The red flour beetle Tribolium castaneum: A model for host-microbiome interactions

C-Type Lectin S Group TcCTL4 Participates in the Immunity of Tribolium castaneum

C-type lectin S (CTL-S) plays a crucial role in pathogen recognition and the activation of immune response. In comparison, the proportion of CTL-S was relatively high in insects, but the study was much smaller than the proportion observed. In this study, we cloned and characterized one CTL-S, TcCTL4, from Tribolium castaneum. Our analysis revealed that TcCTL4 was highly expressed during the early pupal stage, with expression levels exhibiting a tendency to change with developmental stages. Additionally, tissue expression analysis showed a high expression of TcCTL4 in the central nervous system (CNS). Moreover, we observed a significant increase in TcCTL4 transcripts after bacterial challenge. The RNA interference (RNAi) of TcCTL4 before bacterial treatment resulted in a significant reduction in the transcripts of immune factors (IFs) and antimicrobial peptides (AMPs), indicating that TcCTL4 may regulate AMP expression through the activation of the immune signaling pathway. Furthermore, our investigation revealed that the recombinant protein TcCTL4 (rTcCTL4) not only recognized bacteria but also agglutinated bacteria in a Ca2+-dependent manner. Enzyme activity analysis suggested that rTcCTL4 could enhance phenoloxidase activity, implying its potential involvement in the prophenoloxidase activation pathway. In conclusion, these results indicate that TcCTL4 is involved in the immunity of T. castaneum, providing valuable insights into insect CTLs.

Alteration of diet microbiota limits the experimentally evolved immune priming response in flour beetles, but not pathogen resistance

Host-associated microbiota play a fundamental role in the training and induction of different forms of immunity, including inducible as well as constitutive components. However, direct experiments analysing the relative importance of microbiota on diverse forms of evolved immune functions are missing. We addressed this gap by using experimentally evolved lines of Tribolium castaneum that either produced inducible immune memory-like responses (immune priming) or constitutively expressed basal resistance (without priming), as divergent counterstrategies against Bacillus thuringiensis infection. We altered the microbial communities present in the diet (i.e. wheat flour) of these evolved lines using UV irradiation and estimated the impact on the beetle's ability to mount a priming response versus basal resistance. Populations that had evolved immune priming lost the ability to mount a priming response upon alteration of diet microbiota. Microbiota manipulation also caused a drastic reduction in their reproductive output and post-infection longevity. In contrast, in pathogen-resistant beetles, microbiota manipulation did not affect post-infection survival or reproduction. The divergent evolution of immune responses across beetle lines was thus associated with divergent reliance on the microbiome. Whether the latter is a direct outcome of differential pathogen exposure during selection or reflects evolved immune functions remains unclear. We hope that our results will motivate further experiments to understand the mechanistic basis of these complex evolutionary associations between microbiota, host immune strategies and fitness outcomes.

Insect Models in Nutrition Research

Insects are the most diverse organisms on earth, accounting for ~80% of all animals. They are valuable as model organisms, particularly in the context of genetics, development, behavior, neurobiology and evolutionary biology. Compared to other laboratory animals, insects are advantageous because they are inexpensive to house and breed in large numbers, making them suitable for high-throughput testing. They also have a short life cycle, facilitating the analysis of generational effects, and they fulfil the 3R principle (replacement, reduction and refinement). Many insect genomes have now been sequenced, highlighting their genetic and physiological similarities with humans. These factors also make insects favorable as whole-animal high-throughput models in nutritional research. In this review, we discuss the impact of insect models in nutritional science, focusing on studies investigating the role of nutrition in metabolic diseases and aging/longevity. We also consider food toxicology and the use of insects to study the gut microbiome. The benefits of insects as models to study the relationship between nutrition and biological markers of fitness and longevity can be exploited to improve human health.

Oral Immune Priming Treatment Alters Microbiome Composition in the Red Flour Beetle Tribolium castaneum

It is now well-established that the microbiome is relevant for many of an organism’s properties and that its composition reacts dynamically to various conditions. The microbiome interacts with host immunity and can play important roles in the defenses against pathogens. In invertebrates, immune priming, that is, improved survival upon secondary exposure to a previously encountered pathogen, can be dependent upon the presence of the gut microbiome. However, it is currently unknown whether the microbiome changes upon priming treatment. We here addressed this question in a well-established model for immune priming, the red flour beetle Tribolium castaneum exposed to the entomopathogenic bacterium Bacillus thuringiensis (Bt). After priming treatments, the microbiota composition of beetle larvae was assessed by deep sequencing of the V1-V2 region of the bacterial 16S rRNA gene. We compared the effect of two established routes of priming treatments in this system: injection priming with heat-killed Bt and oral priming via ingestion of filtered sterilized bacterial spore culture supernatants. For oral priming, we used several strains of Bt known to vary in their ability to induce priming. Our study revealed changes in microbiome composition following the oral priming treatment with two different strains of Bt, only one of which (Bt tenebrionis, Btt) is known to lead to improved survival. In contrast, injection priming treatment with the same bacterial strain did not result in microbiome changes. Combined with the previous results indicating that oral priming with Btt depends on the larval microbiome, this suggests that certain members of the microbiome could be involved in forming an oral priming response in the red flour beetle.

Susceptibility of Tribolium castaneum (Coleoptera: Tenebrionidae) to the Fumigation of Two Essential Satureja Oils: Optimization and Modeling

Due to the numerous side effects of synthetic pesticides, including environmental pollution, threats to human health, harmful effects on non-target organisms and pest resistance, the use of alternative healthy, available and efficient agents in pest management strategies is necessary. In this paper, the susceptibility of the cosmopolitan, polyphagous, stored-product pest Tribolium castaneum (red flour beetle) to the fumigation of the essential oils of two important medicinal and food additive plants, Satureja hortensis and S. intermedia, was investigated. The insecticidal properties of the essential oils were modeled and optimized using response surface methodology. It was found that a maximum significant mortality of 94.72% and 92.97% could be achieved within 72 h with the applications of 55.15 µL/L of S. hortensis (with the linear model) and 58.82 µL/L of S. intermedia (with the quadratic model), respectively. There were insecticidal terpenes and phenylpropanoids in both essential oils, including thymol (50.8%), carvacrol (11.2%) and p-cymene (13.4%), in the S. intermedia and estragole (68.0%) and methyl eugenol (5.6%) in the S. hortensis. It was suggested that the essential oils of S. hortensis and S. intermedia could be offered as promising pesticidal agents against T. castaneum for further studies in the management of such pests instead of detrimental synthetic pesticides.