Do figs really have no "flowers"? Take you into the magical world of plants

Produced by: Science Popularization China

Author: Qin Yalong (Institute of Botany, Chinese Academy of Sciences, Jiangsu Province, Nanjing Zhongshan Botanical Garden)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

Speaking of figs, the first thing that comes to mind is the fresh or dried figs. Ficus carica Linn., as the model species of the genus Ficus in the Moraceae family, is native to the Mediterranean coast and is one of the earliest domesticated economic crops. Humans have cultivated and eaten figs for more than 11,000 years. In the Bible, Adam and Eve used fig leaves to cover their shame after eating the forbidden fruit.

Fig

(Photo credit: Photo taken by the author)

Figs were introduced from Persia to Xinjiang in the Han Dynasty, and were gradually introduced from Xinjiang to other parts of the Central Plains via the Silk Road during the Tang Dynasty. Today, Xinjiang is still the main producing area of ​​figs in my country, and it is rich in a kind of yellow-skinned figs. The locals are very particular about eating this kind of figs. They wrap the fruit with fig leaves and pat it three times, saying that it is to wake up each sleeping fig, but in fact, by patting, the sugar and juice inside the fruit are distributed more evenly, making it taste better.

In a broad sense, "figs" refer to all plants of the genus Ficus in the Moraceae family, which are mainly distributed in tropical and subtropical areas. The biggest feature of Ficus plants is their unique inflorescence structure. The middle of the swollen inflorescence axis is concave to form a cavity, and countless unisexual small flowers are born on the inner wall. There are tiny holes composed of imbricate receptacles at the top. This unique flower structure is called "hidden head inflorescence". Therefore, people cannot see the small flowers of these "figs" from the beginning to the end, which is also the origin of their name.

The imbricate receptacle of the Ficus microcarpa

(Photo credit: Photo taken by the author)

How are figs pollinated?

Figs, as the model species of the genus Ficus, are divided into male and female. Figs on female plants only have female flowers, while figs on male plants have both male and gall flowers. As insect-pollinated plants, figs are very specific, just like Ficus plants in the Moraceae family. There is only one kind of insect that can pollinate figs - the fig wasp.

The female flowers of the genus Ficus are divided into two types: long-stigma female flowers and short-stigma female flowers. Gall flowers are specialized female flowers with short stigmas, which are used by fig wasps to lay eggs and reproduce. It is easier for fig wasps to lay eggs in the ovary of gall flowers through the short stigmas, stimulating the ovary to swell into galls. Therefore, gall flowers almost never produce seeds, while the long-stigma female flowers of female plants can produce seeds after pollination.

When figs bloom, the imbricate receptacles at the top of the figs loosen slightly and emit unique volatile substances, attracting fig wasps to lay eggs and pollinate. At this time, if the female fig wasps carrying pollen encounter a female plant (as to why they carry pollen and why they are female fig wasps, we will explain later), they will drill into the female fruit through the holes at the top of the figs to pollinate the female flowers.

In the process of trying hard to squeeze in, the female bee sometimes breaks off her antennae and wings, and sometimes even squeezes out some of her internal organs. She finally gets in and completes pollination of the female flower. When the female fig wasp encounters a male plant, she will try her best to squeeze into the male fruit to find gall flowers to lay eggs. The eggs gradually develop as the male fruit develops.

After a few weeks, the male fig wasps in the male fruit hatch first. After drilling out of the gall, they look for female fig wasps still in the gall to complete mating. Then they help the female fig wasps to drill out of the gall. They may even sacrifice their lives to bite a passage (bee hole) for the female to leave the male fruit. In the end, most of them die in the fruit.

The sacrifice of the male fig wasp is exchanged for the female fig wasp carrying the pollen of the male flower to escape from the male fruit and start the next life cycle, which explains why it is the female fig wasp that pollinates and also carries the pollen. In the long-term evolution, figs and fig wasps have formed an absolute mutualistic symbiotic relationship.

Symbiotic relationship between fig and fig wasp

(Photo source: World Knowledge Pictorial)

Will I eat fig wasps when I eat figs?

Some people may worry about whether they will eat fig wasps when eating figs?

It should be noted that even for the native species of figs, when the fig fruit is ripe, the fig wasps that remain in the fruit are basically broken down by secreted enzymes. Even if there are a few remaining, they are high-quality protein that they have sucked from the fig juice since childhood, and it is harmless to the body if they are eaten along with the fruit.

Moreover, the figs sold on the market today are basically female plants that have been carefully cultivated by humans. There are no male plants and therefore no gall flowers. They can produce fruits without pollination by fig pollen, which is scientifically known as "parthenocarpy". Because of this, these figs do not contain mature seeds. What we eat is just the abnormally swollen receptacle of the fig, which can be eaten with confidence.

The Bodhi tree is also a kind of "fig"

The Bodhi tree is known as the sacred tree of Buddhism. It is also a plant of the genus Ficus in the family Moraceae. The top of the heart-shaped leaves has a long dripping tip, which can adapt well to the characteristics of tropical rainforests with heavy rainfall and high humidity. The water film on the surface of the leaves can quickly gather into water droplets and drip from the dripping tip, thus ensuring that the leaves can carry out photosynthesis.

