Specialized acquisition behaviors maintain reliable environmental transmission in an insect-microbial mutualism

Dynamic reciprocal morphological changes in insect hosts and bacterial symbionts

Symbiotic interactions, central to most life on Earth, are interwoven associations that vary in intimacy and duration. Some of the most well-known examples of symbioses occur between animals and gut bacteria. These associations lead to physiological integration of host and symbionts. The diversity of microbes within animal hosts can make studying them technically challenging. Thus, most science heavily focuses on the animal side of symbioses, limiting study of the microbial symbionts to characterization of their genetic and functional diversity. These limitations are minimized in Heteropteran insects that have specialized midguts that separately house single symbiont species away from ingested food. These insect-bacteria associations allow us to address fundamental questions regarding how both hosts and symbionts change to establish a cooperative relationship. In this study, through ex vivo and in vivo observations of cellular behaviors, we explore concurrent structural and cellular dynamics in both the squash bug host (Anasa tristis) and its Caballeronia zhejiangensis symbionts during the initiation of symbiosis. We elucidate how C. zhejiangensis is sequestered within a specialized symbiotic organ within the A. tristis midgut, how the symbiont uses active motility to reach the symbiotic organ, how symbionts colonize host crypts within the organ and how host crypt morphogenesis progresses during the initiation of symbiotic interactions. Our findings provide insight into how dynamic cellular activity and morphological development reciprocally change in both host and symbiont as they establish symbiotic interactions.

Acquisition of an obligate environmental symbiont may be limited in the arboreal environment

Many eukaryotic organisms have environmentally acquired microbial symbionts. In animals, microbes commonly occupy the gut and may supply critical nutrients. The leaf-footed bug, Leptoglossus zonatus (Coreidae), is a true bug that is dependent upon ingestion of the free-living, soilborne bacterium Caballeronia early in development for growth and reproduction. In 2019 and 2020, we tested the ability of second instar L. zonatus to acquire Caballeronia in the canopy of pomegranate trees where L. zonatus are often found. We compared the acquisition rate of Caballeronia in nymphs left to forage for the symbiont to bugs fed Caballeronia in advance. Additionally, we aimed to determine whether the microhabitat of potential symbiont sources influenced acquisition success. We hypothesized that the acquisition rate would be heterogeneous among treatments. In 2019, ∼30% of experimental bugs acquired Caballeronia, compared to 75% of those fed the symbiont. In 2020, only about 4% of experimental bugs acquired any symbiont. The symbiont composition of caged bugs differed, and strain diversity was reduced relative to wild bugs. We concluded that Caballeronia is present in the canopy environment, but nymphs may fail to acquire it in the fragments of habitat represented by caged branches, suggesting a cost to host dependency on environmentally acquired symbionts.

Temperature influences outcomes of an environmentally acquired symbiosis

Microbial symbioses are essential for many animals, but their outcomes are often context dependent. For example, rising temperatures can disrupt symbioses by eliminating thermally sensitive symbionts. The temperature tolerance of a symbiont may therefore limit the temperature range of its host, but switching to a more thermally tolerant partner could expand this range. Eastern leaf footed bugs (Leptoglossus phyllopus) depend on symbiotic Caballeronia bacteria which they must acquire from the environment early in development. Could this result in intergenerational partner switching that improves host outcomes under changing conditions? As a first step towards answering this question, we tested the hypothesis that host outcomes in this symbiosis vary among symbiont strains in a temperature-dependent manner. Nymphs were provided with one of six Caballeronia strains with varying thermal optima and reared at temperatures from 24-40°C. We observed temperature- and strain-dependent tradeoffs in host outcomes, with different strains conferring improved host weight, development time, and survival at cooler versus warmer temperatures. Differences in host outcomes were most pronounced at high temperatures, with some strains imposing severe costs. However, Caballeronia's in vitro thermal optima did not predict in vivo outcomes. Regardless, strain- and temperature- dependent outcomes suggest that environmental symbiont acquisition could mitigate the effects of thermal stress on host populations. It is often assumed that vertical transmission of a beneficial symbiont from parent to offspring is the optimal strategy, but our results suggest that environmental acquisition could offer unique benefits under changing conditions.

