The midgut of the silkmoth Bombyx mori is able to recycle molecules derived from degeneration of the larval midgut epithelium

Anatomical changes of Tenebrio molitor and Tribolium castaneum during complete metamorphosis

In holometabolous insects, extensive reorganisation of tissues and cells occurs at the pupal stage. The remodelling of the external exoskeleton and internal organs that intervenes during metamorphosis has been traditionally studied in many insect species based on histological or ultrastructural methods. This study demonstrates the use of synchrotron X-ray phase-contrast micro-computed tomography as a powerful, non-destructive tool for in situ morphological observation of anatomical structures at the pupal stage in two Tenebrionid beetles, i.e. Tribolium castaneum and Tenebrio molitor, known as important pests, as well as emerging and promising models in experimental biology. Virtual sections and three-dimensional reconstructions were performed on both males and females at early, intermediate, and late pupal stage. The dataset allowed us to observe the remodelling of the gut and nervous system as well as the shaping of the female and male reproductive system at different pupal ages in both mealworm and red flour beetles. Moreover, we observed that the timing and duration pattern of organ development varied between the species analysed, likely related to the species-specific adaptations of the pre-imaginal stages to environmental conditions, which ultimately affect their life cycle. This research provides new knowledge on the morphological modifications that occur during the pupal stage of holometabolous insects and provides a baseline set of information on beetle metamorphosis that may support future research in forensics, physiology, and ecology as well as an image atlas for educational purposes.

Distinct enzyme activities of serine protease p37k in silkworm midgut and molting fluid

Serine proteases possess various biological functions. The serine protease p37k exhibits gelatinolytic activity in the silkworm midgut and degrades cuticular proteins in the molting fluid. In this study, we analyzed the activity changes of recombinant p37k (re-p37k) and p37k in the midgut and molting fluid of Bombyx mori. Firstly, in vitro-expressed re-p37k was activated when a 22 kDa band was observed by western blot. Re-p37k exhibits strong gelatinolytic activity, with the highest activity observed at pH 7.0-9.0 and 45 °C. Compared to p37k in the midgut, re-p37k loses thermal stability but can be restored by midgut extract or ions. E64, AEBSF, and an inhibitor cocktail inhibited the hydrolytic activity of re-p37k on epidermal proteins but did not inhibit the gelatinolytic activity. Subsequently, zymography showed that the positions of gelatinolytic band produced by p37k in the midgut and molting fluid were different, 35 kDa and 40 kDa, respectively. Finally, when heated midgut extract was added to re-p37k or molting fluid, the gelatinolytic band shifted from 40 kDa to 35 kDa, and the proteolytic activity of p37k in the molting fluid was inhibited. Collectively, our results demonstrate that p37k exhibits different activities in various tissues, suggesting its distinct tissue-specific functions during insect metamorphosis.

Insights into midgut cell types and their crucial role in antiviral immunity in the lepidopteran model Bombyx mori

The midgut, a vital component of the digestive system in arthropods, serves as an interface between ingested food and the insect's physiology, playing a pivotal role in nutrient absorption and immune defense mechanisms. Distinct cell types, including columnar, enteroendocrine, goblet and regenerative cells, comprise the midgut in insects and contribute to its robust immune response. Enterocytes/columnar cells, the primary absorptive cells, facilitate the immune response through enzyme secretions, while regenerative cells play a crucial role in maintaining midgut integrity by continuously replenishing damaged cells and maintaining the continuity of the immune defense. The peritrophic membrane is vital to the insect's innate immunity, shielding the midgut from pathogens and abrasive food particles. Midgut juice, a mixture of digestive enzymes and antimicrobial factors, further contributes to the insect's immune defense, helping the insect to combat invading pathogens and regulate the midgut microbial community. The cutting-edge single-cell transcriptomics also unveiled previously unrecognized subpopulations within the insect midgut cells and elucidated the striking similarities between the gastrointestinal tracts of insects and higher mammals. Understanding the intricate interplay between midgut cell types provides valuable insights into insect immunity. This review provides a solid foundation for unraveling the complex roles of the midgut, not only in digestion but also in immunity. Moreover, this review will discuss the novel immune strategies led by the midgut employed by insects to combat invading pathogens, ultimately contributing to the broader understanding of insect physiology and defense mechanisms.

Proteomic Analysis of the Midgut Contents of Silkworm in the Pupal Stage

The silkworm Bombyx mori, a lepidopteran insect, possesses an 8-10-day pupal stage, during which significant changes occur in the midgut, where it first condenses into the yellow body, and then undergoes decomposition. To gain insights into this transformation process, proteomics was performed on Bombyx mori midgut contents on day 2 and day 7 after pupation. The results revealed the identification of 771 proteins with more than one unique peptide. An analysis using AgriGO demonstrated that these proteins were predominantly associated with catalytic activity. Among the identified proteins, a considerable number were found to be involved in carbohydrate metabolism, amino acid metabolism, lipid metabolism, nucleic acid degradation, and energy support. Additionally, variations in the levels of certain proteases were observed between the midgut contents on day 2 and day 7 after pupation. An in-depth analysis of the two-dimensional electrophoresis of the midgut contents on day 7 after pupation led to the identification of twelve protein spots with potential gelatinolytic activity. Among these, six proteases were identified through mass spectrometry, including the p37k protease, vitellin-degrading protease, chymotrypsin-2, etc. These proteases may be responsible for the digestion of the yellow body during the later stages of pupal development.

