Partial incompatibility between ants and symbiotic fungi in two sympatric species of Acromyrmex leaf-cutting ants

We investigate the nature and duration of incompatibility between certain c ombinations of Acromyrmex leaf-cutting ants and symbiotic fungi, taken from sympatric colonies of the same or a related species. Ant-fungus incompatib ility appeared to be largely independent of the ant species involved, but c ould be explained partly by genetic differences among the fungus cultivars. Following current theoretical considerations, we develop a hypothesis, ori ginally proposed by S. A. Frank, that the observed incompatibilities are ul timately due to competitive interactions be, tween genetically different fu ngal lineages, and we predict that the ants should have evolved mechanisms to prevent such competition between cultivars within a single garden. This requires that the ants are able to recognize unfamiliar fungi, and we show that this is indeed the case. Amplified fragment length polymorphism genoty ping further shows that the two sympatric Acromyrmex species share each oth er's major lineages of cultivar, confirming that horizontal transfer does o ccasionally take place. We argue and provide some evidence that chemical su bstances produced by the fungus garden may mediate recognition of alien fun gi by the ants. We show that incompatibility between ants and transplanted, genetically different cultivars is indeed due to active killing of the nov el cultivar by the ants. This incompatibility disappears when ants are forc e-fed the novel cultivar for about a week, a result that is consistent with our hypothesis of recognition induced by the resident fungus and eventual replacement of incompatibility compounds during force-feeding.