After creating mutant Indian jumping ants with no sense of smell, HHMI Investigator Danny Reinberg and colleagues saw profound abnormalities in the ants’ behavior and brains. The results show that the sense of smell is fundamental to maintaining colony harmony.
Without a sense of smell, female ants wander off on their own, mooch crickets caught by others, and refuse to mate. These mutant insects are adrift in society, living lives seemingly separate from their social sisters, Howard Hughes Medical Institute (HHMI) Investigator Danny Reinberg and colleagues report August 10 in the journal Cell.
Their work, one of the first studies of genetically engineered ants, highlights smell’s role in keeping social insect societies running smoothly. Disabling the sense of smell makes ants “space cadets,” says Reinberg, a biochemist at New York University School of Medicine. “They live in Wonderland. They do their own thing.”
Researchers have long known the importance of smell to social insect societies, but until now, no one could do the kind of genetic studies to confirm it. By showing how genes can be permanently tweaked in an unusual species of ant, the research paves the way for ants to ultimately join the ranks of lab mice and fruit flies: extensively-studied organisms that scientists can use to probe everything from the neural basis of behavior to aging.
Reinberg and colleagues’ work is part of a sweeping collection of new papers that reveal the details of how ants’ social lives are governed by chemical signals such as odors.
Making a mutant
As part of a long-running collaboration, Reinberg, Claude Desplan, a developmental neurobiologist at New York University, and colleagues studied Indian jumping ants, Harpegnathos saltator. These stinging ants have lightning-quick mandibles and one particularly unusual trait: any female can become fertile, given the right cues. This is unlike many other ant species, in which the queen is the only female that can lay eggs. But in an underground H. saltator nest, female workers can morph into “pseudoqueens” if the primary queen dies or is lost.
“The species is very special because we can transform any worker into a pseudoqueen,” says study coauthor Desplan. “It’s quite fascinating biology.”
These fertility capabilities are what led Reinberg, Desplan and colleagues to the idea that they could do genetic studies on the species. If a lone queen was the only female that could reproduce, then scientists would need to catch lots of queens for their experiments — a time-consuming and difficult proposition. But with the ability to create many egg-layers, the scientists suspected that they could easily raise enough ants for their research. “We needed a twist,” says Reinberg. “We needed an ant species that we could propagate in the lab.”
The researchers then tweaked the lab-grown ants’ DNA. After years of careful study, the team figured out a technique to produce ants that all carried the same genetic mutations. Their work relied on a gene-editing tool called CRISPR-Cas9 to snip out part of an essential olfactory gene called orco. Because this genetic edit takes place in DNA, it gets passed from generation to generation, creating many orco mutants.
These mutant ants no longer have the genetic instructions for making the Orco protein, which is critical for the sense of smell. Typically, Orco partners up with odor-sensing proteins called olfactory receptors. “The beauty of the system is that every olfactory receptor relies on Orco,” Desplan says. Without Orco, ants lose their ability to smell, a loss that profoundly affects their behavior.
Observations of ants lacking Orco showed that some young ants spent half of their time wandering outside the nest — a very unusual behavior. The mutant ants exhibited other bizarre behaviors, too; for example, they didn’t forage for food, nor did they duel with other females to become pseudoqueens.
Along with these abnormal behaviors came an abnormal brain. The odor-related parts of orco mutants’ brains were small and disorganized. Normal ants have about 275 distinct glomeruli, rounded structures in the antennal lobes of ants’ brains that receive odor signals. Orco mutants, the researchers found, had only 62 glomeruli, and they weren’t cleanly separated. A sense of smell is not only required for ant behavior, says Desplan, it’s also necessary for a normal brain.
Similar results on orco mutants in a different species of ant, Ooceraea biroi, are described in the same issue of Cell by Daniel Kronauer, an HHMI Faculty Scholar at Rockefeller University.
Three female ants that carry no working copies of the orco gene (homozygous) wander outside of the colony — unusual behavior that resulted from their inability to smell. The ants are each painted with an identifying code of three colors. The three orco mutants are painted orange, red, red (ORR), orange, blue, red (OBR) and orange, white, orange (OWO). The movie plays at 4 times the normal speed. Credit: Yan et al./ Cell 2017
The inner lives of ants
Reinberg and colleagues’ mutant ant work was catalyzed by HHMI’s Collaborative Innovation Award (HCIA), a program begun in 2008 aimed at supporting teams of diverse scientists pursuing large and potentially groundbreaking projects. Reinberg and collaborators received the first award in 2008, and a second one in 2012, to untangle the biochemistry that allows ants with the exact same genome to switch jobs, transforming a worker into a queen, for instance. Differences in how genes are used — called epigenetic changes — are probably driving the differences, the team suspects.
Reinberg’s collaborators on the 2012 grant include Desplan, Shelly Berger of the University of Pennsylvania, Jürgen Liebig of Arizona State University, Anandasankar Ray of the University of California, Riverside, and Laurence Zwiebel of Vanderbilt University.
Now, in a suite of papers published this summer, Reinberg and others are unraveling the pivotal roles chemical signals play in ants’ social lives. In a July report in the Proceedings of the National Academy of Sciences, the researchers show that H. saltator odorant receptors detect a wide range of smell molecules. In another study, they pinpoint a signaling molecule that, when injected, can stop worker ants from turning into pseudoqueens. That study began in the Reinberg laboratory with an initial discovery by former postdoctoral fellow Roberto Bonasio, who then developed it independently in his own laboratory at the University of Pennsylvania. It appears in the same issue of Cell as the orco mutant project.
Other soon-to-be-published research describes the molecules detected by specific olfactory receptors, including a pheromone produced by the queen. And it turns out that the queen may not be able to sense her own pheromones. The latter study, published June 16 in Scientific Reports, shows that pseudoqueens lose the ability to sense their own pheromones that prevent others from laying eggs.
With this vast array of results, Reinberg and colleagues have helped illuminate some of the basic biology that helps ants’ highly social colonies hum along. Yet despite the recent flurry of papers, the story is just beginning, Reinberg says.
One particularly intriguing aspect of ant biology is how the insects age, he says. Queens can live up to 10 times as long as workers, even though the ants possess the exact same genome. And this trait can be shifted. When a worker transitions to a pseudoqueen, her lifespan lengthens; when a pseudoqueen transitions back into a worker, her lifespan shortens again.
“We have, right in front of us, a system in which we can study aging,” Reinberg says. These dramatic lifespan fluctuations may reveal insights about the biochemistry of aging in ants, and perhaps even other organisms.
Hua Yan et al. “An Engineered orco Mutation Produces Aberrant Social Behavior and Defective Neural Development in Ants,” Cell. Published online August 10, 2017. doi: 10.1016/j.cell.2017.06.051