Introduction: Fungus-growing ants cultivate specialized fungi in the tribe Leucocoprineae (Lepiotaceae: Basidiomycota) inside their nests. The conspicuous leaf-cutting ants in the genus Atta build huge nests displacing several cubic meters of soil, whereas lower attine genera such as Cyphomyrmex have small nests with a fungus garden the size of a table-tennis ball. Only the leaf-cutting ants are specialized on using fresh leaves as substrate for their fungus gardens, whereas the more basal attine genera use substrates such as dry plant material (leaf litter and small twigs) and also insect feces and insect carcasses. This diverse array of fungal substrates across the attine lineage implies that the symbiotic fungus needs different enzymes to break down the plant material that the ants provide or different efficiencies of enzyme function.
Methods: (1.) We made a literature survey of substrate use in all extant fungus-growing ant genera to know the range of substrates used for any particular ant genus. (2.) Field assays of enzyme activity in fungus gardens of five candidate enzymes (Amylase, proteinase, pectinase, cellulose and xylanase) to indicate differences in enzyme activity between ant groups. (3.) Phylogenetic comparison and divergence estimates of nuclear ribosomal sequences and sequences coding for candidate enzyme genes (work in progress).
Results: Enzyme activity assays showed significant differences in enzyme activity across major fungus-growing ant groups. Notably the fresh leaf feeding species had a higher activity of amylase. The group of higher attines had a higher activity of proteinase and pectinase. In contrast the lower genera had a non-significant trend towards a higher activity of xylanase compared to the higher attine genera. Cellulase activity was uniform across all tested genera.
Discussion: In this study we document that there are differences in fungus garden enzyme activity between the different ant genera. These different enzyme activity profiles can be partially explained by the difference in substrates brought back by the ants to manure the fungus garden. This system can be viewed as ant induced crop optimization similar to human agricultural practices.