A community of ants, fungi, and bacteria: A multilateral approach to studying symbiosis

The ancient and highly evolved mutualism between fungus-growing ants and th eir fungi is a textbook example of symbiosis. The ants carefully tend the f ungus, which serves as their main food source, and traditionally are believ ed to be so successful at fungal cultivation that they are able to maintain the fungus free of microbial pathogens. This assumption is surprising in l ight of theories on the evolution of parasitism, especially for those speci es of ants that have been clonally propagating their cultivars for millions of years. Recent work has established that, as theoretically predicted, th e gardens of fungus-growing ants are host to a specialized, virulent, and h ighly evolved fungal pathogen in the genus Escovopsis. In addition, the ant s have evolved a mutualistic association with filamentous bacteria (actinom ycetes) that produce antibiotics that suppress the growth of Escovopsis. Th us, the attine symbiosis appears to be a coevolutionary "arms race" between the garden parasite Escovopsis on the one hand and the ant-fungus-actinomy cete tripartite mutualism on the other. These recent findings indicate that microbes may be key components in the regulation of other symbiotic associ ations between higher organisms.