Unveiling Ant Colonies’ Response to Alarms: First Transgenic Ants Illuminate Olfactory System

Caption: Two clonal raider ant pupa, one transgenic, displaying green fluorescence in their olfactory sensory neurons. (Credit: Laboratory of Social Evolution and Behavior at The Rockefeller University)

Researchers have successfully engineered transgenic ants, shedding light on the olfactory system of these insects. The study challenges previous understanding of how ants process scent data by revealing that alarm pheromones activate specific areas within their olfactory system. This groundbreaking achievement paves the way for further investigations into ant social behavior.

Ants navigate their fragrant environment through a combination of odor receptors and pheromones, chemical signals used for various activities such as foraging, defense, mating, and caring for their offspring. The significance of this system is evident in the ant brain’s impressive ability to process an abundance of scents. In fact, the olfactory processing center in an ant’s brain contains ten times more subdivisions than that of a fruit fly, despite having similar brain sizes.

However, the encoding of scent data in the ant olfactory system has largely remained a mystery. To unravel this enigma, scientists at Rockefeller University developed the world’s first transgenic ants. These ants were bred with olfactory sensory neurons that emit a green fluorescence in response to odorants. The results of this study were published on June 14 in the journal Cell.

Contrary to previous findings, the study demonstrated that only specific regions of the olfactory system were activated by alarm pheromones, which induce panic and nest evacuation. These findings raise questions about the processing of sensory information in the ant brain and offer intriguing possibilities for understanding the function of hundreds of other odorant receptors.

“Neurogenetic tools have transformed the field of fruit fly neuroscience in recent decades, while research on social insect neuroscience has been somewhat stagnant,” explains Daniel Kronauer, head of the Laboratory of Social Evolution and Behavior at Rockefeller. “With our technical breakthroughs, we can finally utilize these powerful tools to study the social behavior of ants.”

Aromatic Realm

In 1958, E. O. Wilson observed that a secretion from the mandibular gland of harvester ants triggered nestmates to increase their pace and engage in colony defense behaviors, which he referred to as “alarm behavior.” Since then, scientists have discovered that alarm behavior and other complex social activities in ant colonies are regulated by a wide range of pheromones.

Ants possess olfactory receptors located on neurons in their antennae. These receptors transmit signals to the antennal lobes, specialized brain centers responsible for scent processing. Some ants have over 500 glomeruli in their antennal lobes, enabling them to perceive and distinguish between pheromones more effectively. Previous research from Kronauer’s lab demonstrated that ants lacking functional odorant receptors are unable to respond to pheromone signals.

For this study, the researchers genetically modified clonal raider ants—a queenless species composed entirely of blind female workers—by injecting them with genetic material encoding a synthetic protein called GCaMP. This protein emits a green fluorescence when there are changes in calcium levels during cellular activity.

Lead author Taylor Hart, a researcher in Daniel Kronauer’s lab, explains the significance of targeting olfactory sensory neurons with GCaMP: “Our objective was to express GCaMP solely in the olfactory sensory neurons, a specific cell type.” This approach was crucial because the antennal lobe consists of multiple cell types, including sensory neurons, projection neurons that transmit sensory data to other parts of the brain, and lateral interneurons that establish connections between these components. “The involvement of other cell types can lead to poor signal-to-noise ratio due to additional activities, such as computations, information processing, and signal modulation,” says Hart. These factors can obscure the true behavior of olfactory neurons.

Locating the Panic Button

While successfully breeding a small group of ants with GCaMP expression in their olfactory sensory neurons, the team also developed an advanced two-photon calcium imaging technique. This technique allowed them to record neural activity across the entire antennal lobes of live ants for the first time.

The researchers chose to focus on alarm pheromones due to their volatile nature and their ability to elicit strong and consistent behavioral responses. When adult ants detected these scents, they immediately gathered as many eggs as possible in their mandibles and swiftly fled to an adjacent area within the test chamber.

Using their newly developed techniques, Hart and her team monitored the fluorescence levels of GCaMP in the antennal lobes of 22 transgenic ants while exposing them to various odors, including alarm pheromones (which have a fruity scent to humans). The fluorescence clustered in six specific glomeruli within one region, suggesting that this area serves as the “panic button” in the ant brain.

“We expected a significant portion of the antennal lobe to exhibit some response to these alarm pheromones,” says Hart. “Instead, we observed extremely localized responses, with the majority of the antennal lobe showing no response at all.”

These findings shed light on how the ant brain processes sensory input. Researchers have debated whether the activity is privatized, with each glomerulus responding to only one or a few specific stimuli, or distributed, with unique combinations of glomeruli activated by a stimulus. If a brain with over 500 glomeruli were to operate in a distributed manner, with hundreds of sensors firing simultaneously, it would require exceptional computational power for sensory processing, notes Hart.

“Most of the odors we tested activated only a small fraction of the total glomeruli,” she explains. “It appears that privatization is the preferred mode in the ant antennal lobe.”

Tools for Future Explorations

Considering that only six glomeruli out of 500 responded, Hart raises the question: “Why do ants need such a variety of glomeruli when fruit flies manage with just 50?”

This discovery will enable researchers to investigate why ants possess a greater need to differentiate odor stimuli compared to other insects, says Kronauer. Furthermore, Hart has already bred hundreds of transgenic ants that are identical to their wild counterparts, except for their fluorescence signaling ability. This provides a robust resource for future studies.

“The tools developed by Taylor have opened up a wide range of previously inaccessible questions,” adds Kronauer. These questions include identifying specific glomeruli associated with various pheromones used by ants for activities like raiding, recruitment, and distinguishing between nestmates and intruders. Additionally, researchers can explore the developmental aspects of how the ant olfactory system assembles, given its complexity. Since even ant larvae possess olfactory sensory neurons, their sensory capabilities can now be examined.

Reference: “Sparse and stereotyped encoding implicates a core glomerulus for ant alarm behavior” by Taylor Hart, Dominic D. Frank, Lindsey E. Lopes, Leonora Olivos-Cisneros, Kip D. Lacy, Waring Trible, Amelia Ritger, Stephany Valdés-Rodríguez, and Daniel J.C. Kronauer, 14 June 2023, Cell.
DOI: 10.1016/j.cell.2023.05.025

Frequently Asked Questions (FAQs) about transgenic ants

More about transgenic ants

• Research article: “Sparse and stereotyped encoding implicates a core glomerulus for ant alarm behavior” (DOI: 10.1016/j.cell.2023.05.025)
• Rockefeller University: Laboratory of Social Evolution and Behavior
• E. O. Wilson’s study on alarm behavior: Link to the study (Note: Requires access to JSTOR)