Department of Botany and Zoology, Stellenbosch University, Matieland, South Africa.
Ecology and Evolution, Research School of Biology, The Australian National University, Acton, ACT, Australia.
Ann Bot. 2019 Jan 23;123(2):347-354. doi: 10.1093/aob/mcy083.
Selection exerted by pollinators on flowers is predicted to occur along two distinct axes. While pollinator attraction to flowers is governed by pollinator preferences, pollen transfer efficiency is mediated by the mechanical fit of pollinators to flower morphology. Although pollinator attraction in sexually deceptive orchids is typically underpinned by floral odour, morphological traits are expected to play a vital role in mechanical fit during floral contact with pollinators.
Here we utilize a comprehensive and novel procedure to test for pollinator-mediated selection through mechanical fit with the flower labellum in the orchid Chiloglottis trapeziformis. This approach combines detailed pollinator observations related to plant reproductive fitness with complementary experimental manipulation and phenotypic selection analysis.
Experiments with virgin flowers revealed that pollen removal occurs only during vigorous pseudocopulation. This behaviour involves male wasps that grasp the insectiform callus structure on the labellum while probing the labellum tip in a forward orientation. Both orientation and duration of pseudocopulation were significant predictors of pollen removal, confirming a direct relationship between pollinator behaviour and plant fitness. Controlled floral manipulation that either shortened or elongated the distance between the callus and the labellum tip detected no change in pollinator attraction. The duration of pseudocopulation, however, was significantly reduced on flowers with shortened or elongated callus-tip distances, consistent with stabilizing selection. Phenotypic selection analysis confirmed this prediction in natural populations by uncovering evidence for stabilizing selection on the distance between the callus and the labellum tip.
Our experimental manipulations and selection analysis in natural populations thus demonstrate stabilizing selection on the distance from the callus to the labellum tip, and illustrate the utility of employing multiple approaches to confirm selection exerted by pollinators on floral form.
传粉者对花的选择预计沿两个不同的轴发生。虽然传粉者对花的吸引力受传粉者偏好的支配，但花粉传递效率则由传粉者与花形态的机械适合度来调节。尽管有性欺骗兰花的传粉者吸引力通常以花的气味为基础，但形态特征在花与传粉者接触时的机械适合度中预计发挥着至关重要的作用。
在这里，我们利用一种全面而新颖的程序，通过与兰花 Chiloglottis trapeziformis 的唇瓣的机械适合度来测试传粉者介导的选择。这种方法结合了与植物生殖适应性相关的详细传粉者观察结果，以及补充的实验操作和表型选择分析。
对处女花的实验表明，花粉去除仅在剧烈的拟态交配期间发生。这种行为涉及到雄性黄蜂，它们在向前方向探测唇瓣尖端时抓住唇瓣上的昆虫状愈伤组织结构。拟态交配的方向和持续时间都是花粉去除的显著预测因子，这证实了传粉者行为与植物适应性之间的直接关系。对花的控制操作，无论是缩短还是延长愈伤组织和唇瓣尖端之间的距离，都不会改变传粉者的吸引力。然而，在缩短或延长愈伤组织-尖端距离的花朵上，拟态交配的持续时间显著缩短，这与稳定选择一致。自然种群中的表型选择分析证实了这一预测，表明对愈伤组织和唇瓣尖端之间距离的稳定选择。
我们在自然种群中的实验操作和选择分析表明，对愈伤组织到唇瓣尖端的距离存在稳定选择，并说明了采用多种方法来证实传粉者对花形态的选择的有效性。
