The Fig Tree’s Revenge

The insect world brims with crazy examples of the power of natural selection, and how necessity drives it to weave through almost every possible survival niche that exists in nature. Insects thrive in every environment where life exists. They, in differing forms, are both highly social and solitary.  Some care for their young, others do not.  Many live their entire lives underwater, while others survive in the most barren climates.  The insect footprint, while diminutive in size, is massive in the larger study of life.

Insects were some of the first terrestrial herbivores, which placed them in an influential position regarding the future of plant life. Genetic evidence suggests that an evolutionary arms race broke out between insects and plants, each undergoing selection for the best countermeasures to the other.  Starting around the Devonian, plants began to develop toxins to deter these new pests, and over time, insects adapted tolerances to them.  Around 275 million years ago, the first signs of mutually beneficial symbiotic relationships emerged between plants and the insect world.  Around 75 million years later the ancestors of flowering plants started to diverge from gymnosperms, and over the next 150 million years flowering plants slowly became the dominant form of large flora all across the planet.  During this transition, natural selection in the insect world stayed close behind.  Many species of predatory wasp gained a taste for pollen, and eventually gave up their carnivorous ways.  These are the ancestors of modern bees.

One of the more fascinating cases of mutualism between insects and plants is that of the fig wasp and its namesake tree.  Each fig wasp species only interacts with a specific and corresponding fig tree species.  The female fig wasp enters a fig through a small natural opening at the top in order to lay her fertilized eggs. During this process, she moves pollen from the outside of the fruit to the seeds in the core, effectively fertilizing them.  After laying her eggs, the female wasp dies, and her corpse is broken down within the fig. Following the hatching of the eggs, the young males mate with the young females, and then dig their way out of the anterior of the fig, dying shortly thereafter.  The young females emerge from these new tunnels and fly to another fig tree to begin the cycle over again, bringing some of the pollen from their benefactor tree as they go along.

This is a process over 80 million years in the making, so we should see evidence for the evolution of a mechanism to prevent either side from cheating and not fulfilling their end of the symbiotic bargain.  New research from Cornell University provides insight into how this works.  Not all fig wasps are passive pollinators (carrying the pollen on their bodies), some are active pollinators, and must manually grab pollen in order to accumulate it.  This leaves the opportunity for a female to enter the fig, lay her eggs, and die without ever manually pollinating the seeds.  Among fig wasp species of active pollinators, their corresponding fig tree species have developed a countermeasure.  If a fig from one of these trees is used by the wasp for her eggs, but is not fertilized, the fig tree drops the fruit prematurely.  The long fall to the ground below crushes any wasp eggs within.  This retaliatory behavior evolved as the balancing force which encouraged for the selection of wasps that expended the energy necessary to fertilize the figs.  It likely originated from pressures resulting in the widespread prevalence of unfaithful insect pollinators.  This give and take will see-saw back and forth as long as these catalysts remain in flux.

What’s the most interesting to me is that the fig tree gets no short term gain from dropping the unpollinated fruit.  It’s not very likely that what pollen remains on the outside of the fruit is going to fertilize the seeds before they get consumed by life on the jungle floor. It’s even possible that the fruit could be pollinated by another passing wasp if left remaining high in the tree.  Despite this chance, it is more beneficial to the fig tree to destroy the progeny of the non-pollinating female in order to squash the genes promoting that sort of uncooperative behavior.

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~ by Wil Finley on February 2, 2010.

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