Botanists believe that the frilly lip of the white egret orchid evolved to act as a stabilizer for the hawkmoth while pollinating the plant, resulting in increased seed production.
The pure white petals of the wild orchid Habenaria radiata resemble the flight of a white egret (hence its common name white egret orchid). People have loved H. radiata since ancient times, but the adaptive significance of the flower’s distinctive jagged shape has been unknown until now. For three years, a multi-institutional research team has been working to solve this mystery by conducting field experiments with the feather-like fringe removed and detailed behavioral observations of the orchid’s pollinators.
The research collaboration consisted of Associate Professor SUETSUGU Kenji and student ABE Yusuke (who completed his Master’s degree in the 2021 academic year) of Kobe University Graduate School of Science, ASAI Takeshi, and MATSUMOTO Shuji of Himeji Tegarayama Botanical Garden, and HASEGAWA Masahiro of Osaka Museum of Natural History.
According to the findings, white egret orchids with the fringe removed produced fewer healthy seeds per individual fruit in their natural habitat than intact plants. Hawkmoths, which are important pollinators of this orchid, normally grip the fringe with their mid-legs to keep themselves steady while drinking its nectar; however, the researchers discovered that the hawkmoth was frequently unable to do so on plants that had the fringe removed. In other words, the pollen-carrying hawkmoth relies on this fringe for support. Hawkmoths were previously thought to primarily hover while drinking nectar.
Although the white egret orchid utilizes 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 the nectar.
The white egret orchid gets its name from its brilliant white petals, which resemble the bird in flight. According to legend, the soul of a deceased white egret was reborn as the popular white egret orchid. Nonetheless, it is now clear that fringes primarily stabilize the posture of the hawkmoth, increasing pollen transfer.
Professor Suetsugu
Research Background
Around 90% of flowering plants (angiosperms) rely on animals such as bees to pollinate them; when the insect transports pollen between flowers, it is rewarded (nectar etc.). Mutualisms with pollinators are known to play a significant role in flower shape diversity. Numerous orchid species, in particular, have evolved dramatically shaped flowers; this is visible even when looking at orchids found in florists, such as the moth orchid (Phalaenopsis aphrodite).
Orchids have three petals, one of which is large and prominent (*1), and it is thought that this petal formation evolved alongside the insects that transport its pollen. In fact, many orchid species utilize particular kinds of insects as pollinators and it is thought that the variations in dramatic petal structure result from each species of orchid evolving to appeal to specific insect species.
The wetland-growing wild white egret orchid is no exception: it has evolved intricate petals. Its beautiful appearance calls to mind a white egret soaring through 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 had led the white egret orchid’s fringed petal to evolve into such a distinctive shape.
Detailed Explanation of Research
The researchers conducted a fringe removal experiment in a natural setting to determine the extent to which petal fringe shape contributes to the reproductive success of the white egret orchid. In general, petals are thought to serve primarily as a visual attractant. Hawkmoths, the primary pollinators of the white egret orchid, hover in the air while drinking nectar from flowers, so they don’t need a place to rest their legs while feeding. As a result, the researchers hypothesized that the fringe’s primary function is to visually attract the hawkmoth.
Even though the hawkmoth is nocturnal, it can use its vision to recognize flowers, so large flowers with a fringe appeal to it. As a result, the flowers of other plants pollinated by hawkmoths (such as snake gourd) often have deeply divided fringed petals. As a result, it is thought that fringed flowers have adapted to attract hawkmoths (which prefer flowers with large fringes) because flowers with a fringe can conserve more resources than flowers without a fringe of the same diameter.
If the fringe serves as a visual attractant, it can be predicted that specimens with the fringe removed will have a lower fruit production rate, as fruit production is an indicator of pollinator visit frequency. This study, however, found that, contrary to expectations, there was no decrease in fruit production in specimens with the fringe removed. In other words, the fringe had no effect on attracting hawkmoths to the flower of the white egret orchid.
Flowers with the fringe removed, on the other hand, had a lower rate of healthy seeds in their fruits than those with the fringe intact. Furthermore, whether or not they had a fringe, artificially pollinated white egret orchids produced the same number of healthy seeds. This demonstrates that the reduced seed production in fringeless specimens is due to the flower’s mutualism with its pollinators, rather than damage caused by the removal of the fringe.
The researchers conducted detailed behavioral observations of hawkmoths to investigate how this reduction in the number of healthy seeds was related to pollinator behavior. These findings revealed that this important pollinator of white egret orchids did not hover continuously while drinking nectar, but instead grasped the petal fringe with its midlegs. However, without the fringe, the hawkmoth could not grasp the petal in many cases. As a result, it is highly likely that without the stability provided by the fringe, the hawkmoth could not pass on as much pollen to the plant, resulting in fringeless plants receiving fewer pollen grains per visit and producing fewer healthy seeds.
Up until now, research into the function of petals has focused on their role in visually attracting pollinators and other functions beyond this have received little attention. In particular, the results of this study have indicated that contrary to the researchers’ hypothesis, the eye-catching fringe plays more of a role as a foothold while feeding for hawkmoths (that were believed to hover while drinking nectar) than as a visual attractant.
“The white egret orchid gets its name from its brilliant white petals, which resemble the bird in flight. According to legend, the soul of a deceased white egret was reborn as the popular white egret orchid. Nonetheless, it is now clear that fringes primarily stabilize the posture of the hawkmoth (the primary pollinator), increasing pollen transfer. I’m glad we discovered the unexpected adaptive significance at the heart of its distinctive fringe.” Professor Suetsugu weighs in.
[Note: The academic term for the large petal on an orchid is ‘lip’ and it is differentiated from the other petals.]
