This “Useless” Organ Determines Which Ants Grow Into Large Soldiers

Ant colonies have evolved some surprising collective traits over the 150 million years or so that they have crawled the Earth. Fire ants link together to make frighteningly effective rafts, for example, and some army ants seem to instinctively build perfectly efficient ant bridges for collecting food.

These insects also seem to have the ability to control their populations, maintaining a set ratio of soldiers to workers. How such ant caste systems came to be is something of a mystery for entomologists, but a study published in Nature today reveals that whether an ant grows into a soldier or a worker has to do with an organ that was long thought useless—wing discs.

“These caste systems are one of the major unexplained phenomena of phenotypic evolution,” says Scott Powell, an ant biologist at The George Washington University in Washington D.C. “This study really identifies the main underlying control center that produces these castes.”

The wing discs, tiny pouches of cells, are destined to become wings on a queen, but they grow in the larvae and then die during metamorphosis to produce wingless soldiers. The study shows that these rudimentary wing discs are not so rudimentary after all, but rather affect the development of the ant larvae.

“The really important point about this work is that rudimentary organs and vestigial structures, which were long believed to have no function, might actually have major signaling roles during development,” says evolutionary developmental biologist Ehab Abouheif of McGill University, the senior author of the new study. (Disclaimer: I spoke with Abouheif about his research prior to publication and he has included me in the paper's acknowledgements.)

Developing soldier larvae produce large, rudimentary wing discs, while the larvae of workers do not. Previously thought to simply die away during metamorphosis, the wing discs apparently have some influence on the growth of other body parts.

“[The wing discs] do get quite big–it’s kind of surprising,” says Diana Wheeler, an ant biologist at the University of Arizona who has studied ant caste determination since the 1980s. “It’s evidently not something that evolution just forgot to get rid of. It seems like it’s being used for something.”

The apparent function of the ant larvae’s wing discs has significant implications for the insect’s evolutionary history. Early ants produced colonies with a winged queen and a wingless worker caste, and subsequently, in several independent lineages, the worker caste became further differentiated into subcastes. In Pheidole, a hyperdiverse genus that includes more than 1,000 species of ants, the worker caste is divided into minor workers and soldiers. Soldiers have disproportionately big heads which they use for defense and seed processing, while minor workers make up 90 to 95 percent of the colony and do tasks like brood rearing and foraging.

Wheeler’s studies from the 1980s demonstrated that Pheidole ants can regulate the proportion of minor workers and soldiers in the colony. One way this regulation works is through an inhibitory pheromone—a cuticular hydrocarbon—made by the soldiers that suppresses soldier development in larvae if soldiers exceed approximately five percent of the colony’s population.

To investigate the relationship between the wing discs and the soldier subcaste, Abouheif and his team knocked down a gene called vestigial, which causes the wing disc cells to die very early in development. Knocking down vestigial in soldier-destined larvae reduced the head size and body size of the animals, producing minor workers, while knocking down vestigial in minor workers produced no effect.

The team then raised soldier-destined larvae with adult populations of 100 percent minor workers or with 100 percent soldiers. With 100 percent minor workers in the colony, the larvae produced soldiers. But with 100 percent soldiers, giving off the cuticular hydrocarbon pheromone, the larvae’s rudimentary wing disc size was significantly reduced and produced adult ants with smaller heads and bodies.

Of course, the fact that organs affect the development of other organs within the body isn’t new. Previous experiments in insects have demonstrated that developing wings compete for nutrients and growth factors, for instance. Damage to a developing wing can produce signals that delay development across the whole body, allowing the disc time to recover so that growth can proceed in a properly coordinated way.

What’s striking here, according to first author Rajendhran Rajakumar, is that instead of being in competition, the rudimentary wing actually promotes the growth of the head in the soldier subcaste. “It’s even more surprising based on the fact that this tissue doesn’t actually form in the adults. It’s rudimentary, so we really didn’t expect to find the kind of results we did when we perturbed it,” says Rajakumar, who worked in Abouheif’s lab at McGill and is now a researcher at Harvard.

The ants contrast with other social insects like bees and wasps, which have not evolved worker polymorphisms. Queen and worker bees all have wings, and “if you are making a wing that must fly, you can’t really play around with its growth very much, so the growth of those discs is very constrained,” explains Abouheif. That may explain why worker bees, unlike ants, never evolutionarily differentiated further into subcastes.

There aren’t many known examples of rudimentary or vestigial organs driving evolutionary innovation. Other than the wing discs, another intriguing possibility is the ovary of a larval honeybee worker, which is as large as that of a queen during early stages of development. It’s not clear why the larval honeybee workers need ovaries, since they will never reproduce, says Mary Jane West-Eberhard, an evolutionary developmental biologist at the Smithsonian Tropical Research Institute.

“So the question would be: is it important for normal worker development to have the ovary present in the early larva?” she asks. Only future investigation will tell.

“I hope this work will lead people to look at what they think of vestigial organs in a different way, and that they try to probe what those things are actually doing, instead of just dismissing them as just a leftover thing,” Abouheif says. “I think actually they’re playing much bigger roles than we had thought previously.”