Bees and flowers play billiards with pollen

What is it about?

Buzz-pollinated flowers face a dilemma: while pollen is the only floral attractant to bees, it is also the vehicle for male gametes in plants. Bees and these pollen-only species are not ideal partners, and compete for the same pollen grains. Plants have evolved many floral traits that constrain and/or reconcile such excessive pollen collection by bees. One of such responses is displaying two or more types of stamens in a single flower (heteranthery). Stamens are specialized into foraging and pollinating functions, reflecting a remarkable division of labour. Bees grasp a group of short, centrally located, foraging stamens and sonicate the flowers so as to extract pollen. A group of long, curved, lower, pollinating stamens shed pollen on the bee's flanks or back, where it is picked up by the stigma. Many species of Senna, however, brake this rule: the pollinating stamens can be so short as the foraging stamens, and are all located beneath the bee's ventral side. The pistil is deflected sideways, curved and long so that the stigma touches the dorsal side of the bees. Thus, pollen grains must reach the bee's back - but how? Flowers of Senna have evolved an astonishing manner to solve this puzzle. Here we show that when large carpenter bees sonicate a Senna flower, the pollen grains (mainly from the short pollinating stamens) are ejected to a specialized petal. When hitting this petal, the grains must ricochet multiple times like a billiard ball up to the petal tip or to another petal, and eventually reach the bee's back so as to effect pollination.

Featured Image

Photo by Amritanshu Sikdar on Unsplash

Photo by Amritanshu Sikdar on Unsplash

Why is it important?

We show that a division of labour can also be achieved in a more complex way than heteranthery alone, through the synergy between different floral structures, such as stamens and specialized petals. Our findings reinforce the adaptive importance of the division-of-labour hypothesis put forward in the late nineteenth century by naturalists Fritz and Hermman Müller.

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