Research hints at how fungus farming ants keep their gardens healthy

Newswise — The key to maintaining a productive garden lies in weeding diligently and frequently. Surprisingly, some species of ants have also embraced gardening as an integral part of their lives, honing their skills over an impressive span of 50 million years. These remarkable creatures engage in the essential task of weeding their subterranean fungus gardens, yet the mechanism by which they discern between beneficial and harmful fungi has long perplexed scientists. However, a group of interdisciplinary researchers has recently published an enlightening study in the Proceedings of the National Academy of Sciences (PNAS) on June 15, unveiling the ants' ability to differentiate between the desirable and undesirable fungus.

While humans primarily rely on visual cues to identify weeds, ants face a unique challenge as they cultivate their fungus gardens underground, devoid of light. Consequently, they must possess alternative methods to sense unwanted garden intruders. Led by Dr. Jonathan Klassen from the University of Connecticut and Dr. Marcy Balunas from the University of Michigan, a team of researchers has made a significant discovery in this regard. They have determined that ants employ their olfactory abilities to detect diseased fungus by detecting specific chemical compounds known as peptaibols.

The team's research specifically centered around the ant species Trachymyrmex septentrionalis, which thrives in the pine barren ecosystem extending from Long Island to East Texas. These particular ants cultivate their fungus gardens beneath the ground and nourish them with fresh organic debris. The fungus serves as an essential component of the ant colony, functioning akin to an external digestive system. As the colony lays fresh food on top of the fungus, it grows in honeycomb-like structures, digesting the food and providing nutrition to the ants. Subsequently, the fungus secretes waste products, completing the symbiotic cycle.

One of the co-first authors of the study, Katie Kyle, a graduate student from the Klassen Lab, conducted experiments by deliberately introducing Trichoderma, a naturally occurring fungus known to cause disease in the ant gardens. The infected ant nests prompted a remarkable response from the ants—they intensified their efforts to eliminate the infection from their habitats, resulting in an increased production of waste materials. This observation indicated that the ants recognized the presence of the disease-causing fungus and responded by actively combating its spread within their fungus gardens.

During the winter season, when the ants were in a dormant state, the team conducted an analysis of the fungal communities within various ant nests gathered from diverse locations. Interestingly, Trichoderma was discovered in all of the examined nests.

Dr. Sara Puckett, a recent graduate from the Balunas' UConn lab and co-first author of the study, prepared Trichoderma extracts containing the organic compounds found in the fungus. The purpose was to investigate whether the weeding response was initiated by one or more specific compounds within these extracts or merely by the presence of the pathogen's cells.

“We were curious to see if the ants were weeding because of compounds produced by the infecting fungus,” Balunas said.

The team found the Trichoderma extract, when applied to the fungus garden, sent the ants into frenzied weeding activity just as actual Trichoderma infections had.

n collaboration with scientists from the University of California, San Diego, and the University of North Carolina, Greensboro, the researchers made an intriguing discovery. They found that the ant nests contained peptaibols, which are a family of compounds known to be produced by Trichoderma. However, pinpointing the exact peptaibols responsible for triggering ant weeding proved to be a more formidable task, as the extracts contained numerous compounds.

The researchers tested pure peptaibols, including two new compounds called trichokindins VIII and IX.

Surprisingly, the researchers found that all the tested peptaibols exhibited some degree of ant weeding activity. This significant finding suggests that it may not be attributed to a specific peptaibol alone but rather to the collective presence of the entire suite of peptaibols, which can induce the ants to engage in weeding behavior within their garden.

Dr. Balunas highlights that this suite of Trichoderma compounds triggering ant behavior stands in contrast to numerous other natural products, where the observed activity can typically be attributed to a single compound. The unique characteristic of the Trichoderma compounds lies in their collective effect, emphasizing the complexity and multifaceted nature of the interaction between these compounds and the ants.

While the data gathered by the researchers strongly supports peptaibols as a signal for ant weeding, the exact perception mechanism employed by the ants remains unclear. The researchers propose two possibilities. One hypothesis suggests that the invading Trichoderma fungus itself produces the peptaibols, which the ants detect, leading to their weeding behavior. Alternatively, it is also plausible that the ants are sensing a secondary response triggered by the fungus garden itself. Further investigation is required to discern the precise nature of the ant's perception and response to the peptaibols.

The next step is to figure out those details of ant-fungus communication, Klassen says.

Klassen suggests that the fungus might be conveying the message "I'm sick" and possibly detecting the presence of peptaibols. It is crucial for us to further elaborate on the sequence of signaling mechanisms.

The discoveries emphasize a rare instance in which an animal reacts to an ailment affecting its mutually beneficial symbiotic partner, rather than its own body. This intriguing occurrence, termed an "extended defense response" by Balunas and Klassen, underscores the need for further investigation as they eagerly anticipate unraveling its complexities.