Even when the water in the bodhi tree cannot be discharged through transpiration, liquid water will be discharged directly through the leaf tips and leaf edges, and finally gather at the leaf tips and drip. This is called "water spitting phenomenon" in the plant world.

Bodhi tree leaves

(Photo credit: Photo taken by the author)

In September 2020, relevant research pointed out that compared with other plant-derived foods (such as nectar), water droplets exhaled by plants have a long supply cycle and a wide range of supply objects. They are also rich in carbohydrates and proteins, and are a reliable food source for many insects. They can help increase insect populations and adaptability.

Unlike figs, the Bodhi tree is monoecious, so the fruit of the Bodhi tree contains both male and female flowers and gall flowers. The fruit of the Bodhi tree is very small, about the size of a little finger, and when it matures and turns red, it attracts birds to eat.

Bodhi fruit

(Photo credit: Photo taken by the author)

When a bird has had a full meal, it may accidentally fly high in the sky (shit), and the undigested seeds in the bodhi tree fruit will fall on the trunks or branches of other trees.

As the seeds take root and sprout, they will eventually cover the entire tree like a net after years in the tropical rainforest. The trees inside will eventually die and rot due to the external strangulation and lack of nutrients inside the Bodhi tree, turning into spring mud for the Bodhi tree to absorb. This is called the "strangulation phenomenon" in nature. Sometimes, after the Bodhi tree is strangled, a huge strangulation tree hole will be formed, which is also a wonder of the tropical rainforest.

Tuberous fig tree hole

(Photo credit: Photo taken by the author)

Bodhi tree and bodhi seeds

As we mentioned above, the fruit of the Bodhi tree is hollow like a fig, and the countless female flowers inside will eventually develop into seeds. Therefore, the seeds of the real Bodhi tree are very small, even smaller than sesame seeds. You can imagine, how can such small seeds be worn?

In fact, Bodhi seeds have nothing to do with Bodhi trees. There are many kinds of Bodhi seeds on the market, most of which are processed from relatively hard or starch-rich plant seeds or fruit cores. For example, Star and Moon Bodhi and Ivory Bodhi belong to the Arecaceae family, Lotus Bodhi belongs to the Pandanaceae family, Pandanus genus, Vajra Bodhi belongs to the Elaeocarpaceae family, Elaeocarpus plant, and Five-Eyed and Six-Way Bodhi belongs to the Anacardiaceae family, Choerospondias genus, and the pulp can be used to make sour jujube cake.

“There is no Bodhi tree, nor is there a mirror stand. Originally there is nothing, so where can dust gather?” Therefore, when it comes to Bodhi seeds, you can just pick the ones you like to play with, and don’t worry about their relationship with the Bodhi tree.

Banyan tree-fig bee mutualistic symbiosis system

In nature, banyan trees and fig wasps have undergone about 75 million years of co-evolution. Each banyan tree has specific fig wasps that coexist with it, forming a banyan tree-fig wasp mutualistic system, which has now become a model system for studying the specialized co-evolution of plants and pollinating insects.

Fig wasps are the only pollinators of banyan trees. Each banyan tree usually has only one species of fig wasp for pollination. Some studies have also found that some banyan trees have two or more species of fig wasps, and a few fig wasps can pollinate more than one species of banyan trees. However, the relationship between the vast majority of banyan trees and fig wasps is strictly one-to-one.

**What many people don’t know is that fig wasps are divided into pollinating fig wasps and non-pollinating fig wasps. **Sometimes, in addition to a single pollinating fig wasp, there are also several to dozens of non-pollinating fig wasps coexisting in figs. Non-pollinating fig wasps usually pass through the fruit cavity through the long ovipositor outside the fig and lay eggs in the ovary of the female flower, reproducing offspring without pollinating the female flower.

Of course, there are a few non-pollinating fig wasps that can drill into the inflorescence to lay eggs, but they will not actively pollinate female flowers. After a long period of evolution, fig trees, pollinating fig wasps, and non-pollinating fig wasps have formed a complex and stable community.

**During the flowering period, different figs will release different special smells, attracting and allowing specialized pollinating fig wasps to drill in. **When pollinating fig wasps drill into figs to pollinate female flowers, they will lay eggs in some gall flowers, and the female flowers that are not spawned by fig wasps will develop into seeds. Those who have picked figs know that fig plants are rich in white latex, which is their defense mechanism. After a long period of evolution, pollinating fig wasps have also adapted to the host fig tree's chemical defense against invasion, forming a highly specialized symbiotic relationship with it.

White milky discharge from the wound of the fig

(Photo source: veer photo gallery)

The reproduction mechanism of banyan tree

The banyan family can be divided into two major categories: dioecious and monoecious.

In the dioecious banyan tree, male plants produce male fruits, which contain gall flowers (gal flowers are specialized female flowers that almost never produce seeds, with short and straight styles, which are conducive to the laying of eggs by pollinating fig wasps) and male flowers; female plants produce female fruits, which only contain female flowers, with long and curved styles, which make it difficult for pollinating fig wasps to lay eggs in the ovary, and the eggs gradually develop into seeds after pollination.

