The distinctive-looking Arisaema urashima plant is a common sight in the forests of Japan. It has a large spathe (sheath enclosing its flowers) from which hangs a long thread like appendage (which can reach over 50cm in length) resembling a fishing rod and line. Its unusual shape was thought to resemble the Japanese folktale character Urashima Taro fishing, hence the plant’s name. However, the functional significance of this wired appendage remained unknown for many years.
A research collaboration (including Associate Professor SUETSUGU Kenji and graduate student M. NISHIGAKI Hiroki from the Graduate School of Science, Kobe University) spent 3 years conducting field experiments to determine the role that this thread in the attraction of pollinators. Conducting experiments where the plant’s characteristic thread was removed revealed that this affected the plant’s ability to attract fungus gnats, believed to be its main pollinators, however, the attraction of other insect species n was not affected by the withdrawal. In addition, flower fruit production (fruiting rate) was significantly reduced if the wire was removed. In other words, as the pictures surrounding the plant suggest, Arisaema urashima’s “fishing rod” selectively coils the primary pollinator, increasing the plant’s reproductive success.
This research was published online in the international journal Ecology on June 28.
The majority of plants depend on animals such as bees to help them pollinate; when the insect carries pollen between the flowers, it receives a reward (nectar etc.). However, even what appears to be a harmonious relationship between two species at first glance is the result of a strategy of mutual exploitation. In other words, the partners in a mutualistic relationship are not altruistic and simply take actions that further their own interests. Therefore, mutualistic relationships also provide a window for exploitation by parasitic species that may acquire a resource without providing anything in return. For example, there are plants with beautiful flowers that appear to carry nectar but actually exist to trick insects into carrying their pollen. Of the many plants that usually exploit their pollinating insects in this way, the Arisaema plant genus adopts the most cunning strategy. Seeing the water jug-like shape of Arisaema, many people may think that it is a carnivorous plant (Figure 1). Even if not, the similar shape is no coincidence. Carnivorous plants and Arisaema evolved into this form due to their common characteristic; they both trap the insects they attract.
Let’s take a closer look at why plants of the genus Arisaema trap the pollinators they have attracted. Both male and female Arisaema plants have a structure called a spathe, which encloses their many flowers (Figure 1). A cylindrical organ called the spadix emits an odor which attracts pollinators (mainly fungus gnats), bringing them in through the opening at the top of the spathe. However, the inside of the spathe is coated with a slippery wax, which prevents the gnat from coming out the top. Despite this, the structure of the male plant allows the midge to escape. Walking between the flowers at the bottom of the spathe, the midge picks up pollen on its body and eventually finds the small holes in the bottom that allow it to come back outside. In contrast, the female plant has no such outlets. Once inside the female plant, the midge picks up the pollen it has distributed to all the flowers as it struggles inside the flower cluster (inflorescence) to find a way to s’ escape and eventually die (Figure 1).
Another exciting aspect of this plant genus is that some species of Arisaema have a threadlike structure that protrudes from the tip of the spadix. Arisaema urashima is a prime example as it has an appendage over 50cm long that protrudes from the spathe and hangs down, resembling a fishing rod. It gets its name because the shape of the plant resembles the Japanese folktale character Urashima fishing for taro (Figure 2). Additionally, many plants beyond the genus Arisaema are known to have a similar thread-like appendage, including unrelated plants that are also similarly pollinated by flies (e.g. the genus Aristolochia). However, few studies have been conducted on the effect of this distinctive appendage on pollinator behavior and it has been a matter of debate for over 100 years.
Detailed explanation of the research
Seeking to reveal whether or not Arisaema urashima yarn is attractive to pollinators, a formed research group consisting of Associate Professor Suetsugu, Mr. Nishigaki, Mr. FUKUSHIMA Shigeki (Researcher at the Prefectural Research Center for Agriculture and Chiba Forestry), Mr. ISHITANI Eiji (former researcher at the above-mentioned research center), Dr. KAKISHIMA Satoshi (part-time specialist researcher at the National Museum of Nature and Science) and Dr. SUEYOSHI Masahiro (researcher at the Institute of forestry and forest products research).
In 2019 and 2020, they conducted field experiments where they removed different parts of the threadlike appendage from Arisaema urashima plants just before they flowered. The groups were divided as follows: ‘Wire Removal’ where they left part of the appendage so that it resembles that of other typical Arisaema species, ‘Complete Removal’ where the entire appendage has removed, and ‘Intact’, an unmanipulated control group (Note 1). Once the flowering period was over, the researchers examined the number and species of insect individuals trapped inside the inflorescence of plants in each group. The results revealed that simply removing the threadlike part significantly reduced the number of visitors from the midge of the Mycetophila fungus, which was considered to be the main pollinator of the plant (Figure 3). In contrast, there was no change in the frequency of visits from other insect species when the threadlike part was removed. However, complete removal of the appendage, including the swollen basal portion that emits scent, resulted in a decreased rate of overall flower visitation (Figure 3).
As it was revealed that the thread attracts insects considered to be the main pollinators, the researchers conducted a subsequent experiment in 2021 where they left female plants unhandled and female plants with the thread removed until the fruits reach maturity to see if a difference in the percentage of fruit could be observed. The results showed that simply removing the yarn significantly reduced fruit set. Again, wire removal did not reduce the frequency of other insect visitors, but rather the decline in the number of fungus gnat visitors specifically affected the reproductive success of the plant.
In summary, the results of this experiment provide evidence that fungus gnats are the primary pollinator of Arisaema urashima. Looking at the two experiments together, it can be said that the function of the “fishing rod” of Arisaema urashima is to selectively reel the primary pollinator, thereby increasing the reproductive success of the plant (Figure 4). In the future, the researchers hope to investigate the functional significance of similar threadlike appendages in many other fly-pollinated plant species.
- In order to count the total number of insects trapped by Arisaema urashima, the exit holes of all the male inflorescences were plugged with cotton to prevent them from escaping through the bottom holes.
- “Thread-like appendage on Arisaema urashima (Araceae) attracts fungus gnat pollinators”
- Kenji Suetsugu, Hiroki Nishigaki, Shigeki Fukushima, Eiji Ishitani, Satoshi Kakishima and Masahiro Sueyoshi