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The dark red flower hides an unknown pollination syndrome

Akira MOCHIZUKI (Assistant Professor, Botanical Garden)

Atsushi Kawakita (Professor, Department of Biological Sciences, The Botanical Garden)

We are in the mountains of Wulai, Taiwan. I am looking for insects that visit the red flowers of Formosan sweetgum. This is my second attempt in two years.
The sun, which had begun to rise when I started my observation, disappeared deep into the mountains.
I wondered if it would be no luck this time either... The forest at night made me feel even more uneasy.
I gave up and shined my headlights on the flowers, and I saw a small insect buzzing around.
This was the pollinator we had been looking for, the mushroom fly.
This observation, accomplished with a cry of joy, was an important piece of evidence for the existence of an unknown pollinator syndrome in angiosperms.

The flowering plants attract animals to their flowers as a reward for nectar and pollen, and they accomplish inter-individual pollination. Although plants have a wide variety of flowers, plants that have the same pollinators sometimes have similar flowers, such as the gardenia and the tuberose, which are pollinated by the tuberose and have white, aromatic flowers. This is called the pollination syndrome, and classically, 11 types of flowers have been known according to pollinators, such as bumblebees and birds. However, this typology does not describe all floral diversity, and some familiar plants have flowers that do not belong to any one type. In particular, flowers with a deep red color, such as wine, originated in several taxa with different evolutionary lineages, but their relationship to pollinators was largely unknown.

In our previous study, we had reported that seven species of plants in five Graduate Schools, including Aoki, which grow widely in Japanese mountain forests, have flowers similar to each other: "small, flat, dark red flowers with short stamens and exposed nectary glands. Since these plants are all pollinated by the mushroom fly, a dipterous insectNote), he suggested that the common floral traits may be an evolutionary result of pollination by the mushroom fly.

In the present study, we examined the evolutionary relationship between dark-red flowers and the mushroom fly in the western yellow-flowered plants, where dark-red or greenish-white flowers are found. We collected 1,853 flower-visiting insects during 258 hours of field observations in Japan, Taiwan, and the United States, and evaluated the contribution of each insect to pollination based on the pollen on their body surface. The five species with dark-red flowers were found to be pollinated by leafhoppers, and the seven species with greenish-white flowers by bumblebees, large houseflies, and beetles (Fig.) The evolutionary relationship between pollinator type and floral traits was examined, and it was found that pollination mediated by the mushroom fly was correlated with dark red petals, short stamens, and floral odor, mainly acetoin. This suggests that adaptation to the mushroom fly resulted in a coordinated evolution of floral traits, i.e., a pollination syndrome.

Evolutionary patterns of flower color and mode of pollination in the genus Nyctinia. The pie chart on the phylogenetic tree indicates the probability of having either greenish-white or dark red flower color at the time of divergence of the lineage.

In this study, a new type of pollination syndrome has been added, and it is clear that interactions with mushroom flies play an important role in floral evolution. Although dipterous insects, including mushroom flies, are typical flower visitors, their relationship with flower evolution has rarely been examined, making this a valuable empirical example. On the other hand, not all plants with dark red flowers are pollinated by mushroom flies. We hope to clarify the ecology and evolution of dark red flowers through repeated field observations of each one.

This study was published in K. Mochizuki et al. Annals of Botany 132, 319-333 (2023).

Note: Also known as flies. Insect taxonomic group that includes flies, mosquitoes, horseflies, and gadflies.

(Press release, August 24, 2023)

Published in The Rigaku-bu News, January 2024.

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