New insights into peritrophic matrix synthesis, architecture, and function

Physical and Chemical Barriers in the Larval Midgut Confer Developmental Resistance to Virus Infection in Drosophila

Insects can become lethally infected by the oral intake of a number of insect-specific viruses. Virus infection commonly occurs in larvae, given their active feeding behaviour; however, older larvae often become resistant to oral viral infections. To investigate mechanisms that contribute to resistance throughout the larval development, we orally challenged Drosophila larvae at different stages of their development with Drosophila C virus (DCV, Dicistroviridae). Here, we showed that DCV-induced mortality is highest when infection initiates early in larval development and decreases the later in development the infection occurs. We then evaluated the peritrophic matrix as an antiviral barrier within the gut using a Crystallin-deficient fly line (Crys^-/-), whose PM is weakened and becomes more permeable to DCV-sized particles as the larva ages. This phenotype correlated with increasing mortality the later in development oral challenge occurred. Lastly, we tested in vitro the infectivity of DCV after incubation at pH conditions that may occur in the midgut. DCV virions were stable in a pH range between 3.0 and 10.5, but their infectivity decreased at least 100-fold below (1.0) and above (12.0) this range. We did not observe such acidic conditions in recently hatched larvae. We hypothesise that, in Drosophila larvae, the PM is essential for containing ingested virions separated from the gut epithelium, while highly acidic conditions inactivate the majority of the virions as they transit.

Analysis of microbiota in the stomach and midgut of two penaeid shrimps during probiotic feeding

In mammals, the intestine harbors numerous bacteria that play an important role in health. Intestinal microbiota have also been thought to be an important factor in the health of shrimp. However, the barrier systems of the digestive tracts of shrimp seem to be different from those of mammals. In this study, we analyzed the bacterial composition in the stomach and midgut of two species of shrimp during administration of a probiotic, Bacillus amyloliquefaciens strain TOA5001 by analysis of 16S rRNA genes with Illumina sequencing technology. Whiteleg shrimp Litopenaeus vannamei were observed under laboratory conditions and kuruma shrimp Marsupenaeus japonicus were observed in an aquaculture farm. The diversities of bacteria in the stomachs of both shrimps were significantly higher than those in the midgut. Also, the microbiota changed during probiotic feeding. Feeding whiteleg shrimp the probiotic after being challenged with an acute hepatopancreatic necrosis disease (AHPND)-causing strain of Vibrio parahaemolyticus increased their survival compared to the control group, which suggested that the probiotic prevented AHPND. These results appear to show that a probiotic can affect the microbiota throughout digestive tract of penaeid shrimps and that probiotic can have a role in preventing disease.

Plasmodium chitinases: revisiting a target of transmission-blockade against malaria

Malaria is a life-threatening disease caused by one of the five species of Plasmodium, among which Plasmodium falciparum cause the deadliest form of the disease. Plasmodium species are dependent on a vertebrate host and a blood-sucking insect vector to complete their life cycle. Plasmodium chitinases belonging to the GH18 family are secreted inside the mosquito midgut, during the ookinete stage of the parasite. Chitinases mediate the penetration of parasite through the peritrophic membrane, facilitating access to the gut epithelial layer. In this review, we describe Plasmodium chitinases with special emphasis on chitinases from P. falciparum and P. vivax, the representative examples of the short and long forms of this protein. In addition to the chitinase domain, chitinases belonging to the long form contain a pro-domain and chitin-binding domain. Amino acid sequence alignment of long and short form chitinase domains reveals multiple positions containing variant residues. A subset of these positions was found to be conserved or invariant within long or short forms, indicating the role of these positions in attributing form-specific activity. The reported differences in affinities to allosamidin for P. vivax and P. falciparum were predicted to be due to different residues at two amino acid positions, resulting in altered interactions with the inhibitor. Understanding the role of these amino acids in Plasmodium chitinases will help us elucidate the mechanism of catalysis and the mode of inhibition, which will be the key for identification of potent inhibitors or antibodies demonstrating transmission-blocking activity.

Peritrophic matrix-degrading proteins are dispensable virulence factors in a virulent Melissococcus plutonius strain

European foulbrood (EFB) caused by Melissococcus plutonius is a major bacterial disease of honey bees. Strains of the causative agent exhibit genetic heterogeneity, and the degree of virulence varies among strains. In bee larvae orally infected with the highly virulent strains, ingested bacterial cells colonize the larval midgut and proliferate within the sac of the peritrophic matrix (PM), a barrier lining the midgut epithelium. However, the barrier is degraded during the course of infection, and M. plutonius cells eventually directly interact with the midgut epithelium. As M. plutonius possesses genes encoding putative PM-degrading proteins (enhancin, a chitin-binding domain-containing protein and endo-α-N-acetylgalactosaminidase), we constructed PM-degrading protein gene-knockout mutants from a highly virulent M. plutonius strain and investigated their role in the pathogenesis of EFB. In larvae infected with the triple-knockout mutant, which has no PM-degrading protein genes, M. plutonius that proliferated in the larval midguts was confined to the sac of the PM. However, the midgut epithelial cells degenerated over time, and the mutant killed approximately 70-80% of bee brood, suggesting that although the PM-degrading proteins are involved in the penetration of the PM by M. plutonius, they are not indispensable virulence factors in the highly virulent M. plutonius strain.

Interaction of Viruses with the Insect Intestine

In nature, insects face a constant threat of infection by numerous exogeneous viruses, and their intestinal tracts are the predominant ports of entry. Insects can acquire these viruses orally during either blood feeding by hematophagous insects or sap sucking and foliage feeding by insect herbivores. However, the insect intestinal tract forms several physical and immunological barriers to defend against viral invasion, including cell intrinsic antiviral immunity, the peritrophic matrix and the mucin layer, and local symbiotic microorganisms. Whether an infection can be successfully established in the intestinal tract depends on the complex interactions between viruses and those barriers. In this review, we summarize recent progress on virus-intestinal tract interplay in insects, in which various underlying mechanisms derived from nutritional status, dynamics of symbiotic microorganisms, and virus-encoded components play intricate roles in the regulation of virus invasion in the intestinal tract, either directly or indirectly. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Proteomics and ultrastructural analysis of Hermetia illucens (Diptera: Stratiomyidae) larval peritrophic matrix

BACKGROUND

The black soldier fly (Hermetia illucens) has significant economic potential. The larvae can be used in financially viable waste management systems, as they are voracious feeders able to efficiently convert low-quality waste into valuable biomass. However, most studies on H. illucens in recent decades have focused on optimizing their breeding and bioconversion conditions, while information on their biology is limited.

METHODS

About 200 fifth instar well-fed larvae were sacrificed in this work. The liquid chromatography-tandem mass spectrometry and scanning electron microscopy were employed in this study to perform a proteomic and ultrastructural analysis of the peritrophic matrix (PM) of H. illucens larvae.

RESULTS

A total of 565 proteins were identified in the PM samples of H. illucen, of which 177 proteins were predicted to contain signal peptides, bioinformatics analysis and manual curation determined 88 proteins may be associated with the PM, with functions in digestion, immunity, PM modulation, and others. The ultrastructure of the H. illucens larval PM observed by scanning electron microscopy shows a unique diamond-shaped chitin grid texture.

CONCLUSIONS

It is the first and most comprehensive proteomics research about the PM of H. illucens larvae to date. All the proteins identified in this work has been discussed in details, except several unnamed or uncharacterized proteins, which should not be ignored and need further study. A comparison of the ultrastructure between H. illucens larval PM and those of other insects as observed by SEM indicates that the PM displays diverse textures on an ultra-micro scale and we suscept a unique diamond-shaped chitin grid texture may help H. illucens larval to hold more food. This work deepens our understanding of the molecular architecture and ultrastructure of the H. illucens larval PM.

Transcriptomic analysis of the poultry red mite, Dermanyssus gallinae, across all stages of the lifecycle

BACKGROUND

The blood feeding poultry red mite (PRM), Dermanyssus gallinae, causes substantial economic damage to the egg laying industry worldwide, and is a serious welfare concern for laying hens and poultry house workers. In this study we have investigated the temporal gene expression across the 6 stages/sexes (egg, larvae, protonymph and deutonymph, adult male and adult female) of this neglected parasite in order to understand the temporal expression associated with development, parasitic lifestyle, reproduction and allergen expression.

