The making of soldier ants

"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 Ehab Abouheif from McGill's Biology Department, the senior author on the paper.

Rajendhran Rajakumar the first author adds, "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. 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."

Now you see it, now you don't

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

Soldier ants keep the colony in balance

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 90-95 % to 5-10% soldiers. The McGill 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, 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.

A more important role for all rudimentary organs than previously suspected?

Based on his teams' 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?"