Leafcutter ants are known for collecting leaves within their colonies to cultivate a certain fungus, which the ants then eat. To ensure that only the desired fungus grows, the ants can also produce antimicrobial compounds that prevent contamination. In their quest to identify such compounds, a research team instead stumbles on a different kind of discovery—one about how ants communicate with each other. Findings published in Scientific Reports from the University of São Paulo in Brazil reveal that trail pheromone compounds released by leafcutters come from their own microbiome.
"This system is susceptible to infections. The symbiotic bacteria produce compounds that can kill the parasitic fungi without damaging the food source. We set out to identify these compounds," explained study senior author Mônica Tallarico Pupo.
The team isolated and cultured bacteria found on the surface and inside the bodies of the ants. One particular species, Serratia marcescens, emitted a strong aroma that closely resembled the actual smell of the ants. In their curiosity to learn the origin of the scent, the team stumbled on aromatic pyrazine compounds. They accomplished this by absorbing aromatic compounds from the culture plates and analyzing them by gas chromatography-mass spectrometry (GC-MS).
"We decided to investigate the volatile compounds produced by this bacterium and discovered the pyrazines, among which there was a molecule not described in the scientific literature," said Pupo. "We found both pyrazines and bacteria in the ants' poison glands. We don't know for sure if their synthesis is shared: maybe the microorganism produces the aromatic compounds and the ants store them in their glands.”
As it turns out, pyrazines are heterocyclic aromatic compounds that have been shown to act as pheromones for ant communication, such as in defining trails and setting off alarms. In their paper, the team notes this discovery as the first report of a leaf-cutter ant-associated bacterium capable of producing trail pheromone pyrazines. To determine whether or not this type of phenomena is observed in other ants is a future direction for the team.
Image: An Atta sexdens rubropilosa leafcutter worker ant carries its precious leaf. Image courtesy of Eduardo Afonso da Silva Junior.