In the monoecious banyan tree, female flowers, male flowers and gall flowers grow simultaneously in the sycamore inflorescence, which can not only meet the needs of pollinating fig wasps for laying eggs, but also produce seeds.

Pollinating fig wasps are divided into male and female. Male fig wasps live in the closed fig fruit for their entire lives. They have no wings, and their eyes and antennae are completely degenerate. After mating and biting out the beehive, most of them are buried in the fruit. After the female fig wasps emerge in the fig fruit, they will collect pollen through the evolved pollen brushes and pollen bags, and then leave through the beehives with the pollen to find a specific fig fruit to complete pollination.

During the fig pollination period, for monoecious fig trees, figs can release special volatiles during the female flowering period to attract specific pollinating fig wasps to carry pollen into the fruit cavity to pollinate the female flowers and lay eggs in the gall flowers.

For dioecious fig trees, figs also release special volatiles to attract specific pollinating fig wasps to carry pollen into the female fruit cavity for pollination, and then drill into the male fruit to lay eggs in the gall flower. The maturation period of the female and male flowers of the fig tree is several weeks to several months apart, and the female flowers in the same inflorescence have withered when the male flowers mature.

Therefore, whether it is a monoecious or dioecious fig tree, the pollinating fig wasp needs to carry pollen and drill into the new inflorescence in the female flowering period to complete pollination. After the pollinating fig wasp enters, the inflorescence hole will gradually close, providing a stable pollination and egg-laying environment for the fig wasp.

Studies have shown that during the development period of the monoecious blunt-leaved fig, the different volatile compositions of the male and female flowering periods regulate the behavior of pollinating fig wasps, causing them to collect pollen in the figs during the male flowering period and only drill into the fruit cavity of the figs during the female flowering period to pollinate and lay eggs.

Other studies have pointed out that in monoecious fig trees, there are male flowers, female flowers and gall flowers in the inflorescence. The pedicels of gall flowers are long and close to the inflorescence cavity, while the pedicels of female flowers are short and close to the fig wall. Non-pollinating fig wasps can use their long ovipositors to penetrate the fig wall and lay their eggs in the female flowers in the monoecious fig or in the galls where pollinating fig wasps have laid eggs.

**Through the study of dioecious figs, it was found that in addition to the pollinating fig wasps distributed inside the inflorescence cavity, there were three non-pollinating fig wasps distributed near the fig fruit wall in the mature male fruits of figs. **At the same time, the distribution of pollinating fig wasps was found that the gall pedicels of male pollinating fig wasps were the longest and distributed in the innermost part of the inflorescence cavity, so they were least likely to be attacked by non-pollinating fig wasps.

Based on this, the researchers predicted that it might be that the pollinating fig wasps first lay drone eggs in the gall flowers when laying eggs, causing the ovary and pedicel of the drone gall to swell and lengthen first. When the non-pollinating fig wasps penetrate the male fruit to lay eggs after a long egg-laying period, they are more likely to choose to lay eggs in the galls of female pollinating fig wasps with relatively short gall pedicels, thereby ensuring that the male pollinating fig wasps can develop normally and complete the subsequent mating process.

Fruits of the large-fruited fig, with pedicels of different lengths, and holes drilled by fig wasps are obvious

(Photo credit: Photo taken by the author)

In tropical rainforests, external organisms also promote the coexistence of pollinating fig wasps and non-pollinating fig wasps. Many ants mainly feed on non-pollinating fig wasps that lay eggs outside figs. While controlling the population of non-pollinating fig wasps, they also greatly reduce the egg-laying efficiency of non-pollinating fig wasps, thereby ensuring the number of offspring and seeds of pollinating fig wasps and maintaining the stability of the fig tree-fig wasp mutualistic symbiosis system.

Yellow ants preying on fig fruit

(Photo credit: Photo taken by the author)

Fruit of Ficus rhombus

(Photo credit: Photo taken by the author)

At the same time, as an important plant group in the tropical rainforest, in addition to fig wasps, there are also many insects and animals that feed on fig fruits directly or indirectly and help spread seeds, which is also crucial to maintaining the stability of the rainforest ecological form.

(Note: Latin parts in the text should be italicized)

References:

1. Li Xinxin, Wu Minxia, ​​Wu Wenshan, et al. Study on the behavior of non-pollinating fig wasps[J]. 2013.

2. Zhang Yuhao. Effects of non-pollinating fig wasps on seed production of banyan trees[J]. Guangdong Sericulture, 2021.

3.Gu D, Compton SG, Peng Y, et al. 'Push' and 'Pull' responses by fig wasps to volatiles released by their host figs[J]. Chemoecology, 2012, 22(4).

4.Urbaneja-Bernat P, Tena A, Gonzalez-Cabrera J, et al. Plant guttation provides nutrient-rich food for insects[J]. Proceedings of the Royal Society B: Biological Sciences, 2020, 287(1935).

5.Yu H , Compton SG .Moving Your Sons to Safety: Galls Containing Male Fig Wasps Expand into the Center of Figs, Away From Enemies[J].Plos One, 2012, 7.