Investigation of the effects of starvation stress in the midgut of the silkworm Bombyx mori

During their evolution, organisms have developed various mechanisms to adapt to changing nutritional conditions such as mobilization of storage molecules and activation of autophagy. In this study, the mechanism of adaptive responses in the midgut of the silkworm Bombyx mori L., 1758 (Lepidoptera: Bombycidae) larvae, which were starved for different days, was investigated. The study was carried out at the Insect Physiology Research Laboratory and Silkworm Culture Laboratory at Ege University between 2018 and 2020. For this purpose, the histological structure of the midgut was examined using hematoxylin&eosin staining and its protein, sugar, glycogen, and lipid contents were determined. As autophagy markers, lysosomal enzyme activities were measured and expressions of autophagy-related genes (mTOR, ATG8, and ATG12) were analyzed by qRT-PCR. The results showed that, depending on the time of onset of starvation stress, autophagy plays no role as an adaptive response under starvation conditions or occurs at a much more moderate level than autophagy which happens as part of cell death during larval-pupal metamorphosis.

Crosstalk between the microbiota and insect postembryonic development

Insect sequential development evolves from a simple molt towards complete metamorphosis. Like any multicellular host, insects interact with a complex microbiota. In this review, factors driving the microbiota dynamics were pointed out along their development. Special focus was put on tissue renewal, shift in insect ecology, and microbial interactions. Conversely, how the microbiota modulates its host development through nutrient acquisition, hormonal control, and cellular or tissue differentiation was exemplified. Such modifications might have long-term carry-over effects on insect physiology. Finally, remarkable microbe-driven control of insect behaviors along their life cycle was highlighted. Increasing knowledge of those interactions might offer new insights on how insects respond to their environment as well as perspectives on pest- or vector-control strategies.

Autophagy Is Required to Sustain Increased Intestinal Cell Proliferation during Phenotypic Plasticity Changes in Honey Bee (Apis mellifera)

Tissue phenotypic plasticity facilitates rapid adaptation of organisms to biotic and/or abiotic pressure. The reproductive capacity of honey bee workers (Apis mellifera) is plastic and responsive to pheromones produced by broods and the queen. Egg laying workers (ELWs), which could reactivate their ovaries and lay haploid eggs upon queen lost, have been commonly discussed from many aspects. However, it remains unclear whether midgut homeostasis in ELWs is affected during plastic changes. Here, we found that the expression of nutrition- and autophagy-related genes was up-regulated in the midguts of ELWs, compared with that in nurse workers (NWs) by RNA-sequencing. Furthermore, the area and number of autophagosomes were increased, along with significantly increased cell death in the midguts of ELWs. Moreover, cell cycle progression in the midguts of ELWs was increased compared with that in NWs. Consistent with the up-regulation of nutrition-related genes, the body and midgut sizes, and the number of intestinal proliferation cells of larvae reared with royal jelly (RJ) obviously increased more than those reared without RJ in vitro. Finally, cell proliferation was dramatically suppressed in the midguts of ELWs when autophagy was inhibited. Altogether, our data suggested that autophagy was induced and required to sustain cell proliferation in ELWs' midguts, thereby revealing the critical role of autophagy played in the intestines during phenotypic plasticity changes.

A Bombyx mori Infection Model for Screening Antibiotics against Staphylococcus epidermidis

The increasing number of microorganisms that are resistant to antibiotics is prompting the development of new antimicrobial compounds and strategies to fight bacterial infections. The use of insects to screen and test new drugs is increasingly considered a promising tool to accelerate the discovery phase and limit the use of mammalians. In this study, we used for the first time the silkworm, Bombyx mori, as an in vivo infection model to test the efficacy of three glycopeptide antibiotics (GPAs), against the nosocomial pathogen Staphylococcus epidermidis. To reproduce the human physiological temperature, the bacterial infection was performed at 37 °C and it was monitored over time by evaluating the survival rate of the larvae, as well the response of immunological markers (i.e., activity of hemocytes, activation of the prophenoloxidase system, and lysozyme activity). All the three GPAs tested (vancomycin, teicoplanin, and dalbavancin) were effective in curing infected larvae, significantly reducing their mortality and blocking the activation of the immune system. These results corroborate the use of this silkworm infection model for the in vivo studies of antimicrobial molecules active against staphylococci.

