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.

互利共生相互作用的稳定性，大概率会受到竞争同一功能生态位的共生体遗传多样性的影响。种植真菌的切叶蚁（attine）拥有多组复杂的共生体系，因此为探究共生体特异性与多样性相关问题提供了充足的研究契机。其共生伙伴包括放线菌（Actinobacteria）中的假诺卡氏菌属（Pseudonocardia），这类细菌定殖于蚂蚁外骨骼表面以产生抗生素，主要用于抵御共生真菌园的真菌寄生物。此前学界普遍认为这类共生关系是进化稳定性的标志性范例，但培养与测序数据显示其多样性水平高得难以预测，这一观点已受到学界的质疑。我们通过对16S核糖体RNA（16S rRNA）开展454焦磷酸测序（454 pyrosequencing），估算了来自巴拿马甘博阿地区的切叶蚁棘颚顶切叶蚁（Acromyrmex echinatior）及其他种植真菌蚁的体表细菌群落多样性。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年后获取的样本。结果表明，细菌群落具有极强的菌落特异性，且随时间推移保持稳定。绝大多数蚁群（25/26）仅存在单一优势假诺卡氏菌菌株，且17年间在甘博阿种群中仅发现两种菌株，这一结果验证了此前的一项研究成果。新孵化蚂蚁体表的微生物群落几乎仅由单一假诺卡氏菌菌株构成，而在年长的觅食蚁体表则可检测到其他放线菌，其丰度虽存在波动，但通常远低于优势菌株。上述发现与近期理论预测相符：当产抗生素细菌群落由单一垂直传播且资源依赖型菌株主导时，其混合菌群仍可维持互利共生关系。