Red flowers, short stamens, and acetoin: the secret to attracting fungus gnats

Dipteran insects, commonly known as flies, are known pollinators of many angiosperms. However, little is known about how flies affect the evolution of flowers. A new study published in Annals of Botany suggests that fungus gnats may have played a role in the evolution of a unique set of floral traits in the genus Euonymus. 

Some plants that are pollinated by fungus gnats have distinctive floral traits different from other known pollination syndromes, including dark red display, flat form, and short stamens. Researchers tested whether that set of attributes are a pollination syndrome linked to fungus gnat pollination in Euonymus species, known for their colourful flowers.

Ko Mochizuki and colleagues first investigated the pollinators of ten Euonymus species. They found that the red-flowered species were predominantly pollinated by fungus gnats, while the white-flowered species were pollinated by bees, beetles, and other flies. This suggests that flower colour may be an important cue for fungus gnats when choosing flowers to pollinate.

The researchers then used bee and fly colour vision models to evaluate the flower colour of the Euonymus species. They found that both bees and flies can see the difference between red and white flowers, suggesting that both pollinator groups can use flower colour to select Euonymus flowers. 

The team also investigated the floral morphology and scent profile of the ten Euonymus species and found that the fungus gnat-pollinated species had shorter stamens and emitted a stronger scent of acetoin than the bee-pollinated species. Acetoin is a compound that is produced by some fungi and bacteria, and it is thought to attract fungus gnats by mimicking the smell of rotting food. 

A phylogenetically independent contrast was also conducted to test the evolutionary association between fungus gnat pollination and each plant character and suggested that the evolution of fungus gnat pollination is associated with the acquisition of red flowers, short stamens, and acetoin emission. 

The researchers concluded that the observed combination of floral characters is a pollination syndrome associated with the parallel evolution of pollination by fungus gnats. They suggest they could use the red floral display and acetoin emission to locate and pollinate Euonymus flowers. The findings of this study highlight the importance of fungus gnats as potential contributors to floral diversification.

“Before this study, I was not confident that red-flowering euonymus from outside of Japan were pollinated by fungus gnats, as until then I had only observed this behavior within Japan. When I observed a fierce visitation of fungus gnats to a Euonymus laxiflorus bush in Taiwan, I exclaimed ‘pollination syndrome exists!’ The exact role of the dark-red floral display and acetoin emission remains unclear, so as a next step I need to clarify whether they attract fungus gnats or refute other visitors,” said Mochizuki in a press release.

“I believe that our study contributes to understanding the complex interactions between plants and insects in nature. Particularly, it shows the importance of Diptera as a driver of floral evolution, which I hope may change the public notion to see Diptera not as pests but as useful visitors.”

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Mochizuki, K., Okamoto, T., Chen, K.-H., Wang, C.-N., Evans, M., Kramer, A.T. and Kawakita, A. (2023) “Adaptation to pollination by fungus gnats underlies the evolution of pollination syndrome in the genus Euonymus,” Annals of Botany, p. mcad081. Available at: https://doi.org/10.1093/aob/mcad081.