The insecticidal capacity of ethanol extract from Cascabela peruviana (L.) Lippold against fruit fly

Cascabela peruviana (L.) Lippold (C. peruviana) has been extensively used for its antifungal and antibacterial properties. However, its role in anti-insect is still under investigation. To investigate the ability of the ethanol extract of C. peruviana against insects, we used the fruit fly (Drosophila melanogaster) as a model to gain more insight into the toxic effects of this extract. We found that the ethanol extract from the stem and leaves of C. peruviana was effective against insects and contained polyphenol and flavonoid compounds. C. peruviana could induce mortality of 2^nd-instar larvae and reduce growth and reproduction of fruit flies. Interestingly, the toxicity of C. peruviana extract has been remained to affect the development of the next generation of fruit flies. The locomotor activity and feeding ability of the F1 generation of this insect were significantly reduced by C. peruviana. In addition, flavonoids and polyphenols, as well as saponins and tannins were detected in the ethanol extract of C. peruviana. We assume that the ability of the extract of C. peruviana to control insects may be related to the presence of high levels of these compounds. The findings highlighted that the extract from the leaves of Cascabela peruviana has the potential to be used as an insecticide.

A food-ingested sublethal concentration of thiamethoxam has harmful effects on the stingless bee Melipona scutellaris

In recent years, the importance of bee's biodiversity in the Neotropical region has been evidencing the relevance of including native bees in risk assessments. Therefore, the sublethal effects of the insecticide thiamethoxam on the survival and morphological parameters of the stingless bee Melipona scutellaris were investigated in the present study. Cells from both non-target organs (Malpighian tubules and midgut) and target organs (brain) were analyzed for morphological alterations using light microscopy and transmission electron microscopy. The findings showed that when M. scutellaris foragers were exposed to a sublethal concentration of thiamethoxam (LC_50/100 = 0.000543 ng a. i./μL), longevity was not reduced but brain function was affected, even with the non-target organs attempting to detoxify. The cellular damage in all the organs was mostly reflected in irregular nuclei shape and condensed chromatin, indicating cell death. The most frequent impairments in the Malpighian tubules were loss of microvilli, disorganization of the basal labyrinth, and cytoplasmic loss. These characteristics are related to an attempt by the cells to increase the excretion process, probably because of the high number of toxic molecules that reach the Malpighian tubules and need to be secreted. In general, damages that compromise the absorption of nutrients, excretion, memory, and learning processes, which are essential for the survival of M. scutellaris, were found. The present results also fill in gaps on how these bees respond to thiamethoxam exposure and will be useful in future risk assessments for the conservation of bee biodiversity.

Haemocyte-mediated immunity in insects: Cells, processes and associated components in the fight against pathogens and parasites

The host defence of insects includes a combination of cellular and humoral responses. The cellular arm of the insect innate immune system includes mechanisms that are directly mediated by haemocytes (e.g., phagocytosis, nodulation and encapsulation). In addition, melanization accompanying coagulation, clot formation and wound healing, nodulation and encapsulation processes leads to the formation of cytotoxic redox-cycling melanin precursors and reactive oxygen and nitrogen species. However, demarcation between cellular and humoral immune reactions as two distinct categories is not straightforward. This is because many humoral factors affect haemocyte functions and haemocytes themselves are an important source of many humoral molecules. There is also a considerable overlap between cellular and humoral immune functions that span from recognition of foreign intruders to clot formation. Here, we review these immune reactions starting with the cellular mechanisms that limit haemolymph loss and participate in wound healing and clot formation and advancing to cellular functions that are critical in restricting pathogen movement and replication. This information is important because it highlights that insect cellular immunity is controlled by a multilayered system, different components of which are activated by different pathogens or during the different stages of the infection.

Sequential changes in the regulatory mechanism of carbohydrate digestion in larvae of the silkworm, Bombyx mori

Nutritional signals strictly control post-embryonic development in insects. Dietary carbohydrates are hydrolyzed to monosaccharides in the gut and then transported into the hemolymph. These monosaccharides in hemolymph are rapidly taken up by tissues and utilized in glycolysis, the pentose phosphate shunt, and glycogen or trehalose synthesis. These metabolic pathways are essential for nutrient metabolism; therefore, the control of carbohydrate digestion is indispensable for maintaining energy supply during development. Carbohydrate digestion was believed to be controlled by dietary mechanisms. We previously reported that hormonal and developmental controls participate in the regulation of carbohydrate digestion during larval-pupal metamorphosis. However, it is unclear whether this regulatory mechanism also works during larval-larval molting and inter-molt feeding period. Here, we show that control mechanisms of the carbohydrate digestion show sequential changes that are controlled by different mechanisms. In the penultimate larval instar, carbohydrate hydrolysis activity changed depending on developmental progress and dietary state. Maltose- and sucrose-hydrolysis activity were suppressed by ecdysteroid, an insect steroid hormone. During the inter-molt feeding period, carbohydrate hydrolysis activities were grouped as either nutrient-sensitive or nutrient-insensitive. Although the activity in both groups was suppressed by ecdysteroid, this hormonal regulatory machinery remains in an "off-state" because ecdysteroid is scarce during the feeding period, suggesting that the carbohydrate digestion system is exclusively regulated by the dietary state during inter-molt feeding period.

