Department of Zoology, University of Wisconsin, Madison, Wisconsin 53706, USA.
Am Nat. 2012 Nov;180(5):604-17. doi: 10.1086/667886. Epub 2012 Sep 25.
Fungus-growing ants (Attini) are part of a complex symbiosis with Basidiomycetous fungi, which the ants cultivate for food, Ascomycetous fungal pathogens (Escovopsis), which parasitize cultivars, and Actinobacteria, which produce antibiotic compounds that suppress pathogen growth. Earlier studies that have characterized the association between attine ants and their bacterial symbionts have employed broad phylogenetic approaches, with conclusions ranging from a diffuse coevolved mutualism to no specificity being reported. However, the geographic mosaic theory of coevolution proposes that coevolved interactions likely occur at a level above local populations but within species. Moreover, the scale of population subdivision is likely to impact coevolutionary dynamics. Here, we describe the population structure of bacteria associated with the attine Apterostigma dentigerum across Central America using multilocus sequence typing (MLST) of six housekeeping genes. The majority (90%) of bacteria that were isolated grouped into a single clade within the genus Pseudonocardia. In contrast to studies that have suggested that Pseudonocardia dispersal is high and therefore unconstrained by ant associations, we found highly structured ([Formula: see text]) and dispersal-limited (i.e., significant isolation by distance; [Formula: see text], [Formula: see text]) populations over even a relatively small scale (e.g., within the Panama Canal Zone). Estimates of recombination versus mutation were uncharacteristically low compared with estimates for free-living Actinobacteria (e.g., [Formula: see text] in La Selva, Costa Rica), which suggests that recombination is constrained by association with ant hosts. Furthermore, Pseudonocardia population structure was correlated with that of Escovopsis species ([Formula: see text], [Formula: see text]), supporting the bacteria's role in disease suppression. Overall, the population dynamics of symbiotic Pseudonocardia are more consistent with a specialized mutualistic association than with recently proposed models of low specificity and frequent horizontal acquisition.
真菌养殖蚂蚁（Attini）与担子菌真菌形成复杂共生关系，蚂蚁以此为食。此外，共生关系中还包括子囊菌真菌病原体（Escovopsis）和放线菌，前者会寄生在作物上，后者则产生抑制病原体生长的抗生素化合物。此前，针对 Attini 蚂蚁与其细菌共生体之间的关联，已有大量研究采用了广泛的系统发育方法，结论范围从弥散共生进化到没有特异性。然而，共进化的地理镶嵌理论提出，共进化的相互作用可能发生在种群以上但种内的水平上。此外，种群划分的规模可能会影响共进化动态。在这里，我们采用六个看家基因的多位点序列分型（MLST）方法，描述了中美洲 Apterostigma dentigerum 蚂蚁相关细菌的种群结构。分离出的大多数（90%）细菌聚集在假诺卡氏菌属的一个单系群中。与那些表明假诺卡氏菌属的扩散率很高且不受蚂蚁共生体限制的研究不同，我们发现即使在相对较小的范围内（例如巴拿马运河区），种群结构高度结构化（[Formula: see text]）且扩散受限（即显著的隔离距离；[Formula: see text]，[Formula: see text]）。与自由生活的放线菌相比，重组与突变的估计值异常低（例如，在哥斯达黎加的拉塞尔瓦），这表明与蚂蚁宿主的关联限制了重组。此外，假诺卡氏菌的种群结构与 Escovopsis 物种的结构相关（[Formula: see text]，[Formula: see text]），支持了其在疾病抑制中的作用。总的来说，共生假诺卡氏菌的种群动态与专门的互利共生关系更一致，而不是与最近提出的低特异性和频繁水平获得的模型一致。