RESULTS

RNA-seq transcript data for the 6 stages were mapped to the PRM genome creating a publicly available gene expression atlas (on the OrcAE platform in conjunction with the PRM genome). Network analysis and clustering of stage-enriched gene expression in PRM resulted in 17 superclusters with stage-specific or multi-stage expression profiles. The 6 stage specific superclusters were clearly demarked from each other and the adult female supercluster contained the most stage specific transcripts (2725), whilst the protonymph supercluster the fewest (165). Fifteen pairwise comparisons performed between the different stages resulted in a total of 6025 Differentially Expressed Genes (DEGs) (P > 0.99). These data were evaluated alongside a Venn/Euler analysis of the top 100 most abundant genes in each stage. An expanded set of cuticle proteins and enzymes (chitinase and metallocarboxypeptidases) were identified in larvae and underpin cuticle formation and ecdysis to the protonymph stage. Two mucin/peritrophic-A salivary proteins (DEGAL6771g00070, DEGAL6824g00220) were highly expressed in the blood-feeding stages, indicating peritrophic membrane formation during feeding. Reproduction-associated vitellogenins were the most abundant transcripts in adult females whilst, in adult males, an expanded set of serine and cysteine proteinases and an epididymal protein (DEGAL6668g00010) were highly abundant. Assessment of the expression patterns of putative homologues of 32 allergen groups from house dust mites indicated a bias in their expression towards the non-feeding larval stage of PRM.

CONCLUSIONS

This study is the first evaluation of temporal gene expression across all stages of PRM and has provided insight into developmental, feeding, reproduction and survival strategies employed by this mite. The publicly available PRM resource on OrcAE offers a valuable tool for researchers investigating the biology and novel interventions of this parasite.

Mosquito Trilogy: Microbiota, Immunity and Pathogens, and Their Implications for the Control of Disease Transmission

In mosquitoes, the interaction between the gut microbiota, the immune system, and the pathogens that these insects transmit to humans and animals is regarded as a key component toward the development of control strategies, aimed at reducing the burden of severe diseases, such as malaria and dengue fever. Indeed, different microorganisms from the mosquito microbiota have been investigated for their ability to affect important traits of the biology of the host insect, related with its survival, development and reproduction. Furthermore, some microorganisms have been shown to modulate the immune response of mosquito females, significantly shaping their vector competence. Here, we will review current knowledge in this field, focusing on i) the complex interaction between the intestinal microbiota and mosquito females defenses, both in the gut and at humoral level; ii) how knowledge on these issues contributes to the development of novel and targeted strategies for the control of mosquito-borne diseases such as the use of paratransgenesis or taking advantage of the relationship between Wolbachia and mosquito hosts. We conclude by providing a brief overview of available knowledge on microbiota-immune system interplay in major insect vectors.

Regulatory mechanisms of microbial homeostasis in insect gut

Insects live in incredibly complex environments. The intestinal epithelium of insects is in constant contact with microorganisms, some of which are beneficial and some harmful to the host. Insect gut health and function are maintained through multidimensional mechanisms that can proficiently remove foreign pathogenic microorganisms while effectively maintaining local symbiotic microbial homeostasis. The basic immune mechanisms of the insect gut, such as the dual oxidase-reactive oxygen species (Duox-ROS) system and the immune deficiency (Imd)-signaling pathway, are involved in the maintenance of microbial homeostasis. This paper reviews the role of physical defenses, the Duox-ROS and Imd signaling pathways, the Janus kinase/signal transducers and activators of transcription signaling pathway, and intestinal symbiotic flora in the homeostatic maintenance of the insect gut microbiome.

A mixture containing the herbicides Mesotrione and Atrazine imposes toxicological risks on workers of Partamona helleri

A mixture of Mesotrione and Atrazine (Calaris®) has been reported as an improvement of the atrazine herbicides, which are agrochemicals used for weed control. However, its possible harmful effects on non-target organisms, including pollinators, needs to be better understood. In this work, the effects of the mix of herbicides on food consumption, behaviour (walking distance, and meandering), and the morphology of the midgut of the stingless bee Partamona helleri were studied. Foragers were orally exposed to different concentrations of the mix. The concentrations leading to 10% and 50% mortality (LC₁₀ and LC₅₀, respectively) were estimated and used in the analysis of behaviour and morphology. The ingestion of contaminated diets (50% aqueous sucrose solution + mix) led to a reduction in food consumption by the bees when compared to the control, bees fed a non-contaminated diet (sucrose solution). Ingestion of the LC₅₀ diet reduced locomotor activity, increased meandering, induced the degradation of the epithelium and peritrophic matrix, and also changed the number of cells positive for signalling-pathway proteins in the midgut. These results show the potential toxicological effects and environmental impacts of the mix of herbicides in beneficial insects, including a native bee.

Social immunity in honey bees: royal jelly as a vehicle in transferring bacterial pathogen fragments between nestmates

Social immunity is a suite of behavioral and physiological traits that allow colony members to protect one another from pathogens, and includes the oral transfer of immunological compounds between nestmates. In honey bees, royal jelly is a glandular secretion produced by a subset of workers that is fed to the queen and young larvae, and which contains many antimicrobial compounds. A related form of social immunity, transgenerational immune priming (TGIP), allows queens to transfer pathogen fragments into their developing eggs, where they are recognized by the embryo's immune system and induce higher pathogen resistance in the new offspring. These pathogen fragments are transported by vitellogenin (Vg), an egg-yolk precursor protein that is also used by nurses to synthesize royal jelly. Therefore, royal jelly may serve as a vehicle to transport pathogen fragments from workers to other nestmates. To investigate this, we recently showed that ingested bacteria are transported to nurses' jelly-producing glands, and here, we show that pathogen fragments are incorporated into the royal jelly. Moreover, we show that consuming pathogen cells induces higher levels of an antimicrobial peptide found in royal jelly, defensin-1.

Caterpillar gut and host plant phylloplane mycobiomes differ: a new perspective on fungal involvement in insect guts

Compared with the highly diverse microbiota of leaves, herbivorous insects exhibit impoverished gut microbial communities. Research to date has focused on the bacterial component of these gut microbiomes, neglecting the fungal component. As caterpillar gut bacterial microbiomes are derived strongly from their diet, we hypothesized that their mycobiomes would reflect the host leaf mycobiomes. Using the ITS2 rDNA and V5-V6 16S rRNA gene regions for DNA metabarcoding of caterpillar gut and host leaf sample pairs we compared their mycobiome genus diversity and compositions and identified genera associated with caterpillar guts. Leaves and caterpillar guts harbored different mycobiomes with quite low qualitative similarity (Jaccard index = 38.03%). The fungal genera most significantly associated with the caterpillar gut included Penicillium, Mucor and unidentified Saccharomycetales, whereas leaf-associated genera included Holtermanniella, Gibberella (teleomorph of Fusarium) and Seimatosporium. Although caterpillar gut and leaf mycobiomes had similar genus richness overall, this indicator was not correlated for individual duplets. Moreover, as more samples entered the analysis, mycobiome richness increased more rapidly in caterpillar guts than in leaves. The results suggest that the mycobiota of the caterpillar gut differs from that of their feeding substrate; further, the mycobiomes appear to be richer than the well-studied bacterial microbiotas.

The resistance of the cowpea cv. BRS Xiquexique to infestation by cowpea weevil is related to the presence of toxic chitin-binding proteins

The cowpea weevil (Callosobruchus maculatus) is the main pest that attacks cowpea (Vigna unguiculata) seeds during storage, causing nutritional and economic losses in the cowpea crop. Thus, studies aiming to identify resistant cowpea cultivars have been developed. Chitin-binding proteins (CBP), such vicilins and chitinases, have been detected in seeds and related with the toxicity to insects. In this work, we investigated the presence of chitin-binding proteins in the partially resistant cowpea cv. BRS Xiquexique and evaluated their toxicity towards cowpea weevil. The CBP fraction was isolated by chitin affinity chromatography. CBP fraction showed, through 15% SDS PAGE, protein bands with varying molecular masses, mainly below 55 kDa. Proteins present in CBP fraction were identified by Western blotting and mass spectrometry analysis, as vicilins and chitinases. CBP fraction, at 5%, was able to interfere with the development of cowpea weevil, decreasing larval mass and length. A CBV (chitin-binding vicilin) fraction isolated from CBP fraction was toxic, at 2.0%, to C. maculatus, decreasing larval mass and length in 64.3% and 33.23%, respectively. These results suggest that chitin binding proteins, such vicilins and chitinases, may be related to the resistance of cowpea cv. BRS Xiquexique to the infestation by C. maculatus.

