Have you ever seen a cluster fig tree and wondered how the tree suddenly has fruits when it never seems to flower? This is because figs grow clusters of flowers inside round, pot-like structures called syconium that eventually ripen into fruits. Each syconium bears hundreds to thousands of tiny flowers in a secret garden. The only entrance to this secret garden (fig) is a tiny opening well-guarded by overlapping leaf-like flaps. The fig’s secret gardens do not welcome any insect — each species of fig (of which there are ~850) can only be pollinated by a specific species of pollinator fig wasps.
Pollinator wasps have evolved with their fig partners such that the size and shape of the wasps’ bodies and heads allow them to push into the fig. If the wasp is too big, it will get stuck; if it’s too small, it won’t be strong enough to push through.
Cover Photo: A female parasitoid drills into a fig to lay eggs on the larvae of pollinator and galler fig wasps. Some of these wasps have ovipositors (egg-laying organs that look like a tail) that are longer than the wasp itself! Photo: Amith Kiran Menezes
Partners in love
Ficus racemosa or cluster figs, common throughout India, are partners in love with pollinator wasps Ceratosolen fusciceps. Once the cluster fig tree grows its secret garden and has a crop of female flowers (which can grow into seeds) ready for pollination, it pumps out a scent designed to attract its pollinator partner. Each species of fig has its own unique perfume to lure its partner fig wasp. Female pollinator wasps, laden with pollen from another cluster fig, home in on this perfume; some may fly more than a hundred kilometres to find a fig tree to pollinate.
Once a pollinator lands on a fig, she begins to push her way into the fig. This journey is not kind to her. She loses her antennae and wings and may literally bust her guts to get into the secret garden. Even once inside, there is still no rest. The pollinator must now pollinate the flowers and lay her eggs within some of these flowers.
Therefore, pollinator wasps are not only the fig’s pollinating partners, but in a way, they are also parasites that eat up future seeds. Without these wasps, however, figs cannot transport pollen between trees. Similarly, the wasps’ young cannot survive outside of the fig, which is why this partnership straddles the border between mutualism and parasitism.
Betrayals: galler parasites and wasp predators
While the pollinators struggle to enter the fig to pollinate it and lay eggs, some of their sister species do not believe in such self-sacrifice. These parasitic wasps use venom to chemically trick the fig flowers into forming round masses called galls to feed their young and are therefore called galler parasites.
Unlike pollinators, gallers do not enter the fig to lay their eggs. Their egg-laying organs are long, flexible, tube-like drills that can pierce the fig wall and allow them to place their eggs into the fig flowers. Gallers are enemies of both figs and pollinators as they cheat the fig of pollination services and directly compete with pollinators and fig seeds for space and nutrition.
Although the tough walls of the fig, well-irrigated with sticky, foul-tasting sap, keep large animals from eating the growing seeds and wasps, they do not daunt smaller predators. Specialised huntresses of pollinators and gallers, called parasitoids, have long ovipositors (much like the gallers) to drill into the fig. However, unlike gallers (which lay eggs in fig flowers), parasitoids lay their eggs onto the developing larvae of gallers or pollinators. The carnivorous parasitoid larvae literally eat their hosts alive.
Does this make parasitoids heroes or villains to the trees and pollinators? On the one hand, some parasitoids target galler larvae, protecting the next generation of pollinators and figs from cheaters. On the other hand, some parasitoids also target pollinator larvae, which makes them enemies of the fig–pollinator system. Parasitoids are probably the anti-heroes in this situation.
Growing up with enemies
Typically, a cluster fig takes 1–2 months to develop fully. During this time, each pollinator, galler, and parasitoid wasp develops within its own chamber or gall, side-by-side with the fig seeds.
Once the wasps are fully developed, the males chew their way out of their galls and search for females. All the fig wasp species in cluster figs have winged females and wingless and mostly blind males. The males’ sole purpose is to mate with and release the females from their galls. Despite the crowded confines, males of each species somehow recognise galls containing females of their own species. Once the orgy of mating is over and no virgin females remain, most male wasps die; however, the pollinator males still need to perform one last task — to release the females from the fig.
During this time, the fig also prepares for the fig wasps’ exit. The walls of the fig soften and the male flowers in the fig are bursting with pollen. Mated female pollinators collect the fig pollen while the male pollinators work together to chew a hole through the softened fig walls. The relationship between the pollinators and other wasps in this system is fascinating because of the self-limiting nature of parasitism and predation. If the parasites and predators kill off too many pollinators, their progeny will die within the fig as none of the other male wasps have jaws strong enough to chew a hole in the fig wall.
Once the male pollinators chew the exit hole, the female pollinators, gallers, and parasitoids fly out in search of another fig to start the cycle anew.
Meanwhile, the fig, which has housed the wasps and seeds, has one last task. It softens further and turns bright red-orange or yellow, juicy, and fruitily aromatic. This attracts bats, squirrels, and birds to eat the fig and scatter its seeds.
Other partners: Ants, hitchhiking worms, and son-killing bacteria
Many cluster fig trees are also home to the Asian weaver ant (Oecophylla smaragdina), whose colonies often take over entire cluster fig trees.
These ants often “eavesdrop” on the fig tree’s scent-communications system to capture and eat incoming or outgoing pollinators. They can often identify where the male pollinators are chewing holes and will lay in wait to grab the emerging female wasps. The ants also prey on the gallers and parasitoids that come to lay their eggs in the figs. When a galler or parasitoid female has her ovipositor deep inside a fig, probing for a good place to lay her egg, she is easy prey; the ants snip off the ovipositors and carry away the hapless wasps to feed to their nest mates.
To fig wasps, ants are like big scary monsters that indiscriminately kill and eat pollinators, gallers, and parasitoids.
Apart from the dangers lurking outside, the fig interior has dangers. Cluster figs host at least three species of microscopic worms that feed on the fig flowers and use pollinators as taxis. These worms nictate — raise the front parts of their bodies and wave them in the air — to specifically locate female pollinators.
Once they find a pollinator female, the worms will burrow into her abdomen to hitch a ride to a new fig. Once inside a new fig, the worms abandon the dying pollinator and begin to feed and reproduce on fig flowers. The worms are super-specific about which wasps they hitchhike on — they do not get into male wasps or any of the gallers or parasitoids, as these wasps never enter other figs.
Another strange creature found in the fig–fig wasp system is a bacterium known as Wolbachia. Technically, Wolbachia do not live in figs as they live within the cells of galler and parasitoid wasps. These bacteria are parasites that live within the cells of wasps and manipulate their hosts in strange ways. In many species of insects, Wolbachia either kill off males or convert male eggs and embryos into females. Although this bacterium is well studied in other insects (it is currently being used to curb the spread of dengue, chikungunya, and Zika in mosquitoes), its effects on fig wasps are as yet unknown.
A world within a world
The fig is a world within a world, a microcosm. Within it, one can see all of life’s interactions played out in miniature — there is love, sacrifice, cheating, betrayal, predation, parasitism, and even gender discrimination. All in all, life within a fig is a microscopic performance that could be a Bollywood movie!
About the contributor
Anusha Krishnan
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