The Wigglesworth Lecture: Insects for studying fundamental problems in biology

Genome-wide annotation and comparative analysis revealed conserved cuticular protein evolution among non-biting midges with varied environmental adaptability

Chironomidae, non-biting midges, a diverse and abundant insect group in global aquatic ecosystems, represent an exceptional model for investigating genetic adaptability mechanisms in aquatic insects due to their extensive species diversity and resilience to various environmental conditions. The cuticle in insects acts as the primary defense against ecological pressures. Cuticular Proteins (CPs) determine cuticle characteristics, facilitating adaptation to diverse challenges. However, systematic annotation of CP genes has only been conducted for one Chironomidae species, Propsilocerus akamusi, by our team. In this study, we expanded this annotation by identifying CP genes in eight additional Chironomidae species, covering all Chironomidae species with available genome data. We identified a total of 889 CP genes, neatly categorized into nine CP families: 215 CPR RR1 genes, 272 CPR RR2 genes, 23 CPR RR3 genes, 21 CPF genes, 16 CPLCA genes, 19 CPLCG genes, 28 CPLCP genes, 77 CPAP genes, and 37 Tweedle genes. Subsequently, we conducted a comprehensive phylogenetic analysis of CPs within the Chironomidae family. This expanded annotation of CP genes across diverse Chironomidae species significantly contributes to our understanding of their remarkable adaptability.

The transcription factor RUNT-like regulates pupal cuticle development via promoting a pupal cuticle protein transcription

Holometabolous insects undergo morphological remodeling from larvae to pupae and to adults with typical changes in the cuticle; however, the mechanism is unclear. Using the lepidopteran agricultural insect Helicoverpa armigera, cotton bollworm, as a model, we revealed that the transcription factor RUNT-like (encoded by Runt-like) regulates the development of the pupal cuticle via promoting a pupal cuticle protein gene (HaPcp) expression. The HaPcp was highly expressed in the epidermis and wing during metamorphosis and was found being involved in pupal cuticle development by RNA interference (RNAi) analysis in larvae. Runt-like was also strongly upregulated in the epidermis and wing during metamorphosis. Knockdown of Runt-like produced similar phenomena, a failure of abdomen yellow envelope and wing formation, to those following HaPcp knockdown. The insect molting hormone 20-hydroxyecdysonen (20E) upregulated HaPcp transcription via RUNT-like. 20E upregulated Runt-like transcription via nuclear receptor EcR and the transcription factor FOXO. Together, RUNT-like and HaPCP are involved in pupal cuticle development during metamorphosis under 20E regulation.

Silencing NlFAR7 destroyed the pore canals and related structures of the brown planthopper

Fatty acyl-CoA reductase (FAR) is one of the key enzymes, which catalyses the conversion of fatty acyl-CoA to the corresponding alcohols. Among the FAR family members in the brown planthopper (Nilaparvata lugens), NlFAR7 plays a pivotal role in both the synthesis of cuticular hydrocarbons and the waterproofing of the cuticle. However, the precise mechanism by which NlFAR7 influences the formation of the cuticle structure in N. lugens remains unclear. Therefore, this paper aims to investigate the impact of NlFAR7 through RNA interference, transmission electron microscope, focused ion beam scanning electron microscopy (FIB-SEM) and lipidomics analysis. FIB-SEM is employed to reconstruct the three-dimensional (3D) architecture of the pore canals and related cuticle structures in N. lugens subjected to dsNlFAR7 and dsGFP treatments, enabling a comprehensive assessment of changes in the cuticle structures. The results reveal a reduction in the thickness of the cuticle and disruptions in the spiral structure of pore canals, accompanied by widened base and middle diameters. Furthermore, the lipidomics comparison analysis between dsNlFAR7- and dsGFP-treated N. lugens demonstrated that there were 25 metabolites involved in cuticular lipid layer synthesis, including 7 triacylglycerols (TGs), 5 phosphatidylcholines (PCs), 3 phosphatidylethanolamines (PEs) and 2 diacylglycerols (DGs) decreased, and 4 triacylglycerols (TGs) and 4 PEs increased. In conclusion, silencing NlFAR7 disrupts the synthesis of overall lipids and destroys the cuticular pore canals and related structures, thereby disrupting the secretion of cuticular lipids, thus affecting the cuticular waterproofing of N. lugens. These findings give significant attention with reference to further biochemical researches on the substrate specificity of FAR protein, and the molecular regulation mechanisms during N. lugens life cycle.

