Moeller David A, Geber Monica A
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, New York 14853, USA.
Evolution. 2005 Apr;59(4):786-99. doi: 10.1554/04-656.
The repeated evolutionary transition from outcrossing to self-pollination in flowering plants has been suggested to occur because selfing provides reproductive assurance. Reports from biogeographical and ecological surveys indicate that selfing taxa are often associated with stressful and ephemeral environments, situations in which plant abundance is low (e.g., Baker's law) and with novel plant communities, however experimental tests of ecological hypotheses are few. In this study, we examined the ecological context of selection on mating system traits (herkogamy and protandry) in a California annual, Clarkia xantiana, where natural selfing populations differ from outcrossing populations in that they are often of small size or low density and occur mainly outside the range of pollinator-sharing congeners. We constructed artificial populations of plants with broad genetic variation in floral traits and manipulated two ecological factors, plant population size, and the presence versus absence of pollinator-sharing congeners, in the center of the geographic range of outcrossing populations. We found evidence for context-dependent selection on herkogamy and protandry via female fitness in which reduced traits, which promote autonomous selfing, were favored in small populations isolated from congeners whereas selection was comparatively weak in large populations or when congeners were present. In small, isolated populations, the fertility of plants with low herkogamy or protandry was elevated by 66% and 58%, respectively, compared to those with high herkogamy or protandry. The presence of pollinator-sharing congeners augmented bee visitation rates to C. xantiana flowers by 47% for all bees and by 93% for pollen specialists. By facilitating pollinator visitation, congeners mitigated selection on mating system traits in small populations, where outcross mating success is often low (the Allee effect). We also found support for the hypothesis that pollinator availability directly influenced variation in the strength of selection on herkogamy among populations. The striking parallels between our experimental results and patterns of variation in ecological factors across the geographic range of outcrossing and selfing populations suggest that reproductive assurance may play a central role in directing mating system evolution in C. xantiana.
开花植物从异花授粉到自花授粉的反复进化转变被认为是因为自交提供了繁殖保障。生物地理学和生态学调查的报告表明，自交类群通常与压力大且短暂的环境相关，在这些环境中植物丰度较低（例如贝克法则），并且与新的植物群落相关，然而对生态假说的实验检验却很少。在本研究中，我们研究了加利福尼亚一年生植物黄花克拉花（Clarkia xantiana）中交配系统性状（雌雄异位和雄蕊先熟）选择的生态背景，其中自然自交种群与异交种群的不同之处在于，它们通常规模小或密度低，并且主要出现在共享传粉者的同属植物范围之外。我们构建了具有广泛花部性状遗传变异的人工植物种群，并在异交种群地理范围的中心操纵了两个生态因素，即植物种群大小以及共享传粉者的同属植物的有无。我们发现了通过雌性适合度对雌雄异位和雄蕊先熟进行背景依赖选择的证据，其中促进自主自交的性状减少在与同属植物隔离的小种群中受到青睐，而在大种群中或有同属植物存在时选择相对较弱。在小的、隔离的种群中，与雌雄异位或雄蕊先熟程度高的植物相比，雌雄异位或雄蕊先熟程度低的植物的育性分别提高了66%和58%。共享传粉者的同属植物的存在使所有蜜蜂对黄花克拉花的访花率提高了47%，对花粉专性传粉者的访花率提高了93%。通过促进传粉者访花，同属植物减轻了小种群中对交配系统性状的选择，在小种群中异交成功往往较低（阿利效应）。我们还发现支持以下假说的证据，即传粉者可用性直接影响了种群间雌雄异位选择强度的变化。我们的实验结果与异交和自交种群地理范围内生态因素变化模式之间惊人的相似之处表明，繁殖保障可能在指导黄花克拉花的交配系统进化中起核心作用。
