Lignocellulose Metabolism in a Fungus-Gardening Ant Symbiosis

Fungus-gardening ants exhibit a unique interaction with specific fungi in an obligatory symbiosis; ants feed and prune their fungal symbiont in exchange for nourishment. Cellulose is the most abundant plant compound found on earth and may serve as a rich carbon source to ants. Recent studies show that leaf-cutting (Atta and Acromyrmex) ants\u27 fungus does not typically digest cellulose, but there have been few comparisons to the other higher attine fungus gardening ants. In this study, I measured the lignocellulose consumption of the higher fungus-gardening ants Trachymyrmex septentrionalis and Trachymyrmex arizonensis. Colonies were fed either a diet of Texas Red Oak (Quercus buckleyi) catkins (staminate flowers) or tent caterpillar (Malacosoma americanum) frass to assess the ability of the symbiosis to digest plant-based carbohydrates. The Van Soest fiber analysis method was used to evaluate the amount of lignocellulose and hemicellulose content of substrates and fungus garden refuse, i.e., organic matter before and after assimilation in the fungus garden, respectively. The evidence supports the hypothesis that these two species of Trachymyrmex colonies are significant metabolizers of cellulose since levels of lignocellulose and hemicellulose were significantly lower in refuse piles than in the substrates the colonies were fed. Fungus gardens of T. septentrionalis extracted about 46% and 47% of the lignocellulose present in the catkins and frass, respectively, as well as 43% of the hemicellulose found in catkins and 67% of the hemicellulose that was found in frass