Comparative response of Spodoptera litura challenged per os with Serratia marcescens strains differing in virulence

Host plays an important role in influencing virulence of a pathogen and efficacy of a biopesticide. The present study was aimed to characterize the possible factors present in Spodoptera litura that influenced pathogenecity of orally ingested S. marcescens strains, differing in their virulence. Fifth instar larvae of S. litura responded differently as challenged by two Serratia marcescens strains, SEN (virulent strain, LC₅₀ 7.02 10³ cfu/ml) and ICC-4 (non-virulent strain, LC₅₀ 1.19 10¹² cfu/ml). Considerable increase in activity of lytic enzymes protease and phospholipase was recorded in the gut and hemolymph of larvae fed on diet supplemented with S. marcescens strain ICC-4 as compared to the larvae treated with S. marcescens strain SEN. However, a significant up-regulation of antioxidative enzymes SOD (in foregut and midgut), CAT (in the midgut) and GST (in the foregut and hemolymph) was recorded in larvae fed on diet treated with the virulent S. marcescens strain SEN in comparison to larvae fed on diet treated with the non-virulent S. marcescens strain ICC-4. Activity of defense related enzymes lysozyme and phenoloxidase activity were also higher in the hemolymph of larvae fed with diet treated with S. marcescens strain SEN as compared to hemolymph of S. marcescens strain ICC-4 treated larvae. More number of over-expressed proteins was observed in the gut and hemolymph of S. marcescens strains ICC-4 and SEN treated larvae, respectively. Identification of the selected differentially expressed proteins indicated induction of proteins involved in insect innate immune response (Immunoglobulin I-set domain, Apolipophorin III, leucine rich repeat and Titin) in S. marcescens strain SEN treated larvae. Over-expression of two proteins, actin related protein and mt DNA helicase, were noted in S. marcescens treated larvae with very high levels observed in the non-virulent strain. Up-regulation of homeobox protein was noted only in S. marcescens strain ICC-4 challenged larvae. This study indicated that ingestion of non-virulent S. marcescens strain ICC-4 induced strong immune response in insect gut while there was weak response to the virulent S. marcescens strain SEN which probably resulted in difference in their virulence.

Gut microbiota mediate Plutella xylostella susceptibility to Bt Cry1Ac protoxin is associated with host immune response

Insect gut microbiotas have a variety of physiological functions for host growth, development, and immunity. Bacillus thuringiensis (Bt) is known to kill insect pests by releasing insecticidal protoxins, which are activated in the insect midgut. However, the interplay among Bt infection, host immunity, and gut microbiota are still unclear. Here we show that Bt Cry1Ac protoxin interacts with the gut microbiota to accelerate the mortality of P. xylostella larvae. Cry1Ac protoxin was found to cause a dynamic change in the midgut and hemocoel microbiota of P. xylostella, with a significant increase in bacterial load and a significant reduction in bacterial diversity. In turn, loss of gut microbiota significantly decreased the Bt susceptibility of P. xylostella larvae. The introduction of three gut bacterial isolates Enterococcus mundtii (PxG1), Carnobacterium maltaromaticum (PxCG2), and Acinetobacter guillouiae (PxCG3) restored sensitivity to Bt Cry1Ac protoxin. We also found that Cry1Ac protoxin and native gut microbiota can trigger host midgut immune response, which involves the up-regulation of expression of Toll and IMD pathway genes and most antimicrobial peptide genes, respectively. Our findings further shed light on the interplay between insect gut microbiota and host immunity under the Bt toxin killing pressure, and this may provide insights for improving the management of Bt resistance and lead to new strategies for biological control of insect pests.

Interactions between Borrelia burgdorferi and its hosts across the enzootic cycle

The bacterial pathogen Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to humans through an Ixodes tick vector. B. burgdorferi is able to survive in both mammalian and tick hosts through careful modulation of its gene expression. This allows B. burgdorferi to adapt to the environmental and nutritional changes that occur when it is transmitted between the two hosts. Distinct interactions between the spirochete and its host occur at every step of the enzootic cycle and dictate the ability of the spirochete to survive until the next stage of the cycle. Studying the interface between B. burgdorferi, the Ixodes tick vector and the natural mammalian reservoirs has been made significantly more feasible through the complete genome sequences of the organisms and the advent of high throughput screening technologies. Ultimately, a thorough investigation of the interplay between the two domains (and two phyla within one domain) is necessary in order to completely understand how the pathogen is transmitted.

Identification of Peritrophins and Antiviral Effect of Bm01504 against BmNPV in the Silkworm, Bombyx mori

The insect midgut secretes a semi-permeable, acellular peritrophic membrane (PM) that maintains intestinal structure, promotes digestion, and protects the midgut from food particles and pathogenic microorganisms. Peritrophin is an important PM protein (PMP) in the PM. Here, we identified 11 peritrophins with 1–16 chitin binding domains (CBDs) comprising 50–56 amino acid residues. Multiple CBDs in the same peritrophin clustered together, rather than by species. The CBD contained six highly conserved cysteine residues, with the key feature of amino acids between them being CX_11-15CX₅CX_9-14CX_11-12CX_6-7C. Peritrophins with 2 and 4 CBDs (Bm09641 and Bm01504, respectively), and with 1, 8, and 16 CBDs (Bm11851, Bm00185, and Bm01491, respectively) were mainly expressed in the anterior midgut, and throughout the midgut, respectively. Survival rates of transgenic silkworms with Bm01504 overexpression (Bm01504-OE) and knockout (Bm01504-KO) infected with B. morinucleopolyhedrovirus (BmNPV) were significantly higher and lower, whereas expression of the key viral gene, p10, were lower and higher, respectively, compared with wild type (WT). Therefore, Bm01504-OE and Bm01504-KO transgenic silkworms were more and less resistant, respectively, to BmNPV. Bm01504 plays important roles in resisting BmNPV invasion. We provide a new perspective for studying PM function, and reveal how the silkworm midgut resists invasive exogenous pathogenic microorganisms.

A novel tick protein supports integrity of gut peritrophic matrix impacting existence of gut microbiome and Lyme disease pathogens

The peritrophic matrix (PM) is an acellular membrane that covers the gut epithelium in arthropods and physically separates it from the lumen. The structure is thought to play an important role in tick biology. The PM is also known to impact the persistence of tick-borne pathogens like Borrelia burgdorferi, although limited information is available about its molecular constituents or their biological significance. Herein, we characterise a novel PM-associated gut protein in Ixodes scapularis ticks, annotated as Peritrophic Membrane Chitin Binding Protein (PM_CBP), for its role in the integrity and function of the matrix. The PM_CBP displays homology to the chitin deacetylase metalloenzyme, shows upregulation during tick feeding, and is localized at the luminal surface of the gut epithelium. The structural integrity of the PM was impaired both by the knock down of PM_CBP expression via RNA interference and by treatment with anti-PM_CBP antibodies, as revealed by its electron microscopic appearance. Additionally, the duration of tick engorgement on mice and the passage of experimentally-inoculated fluorescent dextran molecules across the PM are affected by the knock down of PM_CBP expression. The transfer of anti-PM_CBP antibodies into the tick gut impacted the overall composition of the resident microbiome, and also influenced B. burgdorferi acquisition in ticks and its transmission to mice. Taken together, these data highlight the biological significance of the Ixodes PM and suggest that the targeting of its molecular constituents may contribute to the development of novel interventions against tick-borne infections.

Challenges and physiological implications of wood feeding in termites

Termites are fascinating insects for a number of reasons, one of which being their specialization on diets of wood lignocellulose. The goal of this review is to consider stress-inducing characteristics of wood and apparent molecular-physiological adaptations in termite guts to overcome these stressors. Defensive factors present in wood include extractive secondary plant metabolites, lignin and related phenolics, crystalline cellulose, and low nitrogen content. Molecular-physiological adaptations of the termite gut to deal with these factors include robust detoxification and antioxidant machinery, the production of a peritrophic matrix and a wide range of cellulases from host and symbiotic sources, and creation of niches available to nitrogen-fixing bacterial symbionts. Considering termite gut physiology and symbioses in the context of stress-response has applied implications. These outcomes can include development of efficient biomass breakdown strategies, protection of microbes during industrial processing applications, and safeguarding wooden structures from termite damage.

