'Anti-bee' and 'pro-bird' changes during the evolution of hummingbird pollination in Penstemon flowers

Floral phenotypes may be as much the result of selection for avoidance of some animal visitors as selection for improving the interaction with better pollinators. When specializing on hummingbird-pollination, Penstemon flowers may have evolved to improve the morphological fit between bird and flower, or to exclude less-efficient bees, or both. We hypothesized how such selection might work on four floral characters that affect the mechanics of pollen transfer: anther/stigma exsertion, presence of a lower corolla lip, width of the corolla tube, and angle of flower inclination. We surgically modified bee-pollinated P. strictus flowers changing one trait at a time to make them resemble hummingbird-pollinated P. barbatus flowers, and measured pollen transfer by bumblebees and hummingbirds. Results suggest that, apart from 'pro-bird' adaptations, specific 'anti-bee' adaptations have been important in shaping hummingbird-flowers. Moreover, some trait changes may have been selected for only if changing in concert with other traits.     

The ultraviolet colour component enhances the attractiveness of red flowers of a bee-pollinated plant

蜂媒花很少呈红色,这可能是由于蜂类的眼睛缺少红光受体,因而较难搜寻红色目标。尽管有实验研究过蜂类对红色的反应,但所用目标多为纯红色。而人类感受到的红色中其实包含细微的色调差别,这些差别还甚少受到关注。我们在本研究中检验了如下假说：红花中的紫外反射组分能够增加其彩调反差,因而增强其对蜂类的吸引力。紫草科的密花滇紫草(Onosma confertum)由熊蜂传粉,其花为红色,但紫外反射明显。本研究利用该植物设计了表型操控实验,在野外检验了紫外反射对传粉者访花行为的影响。我们还在受控环境下检验了无经验熊蜂的色彩偏好。利用两个色彩模型评估熊蜂的色彩感受,用彩调反差和明度反差两个参数进行估计。研究发现,自然状态下及无经验的饲养熊蜂均偏好访问具有紫外反射的红色目标。色彩模型分析表明,具紫外反射的红花对蜂而言色彩更纯,且与叶片背景的彩调反差更大;而它们与无紫外反射的红花在明度反差方面(以绿光受体的反差估计)表现相似。这些结果表明,密花滇紫草红色花朵中的紫外反射组分增加了其彩调反差,增强了其对熊蜂的吸引力。我们推测,由于某些传粉者(例如蜂类)无红光受体,对由它们传粉的红色花而言,光谱中的副反射特征(例如紫外反射)可能是其花色中的必要组分。