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Overlooked organ turns some ants into giant soldiers

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A seemingly unimportant rudimentary organ that only appears briefly during the final stages of larval development may explain why some ants are tiny workers and others are huge soldiers, a new study finds.

The new research answers a question that perplexed Charles Darwin. So much so, that it actually led him to doubt his own theory of evolution.

Darwin wondered, if natural selection works at the level of the individual, fighting for survival and reproduction, how can a single colony produce worker ants that are so dramatically different in size—from the minor workers with their small heads and bodies, to the large-headed soldiers with their huge mandibles—especially if, as in the genus Pheidole, they are sterile?

The answer, according to a paper in Nature, is that the colony itself generates soldiers and regulates the balance between soldiers and minor workers thanks to the seemingly useless organ. This organ appears only briefly during the final stages of larval development and only in some of the ants—the ones that will become soldiers.

“These rudimentary ‘organs’ are not a secondary effect of hormones and nutrition, but are instead responsible for generating the soldiers.”

“It was a completely unexpected finding. People had noticed that during the development of soldiers that a seemingly useless rudimentary ‘organ’ would pop up and then disappear. But they assumed that it was just a secondary effect of the hormones and nutrition that were responsible for turning the larvae into soldiers,” says senior author Ehab Abouheif from McGill University’s biology department.

“What we discovered was that these rudimentary ‘organs’ are not a secondary effect of hormones and nutrition, but are instead responsible for generating the soldiers,” adds first author Rajendhran Rajakumar. “It is their passing presence that regulates the head and body of soldiers to grow at rapid rates, until you get these big-headed soldiers with huge mandibles and big bodies.”

Waiting in the wings

Abouheif has been studying wings in ants for the past twenty-three years. He was curious about the function of the wing imaginal discs which appear, transiently, in the final stages of larval development among the soldier ants—even though the soldier ants never actually develop wings.

He and his team spent nine years in the lab using various techniques (surgical and molecular) to cut away portions of the rudimentary wing discs from the larvae of soldier ants in the widespread and very diverse Pheidole genus. They discovered that by doing so, they affected the growth of the head and the body. Indeed, they found that they were able to scale the size of soldier ants by cutting away differing degrees of the imaginal wing discs, with a corresponding decrease in the size of the heads and bodies of the soldier ants.

It was clear confirmation that the rudimentary wing discs play a crucial role in the development of soldier ants.

Pheromones keep the rank and file

The researchers also discovered that the colony as a whole maintains the balance between soldiers and minor workers by regulating the growth of the rudimentary wing discs in larvae.

Earlier research had shown that the ratio of minor workers to soldiers remains constant in all colonies of the Pheidole genus, with a proportion of minor workers at 90 to 95 percent and soldier ants at 5 to 10 percent. The team has found that the soldier ants maintain this ratio by halting the growth of the rudimentary wing disc with an inhibitory pheromone when there are too many soldiers.

However, the colony is able to ramp up the number of soldier ants very quickly if it is under threat or the numbers of soldiers have dropped for some reason. This is because the rudimentary wing discs that play such a crucial role in regulating the number of soldier ants appear only in the final stages of larval development.

Other overlooked organs?

Based on his team’s discovery in ants, Abouheif proposes that rudimentary organs may play a much larger role in an organism’s development than had previously been imagined.

“Until now, people have assumed that these organs simply offer evidence of evolution and common descent, overlooking any current functions for them. Now that we know the crucial role played in Pheidole ant colonies by the rudimentary wing disc, it means that we will have to go back and look at other rudimentary organs in the same light. Who knows what scientists will discover?”

Konrad Lorenz Institute (KLI) fellowships, the Natural Sciences and Research Council of Canada (NSERC), and a Guggenheim Fellowship funded the research.

Source: McGill University