Back to the future: Supersoldier ants illuminate evolution

Dormant genetic potential facilitates evolutionary change

They look like characters that belong in the Marvel Comic The Hulk, whose body reacts to stress by expanding in size. With huge oblong heads and giant, vicious mandibles, these are supersoldiers of the hyperdiverse ant genus Pheidole. Normally, these supersoldiers occur naturally only in limited geographical regions. But now researchers, led by McGill biology professor Ehab Abouheif, have found ants that are biological anomalies with supersoldier-like characteristics in unexpected regions. And, more importantly, researchers have discovered they can induce supersoldiers in Pheidole ant species that never had them before.

These supersoldier anomalies represent dormant ancestral potential that can be invoked by changes in the environment. They represent, in effect, a source of raw material for natural selection to act upon. The finding, published in the most recent edition of Science, is a significant advance in our understanding of evolutionary processes.

“Birds with teeth, snakes with fingers, and humans with ape-like hair – these are ancestral traits that pop up regularly in nature,” Abouheif explains. “But for the longest time in evolutionary theory, these ancestral traits were thought to go nowhere – slips in the developmental system that reveal things from the past, the Barnum and Bailey of evolution. So they’ve been an unappreciated source of evolutionary variation.”

In Pheidole (big-headed) ant colonies, there are millions of individual workers, including minor workers and soldiers. Typically, depending on the food the ants are fed, certain hormones are triggered in the ant larvae and they either develop into soldiers or minor workers. After unexpectedly finding supersoldier-like anomalies in Pheidole species in Long Island, where they aren’t normally seen, Abouheif and his team knew something unusual was going on. “I’ve been collecting samples there for almost 15 years,” said Abouheif. “But when I saw them, I thought, ‘Holy cow! Those are monstrous looking soldiers!’ They look like the ones that are naturally produced in the American southwest.”

So researchers in Abouheif's lab led by PhD student Rajee Rajakumar and collaborators at the University of Arizona then started trying to artificially induce the production of these supersoldiers. They did so by applying juvenile hormone to the ant larvae at critical stages in their development. And met with immediate success. They were able to produce supersoldier subcastes in at least three species in the genus where they have never been seen before – species that are widely separated in the evolutionary tree of Pheidole.

These findings are groundbreaking for evolutionary theory, according to Abouheif, because they show there is dormant genetic potential that can be locked in place for a very long time. “The kind of environmental stressors that evoke this dormant potential are there all the time – so when the need arises natural selection can take hold of the potential and actualize it,” Abouheif explained. “So what we’re showing is that environmental stress is important for evolution because it can facilitate the development of novel phenotypes. Anytime you have a mismatch between the normal environment of the organism and its genetic potential you can release them – and these things can be locked in place for 30-65 million years.”

The research was funded by Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair in Evolutionary Development Biology, the National Sciences Foundation, Konrad Lorenz Institute Fellowship.

To read an abstract of the paper: http://www.sciencemag.org/

(Photo credit: Alex Wild)