Weevil Carbohydrate Intake Triggers Endosymbiont Proliferation: A Trade-Off between Host Benefit and Endosymbiont Burden

Microbial bases of herbivory in beetles

The ecological radiation of herbivorous beetles is among the most successful in the animal kingdom. It coincided with the rise and diversification of flowering plants, requiring beetles to adapt to a nutritionally imbalanced diet enriched in complex polysaccharides and toxic secondary metabolites. In this review, we explore how beetles overcame these challenges by coopting microbial genes, enzymes, and metabolites, through both horizontal gene transfer (HGT) and symbiosis. Recent efforts revealed the functional convergence governing both processes and the unique ways in which microbes continue to shape beetle digestion, development, and defense. The development of genetic and experimental tools across a diverse set of study systems has provided valuable mechanistic insights into how microbes spurred metabolic innovation and facilitated an herbivorous transition in beetles.

Differences in the abundance and diversity of endosymbiotic bacteria drive host resistance of Philodromus cespitum, a dominant spider of central European orchards, to selected insecticides

The ability of tissue endosymbionts to degrade and detoxify agrochemicals is increasingly recognized as a mechanism supporting the survival of arthropods in agroecosystems. Therefore, tissue endosymbionts have the potential to drive insecticide resistance in agrobiont spiders, i.e., in major generalist predators and pest control agents within agroecosystems. We hypothesized that the abundance and diversity of the endosymbiotic bacteria of Philodromus cespitum, a philodromid spider dominating central European apple orchards, vary with regard to differences in predation capacity and drive host insecticide resistance. We provisioned P. cespitum with diets of varying protein and lipid content and topically exposed them to field-relevant doses of commonly used insecticides, namely Mospilan (acetamiprid), Movento (spirotetramat), Gondola (sulfoxaflor), Decis (deltamethrin), Coragen (chlorantraniliprole), and Benevia (cyantraniliprole). The analyses were based on 16S rDNA profiles from lysates of the cephalothorax and legs of the tested spiders. The application of Benevia, Mospilan, and Movento was partially lethal. The spiders that were resistant to the treatments with Benevia, Mospilan, or Movento were associated with the increased relative abundance of Mycoplasmatota by more than one order of magnitude. Additionally, the abundance of other bacteria differed in Mospilan-resistant and Mospilan-sensitive individuals. In contrast, the diet regimens were not associated with any major differences in the microbiome diversity nor the diversity of endosymbionts. Philodromus cespitum hosts assemblages with unexpectedly high beta diversity of endosymbionts. The OTU identified as the alpha proteobacterium endosymbiont of Coelostomidia zealandica was an obligate endosymbiont of the analyzed P. cespitum population. Wolbachia, Rickettsia, and Spiroplasma endosymbionts were also highly prevalent and differed in their responses to the applied treatments. In conclusion, differences in the abundance and diversity of endosymbiotic bacteria drove the resistance of the spider host to selected insecticides.

microRNA maintains nutrient homeostasis in the symbiont-host interaction

Endosymbionts provide essential nutrients for hosts, promoting growth, development, and reproduction. However, the molecular regulation of nutrient transport from endosymbiont to host is not well understood. Here, we used bioinformatic analysis, RNA-Sequencing, luciferase assays, RNA immunoprecipitation, and in situ hybridization to show that a bacteriocyte-distributed MRP4 gene (multidrug resistance-associated protein 4) is negatively regulated by a host (aphid)-specific microRNA (miR-3024). Targeted metabolomics, microbiome analysis, vitamin B6 (VB6) supplements, 3D modeling/molecular docking, in vitro binding assays (voltage clamp recording and microscale thermophoresis), and functional complementation of Escherichia coli were jointly used to show that the miR-3024/MRP4 axis controls endosymbiont (Serratia)-produced VB6 transport to the host. The supplementation of miR-3024 increased the mortality of aphids, but partial rescue was achieved by providing an external source of VB6. The use of miR-3024 as part of a sustainable aphid pest-control strategy was evaluated by safety assessments in nontarget organisms (pollinators, predators, and entomopathogenic fungi) using virus-induced gene silencing assays and the expression of miR-3024 in transgenic tobacco. The supplementation of miR-3024 suppresses MRP4 expression, restricting the number of membrane channels, inhibiting VB6 transport, and ultimately killing the host. Under aphids facing stress conditions, the endosymbiont titer is decreased, and the VB6 production is also down-regulated, while the aphid's autonomous inhibition of miR-3024 enhances the expression of MRP4 and then increases the VB6 transport which finally ensures the VB6 homeostasis. The results confirm that miR-3024 regulates nutrient transport in the endosymbiont-host system and is a suitable target for sustainable pest control.

