Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT, USA.
School of Forestry and Environmental Studies, Yale University, New Haven, CT, USA.
Ann Bot. 2019 Jan 23;123(2):381-390. doi: 10.1093/aob/mcy117.
Enlarged sterile flowers on the periphery of inflorescences increase the attractiveness of floral displays, and previous studies have generally demonstrated that these have positive effects on insect visitation and/or reproductive success. However, experiments have not specifically been designed to examine the benefits of sterile flowers under conditions that reflect the early stages in their evolution, i.e. when plants that produce sterile flowers are at low frequency.
Over three years, three experiments were performed in natural populations of Viburnum lantanoides, which produces sterile marginal flowers (SMFs). The first experiment established that fruit production in V. lantanoides increases with the receipt of outcross pollen. The second tested the role of SMFs under extant conditions, comparing fruit production in two populations composed entirely of intact plants or entirely of plants with the SMFs removed. The third was designed to mimic the presumed context in which SMFs first evolved; here, SMFs were removed from all but a few plants in a population, and rates of insect visitation and fruit set were compared between plants with intact and denuded SMFs.
In comparing whole populations, the presence of SMFs nearly doubled fruit set. Under simulated 'ancestral' conditions within a population, plants with intact SMFs received double the insect visits and produced significantly more fruits than denuded plants. There was no significant effect of the number of inflorescences or fertile flowers on insect visitation or fruit set, indicating that the presence of SMFs accounted for these differences.
The presence of SMFs significantly increased pollinator attraction and female reproductive success both in contemporary and simulated ancestral contexts, indicating that stabilizing selection is responsible for their maintenance, and directional selection likely drove their evolution when they first appeared. This study demonstrates a novel approach to incorporating historically relevant scenarios into experimental studies of floral evolution.
花序边缘增大的不育花增加了花展示的吸引力，先前的研究普遍表明，这些对昆虫访问和/或生殖成功有积极影响。然而，实验并没有专门设计来研究不育花在反映其早期进化阶段的条件下的好处，即在产生不育花的植物频率较低的情况下。
在三年的时间里，对产生不育边缘花（SMFs）的南烛（Viburnum lantanoides）自然种群进行了三项实验。第一项实验确定了南烛的果实产量随着异交花粉的接收而增加。第二项实验在现存条件下测试了 SMFs 的作用，比较了完全由完整植株或完全由去除 SMFs 的植株组成的两个种群的果实产量。第三项实验旨在模拟 SMFs 最初进化的假定情境；在这里，从一个种群中的除了少数几株植物外，去除了所有的 SMFs，并比较了具有完整和光秃 SMFs 的植物之间的昆虫访问率和果实结实率。
在比较整个种群时，SMFs 的存在使果实结实率几乎增加了一倍。在种群内模拟“祖先”条件下，具有完整 SMFs 的植株所接受的昆虫访问量是光秃植株的两倍，并且产生的果实明显多于光秃植株。花序数量或可育花的数量对昆虫访问或果实结实率没有显著影响，这表明 SMFs 的存在解释了这些差异。
SMFs 的存在显著增加了传粉者的吸引力和雌性生殖成功，无论是在当代还是模拟祖先的情况下，这表明稳定选择是其维持的原因，而当它们首次出现时，定向选择可能推动了它们的进化。这项研究展示了一种将历史相关情景纳入花卉进化实验研究的新方法。
