Pollen competition between morphs in a pollen-color dimorphic herb and the loss of phenotypic polymorphism within populations

Flower color polymorphism is relatively uncommon in natural flowering plants, suggesting that maintenance of different color morphs within populations is difficult. To address the selective mechanisms shaping pollen-color dimorphism, pollinator preferences and reproductive performance were studied over three years in Epimedium pubescens in which some populations had plants with either green or yellow pollen (and anthers). Visitation rate and pollen removal and receipt by the bee pollinator (Andrena emeishanica) did not differ between the two color morphs. Compared to the green morph, siring success of the yellow morph’s pollen was lower, but that of mixtures of pollen from green and yellow morphs was lowest. This difference, corresponding to in and ex vivo experiments on pollen performance, indicated that pollen germination, rather than tube growth, of the green morph was higher than that of the yellow morph and was seriously constrained in both morphs if a pollen competitor was present. A rare green morph may invade a yellow-morph population, but the co-existence of pollen color variants is complicated by the reduced siring success of mixed pollinations. Potential pollen competition between morphs may have discouraged the maintenance of multiple phenotypes within populations, a cryptic mechanism of competitive exclusion.

花色多态性（flower color polymorphism）在自然开花植物中相对罕见，这表明维持种群内不同花色形态颇具难度。为阐明塑造花粉花色二态性（pollen-color dimorphism）的选择机制，研究团队针对部分种群兼具绿色与黄色花粉（及花药）的柔毛淫羊藿（Epimedium pubescens）开展了为期三年的研究，重点考察其传粉者偏好与繁殖表现。作为该植物传粉者的峨眉地蜂（Andrena emeishanica），其访花频率、花粉移除量与柱头上的花粉落置量在两种花色形态间均无显著差异。相较于绿色形态，黄色形态花粉的父本成功率（siring success）更低，而绿色与黄色形态花粉混合后的父本成功率则为三者最低。该结果与花粉性能的体内、体外实验结果相符，表明绿色形态的花粉萌发（pollen germination）率（而非花粉管生长（pollen tube growth））高于黄色形态；且当存在花粉竞争者时，两种形态的花粉萌发均会受到显著抑制。稀有绿色形态或可入侵黄色形态种群，但混合授粉下父本成功率的降低，会使花粉花色变异型的共存过程变得复杂。形态间潜在的花粉竞争或会阻碍种群内多种表型的维持，这是一种隐蔽的竞争排斥机制。