Jiang Xian-Feng, Zhu Xing-Fu, Li Qing-Jun
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, China.
University of Chinese Academy of Sciences, Beijing, China.
AoB Plants. 2018 May 2;10(3):ply022. doi: 10.1093/aobpla/ply022. eCollection 2018 Jun.
Distyly is a widespread floral polymorphism characterized by the flowers within a population showing reciprocal placement of the anthers and stigma. Darwin hypothesizes that distyly evolves to promote precise pollen transfer between morphs. exhibits two types of anther heights, and these two types of anthers show pollen of two different size classes. To understand whether the stigma could capture more pollen grains from the anthers of the pollen donor as the separation between the stigma of pollen receiver and the anther of pollen donor decreased, the present research assessed the source of the pollen load in a series of open-pollinated flowers with continuous variation of style lengths. Individuals with continuous variation of style length were tagged, and the selected flowers in the tagged plants were emasculated the day before dehiscence. The stigma of the emasculated flowers was fixed in fuchsin gel at the end of blooming. We assessed the pollen sources on each stigma by taking photos under a microscope and measured the diameter of each conspecific pollen grain with ImageJ. We found that a shorter distance from the stigmas to the anthers of a pollen donor gave the flower a higher capacity to receive pollen from those anthers. Our result provides a new evidence that distyly could promote the pollen transfer between morphs, which is consistent with Darwin's hypothesis of disassortative pollination. An alternative hypothesis for the evolution of distyly (e.g. selfing avoidance) might also be true, but less likely, because self-incompatibility would greatly avoid self-fertilization for many distylous species.
花柱异长是一种广泛存在的花形态多态现象，其特征是种群内的花朵表现出花药和柱头的相互位置关系。达尔文推测花柱异长的进化是为了促进不同形态间精确的花粉传递。花柱异长表现出两种花药高度类型，且这两种类型的花药呈现出两种不同大小等级的花粉粒。为了了解随着花粉接受者的柱头与花粉供体的花药之间的距离减小，柱头是否能从花粉供体的花药中捕获更多花粉粒，本研究评估了一系列花柱长度连续变化且进行了开放授粉的花朵中花粉负载的来源。对花柱长度连续变化的个体进行标记，并在花药开裂前一天对标记植株中选定的花朵进行去雄。在花朵开放结束时，将去雄花朵的柱头固定在品红凝胶中。我们通过在显微镜下拍照来评估每个柱头上的花粉来源，并用ImageJ测量每个同种花粉粒的直径。我们发现，柱头到花粉供体花药的距离越短，花朵从这些花药中接受花粉的能力就越高。我们的结果提供了新的证据，表明花柱异长能够促进不同形态间的花粉传递，这与达尔文的异花授粉假说一致。花柱异长进化的另一种假说（例如避免自交）可能也是正确的，但可能性较小，因为对于许多花柱异长的物种来说，自交不亲和会极大地避免自花受精。
