spatial distribution of females and hermaphrodites within gynodioecious populations is expected to exert considerable selective pressure on gender fitness through pollen limitation of seed set. If pollen flow is predominantly local, seed set in individual plants may be sensitive to the proximity of pollen donors; pollen limitation of seed set may occur if hermaphrodites are locally rare. Under such circumstances, female fitness will be negatively frequency dependent and hermaphrodite fitness will be positively frequency dependent. Given local seed dispersal, a nonrandom clumped distribution of the genders is expected in gynodioecious populations due to the heritability of gender in gynodioecious species. If gender fitness is frequency dependent, such structure should favor hermaphrodites and select against females. To test this hypothesis, I quantified the distribution of the genders in terms of nearest neighbors and neighborhood sex ratio in two populations of gynodioecious Sidalcea malviflora malviflora. I then measured the effect of neighborhood sex ratio on open-pollinated seed set and pollen limitation in both manipulated and unmanipulated neighborhoods. Results indicate that the genders have a patchy distribution and that both genders are pollen limited and show an increase in seed set with an increase in neighborhood hermaphrodite frequency. The observed population sex structure favors hermaphrodites and disadvantages females. These results highlight the importance that population-level traits can have in determining individual fitness and the evolution of sex ratios in gynodioecious species.
在雌雄异株种群中，雌性和雌雄同体植株的空间分布预计会通过种子结实的花粉限制对性别适合度施加相当大的选择压力。如果花粉传播主要是在本地进行，那么单株植物的种子结实可能会对花粉供体的 proximity 敏感；如果雌雄同体植株在本地较为稀少，就可能会出现种子结实的花粉限制。在这种情况下，雌性适合度将呈负频率依赖，而雌雄同体适合度将呈正频率依赖。考虑到本地种子传播，由于雌雄异株物种中性别具有遗传性，预计雌雄异株种群中性别会呈现非随机的聚集分布。如果性别适合度是频率依赖的，这种结构应该有利于雌雄同体植株，而不利于雌性植株。为了验证这一假设，我在两个雌雄异株的小花西达葵种群中，根据最近邻植株和邻域性别比例对性别分布进行了量化。然后，我在经过处理和未处理的邻域中测量了邻域性别比例对异花授粉种子结实和花粉限制的影响。结果表明，两种性别呈现斑块状分布，并且两种性别都受到花粉限制，且随着邻域雌雄同体频率的增加，种子结实量也会增加。观察到的种群性别结构有利于雌雄同体植株，而不利于雌性植株。这些结果凸显了种群水平特征在决定雌雄异株物种个体适合度和性别比例进化方面可能具有的重要性。
