A remarkable legion of guests: Diversity and host specificity of army ant symbionts

Microbial symbionts are shared between ants and their associated beetles

The transmission of microbial symbionts across animal species could strongly affect their biology and evolution, but our understanding of transmission patterns and dynamics is limited. Army ants (Formicidae: Dorylinae) and their hundreds of closely associated insect guest species (myrmecophiles) can provide unique insights into interspecific microbial symbiont sharing. Here, we compared the microbiota of workers and larvae of the army ant Eciton burchellii with those of 13 myrmecophile beetle species using 16S rRNA amplicon sequencing. We found that the previously characterized specialized bacterial symbionts of army ant workers were largely absent from ant larvae and myrmecophiles, whose microbial communities were usually dominated by Rickettsia, Wolbachia, Rickettsiella and/or Weissella. Strikingly, different species of myrmecophiles and ant larvae often shared identical 16S rRNA genotypes of these common bacteria. Protein-coding gene sequences confirmed the close relationship of Weissella strains colonizing army ant larvae, some workers and several myrmecophile species. Unexpectedly, these strains were also similar to strains infecting dissimilar animals inhabiting very different habitats: trout and whales. Together, our data show that closely interacting species can share much of their microbiota, and some versatile microbial species can inhabit and possibly transmit across a diverse range of hosts and environments.

You are what your host eats: The trophic structure and food chain length of a symbiont community are coupled with the plastic diet of the host ant

Food chain length provides key information on the flow of nutrients and energy in ecosystems. Variation in food chain length has primarily been explained by environmental drivers such as ecosystem size and productivity. Most insights are obtained from theory or aquatic systems, but the importance of these drivers remains largely untested in terrestrial systems. We exploited red wood ant nests markedly differing in size as natural experiments to quantify the drivers of trophic structure and food chain length of their symbiont arthropod communities. Using stable isotopes, we explored the variation in the trophic positions of four symbiont species with the trophic position of the top predator as a proxy for food chain length of the symbiont community. Nest size did not affect food chain length, nor trophic distance between the symbionts. Instead, food chain length and the trophic positions of the symbionts were strongly affected by the host's foraging decisions. When the host diet shifted from predominantly herbivorous to more predacious, the trophic position of the symbionts and food chain length strongly increased. We show for the first time that a food web can be structured by biotic interactions with an engineering species rather than by abiotic environmental variables.

Army ant middens - Home and nursery of a diverse beetle fauna

Army ants provide nourishment to a large variety of animals. This includes birds that feed on animals flushed out by army ant raids, symbiotic arthropods that consume the ants' prey or their brood, and other arthropods that scavenge on army ant refuse deposits. The latter have not received much attention, and the few published studies lack detailed species identifications. Here we provide a first systematic inventory of the beetle fauna associated with refuse deposits of Eciton army ants, with a focus on Eciton burchellii. We collected 8364 adult beetles, 511 larvae, and 24 eggs from 34 deposits at La Selva Biological Station, Costa Rica. We used a combination of DNA barcoding and morphology to identify a subset of 436 specimens to species level. The samples included several new species, and we here formally describe two water scavenger beetles (Hydrophilidae). Refuse deposits harbored a diverse beetle fauna. The identified subset consisted of 91 beetle species from 12 families, with rove beetles being the most abundant and diverse visitors. Of the 85 species found with E. burchellii, 50 species were collected from only one or two refuse deposits. Conversely, seven species were found in 10 or more refuse deposits, indicating a certain level of habitat specialization. We matched adults and immatures for 22 beetle species via DNA barcodes, demonstrating that army ant middens also serve as a beetle nursery. The present survey highlights the significant ecological function of army ants as promoters of biodiversity and their status as keystone species in tropical rainforests.

Ontogeny of collective behaviour

During their lifetime, superorganisms, like unitary organisms, undergo transformations that change the machinery of their collective behaviour. Here, we suggest that these transformations are largely understudied and propose that more systematic research into the ontogeny of collective behaviours is needed if we hope to better understand the link between proximate behavioural mechanisms and the development of collective adaptive functions. In particular, certain social insects engage in self-assemblage, forming dynamic and physically connected architectures with striking similarities to developing multicellular organisms, making them good model systems for ontogenetic studies of collective behaviour. However, exhaustive time series and three-dimensional data are required to thoroughly characterize the different life stages of the collective structures and the transitions between these stages. The well-established fields of embryology and developmental biology offer practical tools and theoretical frameworks that could speed up the acquisition of new knowledge about the formation, development, maturity and dissolution of social insect self-assemblages and, by extension, other superorganismal behaviours. We hope that this review will encourage an expansion of the ontogenetic perspective in the field of collective behaviour and, in particular, in self-assemblage research, which has far-reaching applications in robotics, computer science and regenerative medicine. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Dynamics of gut microflora across the life cycle of Spodoptera frugiperda and its effects on the feeding and growth of larvae

BACKGROUND

Spodoptera frugiperda is an important invasive agricultural pest that causes huge economic losses worldwide. Gut microorganisms play a vital role in host feeding, digestion, nutrition, immunity, growth and insecticide resistance. Illumina high-throughput sequencing was used to study the gut microbial community dynamics across the life cycle (egg, 1st to 6th instar larvae, pupae, and male and female adults) of S. frugiperda fed on maize leaves. Furthermore, the gut microbial community and food intake of the 5th instar S. frugiperda larvae were studied after feeding them antibiotics.