Fates of dietary sterols in the insect alimentary canal

Sterols serve structural and physiological roles in insects. However, insects and other arthropods have lost many genes in the sterol biosynthesis pathway, so they must acquire sterols from their food. Sterols occur naturally as free (unconjugated) molecules, and as conjugated ones (mostly steryl esters). Once sterols are ingested and make their way into the gut, steryl esters can be converted into free sterols by Magro protein, a lipase excreted by enterocytes. Sterols in the free form enter midgut enterocytes through NPC1b and are then transported to the smooth endoplasmic reticulum membrane for possible metabolism. For most insect herbivores, phytosterol dealkylation converts plant sterols into cholesterol. Some ingested sterols may also be consumed by microbiota dwelling inside the insect gut lumen; bacteria use sterols as a source of carbon and energy. Further studies will reveal interesting and exciting discoveries regarding the pathways for the dietary sterols entering the insect alimentary canal.

Mucin family genes are essential for the growth and development of the migratory locust, Locusta migratoria

Mucins are highly glycosylated proteins that are characterized by a higher proportion of threonine, serine, and proline residues in their sequences. Although mucins in humans and vertebrates have been implicated in many biological processes, their roles in growth and development in invertebrates such as in insects remain largely unknown. Based on bioinformatic analyses, we identified eight mucin or mucin-like genes in the migratory locust, Locusta migratoria. RNA interference against these genes demonstrated that three Lmmucin genes were essential for the survival of L. migratoria nymphs, and one Lmmucin was required for adult wing development. Indeed, knockdown of Lmhemomucin and Lmmucin-12 caused lethal phenotypes, with an observed defect of the gastric caeca in which cells were detached from cell junctions. Deficiency of LmIIM3 resulted in lethality of nymphs, with defects of the peritrophic membrane in midgut. Suppression of Lmmucin-17 greatly impaired the structural integrity of the wing cuticle during nymph-adult molting. The present study revealed the significance of mucin and mucin-like genes in insect growth and development, using the orthopteran insect locust as a model.

Interactions between Borrelia burgdorferi and ticks

Borrelia burgdorferi is the causative agent of Lyme disease and is transmitted to vertebrate hosts by Ixodes spp. ticks. The spirochaete relies heavily on its arthropod host for basic metabolic functions and has developed complex interactions with ticks to successfully colonize, persist and, at the optimal time, exit the tick. For example, proteins shield spirochaetes from immune factors in the bloodmeal and facilitate the transition between vertebrate and arthropod environments. On infection, B. burgdorferi induces selected tick proteins that modulate the vector gut microbiota towards an environment that favours colonization by the spirochaete. Additionally, the recent sequencing of the Ixodes scapularis genome and characterization of tick immune defence pathways, such as the JAK–STAT, immune deficiency and cross-species interferon-γ pathways, have advanced our understanding of factors that are important for B. burgdorferi persistence in the tick. In this Review, we summarize interactions between B. burgdorferi and I. scapularis during infection, as well as interactions with tick gut and salivary gland proteins important for establishing infection and transmission to the vertebrate host.

Borrelia burgdorferi has a complex life cycle with several different hosts, causing Lyme disease when it infects humans. In this Review, Fikrig and colleagues discuss how B. burgdorferi infects and interacts with its tick vector to ensure onward transmission.

Complex Relationships at the Intersection of Insect Gut Microbiomes and Plant Defenses

Insect herbivores have ubiquitous associations with microorganisms that have major effects on how host insects may interact in their environment. Recently, increased attention has been given to how insect gut microbiomes mediate interactions with plants. In this paper, I discuss the ecology and physiology of gut bacteria associated with insect herbivores and how they may shape interactions between insects and their various host plants. I first establish how microbial associations vary between insects with different feeding styles, and how the insect host physiology and ecology can shape stable or transient relationships with gut bacteria. Then, I describe how these relationships factor in with plant nutrition and plant defenses. Within this framework, I suggest that many of the interactions between plants, insects, and the gut microbiome are context-dependent and shaped by the type of defense and the isolates present in the environment. Relationships between insects and plants are not pairwise, but instead highly multipartite, and the interweaving of complex microbial interactions is needed to fully explore the context-dependent aspects of the gut microbiome in many of these systems. I conclude the review by suggesting studies that would help reduce the unsureness of microbial interactions with less-defined herbivore systems and identify how each could provide a path to more robust roles and traits.

An Insight into Diversity and Functionalities of Gut Microbiota in Insects

The gut microbiota has long been of research interests due to its nutritional importance for many insects. It has been demonstrated that diversity of gut microbiota in insects can be modulated by many factors, including habitats, feeding preference, etc. Besides, the community structure of gut microbiota could also be altered during the different life stages of host insects. With development of conventional culture-dependent technologies and advanced culture-independent technologies, comprehensive and deep understanding of the functions of gut microbiota and their relationship with host insects were achieved, especially for the nutrient metabolic process mediated by them. In this review, we summarized the gut microbiota composition, major methods for gut microbiota characterization, and vital nutrient metabolic process mediated by gut microbiota in different insects. The increasing knowledge on the modulation of gut microbiota will help us for the comprehension of the contribution of gut microbiota to the nutritional metabolism of insects, prompting their growth and health.

Chemical Modifications of Vicilins Interfere with Chitin-Binding Affinity and Toxicity to Callosobruchus maculatus (Coleoptera: Chrysomelidae) Insect: A Combined In Vitro and In Silico Analysis

Vicilins are related to cowpea seed resistance toward Callosobruchus maculatus due to their ability to bind to chitinous structures lining larval midgut. However, this binding mechanism is not fully understood. Here, we identified chitin binding sites and investigated how in vitro and in silico chemical modifications interfere with vicilin chitin binding and insect toxicity. In vitro assays showed that unmodified vicilin strongly binds to chitin matrices, mainly with acetylated chitin. Chemical modifications of specific amino acids (tryptophan, lysine, tyrosine), as well as glutaraldehyde cross-linking, decreased the evaluated parameters. In silico analyses identified at least one chitin binding site in vicilin monomer, the region between Arg208 and Lys216, which bears the sequence REGIRELMK and forms an α helix, exposed in the 3D structure. In silico modifications of Lys223 (acetylated at its terminal nitrogen) and Trp316 (iodinated to 7-iodine-L-tryptophan or oxidized to β-oxy-indolylalanine) decreased vicilin chitin binding affinity. Glucose, sucrose, and N-acetylglucosamine also interfered with vicilin chitin binding affinity.

Effects of a Reserve Protein on Spodoptera frugiperda Development: A Biochemical and Molecular Approach to the Entomotoxic Mechanism

Talisin is a storage protein from Talisia esculenta seeds that presents lectin-like and peptidase inhibitor properties. These characteristics suggest that talisin plays a role in the plant defense process, making it a multifunctional protein. This work aimed to investigate the effects of chronic intake of talisin on fifth instar larvae of Spodoptera frugiperda, considered the main insect pest of maize and the cause of substantial economic losses in several other crops. The chronic intake of talisin presented antinutritional effects on the larvae, reducing their weight and prolonging the total development time of the insects. In addition, talisin-fed larvae also showed a significant reduction in the activity of trypsin-like enzymes. Midgut histology analysis of talisin-fed larvae showed alterations in the intestinal epithelium and rupture of the peritrophic membrane, possibly causing an increase of aminopeptidase activity in the midgut lumen. Talisin also proved to be resistant to degradation by the digestive enzymes of S. frugiperda. The transcription profile of trypsin, chymotrypsin and aminopeptidase genes was also analyzed through qPCR technique. Talisin intake resulted in differential expression of at least two genes from each of these classes of enzymes. Molecular docking studies indicated a higher affinity of talisin for the less expressed enzymes.

Roles of Symbiotic Microorganisms in Arboviral Infection of Arthropod Vectors

Arthropod vectors serve as native reservoirs and transmitters of hundreds of arboviruses. In arthropod vectors, symbiotic microorganisms residing in the gut lumen and/or hemocoelic tissues maintain complicated relationships with their host and influence multiple aspects of vector physiology. Recently, accumulating evidence has established an important role for symbiotic microorganisms in vector-virus interactions which could potentially be used to control viral transmission. Herein, we review recent progress on symbiotic microbe-arbovirus interactions and summarize the molecular mechanisms by which commensal microbes act on hosts and arboviruses. Understanding the sophisticated interactions among arthropod vectors, microbiota, and arboviruses may offer new strategies for the prevention of arboviral diseases in the future.