A strong priority effect in the assembly of a specialized insect-microbe symbiosis

Specialized host-microbe symbioses are ecological communities, whose composition is shaped by various processes. Microbial community assembly in these symbioses is determined in part by interactions between taxa that colonize ecological niches available within habitat patches. The outcomes of these interactions, and by extension the trajectory of community assembly, can display priority effects-dependency on the order in which taxa first occupy these niches. The underlying mechanisms of these phenomena vary from system to system and are often not well resolved. Here, we characterize priority effects in colonization of the squash bug (Anasa tristis) by bacterial symbionts from the genus Caballeronia, using pairs of strains that are known to strongly compete during host colonization, as well as strains that are isogenic and thus functionally identical. By introducing symbiont strains into individual bugs in a sequential manner, we show that within-host populations established by the first colonist are extremely resistant to invasion, regardless of strain identity and competitive interactions. By knocking down the population of an initial colonist with antibiotics, we further show that colonization success by the second symbiont is still diminished even when space in the symbiotic organ is available and ostensibly accessible for colonization. We speculate that resident symbionts exclude subsequent infections by manipulating the host environment, partially but not exclusively by eliciting tissue remodeling of the symbiont organ.

IMPORTANCE

Host-associated microbial communities underpin critical ecosystem processes and human health, and their ability to do so is determined in turn by the various processes that shape their composition. While selection deterministically acts on competing genotypes and species during community assembly, the manner by which selection determines the trajectory of community assembly can differ depending on the sequence by which taxa are established within that community. We document this phenomenon, known as a priority effect, during experimental colonization of a North American insect pest, the squash bug Anasa tristis, by its betaproteobacterial symbionts in the genus Caballeronia. Our study demonstrates how stark, strain-level variation can emerge in specialized host-microbe symbioses simply through differences in the order by which strains colonize the host. Understanding the mechanistic drivers of community structure in host-associated microbiomes can highlight both pitfalls and opportunities for the engineering of these communities and their constituent taxa for societal benefit.

Gut bacteria of lepidopteran herbivores facilitate digestion of plant toxins

Lepidopterans commonly feed on plant material, being the most significant insect herbivores in nature. Despite plant resistance to herbivory, such as producing toxic secondary metabolites, herbivores have developed mechanisms encoded in their genomes to tolerate or detoxify plant defensive compounds. Recent studies also highlight the role of gut microbiota in mediating detoxification in herbivores; however, convincing evidence supporting the significant contribution of gut symbionts is rare in Lepidoptera. Here, we show that the growth of various lepidopteran species was inhibited by a mulberry-derived secondary metabolite, 1-deoxynojirimycin (DNJ); as expected, the specialist silkworm Bombyx mori grew well, but interestingly, gut microbiota of early-instar silkworms was affected by the DNJ level, and several bacterial species responded positively to enriched DNJ. Among these, a bacterial strain isolated from the silkworm gut (Pseudomonas fulva ZJU1) can degrade and utilize DNJ as the sole energy source, and after inoculation into nonspecialists (e.g., beet armyworm Spodoptera exigua), P. fulva ZJU1 increased host resistance to DNJ and significantly promoted growth. We used genomic and transcriptomic analyses to identify genes potentially involved in DNJ degradation, and CRISPR-Cas9-mediated mutagenesis verified the function of ilvB, a key binding protein, in metabolizing DNJ. Furthermore, the ilvB deletion mutant, exhibiting normal bacterial growth, could no longer enhance nonspecialist performance, supporting a role in DNJ degradation in vivo. Therefore, our study demonstrated causality between the gut microbiome and detoxification of plant chemical defense in Lepidoptera, facilitating a mechanistic understanding of host-microbe relationships across this complex, abundant insect group.

