Army ants harbor a host-specific clade of Entomoplasmatales bacteria

Abstract

In this article, we describe the distributions of Entomoplasmatales bacteria across the ants, identifying a novel lineage of gut bacteria that is unique to the army ants. While our findings indicate that the Entomoplasmatales are not essential for growth or development, molecular analyses suggest that this relationship is host specific and potentially ancient. The documented trends add to a growing body of literature that hints at a diversity of undiscovered associations between ants and bacterial symbionts.

FIG. 1.

Distribution of the Entomoplasmatales across ant taxa. Bar graphs depict the proportion of positive species per ant subfamily (A) or army ant genus (B) based on results from diagnostic screening (pooled data from both the general and army ant screens). Species were declared positive if at least one individual ant screened positive for Entomoplasmatales. Taxa from different subfamilies are given different shading for ease of viewing (black, Aenictinae; dark gray, Dorylinae; light gray, Ecitoninae).

FIG. 2.

16S rRNA phylogeny depicting relatedness of Entomoplasmatales associates from army ants and other organisms. Maximum likelihood was used to construct a phylogeny based on an alignment of 122 16S rRNA sequences from bacteria within the order Entomoplasmatales. The tree was rooted using Mycoplasma genitalium as the outgroup (not shown). Analyzed sequences included nonredundant ant associates from this study (i.e., one representative per species per 1% phylotype), their closest relatives in GenBank (based on BLASTn searches), and selected strains from other arthropod hosts, with an emphasis on those from Drosophila, spiders, and lepidopterans. To better illustrate the main finding—a host-specific clade of microbes exclusively found in army ants (with 100% bootstrap support in parsimony and likelihood searches; “Primary Army Ant Clade”), most clades were collapsed. The full tree (with bootstrap values, strain IDs, and accession numbers but without branch lengths) can be found in Fig. S2 in the supplemental material. Strains from ants are named after their hosts, and the host/environment of origin is indicated for all taxa in parentheses.