Unraveling the Role of Cuticular Protein 3-like (HvCP3L) in the Chitin Pathway through RNAi and Methoxyfenozide Stress Response in Heortia vitessoides Moore

Cuticle proteins (CPs) constitute a multifunctional family; however, the physiological role of Cuticle Protein 3-like (CP3L) in Heortia vitessoides Moore remains largely unclear. In this study, we cloned the HvCP3L gene from the transcriptional library of Heortia vitessoides Moore. RT-qPCR results revealed that HvCP3L exhibited high expression levels during the larval stage of Heortia vitessoides Moore, particularly at the L5D1 stage, observed in both larval and adult heads. Through RNA interference, we successfully silenced the HvCP3L gene, resulting in a significant reduction in the survival rate of Heortia vitessoides Moore, with the survival rate from larvae to adults plummeting to a mere 17.7%, accompanied by phenotypic abnormalities. Additionally, we observed that the knockdown of HvCP3L led to the inhibition of genes in the chitin pathway. Following exposure to methoxyfenozide stress, the HvCP3L gene exhibited significant overexpression, coinciding with phenotypic abnormalities. These findings underscore the pivotal role of HvCP3L in the growth and development of Heortia vitessoides Moore.

Functional importance of groups I and II chitinases, CHT5 and CHT10, in turnover of chitinous cuticle during embryo hatching and post-embryonic molting in the red flour beetle, Tribolium castaneum

Chitinases (CHT) comprise a large gene family in insects and have been classified into at least eleven subgroups. Many studies involving RNA interference (RNAi) have demonstrated that depletion of group I (CHT5s) and group II (CHT10s) CHT transcripts causes lethal molting arrest in several insect species including the red flour beetle, Tribolium castaneum, presumably due to failure of degradation of chitin in their old cuticle. In this study we investigated the functions of CHT5 and CHT10 in turnover of chitinous cuticle in T. castaneum during embryonic and post-embryonic molting stages. RNAi and transmission electron microscopic (TEM) analyses indicate that CHT10 is required for cuticular chitin degradation at each molting period analyzed, while CHT5 is essential for pupal-adult molting only. We further analyzed the functions of these genes during embryogenesis in T. castaneum. Real-time qPCR analysis revealed that peak expression of CHT10 occurred prior to that of CHT5 during embryonic development as has been observed at post-embryonic molting periods in several other insect species. With immunogold-labeling TEM analysis using a fluorescein isothiocyanate-conjugated chitin-binding domain protein (FITC-CBD) probe, chitin was detected in the serosal cuticle but not in any other regions of the eggshell including the chorion and vitelline membrane layers. Injection of double-stranded RNA (dsRNA) for CHT5 (dsCHT5), CHT10 (dsCHT10) or their co-injection (dsCHT5/10) into mature adult females had no effect on their fecundity and the resulting embryos developed normally inside the egg. There were no obvious differences in the morphology of the outer chorion, inner chorion and vitelline membrane among eggs from these dsRNA-treated females. However, unlike dsCHT5 eggs, dsCHT10 and dsCHT5/10 eggs exhibited failure of turnover of the serosal cuticle in which the horizontal chitinous laminae remained intact, resulting in lethal embryo hatching defects. These results indicate that group I CHT5 is essential for pupal-adult molting, whereas group II CHT10 plays an essential role in cuticular chitin degradation in T. castaneum during both embryonic hatching and all of the post-embryonic molts. CHT10 can serve in place of CHT5 in chitin degradation, except during the pupal-adult molt when both enzymes are indispensable to complete eclosion.