Black Soldier Fly Larvae Adapt to Different Food Substrates through Morphological and Functional Responses of the Midgut

Modulation of nutrient digestion and absorption is one of the post-ingestion mechanisms that guarantees the best exploitation of food resources, even when they are nutritionally poor or unbalanced, and plays a pivotal role in generalist feeders, which experience an extreme variability in diet composition. Among insects, the larvae of black soldier fly (BSF), Hermetia illucens, can grow on a wide range of feeding substrates with different nutrient content, suggesting that they can set in motion post-ingestion processes to match their nutritional requirements. In the present study we address this issue by investigating how the BSF larval midgut adapts to diets with different nutrient content. Two rearing substrates were compared: a nutritionally balanced diet for dipteran larvae and a nutritionally poor diet that mimics fruit and vegetable waste. Our data show that larval growth performance is only moderately affected by the nutritionally poor diet, while differences in the activity of digestive enzymes, midgut cell morphology, and accumulation of long-term storage molecules can be observed, indicating that diet-dependent adaptation processes in the midgut ensure the exploitation of poor substrates. Midgut transcriptome analysis of larvae reared on the two substrates showed that genes with important functions in digestion and absorption are differentially expressed, confirming the adaptability of this organ.

Effects of short- and long-term exposure to cadmium on salivary glands and fat body of soil centipede Lithobius forficatus (Myriapoda, Chilopoda): Histology and ultrastructure

Cadmium (Cd) is the most widely studied heavy metal in terms of food-chain accumulation and contamination because it can strongly affect all environments (e.g., soil, water, air). It can accumulate in different tissues and organs and can affect the organism at different levels of organization: from organs, tissues and cells though cell organelles and structures to activation of mechanisms of survival and cell death. In soil-dwelling organisms heavy metals gather in all tissues with accumulation properties: midgut, salivary glands, fat body. The aim of this study was to describe the effects of cadmium on the soil species Lithobius forficatus, mainly on two organs responsible for gathering different substances, the fat body and salivary glands, at the ultrastructural level. Changes caused by cadmium short- and long-term intoxication, connected with cell death (autophagy, apoptosis, necrosis), and the crosstalk between them, were analyzed. Adult specimens of L. forficatus were collected in a natural environment and divided into three experimental groups: C (the control group), Cd1 (cultured in soil with 80 mg/kg of CdCl₂ for 12 days) and Cd2 (cultured in soil with 80 mg/kg of CdCl₂ for 45 days). Transmission electron microscopy revealed ultrastructural alterations in both of the organs analyzed (reduction in the amount of reserve material, the appearance of vacuoles, etc.). Qualitative analysis using TUNEL assay revealed distinct crosstalk between autophagy and necrosis in the fat body adipocytes, while crosstalk between autophagy, apoptosis and necrosis in the salivary glands was detected in salivary glands of the centipedes examined here. We conclude that different organs in the body can react differently to the same stressor, as well as to the same concentration and time of exposure. Different mechanisms at the ultrastructural level activate different types of cell death and with different dynamics.

Mechanisms of programmed cell death in the midgut and salivary glands from Bradysia hygida (Diptera: Sciaridae) during pupal-adult metamorphosis

Programmed cell death is involved with the degeneration/remodeling of larval tissues and organs during holometabolous development. The midgut is a model to study the types of programmed cell death associated with metamorphosis because its structure while degenerating is a substrate for the formation of the adult organ. Another model is the salivary glands from dipteran because their elimination involves different cell death modes. This study aimed to investigate the models of programmed cell death operating during midgut replacement and salivary gland histolysis in Bradysia hygida. We carried out experiments of real-time observations, morphological analysis, glycogen detection, filamentous-actin localization, and nuclear acridine orange staining. Our findings allow us to establish that an intact actin cytoskeleton is required for midgut replacement in B. hygida and nuclear condensation and acridine orange staining precede the death of the larval cells. Salivary glands in histolysis present cytoplasmic blebbing, nuclear retraction, and acridine orange staining. This process can be partially reproduced in vitro. We propose that the larval midgut death involves autophagic and apoptotic features and apoptosis is a mechanism involved with salivary gland histolysis.

