Without pheromone cues, ants become ‘space cadets’

(Credit: Eric Bonkhz/Flickr)

When ants were genetically engineered to lack a “sense of smell,” their antennae and brain circuits failed to fully develop and they couldn’t communicate, forage, or compete to be a queen.

“We found that a species of ant may be the first model to enable in-depth functional analysis of genes that regulate social interaction in a complex society,” says corresponding study author Danny Reinberg, professor of biochemistry and molecular pharmacology at the New York University School of Medicine.

“While ant behavior does not directly extend to humans, we believe that this work promises to advance our understanding of social communication, with the potential to shape the design of future research into disorders like schizophrenia, depression, or autism that interfere with it,” says corresponding author Claude Desplan, professor of biology.

The results are based on the fact that ants communicate through pheromones, secreted chemicals that trigger responses. Such odors are used to spread alarm as a predator approaches, leave a trail to food, indicate social (caste) status, and signal readiness to mate, all within cooperative societies that achieve complex tasks.

Mostly wandering

Ants can receive such signals because they have proteins called odorant receptors on their antennae, with each protein the right shape to bind to a specific odorant chemical.

For any odor or pheromone to be processed in an ant’s brain, however, past studies had shown that both the right odorant receptor protein and a shared, common partner protein called Orco must be present.

As reported in Cell, researchers successfully engineered the genetic loss of the Orco protein, which resulted in ants that could no longer perform some, if not all, pheromone-based social interactions.

Specifically, the altered young ants, unlike their unchanged nestmates, spent much of their time wandering out of the nest. They failed to interact with other members of the colony (a behavior that Reinberg calls “space cadet”), and were unable to forage and bring food back to the nest. Furthermore, mutant females no longer groomed males, a pre-mating behavior.

The study focused on the Indian jumping ant, Harpegnathos saltator, which is unlike many ant species in that only the queen can mate and pass on genes to the next generation. Any Harpegnathos female, adult worker can be converted into a “pseudo-queen” in the absence of the queen.

Ants wear smells like a uniform

That’s because the queen secretes a pheromone that suppresses the ability of workers to mate and lay eggs. If the queen is removed, the most aggressive females, after winning a series of antenna duels, undergo this transition, and can go on to produce progeny, which is essential for colony survival.

Without Orco, females can’t process pheromones, which makes them much less likely to engage in dueling.

Brain ‘blob’

Another study finding came from the fact that each neuronal cell (odorant receptor neuron) capable of processing the presence of a given pheromone on the surface of an ant’s antennae sends out extensions that converge in a specific blob-like brain structure called a glomerulusm where information about the odor is processed.

Past studies have suggested that, in solitary insects like mosquitoes, fruit flies, and moths, the connections between odorant receptors and glomeruli are “hard-wired”—their neural development is independent of receptor activity.

To the contrary, mammals appear to have odorant receptor cells with extensions capable of homing in on the correct glomeruli based on which odorant receptors they express. This makes the function activity-dependent in mice (and humans), in contrast to the hard-wired context of flies.

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The new findings suggest that Harpegnathos ants may also have evolved to have flexible, activity-based patterning of nerve connections, which might have allowed their expanded repertoire of olfactory receptors for detecting pheromones.

This flexibility is required for communication based on the pheromone sensitivity and resultant activity of their olfactory neurons, say the authors. Accordingly, the loss of the Orco protein left female ants, on average, with just 62 of the 275 glomeruli that they would normally develop to process pheromone sensing.

Coauthors are from the University of Pennsylvania, Vanderbilt University, and Arizona State University. The Howard Hughes Medical Institute and the National Institutes of Health supported the work.

Source: NYU