Pri peptides temporally coordinate transcriptional programs during epidermal differentiation

To achieve a highly differentiated state, cells undergo multiple transcriptional processes whose coordination and timing are not well understood. In Drosophila embryonic epidermal cells, polished-rice (Pri) smORF peptides act as temporal mediators of ecdysone to activate a transcriptional program leading to cell shape remodeling. Here, we show that the ecdysone/Pri axis concomitantly represses the transcription of a large subset of cuticle genes to ensure proper differentiation of the insect exoskeleton. The repression relies on the transcription factor Ken and persists for several days throughout early larval stages, during which a soft cuticle allows larval crawling. The onset of these cuticle genes normally awaits the end of larval stages when the rigid pupal case assembles, and their premature expression triggers abnormal sclerotization of the larval cuticle. These results uncovered a temporal switch to set up distinct structures of cuticles adapted to the animal lifestyle and which might be involved in the evolutionary history of insects.

C. elegans epicuticlins define specific compartments in the apical extracellular matrix and function in wound repair

The apical extracellular matrix (aECM) of external epithelia often contains lipid-rich outer layers that contribute to permeability barrier function. The external aECM of nematode is known as the cuticle and contains an external lipid-rich layer, the epicuticle. Epicuticlins are a family of tandem repeat proteins originally identified as components of the insoluble fraction of the cuticular aECM and thought to localize in or near epicuticle. However, there has been little in vivo analysis of epicuticlins. Here, we report the localization analysis of the three C. elegans epicuticlins (EPIC proteins) using fluorescent protein knock-ins to visualize endogenously expressed proteins, and further examine their in vivo function using genetic null mutants. By TIRF microscopy, we find that EPIC-1 and EPIC-2 localize to the surface of the cuticle in larval and adult stages in close proximity to the outer lipid layer. EPIC-1 and EPIC-2 also localize to interfacial cuticles and adult-specific cuticle struts. EPIC-3 expression is restricted to the stress-induced dauer stage, where it localizes to interfacial aECM in the buccal cavity. Strikingly, skin wounding in the adult induces epic-3 expression, and EPIC-3::mNG localizes to wound scars. Null mutants lacking one, two, or all three EPIC proteins display reduced survival after skin wounding yet are viable with low penetrance defects in epidermal morphogenesis. Our results suggest EPIC proteins define specific aECM compartments and have roles in wound repair.

Knickkopf (LmKnk) is required for chitin organization in the foregut of Locusta migratoria

The foregut, located at the front of the digestive tract, serves a vital role in insects by storing and grinding food into small particles. The innermost layer of the foregut known as the chitinous intima, comes into direct contact with the food and acts as a protective barrier against abrasive particles. Knickkopf (Knk) is required for chitin organization in the chitinous exoskeleton, tracheae and wings. Despite its significance, little is known about the biological function of Knk in the foregut. In this study, we found that LmKnk was stably expressed in the foregut, and highly expressed before molting in Locusta migratoria. To ascertain the biological function of LmKnk in the foregut, we synthesized specific double-stranded LmKnk (dsLmKnk) and injected it into locusts. Our findings showed a significant decrease in the foregut size, along with reduced food intake and accumulation of residues in the foregut after dsLmKnk injection. Morphological observations revealed that newly formed intima became thinner and lacked chitin lamella. Furthermore, fluorescence immunohistochemistry revealed that LmKnk was located in the apical region of new intima and epithelial cells. Taken together, this study provides insights into the biological function of LmKnk in the foregut, and identifies the potential target gene for exploring biological pest management strategies.

Degradation of marine crustacean shell wastes through single-stage co-fermentation using proteolytic and chitinolytic bacteria