Connecting and integrating cooperation within and between species

There has long been a fundamental divide in the study of cooperation: researchers focus either on cooperation within species, including but not limited to sociality, or else on cooperation between species, commonly termed mutualism. Here, we explore the ecologically and evolutionarily significant ways in which within- and between-species cooperation interact. We highlight two primary cross-linkages. First, cooperation of one type can change the context in which cooperation of the other type functions, and thus potentially its outcome. We delineate three possibilities: (i) within-species cooperation modulates benefits for a heterospecific partner; (ii) between-species cooperation affects the dynamics of within-species cooperation; and (iii) both processes take place interactively. The second type of cross-linkage emerges when resources or services that cooperation makes available are obtainable either from members of the same species or from different species. This brings cooperation at the two levels into direct interaction, to some extent obscuring the distinction between them. We expand on these intersections between within- and between-species cooperation in a diversity of taxa and interaction types. These interactions have the potential to weave together social networks and trophic dynamics, contributing to the structure and functioning of ecological communities in ways that are just beginning to be explored. This article is part of the theme issue 'Connected interactions: enriching food web research by spatial and social interactions'.

The burrower bug Macroscytus japonensis (Hemiptera: Cydnidae) acquires obligate symbiotic bacteria from the environment

Many plant-feeding stinkbugs belonging to the infraorder Pentatomomorpha possess a specialized symbiotic organ at the posterior end of the midgut, in which mutualistic bacterial symbionts are harbored extracellularly. In species of the superfamily Pentatomoidea, these symbionts typically are verticallytransmitted from host mothers to offspring, whereas in species of the superfamilies Coreoidea and Lygaeoidea they are acquired from the environment. In the pentatomoid family Cydnidae, vertical symbiont transmission has been reported in several species. Here, we report the first case of environmental symbiont acquisition in Cydnidae, observed in the burrower bug Macroscytus japonensis. A comprehensive survey of 72 insect samples from 23 sites across the Japanese archipelago revealed that (1) symbionts exhibit remarkably high diversity, forming six distinct phylogenetic groups within the Enterobacteriaceae of the γ-Proteobacteria, (2) most symbionts are cultivable and closely related to free-living Pantoea-allied bacteria, and (3) symbiont phylogenetic groups do not reflect the host phylogeny. Microbial inspection of eggs revealed the absence of bacteria on the egg surface. These results strongly suggest that symbionts are acquired from the environment, not vertical transmission. Rearing experiments confirmed environmental symbiont acquisition. When environmental symbiont sources were experimentally withheld, nymphs became aposymbiotic and died before molting to the second instar, indicating that nymphs environmentally acquire symbionts during the first-instar stage and that symbionts are essential for nymphal growth and survival. This study highlights Cydnidae as the only pentatomoid family that includes species that environmentally acquire symbionts and those that vertically transmit symbionts, providing an ideal platform for comparative studies of the ecological and environmental factors that influence the evolution of symbiont transmission modes.

Evolution and the critical role of the microbiota in the reduced mental and physical health associated with low socioeconomic status (SES)

The evolution of the gut-microbiota-brain axis in animals reveals that microbial inputs influence metabolism, the regulation of inflammation and the development of organs, including the brain. Inflammatory, neurodegenerative and psychiatric disorders are more prevalent in people of low socioeconomic status (SES). Many aspects of low SES reduce exposure to the microbial inputs on which we are in a state of evolved dependence, whereas the lifestyle of wealthy citizens maintains these exposures. This partially explains the health deficit of low SES, so focussing on our evolutionary history and on environmental and lifestyle factors that distort microbial exposures might help to mitigate that deficit. But the human microbiota is complex and we have poor understanding of its functions at the microbial and mechanistic levels, and in the brain. Perhaps its composition is more flexible than the microbiota of animals that have restricted habitats and less diverse diets? These uncertainties are discussed in relation to the encouraging but frustrating results of attempts to treat psychiatric disorders by modulating the microbiota.

