Banyan Trees and Pollinating Fig Wasps: Facts and Discoveries

A banyan tree named Ficus benghalensis in Hawaii (Photo by Melikamp, via Wikimedia Commons, CC BY-SA 3.0 License)

Impressive and Distinctive Plants

Banyan trees are impressive plants that are known for the aerial roots produced by their branches and their pollination by wasps. They are a type of fig tree and depend on wasps for fruit production. The relationship between the tree and the insect is highly tuned. Scientists have recently discovered some interesting facts about one species of banyan tree and the wasp that pollinates it.

All fig plants belong to the genus Ficus. The genus contains plants that produce an unusual inflorescence and fruit and are pollinated by a wasp in the family Agaonidae. Only some species in the genus are banyan trees, however.

Though all members of the genus Ficus produce figs, the fruits may not be edible for humans. The species that produces the figs commonly bought in grocery stores is Ficus carica, which isn’t a banyan tree.

The Nature of Banyan Trees and Fig Wasps

The name “banyan tree” is commonly applied to Ficus benghalensis, which is the national tree of India, but it’s also used for other plants. The species have a common growth habit. They start their life as an epiphyte, or a plant that grows on another one. The plant is created by a seed that falls into a crevice of a host plant and then germinates.

Banyan plants are sometimes known as strangler figs because they cover their host and eventually kill it. The branches of the banyan tree produce aerial roots that extend downwards and into the soil. These are sometimes known as prop roots. They become woody and can enable the tree to spread over a large area.

The flowers of fig trees (including banyan trees) can’t be seen by a person observing the plants. They are hidden inside the figs. The immature figs of the trees provide a home and food for pollinating fig wasps. The insects pollinate the banyan tree’s flowers inside the immature fig and enable it to mature into a fruit bearing seeds.

Fig wasp species are either pollinating or non-pollinating, I’ve included the photo of the non-pollinating fig wasp below because it shows the long and impressive ovipositor of the insect. The female inserts her eggs into the fig through the ovipositor, where they develop onto new wasps, but she doesn’t help the plant to reproduce. Pollinating fig wasps are described below.

A fig wasp (Philotrypesis sp.) sending her eggs into a fig (Photo by Alandmanson, via Wikimedia Commons, CC BY 4.0 License)

Structure of a Fig Fruit

Most fruits develop from the ovary of a flower. Figs are unusual because they develop from stem tissue, or the receptacle that bears a flower in other plants. The receptacle is usually thickened compared to the rest of the stem.

A fig develops from the receptacle of an inflorescence, or a group of flowers arranged on the same stem. The male and female reproductive structures of each flower are embedded in the stem tissue instead are being borne on top of it. A fig is botanically known as a synconium instead of a fruit.

Pollination of fig trees followed by fruit development is important. Humans may think of edible figs as tasty treats, but they have more value than this in nature. They are nutritious and are a source of energy for forest animals.

Figs are known to sustain at least 1,200 bird and mammal species.  (Quote from Diana Yates, University of Illinois at Urbana-Champaign, via phys.org)

Life Cycle of a Pollinating Fig Wasp

Each species of banyan tree is pollinated by a specific species of pollinating fig wasp. The plant and the insect are so attuned to each other that a fig tree produces a special scent to attract the correct wasp when its flowers are ready for pollination. Slight variations may occur in the life cycle described below, depending on the species of Ficus and wasp that are involved in the relationship.

The fig has a tiny opening called an ostiole that enables the wasp to enter. The opening is so small that it injures her body as she moves through it. The wasp’s wings are torn off her body and her antennae are often damaged. Once she is inside the fig, she lays her eggs. As she moves around, she deposits pollen on her body that was collected from another plant onto the stigma of the female structures. The pollen grains contain the sperm cells needed for fertilization and seed production. The female dies after laying her eggs.

The eggs hatch, producing new wasps. The males have no wings. They look for females inside the fig and fertilize them. The males then die, but not before they have chewed an exit wall in the fig. The young females are able to leave the fig through this hole. They brush against the anthers of the flowers and collect pollen on their bodies before they do so. They then fly to another fig and the cycle begins again.

Do Edible Figs Contain Dead Insects?

People who hear about the interesting method of fertilization and reproduction in fig plants sometimes wonder if there are dead insects in the figs that we eat. It’s a good question. Researchers say that the fig digests the dead insects, so there are unlikely to be any left when we eat the fruit. The crunchy items in a fig are seeds, not insect bodies. If any insect bodies do remain, it’s unlikely to be a serious problem. We ingest parts of insect bodies in many of the foods that we eat.

Two Trees in the Genus Ficus

The recent discoveries about the relationship between a banyan tree and the wasp that pollinates it were discovered in Ficus microcarpa and compared with the features of F. hispida. The first species is a typical banyan tree that develops aerial roots and depends on wasps for pollination and fruit production. A single plant bears both male and female structures. The species is referred to as the Chinese banyan, the Malayan banyan, the Indian laurel, and by other names. Though it produces figs, they aren’t considered to be edible for humans. They are eaten by some animals, though, including certain birds.

F. hispida is a fig tree like F. microcarpa. It doesn’t develop aerial roots, however, which is probably the most common feature that people associate with banyan trees. In addition, the male and the female structures are borne on separate trees.

A Ficus microcarpa tree in Hawaii; aerial roots can be seen near the centre of the photo (Photo by Forest & Kim Starr, via Wikimedia Commons, CC 3.0 License)

Discoveries About the Genome of Ficus microcarpa and a Wasp

Some Chinese researchers have recently studied studied the genome of the two fig species named above. The genome is the complete genetic information of a species. It’s stored in DNA (deoxyribonucleic acid) molecules. The scientists also studied the genome of the pollinating wasp. They found that more segments of DNA were duplicated in the banyan tree (F. microcarpa) than in the other fig species. In fact, the duplicated sections formed 27% of the plant’s genome.

The scientists explored the functions of the genes in the duplicated sections. The sections contained genes for making and transporting plant hormones known as auxins. These hormones stimulate plant growth. The duplicated sections also included genes related to nutrition, immunity, and the production of volatile substances that attract the pollinating wasps.

The researchers discovered that the the level of auxin in the aerial roots of the banyan tree was five times higher than in the leaves of either species. They believe that the auxin triggered the growth of the roots. They found that the copied sections contained a gene that codes for a light receptor molecule that speeds up the production of auxin.

The scientists compared the genome of Eupristina verticillata (the wasp that pollinates F. microcarpa) with the genome of related wasps. They found that E. verticillata had genes for detecting the scent released by the tree’s figs, which the other wasps lacked.

The world of nature contains some fascinating relationships. The one between a banyan tree and a pollinating fig wasp is an example. Understanding more about the relationship between a tree that produces figs and the insect that pollinates it might be very useful for us as well as interesting.

References

  • Ficus microcarpa information from CABI Invasive Species Compendium
  • Fig wasp information from the USDA (United States Department of Agriculture)
  • Life and death of a fig wasp from Arizona State University
  • Genomic study of a banyan tree from the phys.org news service

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