Data from: Specificity and stability of the Acromyrmex-Pseudonocardia symbiosis

The stability of mutualistic interactions is likely to be affected by the genetic diversity of symbionts that compete for the same functional niche. Fungus-growing (attine) ants have multiple complex symbioses and thus provide ample opportunities to address questions of symbiont specificity and diversity. Among the partners are Actinobacteria of the genus Pseudonocardia that are maintained on the ant cuticle to produce antibiotics, primarily against a fungal parasite of the mutualistic gardens. The symbiosis has been assumed to be a hallmark of evolutionary stability, but this notion has been challenged by culturing and sequencing data indicating an unpredictably high diversity. We used 454 pyrosequencing of 16S rRNA to estimate the diversity of the cuticular bacterial community of the leaf-cutting ant Acromyrmex echinatior and other fungus-growing ants from Gamboa, Panama. Both field and laboratory samples of the same colonies were collected, the latter after colonies had been kept under laboratory conditions for up to 10 years. We show that bacterial communities are highly colony-specific and stable over time. The majority of colonies (25/26) had a single dominant Pseudonocardia strain, and only two strains were found in the Gamboa population across 17 years, confirming an earlier study. The microbial community on newly hatched ants consisted almost exclusively of a single strain of Pseudonocardia while other Actinobacteria were identified on older, foraging ants in varying but usually much lower abundances. These findings are consistent with recent theory predicting that mixtures of antibiotic-producing bacteria can remain mutualistic when dominated by a single vertically transmitted and resource-demanding strain.

互利共生互作的稳定性大概率受共享同一功能生态位的共生体遗传多样性影响。培养真菌的菌栖蚁（attine ants）拥有多种复杂的共生关系，可为探究共生体特异性与多样性相关问题提供充足研究契机。其共生伙伴包括假诺卡氏菌属（Pseudonocardia）的放线菌（Actinobacteria）：这类细菌定殖于蚁体表，可合成抗生素，主要用于抵御共生真菌‘花园’的真菌寄生虫。此前该共生关系被认为是进化稳定性的标志性范例，但这一观点受到培养与测序数据的挑战——这些数据显示共生体多样性高得难以预测。本研究通过对16S核糖体RNA（16S rRNA）进行454焦磷酸测序（454 pyrosequencing），对巴拿马甘博阿（Gamboa）地区的埃氏切叶蚁（Acromyrmex echinatior）及其他菌栖蚁的体表细菌群落多样性进行了评估。研究采集了同一蚁群的野外与实验室样本，后者为蚁群在实验室条件下饲养长达10年后采集所得。结果表明，这些细菌群落具有极强的蚁群特异性，且随时间推移保持稳定。26个蚁群中的25个仅存在单一优势假诺卡氏菌菌株，且在17年间的甘博阿种群中仅发现两种菌株，该结果验证了此前的一项研究。新羽化蚁的体表微生物群落几乎仅由单一假诺卡氏菌菌株构成，而在老龄、具有觅食行为的蚁体上可检测到其他放线菌，其丰度虽存在差异，但通常远低于假诺卡氏菌。上述研究结果与近期提出的理论相符：当产抗生素细菌群落由单一垂直传播且依赖宿主资源的菌株占据主导时，该菌群仍可维持互利共生关系。