A Silkworm Infection Model for In Vivo Study of Glycopeptide Antibiotics

Glycopeptide antibiotics (GPAs) are drugs of last resort for treating infections by Gram-positive bacteria. They inhibit bacterial cell wall assembly by binding to the d-Ala-d-Ala terminus of peptidoglycan precursors, leading to cell lysis. Vancomycin and teicoplanin are first generation GPAs, while dalbavancin is one of the few, recently approved, second generation GPAs. In this paper, we developed an in vivo insect model to compare, for the first time, the efficacy of these three GPAs in curing Staphylococcus aureus infection. Differently from previous reports, Bombyx mori larvae were reared at 37 °C, and the course of infection was monitored, following not only larval survival, but also bacterial load in the insect body, hemocyte activity, phenoloxidase activity, and antimicrobial peptide expression. We demonstrated that the injection of S. aureus into the hemolymph of B. mori larvae led to a marked reduction of their survival rate within 24-48 hours. GPAs were not toxic to the larvae and cured S. aureus infection. Dalbavancin was more effective than first generation GPAs. Due to its great advantages (i.e., easy and safe handling, low rearing costs, low antibiotic amount needed for the tests, no restrictions imposed by ethical and regulatory issues), this silkworm infection model could be introduced in preclinical phases-prior to the use of mice-accelerating the discovery/development rate of novel GPAs.

Immune gene regulation in the gut during metamorphosis in a holo- versus a hemimetabolous insect

During metamorphosis, holometabolous insects completely replace the larval gut and must control the microbiota to avoid septicaemia. Rapid induction of bactericidal activity in the insect gut at the onset of pupation has been described in numerous orders of the Holometabola and is best-studied in the Lepidoptera where it is under control of the 20-hydroxyecdysone (20E) moulting pathway. Here, using RNAseq, we compare the expression of immune effector genes in the gut during metamorphosis in a holometabolous (Galleria mellonella) and a hemimetabolous insect (Gryllus bimaculatus). We find that in G. mellonella, the expression of numerous immune effectors and the transcription factor GmEts are upregulated, with peak expression of three antimicrobial peptides (AMPs) and a lysozyme coinciding with delamination of the larval gut. By contrast, no such upregulation was detectable in the hemimetabolous Gr. bimaculatus. These findings support the idea that the upregulation of immune effectors at the onset of complete metamorphosis is an adaptive response, which controls the microbiota during gut replacement. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Cell death during complete metamorphosis

In insects that undergo complete metamorphosis, cell death is essential for reshaping or removing larval tissues and organs, thus contributing to formation of the adult's body structure. In the last few decades, the study of metamorphosis in Lepidoptera and Diptera has provided broad information on the tissue remodelling processes that occur during larva-pupa-adult transition and made it possible to unravel the underlying regulatory pathways. This review summarizes recent knowledge on cell death mechanisms in Lepidoptera and other holometabolous insects, highlighting similarities and differences with Drosophila melanogaster, and discusses the role of apoptosis and autophagy in this developmental setting. This article is part of the theme issue 'The evolution of complete metamorphosis'.

A First Attempt to Produce Proteins from Insects by Means of a Circular Economy

Simple Summary

Protein production for animal husbandry is a crucial ecological problem because of its impact on the environment, as it requires water, energy, and land. These resources are limited and not reusable. In this study, we obtained a continuously regenerating system in which by-products of a process constituted rough material for another one. Leftovers from fruit and vegetable markets were employed as rearing substrate for insects (Black Soldier Fly). Insect biomass was transformed into meal and oil for fish feeding and food/pharmaceutical industry, respectively. The residuals from insect rearing were then used as substrate to grow earthworms, which transformed this material into compost for plants. Therefore, we returned to the starting point of our economic and ecological closed loop, i.e., to soil improvers (nutrient material) for fruit and vegetable production. Moreover, earthworms can be conveniently employed as fishing bites. We also studied a series of physiological parameters of the living organisms involved in this system to verify their health conditions (insects), and growth performances (insects and fish). Microbiological analyses of insects, rearing substrate, and insect meal were conducted to assess their safety for fish and humans. Related technological processes, such as insects drying, grinding, and oil extraction, were also tested.

Abstract

The worldwide growing consumption of proteins to feed humans and animals has drawn a considerable amount of attention to insect rearing. Insects reared on organic wastes and used as feed for monogastric animals can reduce the environmental impact and increase the sustainability of meat/fish production. In this study, we designed an environmentally closed loop for food supply in which fruit and vegetable waste from markets became rearing substrate for Hermetia illucens (BSF— black soldier fly). A vegetable and fruit-based substrate was compared to a standard diet for Diptera in terms of larval growth, waste reduction index, and overall substrate degradation. Morphological analysis of insect organs was carried out to obtain indications about insect health. Processing steps such as drying and oil extraction from BSF were investigated. Nutritional and microbiological analyses confirmed the good quality of insects and meal. The meal was then used to produce fish feed and its suitability to this purpose was assessed using trout. Earthworms were grown on leftovers of BSF rearing in comparison to a standard substrate. Chemical analyses of vermicompost were performed. The present research demonstrates that insects can be used to reduce organic waste, increasing at the same time the sustainability of aquaculture and creating interesting by-products through the linked bio-system establishment.

