Buzz-pollinated pollen flowers have pollen as the primary resource for pollinators and must deal with a conflict between the exploitation of pollen grains by bees and pollination success. It has been hypothesized that heterostemony allows division of labour between stamens as a solution to the pollen dilemma. To test the division of labour hypothesis, we chose Cassia fistula, which has a trimorphic androecium and analysed androecium development, pollen grain release mechanisms and visitor behaviour. We explored the reflectance of floral organs and carried out an exclusion experiment to test the attractiveness of each stamen morph to the bee species. Finally, we explored the structural, ultrastructural and functional variation between the pollen grains, including pollen viability across stamen morphs. The differences among the three stamen morphs, which is developed from two whorls of the stamen, are the first evidence of the division of labour in our study system. Large Bombus and Xylocopa bees actively and exclusively exploited the pollen grains from the central poricidal anthers generating pollen deposition on their bodies. The reflectance pattern of floral organs indicated a targeting of these large bees to the central anthers, corroborated by the anther manipulative experiment where only the exclusion of the anthers positioned in the flower centre, especially the intermediate stamens, reduced bee visits. Both results revealed a division of labour, in which the intermediate stamen morph was responsible for both floral attractiveness and pollen resources. Only the largest stamen morph produced germinable pollen grains, highlighting their role as pollinating stamens. The smallest stamen morph has a less clear function, likely representing an economy in pollen production for feeding function. Our findings suggest that the evolution of the trimorphic androecium is associated with division of labour in large pollen flowers and can represent a strong strategy for circumventing the pollen dilemma, optimizing the feeding function by reducing pollen grain investment from central anthers.

中文翻译：

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决明子（豆科）花三形雄蕊分工的复杂案例

蜂花授粉的花粉花将花粉作为授粉媒介的主要资源，必须处理蜜蜂对花粉粒的利用与授粉成功之间的冲突。已经假设异生茎允许雄蕊之间的劳动分工作为花粉困境的解决方案。为了检验分工假设，我们选择了具有三态雄蕊的决明瘘，并分析了雄蕊的发育、花粉粒释放机制和访客行为。我们探索了花器官的反射率并进行了排除实验以测试每个雄蕊变形对蜜蜂物种的吸引力。最后，我们探讨了花粉粒之间的结构、超微结构和功能变化，包括花粉在雄蕊形态上的活力。三种雄蕊形态之间的差异，它是由两轮雄蕊发育而成的，是我们研究系统中分工的第一个证据。大型熊蜂和木蜂蜂积极且专门地利用来自中央杀虫花药的花粉粒，在它们的身体上产生花粉沉积。花器官的反射模式表明这些大蜜蜂以中央花药为目标，通过花药操作实验证实了这一点，其中仅排除位于花中心的花药，特别是中间雄蕊，减少了蜜蜂的访问。这两个结果都揭示了分工，其中中间雄蕊变形负责花卉的吸引力和花粉资源。只有最大的雄蕊变形产生可发芽的花粉粒，突出了它们作为授粉雄蕊的作用。最小的雄蕊形态具有不太明确的功能，可能代表了花粉生产用于喂养功能的经济性。我们的研究结果表明，三态雄蕊的进化与大花粉花的分工有关，可以代表一种强有力的策略来规避花粉困境，通过减少中央花药的花粉粒投资来优化摄食功能。