Abstract
Leaf-cutting ants live in mutualistic symbiosis with their garden fungus Leucoagaricus gongylophorus that can be attacked by the specialized pathogenic fungus Escovopsis. Actinomyces symbionts from Acromyrmex leaf-cutting ants contribute to protect L. gongylophorus against pathogens. The symbiont Streptomyces sp. Av25_4 exhibited strong activity against Escovopsis weberi in co-cultivation assays. Experiments physically separating E. weberi and Streptomyces sp. Av25_4 allowing only exchange of volatiles revealed that Streptomyces sp. Av25_4 produces a volatile antifungal. Volatile compounds from Streptomyces sp. Av25_4 were collected by closed loop stripping. Analysis by NMR revealed that Streptomyces sp. Av25_4 overproduces ammonia (up to 8 mM) which completely inhibited the growth of E. weberi due to its strong basic pH. Additionally, other symbionts from different Acromyrmex ants inhibited E. weberi by production of ammonia. The waste of ca. one third of Acomyrmex and Atta leaf-cutting ant colonies was strongly basic due to ammonia (up to ca. 8 mM) suggesting its role in nest hygiene. Not only complex and metabolically costly secondary metabolites, such as polyketides, but simple ammonia released by symbionts of leaf-cutting ants can contribute to control the growth of Escovopsis that is sensitive to ammonia in contrast to the garden fungus L. gongylophorus.
Highlights
Leaf-cutting ants belonging to the genus Acromyrmex or Atta (Hymenoptera: Formicidae: Myrmicinae), are particular among ants because they cultivate the fungus Leucoagaricus gongylophorus (Möller, 1893, Agaricales: Agaricaceae) in designated chambers of their nests.Leaf-cutting ants live in mutualistic symbiosis with their garden fungus
As Streptomyces strains are well known to produce a large variety of organic volatile compounds [35,36,37] we screened Streptomyces symbionts from leaf-cutting ants for whether the production of volatile compounds inhibits the growth of the pathogenic fungus
E. weberi and the Streptomyces isolates were cultivated in three compartment Petri dishes so that they grow physically separated from each other but can exchange volatile compounds (Figure 1)
Summary
Leaf-cutting ants belonging to the genus Acromyrmex or Atta (Hymenoptera: Formicidae: Myrmicinae), are particular among ants because they cultivate the fungus Leucoagaricus gongylophorus (Möller, 1893, Agaricales: Agaricaceae) in designated chambers of their nests. The ants cut plant material and bring it to their nests where ant workers process the plant material into small pieces and inoculate their garden fungus with them. In the following years it turned out that the microcosmos of leaf-cutting ants is much more complex than the interactions between a leaf-cutting ant species, its garden fungus, its Pseudonocardia symbiont and the pathogen E. weberi. Many microorganisms are found in the leaf-cutting ants’ microcosomos fulfilling different ecological functions: there are pathogens [5,6,9,10], as well as microbial symbionts protecting and supporting the growth of the garden fungus [19,20,21,22,23,24,25].
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