Structural and Functional Characterization of Hermetia illucens Larval Midgut

The larvae of Hermetia illucens are among the most promising agents for the bioconversion of low-quality biomass, such as organic waste, into sustainable and nutritionally valuable proteins for the production of animal feed. Despite the great interest in this insect, the current literature provides information limited to the optimization of rearing methods for H. illucens larvae, with particular focus on their efficiency in transforming different types of waste and their nutritional composition in terms of suitability for feed production. Surprisingly, H. illucens biology has been neglected and a deep understanding of the morphofunctional properties of the larval midgut, the key organ that determines the extraordinary dietary plasticity of this insect, has been completely overlooked. The present study aims to fill this gap of knowledge. Our results demonstrate that the larval midgut is composed of distinct anatomical regions with different luminal pH and specific morphofunctional features. The midgut epithelium is formed by different cell types that are involved in nutrient digestion and absorption, acidification of the lumen of the middle region, endocrine regulation, and growth of the epithelium. A detailed characterization of the activity of enzymes involved in nutrient digestion and their mRNA expression levels reveals that protein, carbohydrate, and lipid digestion is associated to specific regions of this organ. Moreover, a significant lysozyme activity in the lumen of the anterior and middle regions of the midgut was detected. This enzyme, together with the strong acidic luminal pH of middle tract, may play an important role in killing pathogenic microorganisms ingested with the feeding substrate. The evidence collected led us to propose a detailed functional model of the larval midgut of H. illucens in which each region is characterized by peculiar features to accomplish specific functions. This platform of knowledge sets the stage for developing rearing protocols to optimize the bioconversion ability of this insect and its biotechnological applications.

RNA-Seq Analyses of Midgut and Fat Body Tissues Reveal the Molecular Mechanism Underlying Spodoptera litura Resistance to Tomatine

Plants produce secondary metabolites to provide chemical defense against herbivorous insects, whereas insects can induce the expression of detoxification metabolism-related unigenes in counter defense to plant xenobiotics. Tomatine is an important secondary metabolite in tomato (Lycopersicon esculentum L.) that can protect the plant from bacteria and insects. However, the mechanism underlying the adaptation of Spodoptera litura, a major tomato pest, to tomatine in tomato is largely unclear. In this study, we first found that the levels of tomatine in tomatoes subjected to S. litura treatment were significantly increased. Second, we confirmed the inhibitory effect of tomatine on S. litura by adding moderate amounts of commercial tomatine to an artificial diet. Then, we utilized RNA-Seq to compare the differentially expressed genes (DEGs) in the midgut and fat body tissues of S. litura exposed to an artificial diet supplemented with tomatine. In total, upon exposure to tomatine, 134 and 666 genes were upregulated in the S. litura midgut and fat body, respectively. These DEGs comprise a significant number of detoxification-related genes, including 7 P450 family genes, 8 glutathione S-transferases (GSTs) genes, 6 ABC transport enzyme genes, 9 UDP-glucosyltransferases genes and 3 carboxylesterases genes. Moreover, KEGG analysis demonstrated that the upregulated genes were enriched in xenobiotic metabolism by cytochrome P450s, ABC transporters and drug metabolism by other enzymes. Furthermore, as numerous GSTs were induced by tomatine in S. litura, we chose one gene, namely GSTS1, to confirm the detoxification function on tomatine. Expression profiling revealed that GSTS1 transcripts were mainly expressed in larvae, and the levels were the highest in the midgut. Finally, when larvae were injected with double-stranded RNA specific to GSTS1, the transcript levels in the midgut and fat body decreased, and the negative effect of the plant xenobiotic tomatine on larval growth was magnified. These results preliminarily clarified the molecular mechanism underlying the resistance of S. litura to tomatine, establishing a foundation for subsequent pest control.

The Intestinal Microbiota of Hermetia illucens Larvae Is Affected by Diet and Shows a Diverse Composition in the Different Midgut Regions

The larva of the black soldier fly (Hermetia illucens) has emerged as an efficient system for the bioconversion of organic waste. Although many research efforts are devoted to the optimization of rearing conditions to increase the yield of the bioconversion process, microbiological aspects related to this insect are still neglected. Here, we describe the microbiota of the midgut of H. illucens larvae, showing the effect of different diets and midgut regions in shaping microbial load and diversity. The bacterial communities residing in the three parts of the midgut, characterized by remarkable changes in luminal pH values, differed in terms of bacterial numbers and microbiota composition. The microbiota of the anterior part of the midgut showed the highest diversity, which gradually decreased along the midgut, whereas bacterial load had an opposite trend, being maximal in the posterior region. The results also showed that the influence of the microbial content of ingested food was limited to the anterior part of the midgut, and that the feeding activity of H. illucens larvae did not significantly affect the microbiota of the substrate. Moreover, a high protein content compared to other macronutrients in the feeding substrate seemed to favor midgut dysbiosis. The overall data indicate the importance of taking into account the presence of different midgut structural and functional domains, as well as the substrate microbiota, in any further study that aims at clarifying microbiological aspects concerning H. illucens larval midgut.IMPORTANCE The demand for food of animal origin is expected to increase by 2050. Since traditional protein sources for monogastric diets are failing to meet the increasing demand for additional feed production, there is an urgent need to find alternative protein sources. The larvae of Hermetia illucens emerge as efficient converters of low-quality biomass into nutritionally valuable proteins. Many studies have been performed to optimize H. illucens mass rearing on a number of organic substrates and to quantitatively and qualitatively maximize the biomass yield. On the contrary, although the insect microbiota can be fundamental for bioconversion processes and its characterization is mandatory also for safety aspects, this topic is largely overlooked. Here, we provide an in-depth study of the microbiota of H. illucens larval midgut, taking into account pivotal aspects, such as the midgut spatial and functional regionalization, as well as microbiota and nutrient composition of the feeding substrate.

