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 egret in flight (hence its common name white egret 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 has been 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 during the academic year 2021) from the Graduate School of Science of Kobe University, ASAI Takeshi and MATSUMOTO Shuji from the Himeji Tegarayama Botanical Garden and HASEGAWA Masahiro from the Osaka Museum of Natural History.

Based on 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 primary pollinators of this orchid, normally cling to 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. Hawkmoths were previously thought to hover primarily while 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 has evolved to support the hawkmoth while it feeds on nectar.

These research results were published online in the journal International Ecology on June 2022.

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About 90% of flowering plants (angiosperms) depend animals such as bees to help them pollinate when the insect carries pollen between flowers, it receives a reward (nectar and many others.). It is known that mutualisms with pollinators also play an essential role in the diversity of flower shape. Many orchid species in particular have developed spectacularly shaped flowers that are 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 spectacular versions of petal structure are thought to result from each orchid species evolving to attract specific insect species.

The wild egret orchid that grows in wetlands does not exception: it has developed complex 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. However, until recently, it was unclear what kind 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

In order to find out how much the shape of the petal fringe 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 fringe is to visually attract the sphinx.

Even if the sphinx is nocturnal, it can rely on its vision to some extent to recognize flowers, so large flowers with a fringe attract him. 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 fruit production rate because fruit production is an indicator of the frequency of pollinator visits. However, this study showed that, contrary to this prediction, there was no decrease in fruit production in specimens with the fringe removed. 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. Additionally, the artificially pollinated crested orchids produced the same rate of healthy seeds whether or not they had a fringe. This demonstrates that the result in reduced seed production in specimens without fringes is related to the mutualism of the flower with its pollinators, and not to the damage sustained when the fringe was removed.

To investigate 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).

So far, the search on the function of the 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 researchers’ hypothesis, the eye-catching fringe plays more of a role as a supporting outlet when feeding sphinxes (which were thought to hover while drinking nectar). what an eye-catcher..

“The White Egret Orchid got its name because its brilliant white petals resemble the bird in flight. the soul of a dead white egret is reborn as the much-loved white egret orchid. Nevertheless, it is now evident that the fringes mainly stabilize the posture of the hawkmoth (the primary pollinator), increasing the transfer of pollen. I’m glad we’ve revealed the unexpected adaptive meaning that’s at the heart of her exclusive bangs. Comments by Professor Suetsugu.

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

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