A Bacillus thuringiensis Chitin-Binding Protein is Involved in Insect Peritrophic Matrix Adhesion and Takes Part in the Infection Process

Bacillus thuringiensis (Bt) is used for insect pest control, and its larvicidal activity is primarily attributed to Cry toxins. Other factors participate in infection, and limited information is available regarding factors acting on the peritrophic matrix (PM). This study aimed to investigate the role of a Bt chitin-binding protein (CBPA) that had been previously shown to be expressed at pH 9 in vitro and could therefore be expressed in the alkaline gut of lepidopteron larvae. A ∆cbpA mutant was generated that was 10-fold less virulent than wild-type Bt HD73 towards Ostrinia furnacalis neonate larvae, indicating its important role in infection. Purified recombinant Escherichia coli CBPA was shown to have a chitin affinity, thus indicating a possible interaction with the chitin-rich PM. A translational GFP-CBPA fusion elucidated the localization of CBPA on the bacterial surface, and the transcriptional activity of the promoter PcbpA was immediately induced and confirmed at pH 9. Next, in order to connect surface expression and possible in vivo gut activity, last instar Galleria mellonella (Gm) larvae (not susceptible to Bt HD-73) were used as a model to follow CBPA in gut expression, bacterial transit, and PM adhesion. CBPA-GFP was quickly expressed in the Gm gut lumen, and more Bt HD73 strain bacteria adhered to the PM than those of the ∆cbpA mutant strain. Therefore, CBPA may help to retain the bacteria, via the PM binding, close to the gut surface and thus takes part in the early steps of Bt gut interactions.

Timing and original features of flagellum assembly in trypanosomes during development in the tsetse fly

Background

Trypanosoma brucei exhibits a complex life-cycle alternating between tsetse flies and mammalian hosts. When parasites infect the fly, cells differentiate to adapt to life in various tissues, which is accompanied by drastic morphological and biochemical modifications especially in the proventriculus. This key step represents a bottleneck for salivary gland infection.

Methods

Here, we monitored flagellum assembly in trypanosomes during differentiation from the trypomastigote to the epimastigote stage, i.e. when the nucleus migrates to the posterior end of the cell, by using three-dimensional electron microscopy (focused ion beam scanning electron microscopy, FIB-SEM) and immunofluorescence assays.

Results

The combination of light and electron microscopy approaches provided structural and molecular evidence that the new flagellum is assembled while the nucleus migrates towards the posterior region of the body. Two major differences with well-known procyclic cells are reported. First, growth of the new flagellum begins when the associated basal body is found in a posterior position relative to the mature flagellum. Secondly, the new flagellum acquires its own flagellar pocket before rotating on the left side of the anterior-posterior axis. FIB-SEM revealed the presence of a structure connecting the new and mature flagellum and serial sectioning confirmed morphological similarities with the flagella connector of procyclic cells. We discuss the potential function of the flagella connector in trypanosomes from the proventriculus.

Conclusions

These findings show that T. brucei finely modulates its cytoskeletal components to generate highly variable morphologies.

Using Diverse Model Systems to Define Intestinal Epithelial Defenses to Enteric Viral Infections

The intestine is an essential physical and immunological barrier comprised of a monolayer of diverse and specialized epithelial cells that perform functions ranging from nutrient absorption to pathogen sensing and intestinal homeostasis. The intestinal barrier prevents translocation of intestinal microbes into internal compartments. The microbiota is comprised of a complex community largely populated by diverse bacterial species that provide metabolites, nutrients, and immune stimuli that promote intestinal and organismal health. Although commensal organisms promote health, enteric pathogens, including a diverse plethora of enteric viruses, cause acute and chronic diseases. The barrier epithelium plays fundamental roles in immune defenses against enteric viral infections by integrating diverse signals, including those from the microbiota, to prevent disease. Importantly, many model systems have contributed to our understanding of this complex interface. This review will focus on the antiviral mechanisms at play within the intestinal epithelium and how these responses are shaped by the microbiota.

Regulation of Chitinase in Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) During Infection by Heliothis virescens ascovirus 3h (HvAV-3h)

Insect chitinases play essential roles in the molting and metamorphosis of insects. The virus Heliothis virescens ascovirus 3h (HvAV-3h) can prolong the total duration of the larval stage in its host larvae. In this study, the molecular character and function of chitinase and chitin-binding domain (CBD) were analyzed in larvae of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae). In detecting the chitinase activity of mock-infected and HvAV-3h-infected larval whole bodies and four different larval tissues, the results showed that larval chitinase activity was significantly decreased at 48 h post infection (hpi) and that the chitinase activity of HvAV-3h-infected larval fat body and cuticle was notably decreased at 144 and 168 hpi. The transcription level of S. exigua chitinase 7 (SeCHIT7) was down-regulated at the 6, 9, 12, 48, 72, and 96 hpi sample times, the S. exigua chitinase 11 (SeCHIT11) was down-regulated at 3-96 hpi, while both S. exigua chitinases (SeCHITs) were up-regulated at 120-168 hpi. Further tissue-specific detection of SeCHIT7 and SeCHIT11 transcription showed that SeCHIT7 was down-regulated at 144 and 168 hpi in the fat body and cuticle. SeCHIT11 was down-regulated at 168 hpi in the fat body, midgut, and cuticle. Additionally, the transcription and expression of S. exigua chitin-binding domain (SeCBD) could not be detected in HvAV-3h-infected larvae. The in vitro analyses of SeCHIT7N, SeCHIT11, and SeCBD showed that SeCHIT7N and SeCHIT11 were typical chitinases. Conversely, no chitinase activity was detected with SeCBD. SeCBD, however, could significantly increase the activity of SeCHIT7N and SeCHIT11. In conclusion, HvAV-3h not only interfered with the transcription and expression of SeCHITs but also affected the normal transcription and expression of SeCBD and, in doing so, influenced the host larval chitinase activity. These results will aid in providing a foundation for further studies on the pathogenesis of HvAV-3h.

Graphene quantum dots as cysteine protease nanocarriers against stored grain insect pests

Storing grains remain vulnerable to insect pest attack. The present study developed a biopesticide using biomolecules and their encapsulation in nanoparticles. A 25 kDa cysteine protease extracted from seeds of Albizia procera (ApCP) was encapsulated in graphene quantum dots (GQDs). The insecticidal activity of ApCP, with or without GQDs, against two stored grain insect pests, Tribolium castaneum (Herbst) and Rhyzopertha dominica (Fabricius) was explored. Insects were exposed to three concentrations 7.0, 3.5 and 1.7 mg of ApCP per a gram of wheat flour and grains. The insecticidal activity of ApCP encapsulated with GQDs was improved compared to that of ApCP without GQDs for both insect pests. The number of eggs and larvae of T. castaneum was reduced by 49% and 86%, respectively. Larval mortality was increased to 72%, and adult eclosion of T. castaneum was reduced by 98% at a 7.0 mg/g concentration of ApCP with GQDs compared to that of ApCP without GQDs. Exposure to 7.0 mg/g ApCP with GQDs, the number of R. dominica eggs and larvae was reduced by 72% and 92% respectively, larval mortality was increased by 90%, and eclosion was reduced by 97%. The extraction, purification, characterization, quantification and encapsulation of ApCP with GQDs were also studied. Cysteine protease nanocarriers have the potential to control stored grain insect pests.

Extracellular vesicles restrict dengue virus fusion in Aedes aegypti cells

Aedes aegypti is the primary vector of dengue virus (DENV), and acquires this virus from a vertebrate host during blood feeding. Previous literature has shown that vertebrate blood factors such as complement protein C5a and low-density lipoprotein (LDL) influence DENV acquisition in the mosquito. Here, we show that extracellular vesicles in cell culture medium inhibit DENV infection in mosquito cells. Specifically, extracellular vesicles enter into mosquito cells and inhibit an early stage of infection. Extracellular vesicles had no effect on virus cell attachment or entry. Instead, extracellular vesicles restricted virus membrane fusion. Extracellular vesicles only inhibited DENV infection in mosquito cells and not vertebrate cells. These data highlight a novel virus-vector-host interaction that limits virus infection in mosquito cells by restricting virus membrane fusion.

