Secondary Metabolites from Escovopsis weberi and Their Role in Attacking the Garden Fungus of Leaf-Cutting Ants.

The specialized, fungal pathogen Escovopsis weberi threatens the mutualistic symbiosis between leaf-cutting ants and their garden fungus (Leucoagaricus gongylophorus). Because E. weberi can overwhelm L. gongylophorus without direct contact, it was suspected to secrete toxins. Using NMR and mass spectrometry, we identified several secondary metabolites produced by E. weberi. E. weberi produces five shearinine-type indole triterpenoids including two novel derivatives, shearinine L and shearinine M, as well as the polyketides, emodin and cycloarthropsone. Cycloarthropsone and emodin strongly inhibited the growth of the garden fungus L. gongylophorous at 0.8 and 0.7 µmol, respectively. Emodin was also active against Streptomyces microbial symbionts (0.3 µmol) of leaf-cutting ants. Shearinine L instead did not affect the growth of L. gongylophorus in agar diffusion assays. However, in dual choice behavioral assays Acromyrmex octospinosus ants clearly avoided substrate treated with shearinine L for the garden fungus after a 2 d learning period, indicating that the ants quickly learn to avoid shearinine L.