Orchid helps insects cling

White Egret Orchid has developed a frilly petal to support the pollinating hawkmoth

The pure white petals of the wild orchid Habenaria radiata resemble a white pappus in flight (hence its common name white puff orchid). H. radiata has been enjoyed by people since ancient times, but the adaptive significance of the characteristic serrated shape of the flower was unclear until now. A multi-institutional research group has been working for three years to solve this mystery by conducting field experiments in which the feather-like fringe was removed and detailed observations of the orchid’s pollinator behavior.

The research collaboration consisted of Associate Professor SUETSUGU Kenji and student ABE Yusuke (who completed his master’s degree in the 2021 academic year) from the Graduate School of Science at Kobe University, ASAI Takeshi and MATSUMOTO Shuji from the Himeji Tegarayama Botanical Garden and HASEGAWA Masahiro from the Osaka Museum of Natural History.

From the results, they found that in their natural habitat, white crested orchids with the fringe removed produced fewer healthy seeds per individual fruit than undamaged plants. Hawkmoths, which are the main pollinators of this orchid, normally grab onto the fringe with their middle legs to stabilize themselves when drinking its nectar, but researchers have observed that the Hawkmoth was often unable to do this on plants. whose fringe had been removed. In other words, this fringe functions as a support platform for the pollen-carrying hawk-moth. Sphinxes were previously thought to soar primarily by drinking nectar.

Although the white egret orchid uses hawkmoths to transport its pollen, these important findings indicate that the eye-catching fringe is more than a visual aid for pollinators and evolved to support the hawkmoth while it feeds on nectar.

These research results were published online in the international journal Ecology on June 21, 2022.

Research fund

About 90% of flowering plants (angiosperms) depend on animals such as bees to help pollinate them; when the insect carries pollen between the flowers, it receives a reward (nectar etc.). It is known that mutualisms with pollinators also play an important role in the diversity of flower shape. Many orchid species in particular have developed spectacularly shaped flowers; this is noticeable even if you look at orchids found at florists, such as the moth orchid (Phalaenopsis aphrodite).

Orchids have three petals, one of which is large and protruding (*1) and this petal formation is thought to have evolved alongside the insects that carry its pollen. In fact, many orchid species use particular types of insects as pollinators and the dramatic variations in petal structure are thought to result from each orchid species evolving to attract specific insect species.

The wild crested orchid that grows in wetlands is no exception: it has developed intricate petals. Its handsome appearance is reminiscent of a white egret soaring in the sky and has been a familiar plant in gardens for hundreds of years (Figure 1).

Figure 1: White Egret Orchid (Habenaria radiata) looks like a dancing white egret.

However, until recently, it was unclear what type of mutualism with pollinators led the fringed petal of the white crested orchid to evolve into such a distinctive shape.

Detailed explanation of the research

Figure 2: Results of removing the fringe of white crested orchids in the wild.

Contrary to expectations, only the number of healthy seeds per fruit decreased; there was no reduction in the flower’s rate of fruit production (an indicator of the frequency of pollinator visits).

To find out how much petal fringe shape contributes to the reproductive success of the white egret orchid, researchers conducted a fringe removal experiment in a natural setting. In general, the petals are believed to function primarily as a visual attractant. Hawkmoths, the main pollinators of the White Egret Orchid, tend to hover in the air while drinking nectar from the flowers and therefore do not need a place to rest their legs while feeding. Therefore, the researchers hypothesized that the main function of the bangs is to visually attract the sphinx.

Even though the hawkmoth is nocturnal, it can rely on its vision to some extent to recognize flowers, so large flowers with a fringe attract it. For this reason, the flowers of other plants (such as snake gourd) pollinated by hornworms often have deeply divided fringed petals. Therefore, fringed flowers are believed to have adapted to effectively attract hawkmoths (which prefer flowers with large fringes) because fringed flowers can conserve more resources than non-fringed flowers of the same diameter.

If the fringe functions as a visual attractant, it can be predicted that specimens without fringe would have a reduced rate of fruit production because fruit production is an indicator of the frequency of pollinator visits. However, this study showed that, contrary to this prediction, there was no decline in fruit production in specimens with the fringe removed (Figure 2). In other words, the fringe did not play a significant role in attracting hawkmoths to the flower of the white egret orchid. However, flowers with the fringe removed had a lower rate of healthy seeds in their fruits compared to those with the fringe intact (Figure 2). Additionally, artificially pollinated crested orchids produced the same rate of healthy seeds whether or not they had a fringe. This demonstrates that the cause of reduced seed production in specimens without fringes is related to the flower’s mutualism with its pollinators, not the damage sustained when the fringe was removed.

Figure 3: The adaptive significance of the egret orchid fringe as proposed by this study

Based on the results of the fringe removal experiment in natural populations and detailed observations of pollinator (hawkmoth) behavior.

To study how this reduction in the number of healthy seeds was related to pollinator behavior, the researchers made detailed observations of the behavior of hawkmoths. These results revealed that this major pollinator of white crested orchids did not hover continuously while drinking nectar, but rather clung to the fringe of the petals with its middle legs. However, with the fringe removed, the sphinx could not grasp the petal in many cases. Therefore, it is quite possible that without the stability provided by the fringe, the hornworm could not transmit as much pollen to the plant, thus resulting in plants without fringes receiving fewer pollen grains per visit and producing fewer healthy seeds ( Figure 3).

Until now, research on the function of petals has focused on their role in visual attraction to pollinators, and other functions beyond this have received little attention. In particular, the results of this study indicated that contrary to the hypothesis of the researchers, the eye-catching fringe plays more of a role of a fulcrum when feeding sphinxes (which were thought to hover while drinking nectar) What a visual attractant. .

“The White Egret Orchid got its name because its brilliant white petals resemble the bird in flight. According to legend, the soul of a deceased White Egret Orchid is reborn as the much-loved White Egret Orchid. , it is now apparent that the fringes primarily stabilize the posture of the hawk moth (the primary pollinator), increasing pollen transfer.I am glad we have revealed the unexpected adaptive significance that lies at the heart of its distinctive fringe.Comments by Professor Suetsugu .

To note

1.
The academic term for the large petal of an orchid is “lip” and it is differentiated from the other petals.

Journal Information

Title
“A specialized petal with a visible fringed margin influences reproductive success in Habenaria radiata (Orchidaceae)”
DOI:10.1002/ecy.3781
Authors
Kenji Suetsugu, Yusuke Abe, Takeshi Asai, Shuji Matsumoto and Masahiro Hasegawa
Log
Ecology

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