Intestinal responses of the oriental fruit fly Bactrocera dorsalis (Hendel) after ingestion of an entomopathogenic bacterium strain

BACKGROUND

Bactrocera dorsalis (Hendel) is the main fruit fly pest of tropical and subtropical countries. The application of insecticides to manage this pest has led to serious resistance problems; therefore, new ways to control B. dorsalis are required. Pathogenic bacteria are sources of biocontrol agents for pest management.

RESULTS

We determined that a pathogenic bacterial strain, Serratia marcescens PS-1, isolated from a moribund striped flea beetle (Phyllotreta striolata), was lethal to B. dorsalis adults following ingestion. Histological analyses revealed that PS-1 damaged the intestinal epithelium, resulting in cell death within 24 h. We then generated a gut transcriptomic data set using RNA-Seq at two time points (6 and 24 h) after PS-1 infection. We found that genes encoding the peritrophic matrix constituent were down-regulated, whereas genes involved in lipid and glycan metabolism, and renewal of the gut epithelium, along with genes encoding digestive enzymes and stress response factors, were up-regulated. In addition, 14 cecropin genes were identified and cloned from B. dorsalis. To our knowledge, the number of cecropins identified in the present study is greater than that reported in the insects of earlier studies. Moreover, some of the cecropins identified were significantly down-regulated after PS-1 treatment.

CONCLUSION

Our findings provide new insights into the insect gut response to pathogenic bacterial invasion and may aid the development of new strategies for the biological control of B. dorsalis. © 2019 Society of Chemical Industry.

Gene Expression in the Salivary Gland of Rhipicephalus (Boophilus) microplus Fed on Tick-Susceptible and Tick-Resistant Hosts

The success of cattle tick fixation largely depends on the secretion of substances that alter the immune response of the host. The majority of these substances are expressed by the parasite salivary gland and secreted in tick saliva. It is known that hosts can mount immune responses against ticks and bovine European breeds, and bovine industrial crossbreeds are more susceptible to infestations than are Bos indicus cattle. To identify candidates for the development of novel control strategies for the cattle tick Rhipicephalus (Boophilus) microplus, a salivary gland transcriptome analysis of engorged females fed on susceptible or resistant hosts was performed. Using RNA-Seq, transcriptomes were de novo assembled and produced a total of 235,451 contigs with 93.3% transcriptome completeness. Differential expression analysis identified 137 sequences as differentially expressed genes (DEGs) between ticks raised on tick-susceptible or tick-resistant cattle. DEGs predicted to be secreted proteins include innexins, which are transmembrane proteins that form gap junction channels; the transporters Na⁺/dicarboxylate, Na⁺/tricarboxylate, and phosphate transporter and a putative monocarboxylate transporter; a phosphoinositol 4-phosphate adaptor protein; a cysteine-rich protein containing a trypsin inhibitor-like (TIL) domain; a putative defense protein 3 containing a reeler domain; and an F-actin-uncapping protein LRRC16A with a CARMIL_C domain; these genes were upregulated in ticks fed on tick-susceptible cattle. DEGs predicted to be non-secreted proteins included a small heat shock protein and the negative elongation factor B-like, both acting in a coordinated manner to increase HSP transcript levels in the salivary glands of the ticks fed on tick-susceptible cattle; the 26S protease regulatory subunit 6B and another chaperone with similarity to calnexin, also upregulated in ticks fed on tick-susceptible cattle; an EF-hand calcium binding protein and a serine carboxypeptidase (SCP), both involved in the blood coagulation cascade and upregulated in ticks fed on tick-susceptible cattle; and two ribosomal proteins, the 60S acidic ribosomal protein P2 and the 60S ribosomal protein L19. These results help to characterize cattle tick salivary gland gene expression in tick-susceptible and tick-resistant hosts and suggest new putative targets for the control of tick infestations, as those genes involved in the mechanism of stress response during blood feeding.

A simplified strategy to package foreign proteins into baculovirus occlusion bodies without engineering the viral genome

Polyhedron envelope protein (PEP) is the major component of the calyx that surrounds the baculovirus occlusion body (OB). PEP has been associated with the stabilization and resistance of polyhedra in the environment. Due to the abundant levels of PEP in OBs, we decided to use this protein as a fusion partner to redirect foreign proteins to baculovirus polyhedra. In this study we developed a strategy that involves the generation of a monoclonal transformed insect cell line expressing a protein of interest fused to the the Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) N-terminus of PEP that enables the packaging of foreign proteins into the OBs without generating a recombinant baculovirus. This proved to be an efficient platform that could be exploited to improve wild type baculovirus for their use as bioinsecticides without facing the concerns of releasing genetically modified DNA to the environment and bypassing the associated regulatory issues. We demonstrated, using immunological, proteomic and microscopy techniques, that the envelope of AgMNPV OBs can effectively trap chimeric proteins in an infected insect cell line expressing AgMNPV PEP fused to the enhanced green fluorescent protein (eGFP). Furthermore, packaging of chimeric PEP also took place with heterologous OBs such as those of Autographa californica multiple nucleopolyhedrovirus (AcMNPV), another group I alphabaculovirus.

Gut microbiota is essential in PGRP-LA regulated immune protection against Plasmodium berghei infection

BACKGROUND

Malaria remains to be one of the deadliest infectious diseases and imposes substantial financial and social costs in the world. Mosquitoes rely on the immune system to control parasite infection. Peptidoglycan recognition proteins (PGRPs), a family of pattern-recognition receptors (PRR), are responsible for initiating and regulating immune signaling pathways. PGRP-LA is involved in the regulation of immune defense against the Plasmodium parasite, however, the underlying mechanism needs to be further elucidated.

METHODS

The spatial and temporal expression patterns of pgrp-la in Anopheles stephensi were analyzed by qPCR. The function of PGRP-LA was examined using a dsRNA-based RNA interference strategy. Western blot and periodic acid schiff (PAS) staining were used to assess the structural integrity of peritrophic matrix (PM).

RESULTS

The expression of pgrp-la in An. stephensi was induced in the midgut in response to the rapid proliferating gut microbiota post-blood meal. Knocking down of pgrp-la led to the downregulation of immune effectors that control gut microbiota growth. The decreased expression of these immune genes also facilitated P. berghei infection. However, such dsLA treatment did not influence the structural integrity of PM. When gut microbiota was removed by antibiotic treatment, the regulation of PGRP-LA on immune effectors was abolished and the knock down of pgrp-la failed to increase susceptibility of mosquitoes to parasite infection.

CONCLUSIONS

PGRP-LA regulates the immune responses by sensing the dynamics of gut microbiota. A mutual interaction between gut microbiota and PGRP-LA contributes to the immune defense against Plasmodium parasites in An. stephensi.

Prospects, challenges and current status of RNAi through insect feeding

RNA interference is a phenomenon in which the introduction of double-stranded RNA (dsRNA) into cells triggers the degradation of the complementary messenger RNA in a sequence-specific manner. Suppressing expression of vital genes could lead to insect death, therefore this technology has been considered as a potential strategy for insect pest control. There are three main routes of dsRNA administration into insects: (i) injections to the hemolymph, (ii) topical, and (iii) feeding. In this review, we focus on dsRNA administration through feeding. We summarize novel strategies that have been developed to improve the efficacy of this method, such as the use of nano-based formulations, engineered microorganisms, and transgenic plants. We also expose the hurdles that have to be overcome in order to use this technique as a reliable pest management method. © 2019 Society of Chemical Industry.

