Figs and wasps: Unravelling nature’s intricate relationships9 min read . Updated: 03 Apr 2016, 04:58 PM IST
Scientists are still plumbing the depths of communication between and across animal and plant species
Scientists are still plumbing the depths of communication between and across animal and plant species
The luscious sweet fig is an expert in Machiavellian tactics. It tricks wasps by seducing them with scents for its own benefit, often with the wasp losing out. The ant-plant Humboldtia, on the other hand, is benevolent—the leaves secrete nectar to attract ants and unique arboreal earthworms, which then take shelter in the plants.
Such inter-species communication is the focus of study in the lab of Renee Borges, a professor at the Centre for Ecological Sciences in the Indian Institute of Science (IISc), Bengaluru.
On a hot summer afternoon in March, I visited Borges’s lab, a large spacious room with high ceilings in the Biological Sciences building—tall and spanking new, a far cry from the old stone buildings that housed the biological science labs for many years. Inside, a quadrangle with a cafeteria was abuzz with students. Outside, large trees, sheltering a variety of flora and fauna, stood as a beacon of science.
With 15 scientists and 50 doctoral students, the department’s interests range across the conservation of forest ecosystems and wildlife; the evolution of biological diversity at the genetic, population and community levels; the evolution of social life in organisms; the dynamics of animal and plant populations; and communication between and across animal and plant species.
“Communication across species or biological kingdoms is the most challenging to study. It requires a fair bit of ingenuity to imagine the language between them. It is like hieroglyphics. What are the types of signals sent out by organisms? Who is receiving them?" said Borges.
She hopes to find the metaphorical Rosetta stone in order to decipher these signals and, in turn, unravel some of the mysteries of the natural world. For the past few decades, she has been studying sensory biology—interactions between species through the exchange of sensory inputs (vision, colour, scent or chemicals).
One of the interactions studied by Borges and her team is the 80-million-year relationship between the fig tree and the fig wasp. The tree and the wasp share a mutualistic relationship, with the wasps pollinating fig trees’ flowers and the trees, in return, providing wasps with mating and hatching grounds.
This exchange of resources, either for mutual benefit or for one’s own benefit, is an important part of “mutualism", which plays a key role in ecology. Most of the biological diversity we see today is thought to be a result of mutualism. We humans also engage in mutualism with other species—we shelter and provide food to the flora in our gut, for instance, without which we cannot digest food efficiently.
The term was introduced by Belgian zoologist Pierre Joseph van Beneden in 1876. Known for his work on parasites, he wrote many books—including Les commensaux et les parasites dans le règne animal. His book classified relations between species of animals as parasitism, commensalism and mutualism. He defined commensalism as a relationship between two organisms in which one benefits but the other is not affected and mutualism as one in which both benefit.
But in the fig tree-fig wasp relationship, the barter is not as simple as it seems.
Borges, who has been studying the relationship between the fig and the wasp for more than 10 years, said, “The fig is an intricate microcosm. Each fig species has its own unique fig wasp pollinator; in addition, there are parasitic wasps that inhabit the microcosm. The fig also has nematodes, which are small worms that hitchhike on pollinator fig wasps to move from one fruit to another." The nematodes hone in on only female wasps, which fly from one fruit to another. These savvy hitchhikers know which wasps to target by the smell and taste of each fig species.
Fascinated by the intricacy of the fig and fig wasp system, I asked if I could have a live demonstration. For my benefit, Borges’s research team—doctoral student Satyajeet Gupta, project assistant Anusha Kumble, field assistant G. Yathiraj and post-doctoral fellow Lucy Nongbri—accompanied me to show me the fig trees on the campus, where they are conducting their studies.
A short drive from the lab brought us to the residential part of the campus, where grey stone buildings contain the students’ hostel and canteen. There are 55 fig trees of the species Ficus racemosa spread throughout the IISc campus. The team was leading me to a tree which had recently been penetrated by fig wasps.
In the midst of the buildings, there stood a tall, majestic tree stood, laden with bunches of small spherical green structures, no bigger than grapes. These are called synconia; they contain the flowers of the fig tree. A single syconium can hold hundreds or thousands of flowers.
This tree is monoecious—that is, both male and female flowers are present in the same syconium in the same tree. The female flowers mature first and, when they are ready to receive pollen, volatile fragrances waft out from the syconium, attracting the fig wasp.
Drawn by the scent, a female wasp carrying pollen from its previous host enters the syconium through a tiny opening in the centre. The fig wasp, about 2mm in size, can easily pass through the eye of a needle, but the syconium opening is even narrower. As it enters, its wings break off and its abdomen gets squished as it makes its long slow crawl into the syconium.