RESULTS

Enterobacteriaceae and Enterococcaceae dominated the gut during growth and feeding of the larvae. The relative abundance of Enterobacteriaceae was higher in the 4th and 6th instar larvae. With the increase in larval feeding, the relative abundance of Enterococcaceae gradually increased. In addition, principal coordinate analysis and linear discriminant effect size analysis confirmed differences in the structure of gut microbiota at different developmental stages. After antibiotic treatment, the relative abundance of Firmicutes, Proteobacteria and Fusobacteriota decreased. The relative abundance of Enterococcus and Klebsiella decreased significantly. Antibiotic treatment inhibited the gut flora of S. frugiperda, which decreased larval food intake and body weight gain, and prolonged the larval stage.

CONCLUSION

The composition of the gut bacterial community plays an important role in the growth, development, and feeding of S. frugiperda. The results have a certain theoretical value for the development of bio-pesticides targeting intestinal flora. © 2022 Society of Chemical Industry.

Parallel evolutionary paths of rove beetle myrmecophiles: replaying a deep-time tape of life

The rise of ants over the past ~100 million years reshaped the biosphere, presenting ecological challenges for many organisms, but also opportunities. No insect group has been so adept at exploiting niches inside ant colonies as the rove beetles (Staphylinidae) - a global clade of>64,000 predominantly free-living predators from which numerous socially parasitic 'myrmecophile' lineages have emerged. Myrmecophilous staphylinids are specialized for colony life through changes in behavior, chemistry, anatomy, and life history that are often strikingly convergent, and hence potentially adaptive for this symbiotic way of life. Here, we examine how the interplay between ecological pressures and molecular, cellular, and neurobiological mechanisms shape the evolutionary trajectories of symbiotic lineages in this ancient, convergent system.

Comparison of Gut Bacterial Communities of Fall Armyworm (Spodoptera frugiperda) Reared on Different Host Plants

Spodoptera frugiperda is a highly polyphagous and invasive agricultural pest that can harm more than 300 plants and cause huge economic losses to crops. Symbiotic bacteria play an important role in the host biology and ecology of herbivores, and have a wide range of effects on host growth and adaptation. In this study, high-throughput sequencing technology was used to investigate the effects of different hosts (corn, wild oat, oilseed rape, pepper, and artificial diet) on gut microbial community structure and diversity. Corn is one of the most favored plants of S. frugiperda. We compared the gut microbiota on corn with and without a seed coating agent. The results showed that Firmicutes and Bacteroidetes dominated the gut microbial community. The microbial abundance on oilseed rape was the highest, the microbial diversity on wild oat was the lowest, and the microbial diversity on corn without a seed coating agent was significantly higher than that with such an agent. PCoA analysis showed that there were significant differences in the gut microbial community among different hosts. PICRUSt analysis showed that most of the functional prediction categories were related to metabolic and cellular processes. The results showed that the gut microbial community of S. frugiperda was affected not only by the host species, but also by different host treatments, which played an important role in host adaptation. It is important to deepen our understanding of the symbiotic relationships between invasive organisms and microorganisms. The study of the adaptability of host insects contributes to the development of more effective and environmentally friendly pest management strategies.

Multiple phenotypic traits as triggers of host attacks towards ant symbionts: body size, morphological gestalt, and chemical mimicry accuracy

Background

Ant colonies are plagued by a diversity of arthropod guests, which adopt various strategies to avoid or to withstand host attacks. Chemical mimicry of host recognition cues is, for example, a common integration strategy of ant guests. The morphological gestalt and body size of ant guests have long been argued to also affect host hostility, but quantitative studies testing these predictions are largely missing. We here evaluated three guest traits as triggers of host aggression—body size, morphological gestalt, and accuracy in chemical mimicry—in a community of six Eciton army ant species and 29 guest species. We quantified ant aggression towards 314 guests in behavioral assays and, for the same individuals, determined their body size and their accuracy in mimicking ant cuticular hydrocarbon (CHC) profiles. We classified guests into the following gestalts: protective, myrmecoid, staphylinid-like, phorid-like, and larval-shaped. We expected that (1) guests with lower CHC mimicry accuracy are more frequently attacked; (2) larger guests are more frequently attacked; (3) guests of different morphological gestalt receive differing host aggression levels.

Results

Army ant species had distinct CHC profiles and accuracy of mimicking these profiles was variable among guests, with many species showing high mimicry accuracy. Unexpectedly, we did not find a clear relationship between chemical host similarity and host aggression, suggesting that other symbiont traits need to be considered. We detected a relationship between the guests’ body size and the received host aggression, in that diminutive forms were rarely attacked. Our data also indicated that morphological gestalt might be a valuable predictor of host aggression. While most ant-guest encounters remained peaceful, host behavior still differed towards guests in that ant aggression was primarily directed towards those guests possessing a protective or a staphylinid-like gestalt.

Conclusion

We demonstrate that CHC mimicry accuracy does not necessarily predict host aggression towards ant symbionts. Exploitation mechanisms are diverse, and we conclude that, besides chemical mimicry, other factors such as the guests’ morphological gestalt and especially their body size might be important, yet underrated traits shaping the level of host hostility against social insect symbionts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00427-8.