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

The mucosal immunity in crustaceans: Inferences from other species

Crustaceans such as shrimps and crabs, hold significant ecological significance and substantial economic value within marine ecosystems. However, their susceptibility to disease outbreaks and pathogenic infections has posed major challenges to production in recent decades. As invertebrate, crustaceans primarily rely on their innate immune system for defense, lacking the adaptive immune system found in vertebrates. Mucosal immunity, acting as the frontline defense against a myriad of pathogenic microorganisms, is a crucial aspect of their immune repertoire. This review synthesizes insights from comparative immunology, highlighting parallels between mucosal immunity in vertebrates and innate immune mechanisms in invertebrates. Despite lacking classical adaptive immunity, invertebrates, including crustaceans, exhibit immune memory and rely on inherent "innate immunity factors" to combat invading pathogens. Drawing on parallels from mammalian and piscine systems, this paper meticulously explores the complex role of mucosal immunity in regulating immune responses in crustaceans. Through the extrapolation from well-studied models like mammals and fish, this review infers the potential mechanisms of mucosal immunity in crustaceans and provides insights for research on mucosal immunity in crustaceans.

Exploring the contribution of the salivary gland and midgut to digestion in the swede midge (Contarinia nasturtii) through a genomics-guided approach

The larvae of Contarinia nasturtii (Kieffer) (Diptera: Cecidomyiidae), the swede midge, targets the meristem of brassica crops where they induce the formation of galls and disrupt seed and vegetable production. Previously, we examined the salivary gland transcriptome of newly-hatched first instar larvae as they penetrated the host and initiated gall formation. Here we examine the salivary gland and midgut transcriptome of third instar larvae and provide evidence for cooperative nutrient acquisition beginning with secretion of enzymes and feeding facilitators followed by gastrointestinal digestion. Sucrose, presumably obtained from the phloem, appeared to be a major nutrient source as several α-glucosidases (sucrases, maltases) and β-fructofuranosidases (invertases) were identified. Genes encoding β-fructofuranosidases/invertases were among the most highly expressed in both tissues and represented two distinct gene families that may have originated via horizontal gene transfer from bacteria. The importance of the phloem as a nutrient source is underscored by the expression of genes encoding regucalcin and ARMET (arginine-rich mutated in early stages of tumor) which interfere with calcium signalling and prevent sieve tube occlusion. Lipids, proteins, and starch appear to serve as a secondary nutrient sources. Genes encoding enzymes involved in the detoxification of glucosinolates (myrosinases, arylsulfatases, and glutathione-S-transferases) were expressed indicative of Brassicaceae host specialization. The midgut expressed simple peritrophins and mucins typical of those found in Type II peritrophic matrices, the first such description for a gall midge.

Transcriptomic analysis of Bombyx mori in its early larval stage (2nd instar) of development upon Nosema bombycis transovarial infection

The infection caused by Nosema bombycis often known as pebrine, is a devastating sericulture disease. The infection can be transmitted to the next generation through eggs laid by infected female Bombyx mori moths (transovarial) as well as with N. bombycis contaminated food (horizontal). Most diagnoses were carried out in the advanced stages of infection until the time that infection might spread to other healthy insects. Hence, early diagnosis of pebrine is of utmost importance to quarantine infected larvae from uninfected silkworm batches and stop further spread of the infection. The findings of our study provide an insight into how the silkworm larval host defence system was activated against early N. bombycis transovarial infection. The results obtained from transcriptome analysis of infected 2nd instar larvae revealed significant (adjusted P-value < 0.05) expression of 1888 genes of which 801 genes were found to be upregulated and 1087 genes were downregulated when compared with the control. Pathway analysis indicated activation of the immune deficiency (IMD) pathway, which shows a potential immune defence response against pebrine infection as well as suppression of the melanin synthesis pathway due to lower expression of prophenoloxidase activating enzyme (PPAE). Liquid chromatography mass spectrometry (LC-MS/MS) analysis of haemolymph from infected larvae shows the secretion of serpin binding protein of N. bombycis which might be involved in the suppression of the melanization pathway. Moreover, among the differentially expressed genes, we found that LPMC-61, yellow-y, gasp and osiris 9 can be utilised as potential markers for early diagnosis of transovarial pebrine infection in B. mori. Physiological as well as biochemical roles and functions of many of the essential genes are yet to be established, and enlightened research will be required to characterize the products of these genes.

