Monday, 1 August 2022

Evolution News - A Beautiful Example of Co-Evolution

The white egret orchid, Habenaria radiata, superficially resembles a dancing white egret.
Orchid helps insect get a grip | Research at Kobe

The explanatory power of the Theory of Evolution was demonstrated yet again recently, when five biologists from Japan published a paper in the journal Ecology in which they explained what had been a mystery in the form of a peculiarly shaped orchid flower, the so-called white egret orchid, Habenaria radiata syn. Pecteilis radiata, or sagisō, native to China, Korea and Japan, which has a superficial resemblance to a flying Egret.

The mystery had been the reason for the bizarre, jagged edge to two of the three petals of the flower. Orchids are well known for their adaptation to pollination by specific species of insects, notably bees and moths and for their part in co-evolutionary, co-dependent relationships, often with a single species.

The hawkmoth Theretra oldenlandiae visiting an intact Habenaria radiata flower.
Video credit: Yusuke Abe and Kenji Suetsugu.

The hawkmoth Theretra oldenlandiae visiting Habenaria radiata flower after fringe removal.
Video credit: Yusuke Abe and Kenji Suetsugu.
Now, Associate Professor Suetsuga Kenji and student Abe Yusuke of Kobe University Graduate School of Science, with Asai Takeshi and Matsumoto Shuji of Himeji Tegarayama Botanical Garden and Hasegawa Masahiro of Osaka Museum of Natural History, have shown that the white egret orchid is no exception and the reason for the deep fringes is to enable the pollinating hawkmoth to get a better grip whilst feeding (and so collecting pollen on its body) than had the petals had smooth edges. The fringes are the result of co-evolutionary co-dependency - the hawkmoth for the nectar it gets from the flower and the orchid from the pollen that is transferred during feeding.

Hawkmoths are mainly nocturnal, so it had been assumed that the fringes somehow improved the visual impact of the flowers, but this has turned out not to be the main benefit.
As the Kobe University news release explains:
From the results, [the scientists] discovered that in their natural habitat, white egret orchids with the fringe removed produced fewer healthy seeds per individual fruit than intact plants. Hawkmoths, which are major pollinators of this orchid, normally grasp onto the fringe with their mid-legs to steady themselves when they drink its nectar, however the researchers observed that the hawkmoth was often unable to do this on plants with the fringe removed. In other words, this fringe functions as a supportive platform for the pollen-carrying hawkmoth. It was previously thought that hawkmoths mainly 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.

Research Background

Around 90% of flowering plants (angiosperms) rely on animals such as bees to help them pollinate; when the insect transports pollen between flowers, it receives a reward (nectar etc.). It is known that mutualisms with pollinators also play a significant role in flower shape diversity. Numerous orchid species in particular have evolved dramatically shaped flowers; this is noticeable even if you look at the orchids found in florist’s, such as the moth orchid (Phalaenopsis Aphrodite).

Orchids have three petals, one of which [the 'lip'] is large and stands out 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
Results of removing the fringe from white egret orchids in a natural environment.
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).

In order to discover the extent to which petal fringe shape contributes to the white egret orchid’s reproductive success, the researchers conducted a fringe removal experiment in a natural setting. In general, it is thought that petals mainly function as a visual attractant. Hawkmoths, the primary pollinators of the white egret orchid, tend to hover in the air while drinking nectar from flowers and so do not require a place to rest their legs while feeding. Consequently, the researchers hypothesized that the major function of the fringe is to attract the hawkmoth visually.

Even though the hawkmoth is nocturnal, it can rely on its vision to some extent to recognize flowers, thus big flowers with a fringe appeal to it. For this reason, the flowers of other plants (such as snake gourd) pollinated by hawkmoths often have deeply divided fringed petals. Therefore, it is thought that fringed flowers have adapted to effectively attract hawkmoths (who prefer flowers with big fringes) because flowers with a fringe can conserve more resources than fringeless flowers of the same diameter.

The adaptive significance of the white egret orchid’s fringe as proposed by this study.

Based on the results of the fringe removal experiment in natural populations and the detailed observations of pollinator (hawkmoth) behavior.
If the fringe functions as a visual attractant, it can be predicted that specimens with the fringe removed would have a reduced fruit production rate as 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 white egret orchid’s flower. However, flowers with the fringe removed had a lower rate of healthy seeds in their fruits when compared to those with the fringe intact. Furthermore, artificially pollenated white egret orchids produced the same rate of healthy seeds regardless of whether or not they had a fringe. This demonstrates that the cause of the reduced seed production in fringeless specimens is related to the flower’s mutualism with its pollinators, not due to damage incurred when the fringe was removed.

The white egret orchid was given its name because its brilliant white petals resemble the bird in flight. According to legend, the soul of a white egret that died was reborn as the much-loved white egret orchid. Nevertheless, it is now evident that fringes primarily stabilize the hawkmoth (the primary pollinator)’s posture, increasing pollen transfer. I am pleased that we have revealed the unexpected adaptive significance that is at the heart of its distinctive fringe.

Professor Suetsugu, lead author
Graduate School of Science
Kobe University, Kobe, Japan
To investigate how this reduction in the number of healthy seeds was related to the pollinator behavior, the researchers conducted detailed behavioral observations of hawkmoths. These results revealed that this major pollinator of white egret orchids did not hover continuously while drinking nectar but instead grasped onto the petal fringe with its midlegs. However, with the fringe removed, the hawkmoth could not grasp onto the petal in many cases. Therefore, it is highly possible that without the stability provided by the fringe, the hawkmoth could not pass on as much pollen to the plant, thus 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.
Sadly, the team's paper in Ecology is behind an expensive paywall and has no abstract available.

What it shows is the distinct reproductive advantage of deeply fringed petals compared to smooth-edged petals given the narrow spectrum of the pollinators that are essential to the reproduction of the white egret orchid. The basic driver of evolution is, of course, any variation that gives greater reproductive success. There is no need for a conscious designer in this process since the process itself ensure the outcome - greater reproductive success in the given environment.


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