The family of amphioxus chitin synthases offers insight into the evolution of chitin formation in chordates

Chitin is a very important and widely-used biopolymer in fungi and lower metazoans, but mysteriously disappears in mammals. Recent studies reveal that at least lower vertebrates have chitin synthases (CS) and use them to synthesize endogenous chitin. Amphioxus, a basal chordate, therefore becomes critical to understand the evolution of CS, as it occupies the transitional position from invertebrates to vertebrates, and is considered as a good proxy to the chordate ancestor. Here, by exploiting multiple genome assemblies, high-depth RNA-seq data and synteny relations, we identify 11-12 CS genes for each amphioxus species. It represents the largest CS gene pool ever found in eukaryotes so far. As comparison, most metazoans have one or two CSs. Amphioxus is the only chordate that has both the very ancient type-I CS family and the more broadly distributed type-II CS family. Specifically, amphioxus has only one type-II CS but 10-11 type-I CSs, which means that amphioxus is the only metazoan with a greatly expanded type-I CS family. Further analysis suggests that the chordate ancestor have at least one type-II CS and an expanded of type-I CS family. We hypothesize that: these ancient CSs are mostly retained in amphioxus; but the whole type-I CS family was lost in urochordates and vertebrates; the type-II CS was later duplicated into two lineages in vertebrates and followed by stochastic losses, till all type-II CSs were eventually lost in birds and mammals. Finally, our expression profiling and preliminary gene knockout analysis suggest that amphioxus CSs could have highly diverse but mildly overlapping functions in various tissues and organs. Taken together, these findings not only provide insights into the evolution of chordate CSs, lay a foundation for further functional study of the chordate CSs. After all, it is mysterious that our chordate ancestor needed so many isoenzymes for chitin formation.

Thermal stress responses of Sodalis glossinidius, an indigenous bacterial symbiont of hematophagous tsetse flies

Tsetse flies (Diptera: Glossinidae) house a taxonomically diverse microbiota that includes environmentally acquired bacteria, maternally transmitted symbiotic bacteria, and pathogenic African trypanosomes. Sodalis glossinidius, which is a facultative symbiont that resides intra and extracellularly within multiple tsetse tissues, has been implicated as a mediator of trypanosome infection establishment in the fly’s gut. Tsetse’s gut-associated population of Sodalis are subjected to marked temperature fluctuations each time their ectothermic fly host imbibes vertebrate blood. The molecular mechanisms that Sodalis employs to deal with this heat stress are unknown. In this study, we examined the thermal tolerance and heat shock response of Sodalis. When grown on BHI agar plates, the bacterium exhibited the most prolific growth at 25^oC, and did not grow at temperatures above 30^oC. Growth on BHI agar plates at 31°C was dependent on either the addition of blood to the agar or reduction in oxygen levels. Sodalis was viable in liquid cultures for 24 hours at 30^oC, but began to die upon further exposure. The rate of death increased with increased temperature. Similarly, Sodalis was able to survive for 48 hours within tsetse flies housed at 30^oC, while a higher temperature (37^oC) was lethal. Sodalis’ genome contains homologues of the heat shock chaperone protein-encoding genes dnaK, dnaJ, and grpE, and their expression was up-regulated in thermally stressed Sodalis, both in vitro and in vivo within tsetse fly midguts. Arrested growth of E. coli dnaK, dnaJ, or grpE mutants under thermal stress was reversed when the cells were transformed with a low copy plasmid that encoded the Sodalis homologues of these genes. The information contained in this study provides insight into how arthropod vector enteric commensals, many of which mediate their host’s ability to transmit pathogens, mitigate heat shock associated with the ingestion of a blood meal.

Microorganisms associated with insects must cope with fluctuating temperatures. Because symbiotic bacteria influence the biology of their host, how they respond to temperature changes will have an impact on the host and other microorganisms in the host. The tsetse fly and its symbionts represent an important model system for studying thermal tolerance because the fly feeds exclusively on vertebrate blood and is thus exposed to dramatic temperature shifts. Tsetse flies house a microbial community that can consist of symbiotic and environmentally acquired bacteria, viruses, and parasitic African trypanosomes. This work, which makes use of tsetse’s commensal endosymbiont, Sodalis glossinidius, is significance because it represents the only examination of thermal tolerance mechanisms in a bacterium that resides indigenously within an arthropod disease vector. A better understanding of the biology of thermal tolerance in Sodalis provides insight into thermal stress survival in other insect symbionts and may yield information to help control vector-borne disease.

An influential meal: host plant dependent transcriptional variation in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae)

BACKGROUND

To understand the genetic mechanisms of insect herbivory, the transcriptional response of insects feeding on different host plant species has to be studied. Here, we generated gene expression data of the generalist herbivore Spodoptera exigua (Hübner) feeding on three selected host plant species and a control (artificial diet). The host plant species used in this study -cabbage (Brassica oleracea), maize (Zea mays) and tobacco (Nicotiana tabacum)- are members of different plant families that each employ specific defence mechanisms and toxins.

RESULTS

Spodoptera exigua larvae had a higher growth rate, indicator for herbivore success, when feeding on Z. mays compared to larvae feeding on B. oleracea or N. tabacum. Larvae feeding on the different host plant species showed divergent transcriptional responses. We identified shared and unique gene expression patterns dependent of the host plant species the larvae fed on. Unique gene expression patterns, containing uniquely upregulated transcripts including specific detoxification genes, were found for larvae feeding on either B. oleracea or N. tabacum. No diet-specific gene cluster was identified for larvae feeding on the host for which larvae showed optimal herbivore success, Z. mays, or artificial diet. In contrast, for larvae feeding on hosts for which they showed low herbivore success, specific diet-dependent gene clusters were identified. Functional annotation of these clusters indicates that S. exigua larvae deploy particular host plant-specific genes for digestion and detoxification.

CONCLUSIONS

The lack of a host plant-specific gene activity for larvae feeding on Z. mays and the artificial diet suggest a general and non-specific gene activity for host plants with optimal herbivore success. Whereas the finding of specific gene clusters containing particular digestion and detoxifying genes expressed in larvae feeding on B. oleracea and N. tabacum, with low herbivore success, imply a host plant-specific gene activity for larvae feeding on host plants with suboptimal herbivore success. This observation leads to the conclusion that a polyphagous herbivore is able to feed on a large variation of host plants due to the flexibility and diversity of genes involved in digestion and detoxification that are deployed in response to particular host plant species.

A peritrophin mediates the peritrophic matrix permeability in the workers of the bees Melipona quadrifasciata and Apis mellifera

The permeability of the peritrophic matrix, essential for its function, depends on its chemical composition. The objective was to determine if the permeability of the peritrophic matrix varies along the midgut and in the presence of anti-peritrophin-55 antibody in Melipona quadrifasciata and Apis mellifera bees. The thickness of the peritrophic matrix in both species varies between the anterior and posterior midgut regions in workers. In A. mellifera dextran molecules with 40 kDa cross the peritrophic matrix, whereas those ≥70 kDa are retained in the endoperitrophic space. In M. quadrifasciata the peritrophic matrix permeability was for molecules <40 kDa. Bees fed on anti-peritrophin-55 antibody showed an increase in peritrophic matrix permeability, but survival was not affected. In the bees studied, the peritrophic matrices have morphological differences between midgut regions, but there is no difference in their permeability along the midgut, which is affected by peritrophin 55.

Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitin is a structural component of the arthropod cuticular exoskeleton and the peritrophic matrix of the gut, which play crucial roles in growth and development. In the past few decades, our understanding of the composition, biosynthesis, assembly, degradation, and regulation of chitinous structures has increased. Many chemicals have been developed that target chitin biosynthesis (benzoyphenyl ureas, etoxazole), chitin degradation (allosamidin, psammaplin), and chitin regulation (benzoyl hydrazines), thus resulting in molting deformities and lethality. In addition, proteins that disrupt chitin structures, such as lectins, proteases, and chitinases have been utilized to halt feeding and induce mortality. Chitin-degrading enzymes, such as chitinases are also useful for improving the efficacy of bio-insecticides. Transgenic plants, baculoviruses, fungi, and bacteria have been engineered to express chitinases from a variety of organisms for control of arthropod pests. In addition, RNA interference targeting genes involved in chitin pathways and structures are now being investigated for the development of environmentally friendly pest management strategies. This review describes the chemicals and proteins used to target chitin structures and enzymes for arthropod pest management, as well as pest management strategies based upon these compounds, such as plant-incorporated-protectants and recombinant entomopathogens. Recent advances in RNA interference-based pest management, and how this technology can be used to target chitin pathways and structures are also discussed.

Intestinal Stem Cell Aging: Origins and Interventions

Regenerative processes that maintain the function of the gastrointestinal (GI) epithelium are critical for health and survival of multicellular organisms. In insects and vertebrates, intestinal stem cells (ISCs) regenerate the GI epithelium. ISC function is regulated by intrinsic, local, and systemic stimuli to adjust regeneration to tissue demands. These control mechanisms decline with age, resulting in significant perturbation of intestinal homeostasis. Processes that lead to this decline have been explored intensively in Drosophila melanogaster in recent years and are now starting to be characterized in mammalian models. This review presents a model for age-related regenerative decline in the fly intestine and discusses recent findings that start to establish molecular mechanisms of age-related decline of mammalian ISC function.