Transcriptomic response of citrus psyllid salivary glands to the infection of citrus Huanglongbing pathogen

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is the key vector insect transmitting the Candidatus Liberibacter asiaticus (CLas) bacterium that causes the devastating citrus greening disease (Huanglongbing, HLB) worldwide. The D. citri salivary glands (SG) exhibit an important barrier against the transmission of HLB pathogen. However, knowledge on the molecular mechanism of SG defence against CLas infection is still limited. In the present study, we compared the SG transcriptomic response of CLas-free and CLas-infected D. citri using an illumine paired-end RNA sequencing. In total of 861 differentially expressed genes (DEGs) in the SG upon CLas infection, including 202 upregulated DEGs and 659 downregulated DEGs were identified. Functional annotation analysis showed that most of the DEGs were associated with cellular processes, metabolic processes, and the immune response. Gene ontology and Kyoto Encyclopaedia of Genes and Genomes enrichment analyses revealed that these DEGs were enriched in pathways involving carbohydrate metabolism, amino acid metabolism, the immune system, the digestive system, the lysosome, and endocytosis. A total of 16 DEGs were randomly selected to further validate the accuracy of RNA-Seq dataset by reverse-transcription quantitative polymerase chain reaction. This study provides substantial transcriptomic information regarding the SG of D. citri in response to CLas infection, which may shed light on the molecular interaction between D. citri and CLas, and provides new ideas for the prevention and control of citrus psyllid.

Honey Bee Larval Hemolymph as a Source of Key Nutrients and Proteins Offers a Promising Medium for Varroa destructor Artificial Rearing

Varroa destructor, a major ectoparasite of the Western honey bee Apis mellifera, is a widespread pest that damages colonies in the Northern Hemisphere. Throughout their lifecycle, V. destructor females feed on almost every developmental stage of their host, from the last larval instar to the adult. The parasite is thought to feed on hemolymph and fat body, although its exact diet and nutritional requirements are poorly known. Using artificial Parafilm™ dummies, we explored the nutrition of V. destructor females and assessed their survival when fed on hemolymph from bee larvae, pupae, or adults. We compared the results with mites fed on synthetic solutions or filtered larval hemolymph. The results showed that the parasites could survive for several days or weeks on different diets. Bee larval hemolymph yielded the highest survival rates, and filtered larval plasma was sufficient to maintain the mites for 14 days or more. This cell-free solution therefore theoretically contains all the necessary nutrients for mite survival. Because some bee proteins are known to be hijacked without being digested by the parasite, we decided to run a proteomic analysis of larval honey bee plasma to highlight the most common proteins in our samples. A list of 54 proteins was compiled, including several energy metabolism proteins such as Vitellogenin, Hexamerin, or Transferrins. These molecules represent key nutrient candidates that could be crucial for V. destructor survival.

Hypoxia-induced tracheal elasticity in vector beetle facilitates the loading of pinewood nematode

Many pathogens rely on their insect vectors for transmission. Such pathogens are under selection to improve vector competence for their transmission by employing various tissue or cellular responses of vectors. However, whether pathogens can actively cause hypoxia in vectors and exploit hypoxia responses to promote their vector competence is still unknown. Fast dispersal of pinewood nematode (PWN), the causal agent for the destructive pine wilt disease and subsequent infection of pine trees, is characterized by the high vector competence of pine sawyer beetles (Monochamus spp.), and a single beetle can harbor over 200,000 PWNs in its tracheal system. Here, we demonstrate that PWN loading activates hypoxia in tracheal system of the vector beetles. Both PWN loading and hypoxia enhanced tracheal elasticity and thickened the apical extracellular matrix (aECM) of the tracheal tubes while a notable upregulated expression of a resilin-like mucin protein Muc91C was observed at the aECM layer of PWN-loaded and hypoxic tracheal tubes. RNAi knockdown of Muc91C reduced tracheal elasticity and aECM thickness under hypoxia conditions and thus decreasing PWN loading. Our study suggests a crucial role of hypoxia-induced developmental responses in shaping vector tolerance to the pathogen and provides clues for potential molecular targets to control pathogen dissemination.

Genetic variation and microbiota in bumble bees cross-infected by different strains of C. bombi