Management of crustacean shell waste (SW) through an eco-friendly technique is an environmental obligation to control pollution. The present study showed a novel approach through the simultaneous application of proteolytic and chitinolytic bacteria to effectively degrade unprocessed crustacean SW. For this, the bacteria with concurrent chitinolytic and proteolytic activity (Bacillus subtilis, Priestia megaterium, or Bacillus amyloliquefaciens) were applied either alone or in combination with one proteolytic strain (Paenibacillus alvei) in the unprocessed lobster, crab, and shrimp SW. The method degraded the shells with high deproteinization (> 90%) and demineralization efficiency (> 90%). The degradation was confirmed through scanning electron microscopy. The highest weight loss achieved with shrimp, crab, and lobster shells was 93.67%, 82.60%, and 83.33%, respectively. B. amyloliquefaciens + P. alvei combination produced the highest weight loss in crab and lobster SW, whereas all combinations produced statistically similar weight loss in shrimp SW. There was a concurrent production of N-acetyl glucosamine (up to 532.89, 627.87, and 498.95 mg/g of shrimp, lobster, and crab shell, respectively, with P. megaterium + P. alvei and B. amyloliquefaciens + P. alvei in all SW) and amino acids (4553.8, 648.89, 957.27 μg/g of shrimp, lobster, and crab shells, respectively with B. subtilis + P. alvei in shrimp and B. amyloliquefaciens + P. alvei in crab and lobster). Therefore, it is concluded that, for the first time, efficient degradation of crustacean shell waste was observed using chitinolytic and proteolytic bacterial fermentation with the obtention of byproducts, providing a basis for further application in SW management.

The influences of green light on locomotion, growth and reproduction in the brown planthopper Nilaparvata lugens

BACKGROUND

Light stimulation at a specific wavelength triggers various responses in insects and can be used for pest control. To develop efficient and ecofriendly photophysical pest control methods, the effects of green light on locomotion, growth (molting and eclosion) and reproduction in Nilaparvata lugens (Stål) (BPH), a major rice pest, were studied. Transcriptomics and transmission electron microscopy (TEM) were used to investigate the mechanisms involved.

RESULTS

BPH adults showed disrupted daily locomotion patterns following green light treatment at night and exhibited abnormal locomotion peaks. Total 6-day locomotion of brachypterous adults was significantly greater than in the control group. The durations of growth stages 1-4 were all shorter under green light treatment than in the control, whereas the time from fourth molting to eclosion (stage 5) was significantly longer. When BPH adults under green light treatment began laying eggs, the egg hatching ratio (36.69%) was significantly lower than in the control (47.49%). Moreover, in contrast to the control, BPH molting and eclosion events tended to happen more at night. Transcriptome analysis proved that green light significantly affected the expression of genes involved in cuticular proteins, chitin deacetylase and chitinase, which are related to cuticular development. TEM observations confirmed abnormal cuticular development in nymph and adult BPHs (endocuticle, exocuticle and pore canals) under green light treatment.

CONCLUSION

Green light treatment at night notably affected locomotion, growth and reproduction in BPH, thus providing a novel idea for controlling this pest. © 2023 Society of Chemical Industry.

Tweedle gene family of Drosophila suzukii (Matsumura) larva enhances the basal tolerance to cold and hypoxia

BACKGROUND

Drosophila suzukii (Matsumura) is considered a quarantine pest in the A2 list because it causes serious infection and huge economic losses. Cold and controlled atmosphere treatments have been used to control immature stage pests in fresh fruits. Herein, the basal tolerance response of D. suzukii egg, larva and pupa to cold and hypoxia stress were studied, and underlying transcriptome mechanisms in the larva were pinpointed.

RESULTS

The third instar was more tolerant than 12-h-old egg and 8-day-old pupa when treated at 3 °C + 1% O2 for 7 days, with 34.00% ± 5.22% larval survival. Hypoxia influenced the effect of cold treatment on D. suzukii. Larval survival decreased at 3 °C + 1% O2 , but increased at 0 °C + 1% O2 . Survival increased with temperature between 0 and 5 °C + 1% O2 , but decreased significantly at 25 °C + 1% O2 . RNA-sequencing results showed that the Tweedle (Twdl) family was upregulated and uniquely enriched in larvae treated at 3 °C + 1% O2 . In addition, RNA interference-mediated silencing of a key Twdl gene reduced the survival rate after cold and hypoxia treatment.

CONCLUSION

Hypoxia was able to influence the effect of cold treatment on the survival of D. suzukii positively or negatively. Structural constituents of the chitin-based cuticle, in particular Twdl genes, body morphogenesis, and ATP synthesis-coupled proton transport were involved in the tolerance to cold and hypoxia. In future, the Twdl gene could be used as a nanocarrier delivering RNA pesticides to control D. suzukii in the field and so prevent its worldwide spread. © 2023 Society of Chemical Industry.

