Disentangling the assembly mechanisms of ant cuticular bacterial communities of two Amazonian ant species sharing a common arboreal nest

Bacteria living on the cuticle of ants are generally studied for their protective role against pathogens, especially in the clade of fungus-growing ants. However, little is known of the diversity of cuticular bacteria in other ant host species, as well as of the mechanisms leading to the composition of these communities. Here, we used 16S rRNA gene amplicon sequencing to study the influence of host species, species interactions, and the pool of bacteria from the environment on the assembly of cuticular bacterial communities on two phylogenetically distant Amazonian ant species that frequently nest together inside the roots system of epiphytic plants, Camponotus femoratus and Crematogaster levior. Our results show that 1) the vast majority of the bacterial community on the cuticle is shared with the nest, suggesting that most bacteria on the cuticle are acquired through environmental acquisition, 2) 5.2% and 2.0% OTUs are respectively specific to Camponotus femoratus and Crematogaster levior, likely representing their respective core cuticular bacterial community, and 3) 3.6% of OTUs are shared between the two ant species. Additionally, mass spectrometry metabolomics analysis of metabolites on the cuticle of ants, which excludes the detection of cuticular hydrocarbons produced by the host, were conducted to evaluate correlations among bacterial OTUs and m/z ion mass. Although some positive and negative correlations are found, the cuticular chemical composition was weakly species specific which supports that cuticular bacterial communities are prominently environmentally acquired. Overall, our results suggest that the environment is the dominant source of bacteria found on the cuticle of ants.

蚂蚁体表表皮（cuticle）上定殖的细菌，通常被研究其抵御病原菌的保护作用，尤其在培养真菌的蚂蚁类群中。然而，学界对其他蚂蚁宿主物种体表细菌的多样性，以及塑造这些群落组成的机制仍知之甚少。本研究采用16S rRNA基因扩增子测序（16S rRNA gene amplicon sequencing）技术，探究宿主物种、物种互作以及环境细菌库对两种系统发育关系较远的亚马孙蚂蚁体表细菌群落组装的影响；这两种蚂蚁常共巢于附生植物的根系系统中，分别为弗氏巨首蚁（Camponotus femoratus）和平滑举腹蚁（Crematogaster levior）。本研究结果显示：1）体表细菌群落的绝大多数类群与蚁巢内的细菌类群一致，表明蚂蚁体表的大部分细菌通过环境获取获得；2）分别有5.2%和2.0%的操作分类单元（Operational Taxonomic Unit，OTU）仅存在于弗氏巨首蚁和平滑举腹蚁体表，这两类OTU可能分别代表二者的核心体表细菌群落；3）两种蚂蚁共享3.6%的OTU。此外，本研究还开展了蚂蚁体表代谢物的质谱代谢组学分析（该分析剔除了宿主自身产生的表皮碳氢化合物），以探究细菌OTU与质荷比（m/z）离子峰之间的相关性。尽管本研究发现了部分正负相关关系，但蚂蚁体表的化学组成仅表现出微弱的宿主物种特异性，这一结果支持蚂蚁体表细菌群落主要通过环境获取的结论。综上，本研究结果表明，环境是蚂蚁体表细菌的主要来源。