Tsetse peritrophic matrix influences for trypanosome transmission

Tsetse flies are important vectors of parasitic African trypanosomes, agents of human and animal trypanosomiasis. Easily administrable and effective tools for disease control in the mammalian host are still lacking but reduction of the tsetse vector populations can reduce disease. An alternative approach is to reduce the transmission of trypanosomes in the tsetse vector. The gut peritrophic matrix (PM) has emerged as an important regulator of parasite transmission success in tsetse. Tsetse has a Type II PM that is constitutively produced by cells in the cardia organ. Tsetse PM lines the entire gut and functions as an immunological barrier to prevent the gut epithelia from responding to commensal environmental microbes present in the gut lumen. Tsetse PM also functions as a physical barrier to trypanosome infections that enter into the gut lumen in an infective blood meal. For persistence in the gut, African trypanosomes have developed an adaptive manipulative process to transiently reduce PM efficacy. The process is mediated by mammalian trypanosome surface coat proteins, Variant Surface Glycoproteins (VSGs) which are shed in the gut lumen and taken up by cardia cells. The mechanism of PM reduction involves a tsetse microRNA (miR-275) which acts thru the Wnt signaling pathway. The PM efficacy is once again reduced later in the infection process to enable the gut established parasites to reenter into the gut lumen to colonize the salivary glands, an essential process for transmission. The ability to modulate PM integrity can lead to innovative approaches to reduce disease transmission.

Spinosad-mediated effects in the post-embryonic development of Partamona helleri (Hymenoptera: Apidae: Meliponini)

The use of insecticides based on metabolites found in live organisms, such as the insecticide spinosad, has been an option for the control of agricultural pests because of the allegedly low toxicological risk for nontarget arthropods, such as stingless bees. In the current study, we evaluate the effects of chronic oral exposure to spinosad during the larval phase on survival, developmental time, body mass, midgut epithelial remodeling, and the peritrophic matrix (PM) of Partamona helleri stingless bee workers. Worker larvae that were raised in the laboratory were orally exposed to different concentrations (0, 6.53, 13.06, 32.64, and 3,264 ng. a.i. bee^-1) of spinosad (formulation), and the resulting survival, developmental time, and body mass were studied. The concentration of spinosad recommended for use in the field (3,264 ng. a.i. bee^-1) reduced the survival of workers during development. Also, sublethal concentrations of spinosad delayed the development and caused morphological changes in the midgut epithelium. Finally, the chronic exposure of larvae to 32.64 ng. a.i. bee^-1 spinosad also altered the remodeling of the midgut during metamorphosis and affected the organization of the PM of larvae, pupae, and adults. Our data suggest possible environmental risks for using spinosad in cultures that are naturally pollinated by stingless bees.

Interaction of a Densovirus with Glycans of the Peritrophic Matrix Mediates Oral Infection of the Lepidopteran Pest Spodoptera frugiperda

The success of oral infection by viruses depends on their capacity to overcome the gut epithelial barrier of their host to crossing over apical, mucous extracellular matrices. As orally transmitted viruses, densoviruses, are also challenged by the complexity of the insect gut barriers, more specifically by the chitinous peritrophic matrix, that lines and protects the midgut epithelium; how capsids stick to and cross these barriers to reach their final cell destination where replication goes has been poorly studied in insects. Here, we analyzed the early interaction of the Junonia coenia densovirus (JcDV) with the midgut barriers of caterpillars from the pest Spodoptera frugiperda. Using combination of imaging, biochemical, proteomic and transcriptomic analyses, we examined in vitro, ex vivo and in vivo the early interaction of the capsids with the peritrophic matrix and the consequence of early oral infection on the overall gut function. We show that the JcDV particle rapidly adheres to the peritrophic matrix through interaction with different glycans including chitin and glycoproteins, and that these interactions are necessary for oral infection. Proteomic analyses of JcDV binding proteins of the peritrophic matrix revealed mucins and non-mucins proteins including enzymes already known to act as receptors for several insect pathogens. In addition, we show that JcDV early infection results in an arrest of N-Acetylglucosamine secretion and a disruption in the integrity of the peritrophic matrix, which may help viral particles to pass through. Finally, JcDV early infection induces changes in midgut genes expression favoring an increased metabolism including an increased translational activity. These dysregulations probably participate to the overall dysfunction of the gut barrier in the early steps of viral pathogenesis. A better understanding of early steps of densovirus infection process is crucial to build biocontrol strategies against major insect pests.

The Intestine of Drosophila melanogaster: An Emerging Versatile Model System to Study Intestinal Epithelial Homeostasis and Host-Microbial Interactions in Humans

In all metazoans, the intestinal tract is an essential organ to integrate nutritional signaling, hormonal cues and immunometabolic networks. The dysregulation of intestinal epithelium functions can impact organism physiology and, in humans, leads to devastating and complex diseases, such as inflammatory bowel diseases, intestinal cancers, and obesity. Two decades ago, the discovery of an immune response in the intestine of the genetic model system, Drosophila melanogaster, sparked interest in using this model organism to dissect the mechanisms that govern gut (patho) physiology in humans. In 2007, the finding of the intestinal stem cell lineage, followed by the development of tools available for its manipulation in vivo, helped to elucidate the structural organization and functions of the fly intestine and its similarity with mammalian gastrointestinal systems. To date, studies of the Drosophila gut have already helped to shed light on a broad range of biological questions regarding stem cells and their niches, interorgan communication, immunity and immunometabolism, making the Drosophila a promising model organism for human enteric studies. This review summarizes our current knowledge of the structure and functions of the Drosophila melanogaster intestine, asserting its validity as an emerging model system to study gut physiology, regeneration, immune defenses and host-microbiota interactions.

Polyhedra, spindles, phage nucleus and pyramids: Structural biology of viral superstructures

Viral infection causes comprehensive rearrangements of the cell that reflect as much host defense mechanisms as virus-induced structures assembled to facilitate infection. Regardless of their pro- or antiviral role, large intracellular structures are readily detectable by microscopy and often provide a signature characteristic of a specific viral infection. The structural features and localization of these assemblies have thus been commonly used for the diagnostic and classification of viruses since the early days of virology. More recently, characterization of viral superstructures using molecular and structural approaches have revealed very diverse organizations and roles, ranging from dynamic viral factories behaving like liquid organelles to ultra-stable crystals embedding and protecting virions. This chapter reviews the structures, functions and biotechnological applications of virus-induced superstructures with a focus on assemblies that have a regular organization, for which detailed structural descriptions are available. Examples span viruses infecting all domains of life including the assembly of virions into crystalline arrays in eukaryotic and bacterial viruses, nucleus-like compartments involved in the replication of large bacteriophages, and pyramid-like structures mediating the egress of archaeal viruses. Among these superstructures, high-resolution structures are available for crystalline objects produced by insect viruses: viral polyhedra which function as the infectious form of occluded viruses, and spindles which are potent virulence factors of entomopoxviruses. In turn, some of these highly symmetrical objects have been used to develop and validate advanced structural approaches, pushing the boundary of structural biology.

An easy, inexpensive, and sensitive method for the quantification of chitin in insect peritrophic membrane by image processing

Chitin, poly (β-(1→4)-N-acetyl-d-glucosamine), is an important biopolymer for insects that is utilized as a major component of peritrophic membrane. The chitin content in peritrophic membrane is of expedient interest from a pest control perspective, although it is hard to quantify chitin. In this study, we establish a facile method for the quantification of chitin in peritrophic membrane by image processing. In this method, chitin was indirectly quantified using chitosan–I3− complex, which exhibited a specific red-purple color. A calibration curve using a chitosan solution showed good linearity in a concentration range of 0.05–0.5 μg/μL. We quantified the amount of chitin in peritrophic membrane of Spodoptera litura (Lepidoptera: Noctuidae) larvae using this method. Throughout the study, only common inexpensive regents and easily attainable apparatuses were employed. This method can be easily applied to the sensitive quantification of the amounts of chitin and chitosan in materials by wide range of researchers.

Abbreviations: LOD: limit of detection; LOQ: limit of quantification; ROI: region of interest; RSD: relative standard deviation.