Currie Lab

Symbiosis in Grass-cutting Ants

USDA HATCH GrantSymbiosis likely influences the biology of every living organism, as unrelated organisms unavoidably live in intimate association with one another. These interspecific interactions greatly shape biological diversity and have facilitated many of the major evolutionary transitions over the history of life. The formation of beneficial symbiotic associations can serve as a form of evolutionary innovation by establishing a symbiosis with another organism, a host can rapidly gain access to unique ‘goods’ and/or ‘services’ previously unavailable to them. This can increase the competitive abilities of a host within their niche, or even allow them to invade new niches. Symbiosis as an evolutionary innovation is illustrated by the abundance and diversity of lineages of organisms that readily form symbioses.

Fungus-growing ants are a paradigmatic example of evolutionary innovation through symbiosis; gaining indirect access to plant biomass through the cultivation of fungi in specialized gardens. Their specialized fungal cultivar serves as the primary food source for colonies, and in combination with a recently identified bacterial community, serve as an external digestive system, converting plant biomass into usable energy for the ants. In exchange, the fungus and bacterial symbionts are transmitted from parent to offspring nest, carefully tended and protected by workers, and provided plant material that supports their growth. The most phylogenetically derived fungus-growing ants are leaf-cutters in the genus Atta, which utilize mostly fresh leaf material. Mature colonies of leaf-cutter ants in the genus Atta form massive colonies composed of millions of workers. To support these ant megalopolises, workers harvest hundreds of kilograms of leaves every year. In total, leaf-cutter colonies consume as much as 17% of the leaf biomass produced in some ecosystems and forage on ~30% of plant species (6, 19, 20). Recent work lead by our lab has identified that the fungal cultivar, Leucoagricus gonglyophorous, produces a diverse array of plant biomass deconstructing enzymes that facilitate energy transfer from these recalcitrant materials. In the draft genome of L. gongylophorus, we identified 145 predicted biomass degrading enzymes, including 81 glycoside hydrolases (GH), 6 polysaccharide lyases (PL), 9 carbohydrate esterases (CE), 9 laccases, 5 glyoxal oxidases, 4 aryl-alcohol oxidases, and 26 secreted proteases. Furthermore, metaproteomic analysis revealed 15 CAZymes, 4 FOLymes, and 13 proteases, comprised of 10 GHs, 3 PLs, 2 CEs, and 2 laccases that varied through the layers of the fungus gardens.

Within the leaf-cutter genus Atta, some species have evolved to strictly collect grasses to feed their fungus gardens. It has been shown that grass-cutter ants are morphologically and behaviorally distinct from the other dicot specialized leaf-cutters to specialize on grasses. They have shorter and stronger mandibles than their dicot-cutting counterparts and they cut and process the leaves differently. These adaptations are thought to help the ant overcome the tougher cuticle of grasses as well as to make cutting the grass blades more efficient. Grasses also differ from dicots in their cell wall structure, lignin content and sugar composition. Together these biochemical and structural differences represent a substantially different substrate for enzymatic hydrolysis. Thus, efficient deconstruction of grasses is expected to require unique mechanisms that differ from those optimized for dicots. In fact, studies show that grass pathogens produce enzymes that are specific to grasses. For example, cereal pathogens Rhizoctonia cerealis, Fusarium culmorum and Pseudocercosporella herpotrichoides all secrete similar cell wall degrading enzymes when grown on wheat. We hypothesize that the microbial community of the fungus garden in grass-cutter ants have specifically adapted for the efficient breakdown of grasses. Here we propose characterizing the cellulolytic capacity of the fungal and bacterial symbionts associated with grass-cutter ants.

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