Published: 06.05.09
Evolution of floral biology

Is poisonous pollen enough to put bees off their dinner?

Chemical weapons against uninvited dinner guests: ETH Zurich scientists test whether the pollen of certain flowers contains toxins that give bees an upset stomach and protects the plant from the diligent pollen gatherers.

Peter Rüegg
A colletes bee gathering pollen from a fleabane, a member of the aster family. (photo: Andreas Müller / Applied Entomology ETH Zürich)
A colletes bee gathering pollen from a fleabane, a member of the aster family. (photo: Andreas Müller / Applied Entomology ETH Zürich) (large view)

Bees and flowers: a harmonious interdependency with equal benefits for both. The insects obtain nectar and pollen, pollinate the flower and ensure the plant’s propagation. Even humans benefit as without bees there would be no apples and no honey – a morally immaculate win-win situation.

Andreas Müller, Curator of the ETH Zurich’s entomological collection in Professor Silvia Dorn’s applied entomology group, however, now sees this relationship somewhat more soberly. Bees – besides the honey bee there are over 600 species of wild bees in Switzerland – are herbivores that have specialized in high-protein pollen as their staple diet in the course of evolution. Bees need an enormous amount of pollen – often the entire pollen content of several hundred flowers just to produce one single offspring. And that can be a considerable disadvantage for plants, stresses the researcher. Every grain of pollen that disappears in a bee’s brood cell is really one potential seed less. The more the bees gather pollen randomly from many different plant species and families, the greater the danger that it will not reach its destination – the stigma of the right type of flower – and fertilize it. Moreover, pollen production also uses up a lot of the plants’ energy.

It is therefore in the plants’ interest to reduce the number of pollen eaters in order to prevent too much pollen from going astray. Flowering plants have therefore developed special forms of flower in the course of time to make gathering pollen more difficult for bees – such as the keel flowers of the pea. However, certain kinds of bees have in turn adapted their bodies especially to suit such flower shapes.

Poison keeps bees at bay

Andreas Müller’s team of researchers has now discovered a new mechanism that plants use to ward off pollen eaters. To their surprise, they found that a number of bees belonging to the genus Colletes specialize in the aster family; generalists within the same genus that gather pollen randomly, however, steer clear of this plant family although it is rich in species. Furthermore, the aster family – unlike members of the Fabaceae family such as peas – makes it easy for the guests to gather the pollen.

The notion of defending pollen chemically therefore seemed the obvious explanation to Müller. “Plants often stop themselves from being eaten by insects by storing toxins in leaves. Why should pollen be any different?” asks the researcher, who described his idea as the “aster paradox” in a recent scientific publication.

The insect researchers at ETH Zurich began a series of experiments to test their theory. For example, for his ETH-Zurich-medal winning doctoral thesis, Christophe Praz fed the larvae of specialized bees with pollen from the aster family, which was not part of their normal diet. Although the larvae ate the wrong food for up to 30 days, they did not grow. Not one species managed to develop from a larva into a bee – apart from the specialists for this plant family.

Is aster pollen inedible?

Claudio Sedivy, a PhD student in the applied entomology group, is now collaborating with chemist Rafal Piskorski and the student Claude Hüsser to test whether pollen from members of the aster family contains toxins and whether the corresponding bees have adapted their metabolism especially in order to use the pollen.

With their research, the ETH Zurich scientists are entering unknown territory. The evolutionary research appears to have ignored this aspect of floral biology, as Müller points out: “The chemical protection of pollen must have had an enormous impact on the evolution of the relationships between insects and flowers.”

 
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