Fungus-gardening ants prefer native fungal species: do ants control their crops?

Fundamental features of mutualisms are solutions to conflict. The mutualism between fungus-gardening ants and their fungi is characterized by pronounced evolutionary ant-fungus fidelity. Ant-fungus fidelity is thought to be controlled by the fungus, which secretes somatic incompatibility compounds that suppress incompatible fungal strains that may invade gardens. Accordingly, the ants are thought to perceive incompatibilities and then reject invading cultivars. During cultivar-switch experiments, we discovered that if switched colonies of the ant Trachymyrmex septentrionalis are successful in retaining a minute fragment of their original garden, the ants will gradually co-cultivate the novel and the original fungus strains in a chimeric (polycultural) garden; this chimeric garden will revert after 1–2 weeks completely to a monoculture of the original fungal strain. Experimental examination of worker preference suggests that symbiont rejection behavior seems to be an innate response toward foreign fungi rather than mediated solely by incompatibility compounds secreted by the fungus. These observations suggest that the fungal strain cultivated by a colony imprints the attending ants, and this imprinting modifies ant behavior so that the ants prefer their original cultivar even when experimentally forced to grow a foreign cultivar for several weeks. Fungal monocultures and ant-fungus fidelities in this symbiosis therefore seem to be reinforced by factors intrinsic to both the ants and the fungi.