The fig tree then closes the opening and seals it with an antiseptic liquid to prevent other insects from getting in—in the process, turning it into a death trap for the wasp. Once inside, the wasp lays its eggs in the female flowers, simultaneously depositing the pollen. Tired and wounded from the long gruelling journey down the syconium, the wasp is on its last leg of life. Before it dies, it creates nurseries by injecting venom inside the flower, which grow into fat rounded structures called “galls". After the eggs hatch, these galls provide food and shelter for the young offspring.
Gupta, the doctoral student, plucked one of the synconia from the tree, deftly splitting it into two. Inside, I could see hundreds of fat swollen galls, each probably nurturing eggs inside. A few that had already hatched were feeding on the galls.
Once these offspring attain adulthood, the male and female wasps mate within the fig; the blind and wingless male wasps then cut an opening, allowing the female wasp to fly out. Around the same time, the male flowers are getting ready. The female wasps collect the pollen from the male flower and leave, starting another life cycle.
The syconium now contains only lifeless and wingless male wasps and goes on to ripen. The mature fruit containing fully developed seeds are consumed by birds and monkeys, which also help in the dispersal of seeds.
This entire cycle lasts for about two months. So, the research team surveys all the fig trees on campus once a week in order to keep a check on the phase of the tree. This is important for them to design and conduct experiments and to avoid missing the stages of both the wasp and syconium development.
Borges and her team have been studying the scents that emanate from the syconium, which play a vital role in attracting the fig wasps. In their latest research, the team found that certain type of fig trees have evolved a different strategy—one that tilts the balance of the relationship in their favour.
These fig trees are dioecious—the male and female flowers are present separately on different trees, unlike the monoecious trees where both male and female flowers are present in the same tree. In these species, wasps develop in male flowers but cannot develop in female flowers as they have long stalks inside, so the female wasp cannot reach into the ovaries to lay eggs and gall the flowers.
Borges and her team found that in these fig trees, when the male and female trees flower synchronously, the female figs deceptively attract wasps by mimicking the scent of the male flowers. The female wasps are tricked into entering the synocium, but they cannot lay eggs and end up dying in vain. The female flower, however, is pollinated and goes on to produce seeds.
The fig tree is commonly found in tropical and sub-tropical forests. At any given time, fig trees are always producing fruit, and sometimes are the only fruit available to animals and birds. In African jungles, where the fig tree is also known as the “queen of trees", the fig fruit is eaten by monkeys, squirrels, more than 100 varieties of birds and even elephants.
The fig is a keystone species—it plays a unique and crucial role in maintaining biodiversity and population density. Like the stone at the centre of the arch that keeps it from crumbling, without the keystone species, the ecosystem could change or even fall apart.
And the propagation of the fig tree depends on its mutualistic relationship with the fig wasp. For a stable mutualism to exist, the overall net benefit has to be greater than the cost. The fig has a feedback system: like a bookkeeper keeping checks on business transactions—credits and debits—the fig tree keeps a balance sheet for services received and given. If a greedy wasp tries to lay far too many eggs in a syconium and provide far too little in the way of pollination services, the fig tree punishes the wasp by imposing “host sanctions"—it aborts that syconium.
The fig tree can also regulate the number of wasps it lets into the syconium, but it is not yet clear how. The fig-wasp system is complex, one not completely understood yet. One of the many challenges of working in the field of ecological biology is that a lot of factors are not under researchers’ control, said Borges. “We cannot will something to happen, unlike the controlled systems in the lab. And sometimes, our experiments are vandalized by monkeys or natural forces such as wind, drought, etc."
In order to avoid disturbances from external factors, the team put bags on the syconium for certain experiments. Kumble, the project assistant, tells me, “We start our work at five in the morning—one of us collects samples from the bag, another stands guard to ensure that other wasps do not enter and contaminate the experiment."
But, by and large, working in the field is fun. Borges recollects the time she spent at the Bhimashankar Wildlife Sanctuary near Pune. Her work involved studying the chemistry of trees and how it related to the feeding habits of 1m long giant squirrels, almost as big as monkeys, without interfering in their daily life.
“After many days of quietly observing a set of squirrels, one female who had a pup came down the tree, sniffed me as she circled around and finally went back up to the tree. It was a special moment, as they had accepted me as part of their system—their natural surroundings," she said.
This study was part of her doctoral degree, which she obtained at the University of Miami. Before that, she studied at St. Xavier’s College, Mumbai, which she says was a “liberating experience". “Growing up in a cosmopolitan city like Bombay, one’s outlook is not parochial, but broad and there’s a certain level of professionalism there that’s unmatched," she said.
Borges went on to become the deputy director of the Bombay Natural History Society in 1992, before joining IISc as an assistant professor in 1997.
Currently the director of the Centre for Ecological Sciences, she is mentoring a new generation of ecological scientists. She says an important virtue for any natural biologist to have is patience and powers of observation. “I often tell my students—think like a fig wasp, think like a nematode. Only if we enter their world, without imposing our human perceptions, we can uncover their secrets."
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