Development, feeding, and sex shape the relative quantity of the nutritional obligatory symbiont Wolbachia in bed bugs

The common bed bug, Cimex lectularius, is a hemipteran insect that feeds only on blood, and whose bites cause public health issues. Due to globalization and resistance to insecticides, this pest has undergone a significant and global resurgence in recent decades. Blood is an unbalanced diet, lacking notably sufficient B vitamins. Like all strict hematophagous arthropods, bed bugs host a nutritional symbiont supplying B vitamins. In C. lectularius, this nutritional symbiont is the intracellular bacterium Wolbachia (wCle). It is located in specific symbiotic organs, the bacteriomes, as well as in ovaries. Experimental depletion of wCle has been shown to result in longer nymphal development and lower fecundity. These phenotypes were rescued by B vitamin supplementation. Understanding the interaction between wCle and the bed bug may help to develop new pest control methods targeting the disruption of this symbiotic interaction. The objective of this work was thus to quantify accurately the density of wCle over the life cycle of the host and to describe potential associated morphological changes in the bacteriome. We also sought to determine the impact of sex, feeding status, and aging on the bacterial population dynamics. We showed that the relative quantity of wCle continuously increases during bed bug development, while the relative size of the bacteriome remains stable. We also showed that adult females harbor more wCle than males and that wCle relative quantity decreases slightly in adults with age, except in weekly-fed males. These results are discussed in the context of bed bug ecology and will help to define critical points of the symbiotic interaction during the bed bug life cycle.

Coordination of host and endosymbiont gene expression governs endosymbiont growth and elimination in the cereal weevil Sitophilus spp

BACKGROUND

Insects living in nutritionally poor environments often establish long-term relationships with intracellular bacteria that supplement their diets and improve their adaptive and invasive powers. Even though these symbiotic associations have been extensively studied on physiological, ecological, and evolutionary levels, few studies have focused on the molecular dialogue between host and endosymbionts to identify genes and pathways involved in endosymbiosis control and dynamics throughout host development.

RESULTS

We simultaneously analyzed host and endosymbiont gene expression during the life cycle of the cereal weevil Sitophilus oryzae, from larval stages to adults, with a particular emphasis on emerging adults where the endosymbiont Sodalis pierantonius experiences a contrasted growth-climax-elimination dynamics. We unraveled a constant arms race in which different biological functions are intertwined and coregulated across both partners. These include immunity, metabolism, metal control, apoptosis, and bacterial stress response.

CONCLUSIONS

The study of these tightly regulated functions, which are at the center of symbiotic regulations, provides evidence on how hosts and bacteria finely tune their gene expression and respond to different physiological challenges constrained by insect development in a nutritionally limited ecological niche. Video Abstract.

Sex-specific differences in symbiotic microorganisms associated with an invasive mealybug (Phenacoccus solenopsis Tinsley) based on 16S ribosomal DNA

The ability of Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) to utilize a wide range of host plants is closely related to the symbiotic bacteria within its body. This study investigated the diversity of symbiotic microorganisms associated with the sap-sucking hemipteran insect. Using deep sequencing of the 16S rDNA gene and subsequent analysis with the Qiime software package, we constructed a comprehensive library of bacterial operational taxonomic units (OTUs). We compared the microbial communities of female and male adult mealybugs. Our results showed significant differences in bacterial composition between the sexes, with Proteobacteria, Firmicutes, and Bacteroidetes being the dominant phyla in both female and male mealybugs. These results suggest that the diverse assemblage of symbiotic bacteria in P. solenopsis may be critical in enabling this insect to utilize a wide range of host plants by facilitating carbohydrate digestion and energy uptake.

Endosymbiont-containing germarium transcriptional survey in a cereal weevil depicts downregulation of immune effectors at the onset of sexual maturity

Insects often establish long-term relationships with intracellular symbiotic bacteria, i.e., endosymbionts, that provide them with essential nutrients such as amino acids and vitamins. Endosymbionts are typically confined within specialized host cells called bacteriocytes that may form an organ, the bacteriome. Compartmentalization within host cells is paramount for protecting the endosymbionts and also avoiding chronic activation of the host immune system. In the cereal weevil Sitophilus oryzae, bacteriomes are present as a single organ at the larval foregut-midgut junction, and in adults, at the apex of midgut mesenteric caeca and at the apex of the four ovarioles. While the adult midgut endosymbionts experience a drastic proliferation during early adulthood followed by complete elimination through apoptosis and autophagy, ovarian endosymbionts are maintained throughout the weevil lifetime by unknown mechanisms. Bacteria present in ovarian bacteriomes are thought to be involved in the maternal transmission of endosymbionts through infection of the female germline, but the exact mode of transmission is not fully understood. Here, we show that endosymbionts are able to colonize the germarium in one-week-old females, pinpointing a potential infection route of oocytes. To identify potential immune regulators of ovarian endosymbionts, we have analyzed the transcriptomes of the ovarian bacteriomes through young adult development, from one-day-old adults to sexually mature ones. In contrast with midgut bacteriomes, immune effectors are downregulated in ovarian bacteriomes at the onset of sexual maturation. We hypothesize that relaxation of endosymbiont control by antimicrobial peptides might allow bacterial migration and potential oocyte infection, ensuring endosymbiont transmission.