Leaf-cutting ants of the genera Acromyrmex and Atta (Family Formicidae: Subfamily Myrmicinae: Tribe Attini) live in mutualistic symbiosis with the basidiomycete Leucocoprinus gongylophorus (Agaricaceae). The ants cultivate the mycobiont mycelium in ‘fungal gardens’ where they brought freshly cut and chew leaves. They apply fecal droplets to the leaf pulp before depositing this mixed substrate to the top of the garden. The fecal fluid contains a large range of hydrolytic enzymes (proteases, pectinases, carbohydrate degrading enzymes) able to efficiently degrade the plant cell wall and cell material. Released carbohydrates serve as a primary source of nutrient for the fungus which then differenciate clusters of a unique tissular structure so-called the ‘gongylidia‘. This massive hyphal swelling are the main food source of the farming leaf-cutting ants. In ant agriculture,the attine ants actively propagate, nurture and defend the basidiomycete cultivar. This mutualistic symbiosis is thought to have originated in the basin of the Amazon rainforest some 50–65 million years ago. The molecular mechanisms driving this ant-fungus mutualism are poorly know.
In their study published in BMC Biology, Schiøtt et al. showed that the pectinolytic enzymes present in the ant fecal droplets are produced by the fungus. The genes encoding the hydrolytic enzymes are induced in the gongylidia mycelium, ingested by the feeding ants, transported throughout the ant gut before being released in fecal fluids on the top of the fungal garden. It is suggested by the authors that the fungal enzymes evolved to survive the harsh conditions of the ant gut. The on-going sequencing of the genome of Leucocoprinus gongylophorus will undoubtly provide novel insights on the evolution from saprotrophism to this unique mutualistic symbiosis.
Figure by Schiøtt et al. BMC Biology 2010 8:156 doi:10.1186/1741-7007-8-156
Recommended reading: Fungus-Ant mutualism
Photo: © http://www.zsl.org/zsl-london-zoo/animals/inverts/leaf-cutting-ant,59,AN.html