By their own devices: invasive Argentine ants have shifted diet without clear aid from symbiotic microbes. ant metagenome

The functions and compositions of symbiotic bacterial communities often correlate with host ecology. Yet cause-and-effect relationships, and the order of symbiont versus host change, remain unclear in the face of ancient symbioses and conserved host ecology. Several groups of ants exemplify this challenge, as their low-nitrogen diets and specialized symbioses appear conserved and ancient. To address whether nitrogen-provisioning symbionts might be important in the early stages of ant trophic shifts we studied bacteria from the Argentine ant, Linepithema humile—an invasive species that has transitioned toward greater consumption of sugar-rich, nitrogen-poor foods in parts of its introduced range. Bacteria were present at low densities in most L. humile workers, and among those yielding quality 16S rRNA amplicon sequencing data we found just three symbionts to be common and dominant. Two, a Lactobacillus and an Acetobacteriaceae species, were shared between native and introduced populations. The other, a Rickettsia, was found only in two introduced supercolonies. Across an eight-year period of trophic reduction in one introduced population, we found no change in symbionts, arguing against a relationship between natural dietary change and microbiome composition. Overall our findings thus, argue against major changes in symbiotic bacteria in association with the invasion and trophic shift of L. humile. In addition, genome content from close relatives of the identified symbionts suggest that just one can synthesize most essential amino acids; this bacterium was only modestly abundant in introduced populations, providing little support for a major role of nitrogen-provisioning symbioses in Argentine ant’s dietary shift.

共生细菌群落（symbiotic bacterial communities）的功能与组成通常与宿主生态（host ecology）相关联。然而面对古老的共生关系与保守的宿主生态，二者的因果关联以及共生体与宿主的演化变化顺序仍不明确。多类蚂蚁堪称这一难题的典型范例：它们的低氮饮食与特化共生关系均呈现保守且古老的特征。为探究固氮共生体是否在蚂蚁营养生态位转变的早期阶段发挥重要作用，我们对阿根廷蚁（Linepithema humile）体内的细菌展开了研究——该入侵物种在其部分入侵分布区内已转向更多取食高糖低氮的食物。多数阿根廷蚁工蚁体内的细菌密度较低；在获得合格16S rRNA扩增子测序（16S rRNA amplicon sequencing）数据的样本中，我们仅发现三种常见且占优势的共生体。其中两种分别为乳杆菌属（Lactobacillus）物种与醋杆菌科（Acetobacteriaceae）物种，在原生种群与入侵种群中均有分布；另一种为立克次体属（Rickettsia）物种，仅在两个入侵超级群落中被检出。在一个入侵种群长达八年的营养摄入缩减过程中，我们未观察到共生体组成发生显著变化，这一结果不支持自然饮食改变与微生物组（microbiome）组成之间存在关联。综上，我们的研究发现不支持阿根廷蚁在入侵与营养生态位转变过程中，其共生细菌发生了显著改变。此外，对所鉴定共生体近缘物种的基因组分析显示，仅有一种共生体能够合成大部分必需氨基酸；但该细菌在入侵种群中的丰度仅处于中等水平，这几乎无法为固氮共生体在阿根廷蚁饮食转变中发挥主要作用这一假说提供支撑。