Research hints at how fungivorous ants keep their gardens healthy

„Weed early and often” is the key to a productive garden. Interestingly, some species of ants are also avid gardeners, a practice that has been refined for over 50 million years. They also pick weeds in their underground fungal gardens, but how they know what weeds to pick remains a mystery. Now, a multidisciplinary team of scientists is reporting PNAS June 15 How do ants distinguish good fungus from bad?

People rely on sight to identify weeds, but ants grow underground fungi in the dark, and there must be other ways to sense unwelcome gardeners. A team led by Jonathan Klassen, Ph.D., of the University of Connecticut, and Marcy Balunas, Ph.D., of the University of Michigan, discovered that ants excrete the diseased fungus by detecting chemicals called peptibols.

The team focused on the ant species Trachymyrmex septendraionalis, which adapts to pine barren ecosystems from Long Island south to East Texas. Trachymyrmex ants grow their fungus below ground and provide it with fresh organic toxins. The fungus acts almost like an external gut for the ant colony; The fungus grows around the fresh food laid on top of it in a honeycomb pattern, producing digestible food for the ants as it grows and then excreting waste.

Classen lab graduate student Katie Kyle, co-first author on the paper, infected ant nests with trichoderma, a naturally occurring, disease-causing fungus that infects ant colonies and found that the ants worked longer hours. Nests increase their waste production.

During the winter, when the ants were dormant, the team analyzed the fungal biota of different ant nests collected from different locations and found Trichoderma in all of them.

Co-first author Sarah Puckett, Ph.D., a recent graduate from Balunas’s UConn lab, prepared extracts of Trichoderma containing organic compounds from the fungus to determine whether weeding was triggered by one or more of these compounds. Presence of pathogen cells.

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

When Trichoderma extract was applied to the fungus garden, the team found that it sent the ants into frenzied weeding activity just as they had real Trichoderma infections.

Working with scientists from the University of California, San Diego and the University of North Carolina, Greensboro, they discovered that it contains peptibols, a family of compounds produced by Trichoderma. However, identifying which specific peptibols cause ant weeding is more challenging because these extracts contain many compounds.

The researchers tested pure peptibols, including two new compounds called trichoquintins VIII and IX.

All tested peptipols caused some level of ant weeding, indicating that it is not a specific peptipol, but rather the entire set of peptipols that induces ants to weed their garden.

„This collection of Trichoderma compounds that stimulate ant behavior is in contrast to many other natural products whose activity can be attributed mostly to a single compound,” Balunas says.

Although their data support peptipoles as a signal for weed removal, it is unclear what exactly the ants sense. The invading Trichoderma fungus produces peptipoles and ants can detect and then eradicate them, the researchers note. Or the ants may have detected a secondary response from the fungus garden itself.

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

„Maybe the fungus is signaling 'I’m sick.’ Maybe the fungus is detecting peptibols. We need to unravel the signaling chain,” Klassen says.

The findings highlight one of the few known systems in which an animal responds to the illness of its beneficial symbiotic partner instead of its own, a phenomenon Balunas and Klassen call the extended immune response, and they anticipate. Constant teasing.