The role of animal hosts in shaping gut microbiome variation

Millions of years of co-evolution between animals and their associated microbial communities have shaped and diversified the nature of their relationship. Studies continue to reveal new layers of complexity in host-microbe interactions, the fate of which depends on a variety of different factors, ranging from neutral processes and environmental factors to local dynamics. Research is increasingly integrating ecosystem-based approaches, metagenomics and mathematical modelling to disentangle the individual contribution of ecological factors to microbiome evolution. Within this framework, host factors are known to be among the dominant drivers of microbiome composition in different animal species. However, the extent to which they shape microbiome assembly and evolution remains unclear. In this review, we summarize our understanding of how host factors drive microbial communities and how these dynamics are conserved and vary across taxa. We conclude by outlining key avenues for research and highlight the need for implementation of and key modifications to existing theory to fully capture the dynamics of host-associated microbiomes. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Ecological drift during colonization drives within-host and between-host heterogeneity in an animal-associated symbiont

Specialized host-microbe symbioses canonically show greater diversity than expected from simple models, both at the population level and within individual hosts. To understand how this heterogeneity arises, we utilize the squash bug, Anasa tristis, and its bacterial symbionts in the genus Caballeronia. We modulate symbiont bottleneck size and inoculum composition during colonization to demonstrate the significance of ecological drift, the noisy fluctuations in community composition due to demographic stochasticity. Consistent with predictions from the neutral theory of biodiversity, we found that ecological drift alone can account for heterogeneity in symbiont community composition between hosts, even when 2 strains are nearly genetically identical. When acting on competing strains, ecological drift can maintain symbiont genetic diversity among different hosts by stochastically determining the dominant strain within each host. Finally, ecological drift mediates heterogeneity in isogenic symbiont populations even within a single host, along a consistent gradient running the anterior-posterior axis of the symbiotic organ. Our results demonstrate that symbiont population structure across scales does not necessarily require host-mediated selection, as it can emerge as a result of ecological drift acting on both isogenic and unrelated competitors. Our findings illuminate the processes that might affect symbiont transmission, coinfection, and population structure in nature, which can drive the evolution of host-microbe symbioses and microbe-microbe interactions within host-associated microbiomes.

Coconut rhinoceros beetle digestive symbiosis with potential plant cell wall degrading microbes

Coconut rhinoceros beetle (CRB, Oryctes rhinoceros) is an invasive palm pest whose larvae eat wood, yet lack the necessary digestive enzymes. This study confirmed endogenous CRB cellulase is inactive, suggesting microbial fermentation. The inner lining of the CRB hindgut has tree-like structures covered with a conspicuous biofilm. To identify possible symbionts, 16 S rRNA amplicon sequencing was used on individuals from across Taiwan. Several taxa of Clostridia, an anaerobic class including many cellulolytic bacteria, were highly abundant in most individuals from all locations. Whole metagenome sequencing further confirmed many lignocellulose degrading enzymes are derived from these taxa. Analyses of eggs, larvae, adults, and soil found these cellulolytic microbes are not transmitted vertically or transstadially. The core microbiomes of the larval CRB are likely acquired and enriched from the environment with each molt, and enable efficient digestion of wood.

Extracellular symbiont colonizes insect during embryo development

Insects typically acquire their beneficial microbes early in development. Endosymbionts housed intracellularly are commonly integrated during oogenesis or embryogenesis, whereas extracellular microbes are only known to be acquired after hatching by immature instars such as larvae or nymphs. Here, however, we report on an extracellular symbiont that colonizes its host during embryo development. Tortoise beetles (Chrysomelidae: Cassidinae) host their digestive bacterial symbiont Stammera extracellularly within foregut symbiotic organs and in ovary-associated glands to ensure its vertical transmission. We outline the initial stages of symbiont colonization and observe that although the foregut symbiotic organs develop 3 days prior to larval emergence, they remain empty until the final 24 h of embryo development. Infection by Stammera occurs during that timeframe and prior to hatching. By experimentally manipulating symbiont availability to embryos in the egg, we describe a 12-h developmental window governing colonization by Stammera. Symbiotic organs form normally in aposymbiotic larvae, demonstrating that these Stammera-bearing structures develop autonomously. In adults, the foregut symbiotic organs are already colonized following metamorphosis and host a stable Stammera population to facilitate folivory. The ovary-associated glands, however, initially lack Stammera. Symbiont abundance subsequently increases within these transmission organs, thereby ensuring sufficient titers at the onset of oviposition ~29 days following metamorphosis. Collectively, our findings reveal that Stammera colonization precedes larval emergence, where its proliferation is eventually decoupled in adult beetles to match the nutritional and reproductive requirements of its host.