Transgenic ants shed light on insects’ sense of smell

Researchers have made transgenic ants whose antennae glow green under a microscope, revealing how the insects’ brains process alarming smells.

The findings identify three unique brain regions that respond to alarm signals. In these areas, called glomeruli, the ants’ nerve endings intersect. The work was posted on the bioRxiv preprint server on 29 December 2022¹ and has not yet been peer reviewed.

“Ants are like little walking chemical factories,” says study co-author Daniel Kronauer, a biologist at the Rockefeller University in New York City. Previous research has focused on identifying the chemicals that ants release or analysing the insects’ behavioural responses to these odours, but “how ants can actually smell the pheromones is really only now becoming a little bit clearer”, says Kronauer.

“This is the first time that, in a social insect, a particular glomerulus has been associated very strongly with a particular behaviour,” he adds.

Smelly signals

Ants are social animals that communicate with each other by releasing scented chemicals called pheromones. The clonal raider ants (Ooceraea biroi) that the researchers studied are blind. “They basically live in a world of smells,” says Kronauer. “So the vast amount of their social behaviour is regulated by these chemical compounds.”

When an ant perceives danger, it releases alarm pheromones from a gland in its head to warn its nestmates. Other ants respond to this signal by picking up their larvae and evacuating the nest. “Instead of having dedicated brain areas for face recognition or language processing, ants have a massively expanded olfactory system,” says Kronauer.

The researchers created transgenic clonal raider ants by injecting the insects’ eggs with a vector carrying a gene for a green fluorescent protein combined with one that expresses a molecule that indicates calcium activity in the brain.

The process was helped by the animals’ unique biology, Kronauer says. “They’re asexual, they don’t have queens, so we can basically clonally propagate any transgenic insertion from any individual.”

The team exposed 13 transgenic ants to 4 alarm pheromones while imaging the animals’ brains to examine how they process danger signals. The scientists were surprised to find that the four pheromones activated fewer than six glomeruli in the whole brain. “It’s very different from what we thought we would see going in, because of all these previous studies that have suggested that these responses are very broad,” says Kronauer.

“We were expecting that maybe 20% of the glomeruli would respond, which would have been 100 glomeruli,” says study co-author Taylor Hart, a biologist at Rockefeller. But “it turned out that actually most of the time there’s three”.

The authors identified a specific glomerulus — which they named ‘panic glomerulus, broad’ or PG_b — that strongly responded to three of the pheromones, which all provoked escape behaviour. They also identified two neighbouring glomeruli: one that responds to the fourth pheromone, which caused the ants to leave the nest without their larvae, and another that feeds signals from the three alarm-provoking pheromones to the PG_b.

Transgenic tools

Scientists have been making transgenic fruit flies (Drosophila melanogaster) since the 1980s and transgenic mosquitoes of various species since the 1990s. But genetic modification of insects with complex social systems, such as ants, only began in the past decade. The authors say that the work will improve in understanding of ants’ sophisticated biology.

These findings help to “understand how olfaction works at a molecular level, how it’s processed, and how the individual responds and how this leads to a society response in their natural world”, says Juergen Liebig, a biologist at Arizona State University in Tempe. “One can now use these transgenic tools and make transgenic ants to study the function of specific genes,” he says, “and how this potentially affects the organization of ant society.”

Silke Sachse, a neurobiologist at the Max Planck Institute for Chemical Ecology in Jena, Germany, says that future studies could use similar transgenic tools in other ants to identify the olfactory receptors that respond to alarm pheromones. “It would be interesting to figure out whether these receptors are conserved between different species,” she says.