Wild orchid Habenaria radiantThe pure white petals resemble a white heron in flight (hence its common name, white heron orchid). H. radiata has been loved by humans since ancient times, but the adaptive significance of the flower’s characteristic serrated shape has been unclear until now. A multi-institutional research team worked for three years to solve this mystery, conducting field experiments in which the feather-like fringes were removed and detailed observations of the behavior of the orchid’s pollinators.
The research collaboration consisted of SUETSUGU Associate Professor Kenji and ABE student Yusuke (who received his master’s degree in the 2021 academic year) from Kobe University Graduate School of Science, ASAI Takeshi and MATSUMOTO Shuji from Himeji Tegaraya Botanical Garden, and Masahiro HASEGAWA from the Osaka Museum of Natural History.
According to the results, they found that in their natural habitat, white heron orchids with the fringe removed produce fewer healthy seeds per individual fruit than whole plants. Hawkmoths, which are the main pollinators of this orchid, usually grab the fronds with their middle legs to hold on when they drink its nectar, but the researchers noticed that the hawkmoths were often unable to do so on plants with the fronds removed. In other words, this fringe functions as a support platform for the pollen-carrying hawkmoth. It used to be thought that hawkmoths mostly hovered, drinking nectar.
Although the white heron orchid uses hawk moths to transfer pollen, these important discoveries show that the attractive fringe is more than a visual aid for pollinators, and has evolved to support the moth as it feeds on nectar.
These research results were published online in an international journal Ecology June 21, 2022
About 90% of flowering plants (angiosperms) rely on pollination by animals such as bees; when an insect transfers pollen between flowers, it receives a reward (nectar, etc.). Mutualisms with pollinators are also known to play a significant role in the diversity of flower form. Many species of orchids, in particular, have spectacular flower shapes; this is noticeable even if you look at orchids that can be found at florists, such as the moth orchid (Phalaenopsis Aphrodite).
Orchids have three petals, one of which is large and prominent (*1), and this petal formation is thought to have evolved with the insects that carry its pollen. In fact, many species of orchids use specific types of insects as pollinators, and it is believed that the variation in dramatic petal structure is the result of each orchid species evolving to appeal to specific types of insects.
The wild white heron orchid that grows in the marshes is no exception: it has developed complex petals. Its beautiful appearance resembles a white heron soaring in the sky and has been a familiar plant in gardens for hundreds of years. However, until recently, it was not clear what kind of mutualism with pollinators caused the fringed petal of the white heron orchid to take on such a distinctive shape.
Detailed explanation of research
To find out to what extent the shape of the petal fringe contributes to the reproductive success of the white heron orchid, the researchers conducted an experiment to remove the fringes under natural conditions. In general, it is believed that the petals mainly act as a visual attractant. Hawkmoths, the main pollinators of the white heron orchid, tend to hover in the air while drinking nectar from the flowers, and therefore do not require a place to rest their legs while feeding. Therefore, the researchers hypothesized that the main function of the fringe is to visually attract the hawksbill.
Although the ladybird is nocturnal, it can rely to some extent on its eyesight to recognize flowers, so large flowers with fringes appeal to it. For this reason, flowers of other petal-pollinated plants (such as squash) often have deeply divided, fringed petals. Therefore, fringed flowers are thought to have adapted to effectively attract goshawks (which prefer large fringed flowers) because fringed flowers can save more resources than non-fringed flowers of the same diameter.
If fringes function as a visual attractant, samples with fringe removed would be predicted to have lower fruit production, as fruit production is an indicator of pollinator visitation frequency. However, this study showed that, contrary to this prediction, there was no reduction in fruit production in specimens with the fringe removed. In other words, fringes did not play a significant role in attracting hawkmoths to the white heron orchid flower. However, flowers with removed fringes had lower levels of healthy seeds in fruit compared to flowers with intact fringes. In addition, artificially pollinated white heron orchids produced the same number of healthy seeds whether or not they were bordered. This demonstrates that the cause of reduced seed production in non-fringed specimens is due to mutualism between the flower and its pollinators and not due to damage caused by edging.
To find out how the decline in the number of healthy seeds was related to the behavior of pollinators, the researchers made detailed observations of the behavior of hawksbills. These results indicated that this primary pollinator of white heron orchids did not hover continuously while drinking nectar, but instead grasped the petal margins with its median legs. However, when the fringes were removed, the hawkmoth could not grasp the petal in many cases. Therefore, it is very likely that without the stability provided by the fringes, the hawkmoth cannot transfer as much pollen to the plant, resulting in plants without fringes receiving fewer pollen grains per visit and producing fewer healthy seeds (Figure 3).
Until now, studies of petal function have focused on their role in visual pollinator attraction, with little attention given to functions other than this. In particular, the results of this study showed that, contrary to the researchers’ hypothesis, attention-grabbing fringes played more of a support role during feeding in hawkmoths (which were thought to hover when drinking nectar) than a visual attractant.
“The white heron orchid got its name because its brilliant white petals resemble a bird in flight. According to legend, the soul of a fallen white heron was reborn as the white heron’s much-loved orchid. However, it is now clear that fringes primarily stabilize the position of the hawkmoth (the primary pollinator) by increasing pollen transfer. I am delighted that we have discovered an unexpected adaptive significance underlying its distinctive fringe.” Professor Suetsugu comments.
A note 1. The academic term for the large petal on an orchid is “lip” and it is different from the other petals.