RR-1 cuticular protein TcCPR69 is required for growth and metamorphosis in Tribolium castaneum

Cuticle is not only critical for protecting insects from noxious stimuli but is also involved in a variety of metabolic activities. Cuticular proteins (CPs) affect cuticle structure and mechanical properties during insect growth, reproduction, and environmental adaptation. Here, we describe the identification and characterization of a member of the RR-1 subfamily of CPs with an R&R consensus (CPR) in Tribolium castaneum (TcCPR69). Although it was previously reported to be highly expressed in the wings, we found that knocking down TcCPR69 by RNA interference (RNAi) did not cause obvious wing abnormalities but markedly disrupted the growth and metamorphosis of beetles with 100% cumulative mortality; additionally, the chitin content of the pharate adult was decreased and the new abdominal cuticle was significantly thinner before molting. TcCPR69 showed chitin-binding ability and the expression levels of key genes involved in chitin metabolism (trehalase [TcTRE], chitin synthase [TcCHSA and TcCHSB], and chitinase [TcCHT5 and TcCHT10]) were also decreased by TcCPR69 knockdown. TcCPR69 gene expression peaked shortly after molting and was increased 2.61 fold at 12 h after 20-hydroxyecdysone (20E) injection. This was reversed by RNAi of the ecdysone-related genes ecdysone receptor (TcECR) and fushi tarazu transcription factor 1 (TcFTZ-F1). These results indicate that TcCPR69 is positively regulated by 20E signaling to contribute to cuticle formation and maintain chitin accumulation during the growth and metamorphosis of beetles.

Nuclear receptor HR3 mediates transcriptional regulation of chitin metabolic genes during molting in Tribolium castaneum

BACKGROUND

Chitin, a major component of insect cuticles, plays a critical role in insect molting and morphogenesis. Thus, coordination of chitin remodeling during insect development requires tight transcriptional control of the chitin metabolism genes involved in chitin synthesis, assembly and degradation. However, the molecular mechanism underlying transcriptional coordination of chitin metabolism genes during beetle development is not yet completely understood.

RESULTS

We cloned the full-length cDNA encoding hormone receptor 3 (TcHR3) from Tribolium castaneum and showed a critical role of TcHR3 in modulating chitin metabolism gene expression during molting. Genome-wide transcriptome analysis of HR3-deficient old larvae using RNA sequencing analysis revealed a positive correlation between TcHR3 and transcription of chitin metabolism genes involved in chitin synthesis and degradation. In addition, HR3 overexpression significantly induced the gene promoter activity of N-acetylglucosaminidase 1 (NAG1) involved in chitin degradation and UDP-N-acetylglucosamine pyrophosphorylase 1 (UAP1) involved in chitin synthesis. Chromatin immunoprecipitation analysis revealed that HR3 could directly bind to HR3-response element of NAG1 and UAP1 promoters. Finally, HR3-deficient late instar larvae and prepupae exhibited defects in larval-larval and larval-pupal molting, respectively, leading to eventual larval death because developing larvae were trapped inside the old cuticle as a result of abnormal chitin metabolism.

CONCLUSION

TcHR3 is a transcriptional regulator of chitin metabolic genes for molting of T. castaneum. Controlling the molting system by TcHR3 might be a new management strategy for selective control of red flour beetle infestation. © 2022 Society of Chemical Industry.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

Ultrastructural analysis of beetle larva cuticles during infection with the entomopathogenic fungus, Beauveria bassiana

BACKGROUND

Beauveria bassiana is one of the commercially available entomopathogenic fungi (EPF), and a number of isolates with high virulence and broad host spectrum have been used to control agricultural and forest pests. Although the functional importance of genes in EPFs' pathogenesis have been extensively studied, the precise ultrastructural mechanism of the fungal infection, particularly penetration of the host insect cuticles, is not well understood.