The bumblebee Bombus terrestris is commonly infected by a trypanosomatid gut parasite Crithidia bombi. This system shows a striking degree of genetic specificity where host genotypes are susceptible to different genotypes of parasite. To a degree, variation in host gene expression underlies these differences, however, the effects of standing genetic variation has not yet been explored. Here we report on an extensive experiment where workers of twenty colonies of B. terrestris were each infected by one of twenty strains of C. bombi. To elucidate the host's genetic bases of susceptibility to infection (measured as infection intensity), we used a low-coverage (~2 x) genome-wide association study (GWAS), based on angsd, and a standard high-coverage (~15x) GWAS (with a reduced set from a 8 x 8 interaction matrix, selected from the full set of twenty). The results from the low-coverage approach remained ambiguous. The high-coverage approach suggested potentially relevant genetic variation in cell surface and adhesion processes. In particular, mucin, a surface mucoglycoprotein, potentially affecting parasite binding to the host gut epithelia, emerged as a candidate. Sequencing the gut microbial community of the same bees showed that the abundance of bacterial taxa, such as Gilliamella, Snodgrassella, or Lactobacillus, differed between 'susceptible' and 'resistant' microbiota, in line with earlier studies. Our study suggests that the constitutive microbiota and binding processes at the cell surface are candidates to affect infection intensity after the first response (captured by gene expression) has run its course. We also note that a low-coverage approach may not be powerful enough to analyse such complex traits. Furthermore, testing large interactions matrices (as with the full 20 x 20 combinations) for the effect of interaction terms on infection intensity seems to blur the specific host x parasite interaction effects, likely because the outcome of an infection is a highly non-linear process dominated by variation in individually different pathways of host defence (immune) responses.

AA15 lytic polysaccharide monooxygenase is required for efficient chitinous cuticle turnover during insect molting

Microbial lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of crystalline polysaccharides including chitin and cellulose. The discovery of a large assortment of LPMO-like proteins widely distributed in insect genomes suggests that they could be involved in assisting chitin degradation in the exoskeleton, tracheae and peritrophic matrix during development. However, the physiological functions of insect LPMO-like proteins are still undetermined. To investigate the functions of insect LPMO15 subgroup I-like proteins (LPMO15-1s), two evolutionarily distant species, Tribolium castaneum and Locusta migratoria, were chosen. Depletion by RNAi of T. castaneum TcLPMO15-1 caused molting arrest at all developmental stages, whereas depletion of the L. migratoria LmLPMO15-1, prevented only adult eclosion. In both species, LPMO15-1-deficient animals were unable to shed their exuviae and died. TEM analysis revealed failure of turnover of the chitinous cuticle, which is critical for completion of molting. Purified recombinant LPMO15-1-like protein from Ostrinia furnacalis (rOfLPMO15-1) exhibited oxidative cleavage activity and substrate preference for chitin. These results reveal the physiological importance of catalytically active LPMO15-1-like proteins from distant insect species and provide new insight into the enzymatic mechanism of cuticular chitin turnover during molting.

A mucin protein predominantly expressed in the female-specific symbiotic organ of the stinkbug Plautia stali

Diverse insects are obligatorily associated with microbial symbionts, wherein the host often develops special symbiotic organs and vertically transmits the symbiont to the next generation. What molecular factors underpin the host-symbiont relationship is of great interest but poorly understood. Here we report a novel protein preferentially produced in a female-specific symbiotic organ of the stinkbug Plautia stali, whose posterior midgut develops numerous crypts to host a Pantoea-allied bacterial mutualist. In adult females, several posteriormost crypts are conspicuously enlarged, presumably specialized for vertical symbiont transmission. We detected conspicuous protein bands specific to the female’s swollen crypts by gel electrophoresis, and identified them as representing a novel mucin-like glycoprotein. Histological inspections confirmed that the mucin protein is localized to the female’s swollen crypts, coexisting with a substantial population of the symbiotic bacteria, and excreted from the swollen crypts to the midgut main tract together with the symbiotic bacteria. Using RNA interference, we successfully suppressed production of the mucin protein in adult females of P. stali. However, although the mucin protein was depleted, the symbiont population persisted in the swollen crypts, and vertical symbiont transmission to the next generation occurred. Possible biological roles and evolutionary trajectory of the symbiosis-related mucin protein are discussed.

Distinct molecular impact patterns of abamectin on Apis mellifera ligustica and Apis cerana cerana

The effects of insecticides on bee health are a topic of intensive research. Although abamectin is toxic to bees, the molecular impact of abamectin needs to be clarified. Here, we found that Apis cerana cerana exhibited a higher mortality rate when exposed to abamectin than Apis mellifera ligustica. In addition, A. cerana cerana had markedly higher numbers of differentially expressed genes (DEGs), differentially expressed proteins (DEPs) and differentially expressed metabolites (DEMs) than A. mellifera ligustica during exposure to abamectin. These results indicate that abamectin exposure exerts stronger effects on A. cerana cerana than on A. mellifera ligustica. In addition, six DEGs, two DEPs and two DEMs overlapped between the two bee species under abamectin exposure; however, some genes or proteins from the zinc finger protein, superoxide dismutase and peroxiredoxin families and the energy metabolism pathway were only unregulated in A. cerana cerana, which indicates a significant difference in the impact of abamectin on the two bee species. Despite these differences, several of the same gene families, such as heat shock proteins, cytochrome P450, odorant-binding proteins and cuticle proteins, and pathways, including the carbohydrate metabolism, immune system, lipid metabolism, amino acid metabolism, sensory system, locomotion and development pathways, were influenced by abamectin exposure in both A. cerana cerana and A. mellifera ligustica. Together, our results indicate that abamectin causes adverse effects on bees and thus poses a risk to bee populations and that abamectin exposure affects A. cerana cerana more strongly than A. mellifera ligustica. These findings improve our understanding of the behavioural and physiological effects of abamectin on bees.

