title: Specificity in the symbiotic association between fungus-growing ants and protective Pseudonocardia bacteria.
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.
authors.affiliation: Department of Bacteriology, University of Wisconsin, Madison, 4325 Microbial Sciences Building, 1550 Linden Drive, Madison, WI 53706, USA.
authors.forename: Matías J
authors.initials: MJ
authors.lastname: Cafaro
authors.forename: Michael
authors.initials: M
authors.lastname: Poulsen
authors.forename: Ainslie E F
authors.initials: AE
authors.lastname: Little
authors.forename: Shauna L
authors.initials: SL
authors.lastname: Price
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Gerardo
authors.forename: Bess
authors.initials: B
authors.lastname: Wong
authors.forename: Alison E
authors.initials: AE
authors.lastname: Stuart
authors.forename: Bret
authors.initials: B
authors.lastname: Larget
authors.forename: Patrick
authors.initials: P
authors.lastname: Abbot
authors.forename: Cameron R
authors.initials: CR
authors.lastname: Currie
mesh_terms: D000192:Actinomycetales / Q000145:classification* / Q000235:genetics / Q000502:physiology; D000818:Animals; D001000:Ants / Q000382:microbiology*; D001426:Bacterial Proteins / Q000235:genetics; D044822:Biodiversity; D005075:Biological Evolution; D006999:Hypocreales / Q000502:physiology; D010444:Peptide Elongation Factor Tu / Q000235:genetics; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D013559:Symbiosis
chemical_list: D001426:Bacterial Proteins; D012336:RNA, Ribosomal, 16S; D010444:Peptide Elongation Factor Tu