RESULTS

In this study, we investigated the morphology and ultrastructure of the larval cuticle of the red flour beetle, Tribolium castaneum, after treatment with B. bassiana ERL1170 expressing an enhanced green fluorescent protein (Bb-eGFP). The Bb-eGFP showed high virulence against the larvae, with approximately 90% mortality at 48 h after treatment (HAT) and 100% at 72 HAT under our infection conditions. In these larvae, the regions of the body wall with flexible cuticles, such as the ventral and ventrolateral thorax and abdomen, became darkly melanized, but there was little to no melanization in the rigid dorsal cuticular structures. Confocal microscopy and transmission electron microscopy (TEM) indicated that germinated conidia on the surface of the larval cuticle were evident at 6 HAT, which formed penetration pegs and began to penetrate the several cuticle layers/laminae by 12 HAT. The penetration pegs then developed invading hyphae, some of which passed through the cuticle and reached the epidermal cells by 24 HAT. The larval cuticle was aggressively and extensively disrupted by 48 HAT, and a number of outgrowing hyphae were observed at 72 HAT.

CONCLUSIONS

Our results indicate that Bb-eGFP is capable of infection and penetrating T. castaneum larvae shortly after inoculation (~24 HAT) at the body regions with apparently flexible and membranous cuticles, such as the ventral intersegmental regions and the ventrolateral pleura. This study provides details on the histopathogenesis of the host cuticle by infection and penetration of EPFs, which can facilitate the management of insect pests. © 2022 Society of Chemical Industry.

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 nuclear receptor HR4 is essential for the formation of epidermal cuticle in the migratory locust, Locusta migratoria

Nuclear receptors (NRs) function as key factors in diverse signaling and metabolic pathways. Previous studies have focused on the roles of a nuclear receptor, hormone receptor 4 (HR4), mainly in holometabolous insects, while current knowledge of its function in hemimetabolous insects is still limited. In this study, we identified a HR4 gene in the orthopteran species Locusta migratoria. The full-length open reading frame of LmHR4 comprises 2694-nucleotides encoding a polypeptide of 897 amino acids, which contained a DNA-binding and a ligand-binding domain. Analyzing LmHR4 expression by quantitative reverse-transcription PCR (RT-qPCR) revealed that LmHR4 was highly expressed in integument, hindgut and fat body. During development from 3rd and 5th nymphal instars, the expression of LmHR4 reached maximal levels before ecdysis. We further demonstrated that LmHR4 expression is induced by 20-hydroxyecdysone (20E) and suppressed by silencing LmEcR, suggesting that LmHR4 expression is controlled by 20E signaling. The dsLmHR4-injected nymphs failed to molt and remained in the nymphal stage until death. Hematoxylin and eosin staining of the integument indicated that apolysis in the dsLmHR4-injected insects was delayed compared to that in control insects. Chitin staining and ultra-structural analysis showed that both the synthesis of the new cuticle and the degradation of the old cuticle were blocked in dsLmHR4-injected insects. Silencing LmHR4 decreased 20E titer and down-regulated the transcript levels of genes involved in chitin synthesis and degradation. Taken together, these results suggest that LmHR4 is essential for the formation of epidermal cuticle by mediating the 20E signaling to regulate the expression of chitin synthesis and degradation genes.

Unveiling characteristic proteins for the structural development of beetle elytra

Beetles possess a set of highly modified and tanned forewings, elytra, which are lightweight yet rigid and tough. Immediately after eclosion, the elytra are initially thin, pale and soft. However, they rapidly expand and subsequently become hardened and often dark, resulting from both pigmentation and sclerotization. Here, we identified changes in protein composition during the developmental processes of the elytra in the Japanese rhinoceros beetle, Trypoxylus dichotomus. Using mass spectrometry, a total of 414 proteins were identified from both untanned and tanned elytra, including 31 cuticular proteins (CPs), which constitute one of the major components of insect cuticles. Moreover, CPs containing Rebers and Riddiford motifs (CPR), the most abundant CP family, were separated into two groups based on their expression and amino acid sequences, such as a Gly-rich sequence region and Ala-Ala-Pro repeats. These protein groups may play crucial roles in elytra formation at different time points, likely including self-assembly of chitin nanofibers that control elytral macro and microstructures and dictate changes in other properties (i.e., mechanical property). Clarification of the protein functions will enhance the understanding of elytra formation and potentially benefit the development of lightweight materials for industrial and biomedical applications. STATEMENT OF SIGNIFICANCE: The beetle elytron is a light-weight natural bio-composite which displays high stiffness and toughness. This structure is composed of chitin fibrils and proteins, some of which are responsible for architectural development and hardening. This work, which involves insights from molecular biology and materials science, investigated changes in proteomic, architectural, and localized mechanical characteristics of elytra from the Japanese rhinoceros beetle to understand molecular mechanisms driving elytra development. In the present study, we identified a set of new protein groups which are likely related to the structural development of elytra and has potential for new pathways for processing green materials.