The effect of the brood and the queen on early gene expression in bumble bee workers' brains

Worker reproduction in social insects is often regulated by the queen, but can be regulated by the brood and nestmates, who may use different mechanisms to induce the same outcomes in subordinates. Analysis of brain gene expression patterns in bumble bee workers (Bombus impatiens) in response to the presence of the queen, the brood, both or neither, identified 18 differentially expressed genes, 17 of them are regulated by the queen and none are regulated by the brood. Overall, brain gene expression differences in workers were driven by the queen’s presence, despite recent studies showing that brood reduces worker egg laying and provides context to the queen pheromones. The queen affected important regulators of reproduction and brood care across insects, such as neuroparsin and vitellogenin, and a comparison with similar datasets in the honey bee and the clonal raider ant revealed that neuroparsin is differentially expressed in all species. These data emphasize the prominent role of the queen in regulating worker physiology and behavior. Genes that serve as key regulators of workers’ reproduction are likely to play an important role in the evolution of sociality.

Integrative Analysis of lncRNA-mRNA Co-expression Provides Novel Insights Into the Regulation of Developmental Transitions in Female Varroa destructor

Varroa destructor is a major pathogenic driver of the Western honeybee colony losses globally. Understanding the developmental regulation of V. destructor is critical to develop effective control measures. Development is a complex biological process regulated by numerous genes and long non-coding RNAs (lncRNAs); however, the underlying regulation of lncRNAs in the development of V. destructor remains unknown. In this study, we analyzed the RNA sequencing (RNA-Seq) data derived from the four stages of female V. destructor in the reproductive phase (i.e., egg, protonymph, deutonymph, and adult). The identified differentially expressed mRNAs and lncRNAs exhibited a stage-specific pattern during developmental transitions. Further functional enrichment established that fat digestion and absorption, ATP-binding cassette (ABC) transporters, mitogen-activated protein kinase (MAPK) signaling pathway, and ubiquitin-proteasome pathway play key roles in the maturation of female V. destructor. Moreover, the lncRNAs and mRNAs of some pivotal genes were significantly upregulated at the deutonymph stage, such as cuticle protein 65/6.4/63/38 and mucin 5AC, suggesting that deutonymph is the key stage of metamorphosis development and pathogen resistance acquisition for female V. destructor. Our study provides novel insights into a foundational understanding of V. destructor biology.

Characteristics of the Peritrophic Matrix of the Silkworm, Bombyx mori and Factors Influencing Its Formation

Simple Summary

The insect midgut is an important digestive organ with the peritrophic matrix (PM) being a semi-permeable membrane secreted by the midgut cells. The PM plays an important role in improving midgut digestion efficiency and protecting the midgut from food particles and exogenous pathogens. The silkworm, Bombyx mori, is an economically important insect. Understanding the structure of the PM is necessary for studying its function, but characteristics of PM in B. mori have been rarely reported. In this study, we conducted a comprehensive study on the PM structure of the PM in silkworms and found its thickness increased gradually during growth, but there was no difference in the thickness comparing the anterior, middle, and posterior regions. Permeability of the PM gradually decreased from the anterior to posterior regions. In addition, we found the formation of the PM was influenced by food ingestion and the gut microbiota.

Abstract

The peritrophic matrix (PM) secreted by the midgut cells of insects is formed by the binding of PM proteins to chitin fibrils. The PM envelops the food bolus, serving as a barrier between the content of the midgut lumen and its epithelium, and plays a protective role for epithelial cells against mechanical damage, pathogens, toxins, and other harmful substances. However, few studies have investigated the characteristics and synthesis factors of the PM in the silkworm, Bombyx mori. Here, we examined the characteristics of the PM in the silkworms. The PM thickness of the silkworms increased gradually during growth, while there was no significant difference in thickness along the entire PM region. Permeability of the PM decreased gradually from the anterior to posterior PM. We also found that PM synthesis was affected by food ingestion and the gut microbiota. Our results are beneficial for future studies regarding the function of the PM in silkworms.