The fungal cultivar of leaf‐cutter ants produces specific enzymes in response to different plant substrates

Abstract

Herbivores use symbiotic microbes to help derive energy and nutrients from plant material. Leaf-cutter ants are a paradigmatic example, cultivating their mutualistic fungus Leucoagaricus gongylophorus on plant biomass that workers forage from a diverse collection of plant species. Here, we investigate the metabolic flexibility of the ants’ fungal cultivar for utilizing different plant biomass. Using feeding experiments and a novel approach in metaproteomics, we examine the enzymatic response of L. gongylophorus to leaves, flowers, oats, or a mixture of all three. Across all treatments, our analysis identified and quantified 1,766 different fungal proteins, including 161 putative biomass-degrading enzymes. We found significant differences in the protein profiles in the fungus gardens of sub-colonies fed different plant substrates. When provided with leaves or flowers, which contain the majority of their energy as recalcitrant plant polymers, the fungus gardens produced more proteins predicted to break down cellulose: endoglucanase, exoglucanase, and β-glucosidase. Further, the complete metaproteomes for the leaves and flowers treatments were very similar, while the mixed substrate treatment closely resembled the treatment with oats alone. This indicates that when provided a mixture of plant substrates, fungus gardens preferentially break down the simpler, more digestible substrates. This flexible, substrate-specific enzymatic response of the fungal cultivar allows leaf-cutter ants to derive energy from a wide range of substrates, which likely contributes to their ability to be dominant generalist herbivores.

Metagenomics reveals diet-specific specialization of bacterial communities in fungus gardens of grass- and dicot-cutter ants

“…Thus, we can conclude that garden bacteria do not respond to changes in cell wall structure between grasses and dicots. Instead, it is likely that the genome or gene expression in the fungus from these two systems would show differences, especially since the fungus is the primary degrader of plant biomass in leaf-cutter ant fungus gardens (Aylward et al , 2013; Khadempour et al , 2016; Nagamoto et al , 2011; Grell et al , 2013).…”

“…First, the steps of transcription and translation lie between the number of gene copies and ultimate function. However, previous efforts to examine the bacterial community in situ , both through metaproteomics and metatranscriptomics, have not been fruitful, since the fungal biomass, transcripts, and proteins swamp out the bacterial portion (Khadempour et al , 2016; Moreira-Soto, 2016). Therefore, to date, this work provides the most comprehensive picture of the bacterial community in leaf-cutter ant fungus gardens and the potential roles they may play in the system.…”

“…Leaf-cutter ants lack the capacity to break down recalcitrant plant material. Instead, they gain access to the nutrients in plant biomass by farming a fungus, Leucoagaricus gongylophorus , which enzymatically breaks down recalcitrant biomass in the leaf material that the ants forage (Aylward et al , 2013; Khadempour et al , 2016; Suen et al , 2011a; Grell et al , 2013; Kooij et al , 2011; Nagamoto et al , 2011). Leucoagaricus gongylophorus produces gongylidia, specialized hyphal swellings that contain an abundance of sugars and lipids, that the ants consume and feed to larvae (Bass and Cherrett, 1995; North et al , 1997).…”

Fungal Genomes and Insights into the Evolution of the Kingdom

“…The “domesticated” fungus cultivated by leaf cutter ants, Leucoagaricus gongylophorus , has an expansion of CAZymes related to polysaccharide degradation. These enzymes are differentially expressed depending on the plant substrates ‘fed’ to the fungus garden by leaf cutter farming ants [143–149]. The specialized gongylidia that are swellings of the hyphal tip have evolved to provide a sugar nutrient source for the ants in exchange for the the input of leaf and plant material into the fungal garden.…”

“…The specialized gongylidia that are swellings of the hyphal tip have evolved to provide a sugar nutrient source for the ants in exchange for the the input of leaf and plant material into the fungal garden. The expansion and specialization of the fungal enzyme families necessary for rapid extraction of carbohydrates from plant material has likely been driven by this highly mutualistic symbiosis [147–149]. …”