Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, M5S 3B2, Canada.
Biol Rev Camb Philos Soc. 2021 Oct;96(5):2146-2163. doi: 10.1111/brv.12745. Epub 2021 Jun 2.
Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence-induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal- and wind-pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.
动植物之间从动物传粉到风媒传粉的进化转变在被子植物的历史中反复发生，但选择机制仍难以捉摸。在这里，我们提出，了解花粉释放生物力学对于理解支持这种传粉模式转变的生态和进化过程至关重要。花粉释放是风媒传粉（风媒传粉）的关键第一阶段，因此，在转变的早期，雄蕊的特性很可能受到强烈的选择。我们描述了当前对花粉释放生物力学的理解，以提供对风媒传粉的表型和生态驱动因素的见解。当分离力支配保持花粉在花药内的阻力时，花粉就会释放。分离力可以是主动的，也可以是被动的，这取决于它们是否需要来自环境的能量输入。被动释放在风媒传粉物种中更为普遍，涉及由稳态或非稳态空气动力或湍流引起的振动驱动的过程，这些振动会使花粉从花药中脱落。我们回顾了一些实证和理论研究，这些研究表明，雄蕊振动可能是花粉释放的关键机制。振动响应受雄蕊的形态和生物力学特性的控制，在有无传粉者的情况下，这些特性可能会经历趋异选择。很少有研究调查过花药内花粉的阻力，但它可能对环境条件敏感，并且取决于花的年龄，在动物传粉和风媒传粉物种之间系统地变化。动物传粉和风媒传粉传统上被视为两种截然相反的选择，因为它们通常与截然不同的传粉综合征相关联。但这种观点使人们对更微妙、连续变化的特征的注意力转移了，这些特征介导了花粉释放的流体动力学过程。将花重新解释为对波动环境力做出反应的生物力学实体，可能是一个有前途的前进方向。我们最后确定了未来研究的几个有利领域，以更深入地了解风媒传粉的进化。
