Study reveals evolutionary mechanism that could lead to control of leafcutter ants

Researchers investigated the ants' symbiotic and parasitic relationships with two genera of fungi.

By José Tadeu Arantes  |  Agência FAPESP – Biological control of leafcutting ants is the focus of a scientific paper recently published in the journal Royal Society Open Science (RSOS).

The study reported in the paper is directly linked to two projects supported by FAPESP: “Phylogeny and systematics of the parasite Escovopsis associated with attine ants” and “DNA barcoding and biotechnological potential of microfungi associated with leafcutting ants”.

“The study opens up a great many possibilities both for deepening our knowledge of Escovopsis fungi, which are strong candidates for biological control of ants, and for implementing this control, properly speaking,” said biologist André Rodrigues, a professor in the Department of Biochemistry & Microbiology of São Paulo State University’s Bioscience Institute in Rio Claro (IBRC-UNESP) and head of the same university’s Fungal Ecology & Systematics Laboratory.

Rodrigues is supervising the PhD research of Lucas Andrade Meirelles, first author of the paper published in RSOS and principal investigator for the fungal study. Researchers from the University of Texas at Austin, USA, where Meirelles did part of his master’s research with support from a scholarship awarded by FAPESP under its Research Internship Abroad Program (BEPE), also took part in the project.

The possibility of biological control is based on intricate symbiotic and parasitic relationships established between 65 million and 50 million years ago. Fungi of the Escovopsis genus are parasites of – and ultimately may destroy – fungi of the species Leucoagaricus gongylophorus, which live in symbiosis with ants of the tribe Attini, such as Acromyrmex spp. and Atta spp.

“These ants cultivate L. gongylophorus inside their colonies,” Rodrigues told Agência FAPESP. “The fungus grows in chambers inside the colony in the form of mycelium, a filamentous structure that is reminiscent of cotton. The ants feed their fungal partner with the plants they cut and appropriate small vesicles full of nutrients formed by the fungus in its filaments. The worker ants extract these vesicles and use them to feed the larvae.”

Technically speaking, L. gongylophorus is a mushroom, but the ants try to inhibit the formation of mushrooms because this is how the fungus reproduces and creates genetic diversity. Thus, when L. gongylophorus begins to form mushrooms, the ants rapidly eliminate the variants to ensure that the fungus retains its mycelial form. “There’s an evolutionary hypothesis according to which the ants force the fungus to retain the same form in all colonies by discarding variants that might not be favorable to the associative process,” Rodrigues said.

Parasitism

It may be that the parasitism of Escovopsis fungi is as ancient as the “domestication” of L. gongylophorus by leafcutter ants, possibly as old as tens of millions of years. However, the relationship was only confirmed in the late 1990s.

“Escovopsis fungi parasitize L. gongylophorus, and laboratory research shows that ant colonies infected by Escovopsis eventually die,” Rodrigues said.

Possible human use of this parasitism for biological control of leafcutter ants is favored by the fact that although Escovopsis has been sought in many places, it has never been found in any environment other than the colonies of these ants.

“The fungi appear not to affect any organisms apart from their targets, and this should make their use for biological control fairly safe,” Rodrigues said. “In addition, research has shown that this highly specific action is due to their joint evolution with ants and L. gongylophorus, an association that dates back tens of millions of years.”

Precisely because the association is so old, however, using Escovopsis for biological control would not be as straightforward as it appears at first sight. Over time, the ants have developed ways to rid themselves of these undesirable parasites. They push the fungi out of the nest by mechanical means, using their mandibles, or eliminate them chemically by secreting a fungicidal substance.

The fungicide is produced by bacteria carried by the ants in their cuticle, the external layer of their body. This brings into play a fourth organism, the bacterium Pseudonocardia, which produces the fungicide, making this intricate evolutionary jigsaw puzzle even more fascinating.

“They’re running an unending evolutionary race,” Rodrigues said. “The ants develop associations with bacteria to remove pathogens from their colonies, and the pathogens strive to overcome the ants’ defense mechanisms.”

A possible human stratagem to nudge this evolutionary race in our favor could take advantage of the fact that there are so many species of Escovopsis. “Previous studies have identified and described seven species. By collecting material from various parts of Brazil, we compiled the largest sample of this parasite available since records began, with some 20 species in addition to those already described,” Rodrigues said.

Fungus gardens

From a taxonomic standpoint, the descriptions of these newly discovered species offer a vast amount of material for scientists to process. In fact, Rodrigues is currently leading a project entitled “Phylogeny of fungal parasites in gardens of attine ants” to explore this possibility, also with support from FAPESP. From the standpoint of evolutionary theory, much can be learned from this intricate association between insects, fungi belonging to two different genera, and bacteria. “Our study opened up several windows for investigation,” Rodrigues said.

One of the most intriguing aspects of the field work done by scholars like Rodrigues in exploring the nests of leafcutters is finding what they call “fungus gardens,” sponge-like structures consisting of L. gongylophorus mycelium and a substrate of leaves and flowers cut up and carried into the nest by the ants.

“Leafcutter colonies house millions of ants, with labyrinthine complexes of tunnels and chambers often as deep as ten meters underground. When we’re exploring a colony, we dig a trench two to three meters deep and then excavate the side wall of the trench next to the nest in order to reach the fungal gardens nearest the surface. Here, we find both L. gongylophorus and the Escovopsis parasites,” Rodrigues said.

Leafcutter ants’ reputation as hateful pests, especially among farmers, is a consequence of monoculture, he added. These ants have lived on Earth for millions of years and have always foraged the leaves of many plant species. When all other plants are removed from the land in preparation for growing a single crop or agricultural variety, such as oranges, the ants are left with no alternative to cutting the leaves of that variety. “They aren’t naturally pests. They only appear as enemies to farmers because of the predominant agricultural model based on monoculture,” Rodrigues said.

He stressed that this recent study in which he took part as well as the Escovopsis fungus collection built up over the years was made possible thanks to other projects supported by FAPESP at UNESP’s Social Insect Research Center, and especially the Thematic Projects “Study of the potential of some plant species and natural and synthetic products for the control of leafcutter ants” and “Leaf cutting ants control, integrated studies”.