Post-embryonic development of the Malpighian tubules in Apis mellifera (Hymenoptera) workers: morphology, remodeling, apoptosis, and cell proliferation

The honeybee Apis mellifera has ecological and economic importance; however, it experiences a population decline, perhaps due to exposure to toxic compounds, which are excreted by Malpighian tubules. During metamorphosis of A. mellifera, the Malpighian tubules degenerate and are formed de novo. The objective of this work was to verify the cellular events of the Malpighian tubule renewal in the metamorphosis, which are the gradual steps of cell remodeling, determining different cell types and their roles in the excretory activity in A. mellifera. Immunofluorescence and ultrastructural analyses showed that the cells of the larval Malpighian tubules degenerate by apoptosis and autophagy, and the new Malpighian tubules are formed by cell proliferation. The ultrastructure of the cells in the Malpighian tubules suggest that cellular remodeling only occurs from dark-brown-eyed pupae, indicating the onset of excretion activity in pupal Malpighian tubules. In adult forager workers, two cell types occur in the Malpighian tubules, one with ultrastructural features (abundance of mitochondria, vacuoles, microvilli, and narrow basal labyrinth) for primary urine production and another cell type with dilated basal labyrinth, long microvilli, and absence of spherocrystals, which suggest a role in primary urine re-absorpotion. This study suggests that during the metamorphosis, Malpighian tubules are non-functional until the light-brown-eyed pupae, indicating that A. mellifera may be more vulnerable to toxic compounds at early pupal stages. In addition, cell ultrastructure suggests that the Malpighian tubules may be functional from dark-brown-eyed pupae and acquire greater complexity in the forager worker bee.

Azadirachtin Affects the Growth of Spodoptera litura Fabricius by Inducing Apoptosis in Larval Midgut

Azadirachtin, the environmentally friendly botanical pesticide, has been used as an antifeedant and pest growth regulator in integrated pest management for decades. It has shown strong biological activity against Spodoptera litura, but the mechanism of toxicity remains unclear. The present study showed that azadirachtin inhibited the growth of S. litura larvae, which was resulted by structure destroy and size inhibition of the midgut. Digital gene expression (DGE) analysis of midgut suggested that azadirachtin regulated the transcriptional level of multiple unigenes involved in mitogen-activated protein kinase (MAPK) and calcium apoptotic signaling pathways. Simultaneously, the expression patterns of some differentially expressed unigenes were verified by quantitative real time-PCR (qRT-PCR). In addition, the enhanced terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) staining, the increased expression of caspase family members and apoptosis-binding motif 1 (IBM1) on both gene and protein level and the release of cytochrome c from mitochondria to cytoplasm were induced in midgut after azadirachtin treatment. These results demonstrated that azadirachtin induced structural alteration in S. litura larval midgut by apoptosis activation. These alterations may affect the digestion and absorption of nutrients and eventually lead to the growth inhibition of larvae.

Methods for Monitoring Autophagy in Silkworm Organs

In holometabolous insects, various larval organs are remodeled by autophagy during metamorphosis. Although moths and butterflies are among the first animal models in which this self-eating process was described, only in recent years autophagy has been analyzed in detail in these insects. In particular, the silkworm Bombyx mori, which represents a well-studied model among Lepidoptera, provides a wide repertoire of cellular and molecular tools useful for studying the occurrence of autophagy and for evaluating its role in postembryonic development. Here, we describe some morphological, biochemical, and molecular methods to monitor autophagy in silkworm organs.

Comparative genomics analysis of triatomines reveals common first line and inducible immunity-related genes and the absence of Imd canonical components among hemimetabolous arthropods

Background

Insects operate complex humoral and cellular immune strategies to fend against invading microorganisms. The majority of these have been characterized in Drosophila and other dipterans. Information on hemipterans, including Triatominae vectors of Chagas disease remains incomplete and fractionated.

Results

We identified putative immune-related homologs of three Triatominae vectors of Chagas disease, Triatoma pallidipennis, T. dimidiata and T. infestans (TTTs), using comparative transcriptomics based on established immune response gene references, in conjunction with the predicted proteomes of Rhodnius prolixus, Cimex lecticularis and Acyrthosiphon pisum hemimetabolous. We present a compressive description of the humoral and cellular innate immune components of these TTTs and extend the immune information of other related hemipterans. Key homologs of the constitutive and induced immunity genes were identified in all the studied hemipterans.

