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

Abstract

The functions and compositions of symbiotic bacterial communities often correlate with host ecology. Yet cause-effect relationships and the order of symbiont vs. 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 towards 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 Acetobacteraceae 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 suggests 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.

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“…Many of our screened ant workers lacked detectable levels of bacteria, and overall, we only observed seven core OTUs (Figure ). The observed low bacterial density and diversity in Lasius ants were expected given past work on other ant microbiomes, which display similar patterns (Hu et al., ; Moreau & Rubin, ; Ramalho, Bueno, & Moreau, ; Russell et al., , ). While there are exceptions representing a number of highly specialized ant–microbe associations in a handful of specific ant clades (Anderson et al., ; Lukasik et al., ; Russell et al., , ; Sanders et al., ), overall this low density and diversity is in line with previous work (Sanders et al., ).…”

“…These bacteria are generally found in hosts with sugar‐rich diets, such as the honeydew on which these Lasius ants predominantly feed (Ano, Toyama, Adachi, & Matsushita, ). Indeed, the observed OTUs are closely related to those previously published from the guts of other ants feeding on carbohydrate‐rich diets, including honeydew‐feeding Camponotus carpenter ants, Formica wood ants and Linepithema Argentine ants (Brown & Wernegreen, ; Hu et al., ; Russell et al., ) (Figure , Tables S2 and S3 in Appendix ). In addition, these bacteria are related to Asaia bacteria, recently found in Pseudomyrmex and Tetraponera ants, which both feed on another sugar‐rich diet of extra floral nectar (Kautz, Rubin, & Moreau, ; Samaddar et al., ).…”

“…The closest ncbi genbank matches to these OTUs are to Acetobacteraceae found in other ants with sugar‐rich diets. Inclusion of these sequences in our phylogeny reveals close relationship between Acetobacteraceae found across these different ants, with aab1 and aab2 from Camponotus chromaiodes matching closer than 97% to our observed strains aab_L1 and aab_L2, respectively, and thus being the same strains, under the 97% identity definition, as those found in Lasius ants (Figure b and Table S3 in Appendix ; Brown & Wernegreen, ; Hu et al., ; Russell et al., ). Interestingly, the three most common ant species L. claviger, L. nearcticus and L. umbratus share the same three Acetobacteraceae strains (aab_L1, aab_L2, aab_L3), while the three analysed individuals of L. flavus exclusively harboured strains aab_L4 and aab_L5 (Figure a,c).…”

“…For complete reads of individual ant workers and separate OTU s, see Figures S2 and S3 in Appendix . (b) NJ ‐tree of Tamura–Nei distances (scale bar) between 293 bp and 1,426 bp bacterial 16S rRNA fragments showing close phylogenetic relationship between the five clusters of Acetobacteraceae OTU s (here named strains aab_L1‐L5, colour coded) found during three MiSeq runs ( MS 1, MS 2 MS 3, MS 4) in ants in this study, and those previously described from other ant guts (aab1 and aab2 from Camponotus chromaiodes , A_53_40_5_16S from Linepithema humile, Hu et al., ; #128 from Formica occulta, Russell et al., ). (c) Rooted NJ ‐tree (outgroup removed for clarity) based on Tamura–Nei distances (scale bar) of 685 bp COI mitochondrial gene fragments ( n = 60) shows five species of ants (four Lasius and one Brachymyrmex ) , each represented by colour‐coded clades (original COI NJ ‐tree with sample labels and outgroup in Figure S14 in Appendix ).…”

“…In addition, these bacteria are related to Asaia bacteria, recently found in Pseudomyrmex and Tetraponera ants, which both feed on another sugar‐rich diet of extra floral nectar (Kautz, Rubin, & Moreau, ; Samaddar et al., ). Their prevalence among these specific ants, in combination with shown experimental increase in bacterial density with sugar‐rich diets, suggests that Acetobacteraceae aid in the ants’ digestion of sugary honeydew, although more functional work is needed (Hu et al., ). In addition to sugar‐processing abilities, Asaia found in Tetraponera ants has been suggested to play a role in nitrogen fixation (Samaddar et al., ).…”