Biomass conversion in the fungal garden of the leaf-cutter ant Acromyrmex echinatior

It has been demonstrated that fungal enzymes play a significant role in the fungal garden conversion of the fresh-cut leaves into accessible food for the ant larvae (Schiøtt et al. 2008, BMC Microbiol, 8:40; Licht et al. 2010, Evolution 64: 2055-2069). However, so far specific documentation of conversion of also the cellulose fibers itself has been scarce. In the current study, we have taken an experimental approach allowing us to discover which genes are specifically expressed in the upper, middle, and bottom layers of the fungal garden. Using the DeepSAGE technique (Nielsen et al. 2006, Nucleic Acids Res 34:e133) short cDNA tags of the mRNA molecules produced in each of the fungal garden layers were achieved. Subtractive comparisons were made, identifying the genes at least 2x over-expressed in the bottom layer compared to the upper layer. Extended sequence of the selected genes were acquired through matching to an EST library produced from the same fungal garden material and 454 genome sequencing data of the fungal symbiont Leucoagaricus gongylophorus. Based on Blast searches, the deducted function of the identified genes was achieved. The results provided interesting new knowledge to understand biomass degradation in the leaf-cutter ant fungal garden: We found the full spectrum of cellulose degrading enzymes among the over-expressed genes in the bottom layer of the fungal garden: Glycoside hydrolases of families GH3, GH5, GH6, GH7, and GH61, and additional proteins containing carbohydrate-binding module CBM1. In conclusion, also the cellulose fibers of the ant assembled leaf cuts are degraded by fungal enzymes. The degradation appears to take place primarily in the bottom layer of the fungal garden.