Specificity in the symbiotic association between fungus-growing ants and protective
Pseudonocardia
bacteria
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abstract
Fungus-growing ants (tribe Attini) engage in a mutualism with a fungus that serves as the ants' primary food source, but successful fungus cultivation is threatened by microfungal parasites (genus
Escovopsis
). Actinobacteria (genus
Pseudonocardia
) associate with most of the phylogenetic diversity of fungus-growing ants; are typically maintained on the cuticle of workers; and infection experiments, bioassay challenges and chemical analyses support a role of
Pseudonocardia
in defence against
Escovopsis
through antibiotic production. Here we generate a two-gene phylogeny for
Pseudonocardia
associated with 124 fungus-growing ant colonies, evaluate patterns of ant–
Pseudonocardia
specificity and test
Pseudonocardia
antibiotic activity towards
Escovopsis
. We show that
Pseudonocardia
associated with fungus-growing ants are not monophyletic: the ants have acquired free-living strains over the evolutionary history of the association. Nevertheless, our analysis reveals a significant pattern of specificity between clades of
Pseudonocardia
and groups of related fungus-growing ants. Furthermore, antibiotic assays suggest that despite
Escovopsis
being generally susceptible to inhibition by diverse Actinobacteria, the ant-derived
Pseudonocardia
inhibit
Escovopsis
more strongly than they inhibit other fungi, and are better at inhibiting this pathogen than most environmental
Pseudonocardia
strains tested. Our findings support a model that many fungus-growing ants maintain specialized
Pseudonocardia
symbionts that help with garden defence.