Conclusions

Our results in the TTTs extend previous observations in other hemipterans lacking several components of the Imd signaling pathway. Comparison with other hexapods, using published data, revealed that the absence of various Imd canonical components is common in several hemimetabolous species.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2561-2) contains supplementary material, which is available to authorized users.

A midgut-specific serine protease, BmSP36, is involved in dietary protein digestion in the silkworm, Bombyx mori

Serine proteases play important roles in digestion and immune responses during insect development. In the present study, the serine protease gene BmSP36, which encodes a 292-residue protein, was cloned from the midgut cells of Bombyx mori. BmSP36 contains an intact catalytic triad (H57, D102 and S195) and a conserved substrate-binding site (G189, H216 and G226), suggesting that it is a serine protease with chymotrypsin-like specificity. The temporal and spatial expression patterns of BmSP36 indicated that its messenger RNA and protein expression mainly occurred in the midgut at the feeding stages. Western blotting, immunofluorescence and liquid chromatography-tandem mass spectrometry analyses revealed secretion of BmSP36 protein from epithelial cells into the midgut lumen. The transcriptional and translational expression of BmSP36 was down-regulated after starvation but up-regulated after refeeding. Moreover, expression of the BmSP36 gene could be up-regulated by a juvenile hormone analogue. These results enable us to better define the potential role of BmSP36 in dietary protein digestion at the feeding stages during larval development.

Timing of autophagy and apoptosis during posterior silk gland degeneration in Bombyx mori

Over the years, the silkworm, Bombyx mori, has been manipulated by means of chemical and genetic approaches to improve silk production both quantitatively and qualitatively. The silk is produced by the silk gland, which degenerates quickly once the larva has finished spinning the cocoon. Thus, interfering with this degeneration process could help develop new technologies aimed at ameliorating silk yield. To this end, in this work we studied the cell death processes that lead to the demise of the posterior silk gland of B. mori, directing in particular our attention to autophagy and apoptosis. We focused on this portion of the gland because it produces fibroin, the main component of the silk thread. By using multiple markers, we provide a morphological, biochemical and molecular characterization of the apoptotic and autophagic processes and define their timing in this biological setting. Our data demonstrate that the activation of both autophagy and apoptosis is preceded by a transcriptional rise in key regulatory genes. Moreover, while autophagy is maintained active for several days and progressively digests silk gland cells, apoptosis is only switched on at a very late stage of silk gland demise.

Roles and regulation of autophagy and apoptosis in the remodelling of the lepidopteran midgut epithelium during metamorphosis

We previously showed that autophagy and apoptosis occur in the removal of the lepidopteran larval midgut during metamorphosis. However, their roles in this context and the molecular pathways underlying their activation and regulation were only hypothesized. The results of the present study better clarify the timing of the activation of these two processes: autophagic and apoptotic genes are transcribed at the beginning of metamorphosis, but apoptosis intervenes after autophagy. To investigate the mechanisms that promote the activation of autophagy and apoptosis, we designed a set of experiments based on injections of 20-hydroxyecdysone (20E). Our data demonstrate that autophagy is induced at the end of the last larval stage by the 20E commitment peak, while the onset of apoptosis occurs concomitantly with the 20E metamorphic peak. By impairing autophagic flux, the midgut epithelium degenerated faster, and higher caspase activity was observed compared to controls, whereas inhibiting caspase activation caused a severe delay in epithelial degeneration. Our data demonstrate that autophagy plays a pro-survival function in the silkworm midgut during metamorphosis, while apoptosis is the major process that drives the demise of the epithelium. The evidence collected in this study seems to exclude the occurrence of autophagic cell death in this setting.

Midgut epithelium in molting silkworm: A fine balance among cell growth, differentiation, and survival

The midgut of insects has attracted great attention as a system for studying intestinal stem cells (ISCs) as well as cell death-related processes, such as apoptosis and autophagy. Among insects, Lepidoptera represent a good model to analyze these cells and processes. In particular, larva-larva molting is an interesting developmental phase since the larva must deal with nutrient starvation and its organs are subjected to rearrangements due to proliferation and differentiation events. Several studies have analyzed ISCs in vitro and characterized key factors involved in their division and differentiation during molt. However, in vivo studies performed during larva-larva transition on these cells, and on the whole midgut epithelium, are fragmentary. In the present study, we analyzed the larval midgut epithelium of the silkworm, Bombyx mori, during larva-larva molting, focusing our attention on ISCs. Moreover, we investigated the metabolic changes that occur in the epithelium and evaluated the intervention of autophagy. Our data on ISCs proliferation and differentiation, autophagy activation, and metabolic and functional activities of the midgut cells shed light on the complexity of this organ during the molting phase.