Generalized antifungal activity and 454-screening ofPseudonocardiaandAmycolatopsisbacteria in nests of fungus-growing ants

Abstract

In many host-microbe mutualisms, hosts use beneficial metabolites supplied by microbial symbionts. Fungus-growing (attine) ants are thought to form such a mutualism with Pseudonocardia bacteria to derive antibiotics that specifically suppress the coevolving pathogen Escovopsis, which infects the ants' fungal gardens and reduces growth. Here we test 4 key assumptions of this PseudonocardiaEscovopsis coevolution model. Culture-dependent and cultureindependent (tag-encoded 454-pyrosequencing) surveys reveal that several Pseudonocardia species and occasionally Amycolatopsis (a close relative of Pseudonocardia) co-occur on workers from a single nest, contradicting the assumption of a single pseudonocardiaceous strain per nest. Pseudonocardia can occur on males, suggesting that Pseudonocardia could also be horizontally transmitted during mating. Pseudonocardia and Amycolatopsis secretions kill or strongly suppress ant-cultivated fungi, contradicting the previous finding of a growth-enhancing effect of Pseudonocardia on the cultivars. Attine ants therefore may harm their own cultivar if they apply pseudonocardiaceous secretions to actively growing gardens. Pseudonocardia and Amycolatopsis isolates also show nonspecific antifungal activities against saprotrophic, endophytic, entomopathogenic, and gardenpathogenic fungi, contrary to the original report of specific antibiosis against Escovopsis alone. We conclude that attine-associated pseudonocardiaceous bacteria do not exhibit derived antibiotic properties to specifically suppress Escovopsis. We evaluate hypotheses on nonadaptive and adaptive functions of attine integumental bacteria, and develop an alternate conceptual framework to replace the prevailing Pseudonocardia-Escovopsis coevolution model. If association with Pseudonocardia is adaptive to attine ants, alternate roles of such microbes could include the protection of ants or sanitation of the nest. mutualism ͉ symbiosis ͉ Attini ͉ Actinomycete ͉ Escovopsis

Chemical basis of the synergism and antagonism in microbial communities in the nests of leaf-cutting ants

“…In addition, we observed a strongly antagonistic effect of antimycins A 1 -A 4 (4-7) against the mutualistic fungus L. gongylophorus. This provides a chemical explanation for recent observations, namely that the microbial symbionts of leaf-cutting ants have negative as well as positive effects on the ant colony (18,23). In the case of fungal infection, the cost of inhibiting growth of the garden fungus L. gongylophorus may be outweighed by the benefit of preventing the spread of infection in the nest.…”

“…Av25_2 compete with neighboring microorganisms. Thus, with actinomycins identified as being synthesized by a Streptomyces strain isolated from leaf-cutting ants, we provide the chemical basis for the observation of Sen et al (18), namely that ant-associated microorganisms play different roles in this ecosystem, ranging from mutualistic to detrimental interactions. Competition between their different microbial symbionts may, at first glance, be disadvantageous for the leaf-cutting ants; nevertheless, it may provide a pathway for the selection of potent defenders by inducing an evolutionary arms race between them.…”

“…S1). S. albidoflavus, which was recently isolated from mangroves, has been shown to produce antimycin A 18 (31), suggesting the possible production of antimycins (4-7) by Streptomyces Ao10.…”

“…These symbionts fulfill diverse functions, including defense against pathogens or promotion of the growth of the garden fungus (e.g., by nitrogen fixation) (10)(11)(12)(13)(15)(16)(17). Because some bacterial symbionts of leaf-cutting ants can be also detrimental to the growth of the mutualistic fungus L. gongylophorus, the onesided view of leaf-cutting ants and Actinomyces symbionts as mutualistic partners of the leaf-cutting ants should be challenged (18).…”

Rethinking crop-disease management in fungus-growing ants

“…The attine ants are rivaled in their farming sophistication only by the Macrotermitinae, which evolved a similar obligate dependence on fungus gardens (6). The discovery of new antagonistic and mutualistic partners in the attine symbiosis has attracted wide attention, but the prevailing coevolutionary paradigm behind key components of this mutualism is now challenged by a study by Sen et al in this issue of PNAS (7).…”