Data from: Specificity in the symbiotic association between fungus-growing ants and protective Pseudonocardia bacteria

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.

切叶蚁族（Attini）的真菌栽培蚁与作为其主要食物来源的真菌形成互利共生关系，但成功的真菌栽培却受到微真菌寄生虫埃斯科沃氏霉属（Escovopsis）的威胁。与多数具有系统发育多样性的真菌栽培蚁共生的放线菌（Actinobacteria）多为假诺卡氏菌属（Pseudonocardia），这类菌通常定殖于工蚁的外骨骼表皮；感染实验、生物测定验证与化学分析均证实，假诺卡氏菌可通过产生抗生素发挥抵御埃斯科沃氏霉的作用。本研究构建了与124个真菌栽培蚁群落共生的假诺卡氏菌的双基因系统发育树，评估了蚁类与假诺卡氏菌的共生特异性模式，并检测了假诺卡氏菌对埃斯科沃氏霉的抗生素活性。研究结果显示，与真菌栽培蚁共生的假诺卡氏菌并非单系群：在该共生关系的进化历程中，蚁类多次获得了自由生活的菌株。尽管如此，本分析仍揭示了假诺卡氏菌演化支与相关类群真菌栽培蚁之间存在显著的特异性关联模式。此外，抗生素活性测定结果表明，尽管埃斯科沃氏霉普遍对多种放线菌的抑制作用敏感，但源自蚁类的假诺卡氏菌对埃斯科沃氏霉的抑制效果强于其对其他真菌的抑制能力，且相较于绝大多数受试环境假诺卡氏菌菌株，这类菌对该病原菌的抑制效果更为出色。本研究结果支持如下模型：许多真菌栽培蚁可维持具有特化特性的假诺卡氏菌共生体，以协助其抵御真菌花园的病原菌。