Improving Polysaccharide-Based Chitin/Chitosan-Aerogel Materials by Learning from Genetics and Molecular Biology

Improved wound healing of burnt skin and skin lesions, as well as medical implants and replacement products, requires the support of synthetical matrices. Yet, producing synthetic biocompatible matrices that exhibit specialized flexibility, stability, and biodegradability is challenging. Synthetic chitin/chitosan matrices may provide the desired advantages for producing specialized grafts but must be modified to improve their properties. Synthetic chitin/chitosan hydrogel and aerogel techniques provide the advantages for improvement with a bioinspired view adapted from the natural molecular toolbox. To this end, animal genetics provide deep knowledge into which molecular key factors decisively influence the properties of natural chitin matrices. The genetically identified proteins and enzymes control chitin matrix assembly, architecture, and degradation. Combining synthetic chitin matrices with critical biological factors may point to the future direction with engineering materials of specific properties for biomedical applications such as burned skin or skin blistering and extensive lesions due to genetic diseases.

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

Many organisms rely on directional water transport schemes for the purpose of water retention and collection. Directional transport of water and other fluids is also technologically relevant, for example to harvest water, in separation processes, packaging solutions, functional clothing, and many other applications. One strategy to promote mass transport along a preferential direction is to create compositionally asymmetric, multi-layered, or compositionally graded architectures. In recent years, the investigation of natural and artificial membranes based on this design has attracted growing interest and allowed researchers to develop a good understanding of how the properties of such membranes can be tailored to meet the demands of particular applications. Here a summary of theoretical works on mass transport through dense asymmetric membranes, comprehensive reviews of biological and artificial membranes featuring this design, and a discussion of applications, remaining questions, and opportunities are provided.

Chitin deacetylase 2 is essential for molting and survival of Tetranychus urticae

Chitin metabolism has long been considered promising targets for development of biorational pesticides. Considering the increasing challenges of controlling the twospotted spider mite, Tetranychus urticae Koch, the roles of chitin deacetylases (CDAs) during molting process and mite development are explored. TuCDA1 and TuCDA2 differ in expression patterns during the development process. Feeding of double-strand RNA (dsRNA) against TuCDA1 or TuCDA2 has lethal effects on the mites. Especially TuCDA2 displays a much stronger phenotype than TuCDA1 (p = 0.0003). The treated mites fail to shed the old cuticle and are trapped within exuviate until they die. The aberrant cuticle structure observed by scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM) may be responsible for the lethal phenotype of TuCDA1 and TuCDA2 knocked down mites. However, treatment with both dsRNA-CDA1 and dsRNA-CDA2 cannot significantly enhance the lethal effects of dsRNA-CDA2, which indicates partially redundant function of TuCDA1 and TuCDA2. TuCDA2 may play a key role during the molting and development process. Chitin-modifying enzyme such as TuCDA2 is potential target of RNA interference through feeding.

Identification and expression analysis of cuticular protein genes in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

Structural cuticular proteins (CPs) are major components of the insect cuticle, and they play critical roles in insect development and insecticide resistance. Here, a total of 196 CP genes were successfully annotated in the Plutella xylostella genome. On the basis of motif analysis, these CPs were classified into 10 different families, including 122 CPR, 12 CPAP1, 8 CPAP3, 9 CPLCP, 2 Tweedle, 1 CPF, 1 CPFL, 1 CPCFC, 17 CPG and 2 18 aa proteins, and the remaining 21 unclassified CPs were classed as cuticular proteins hypothetical (CPH). A phylogenetic analysis of CPs from different insects revealed species-specific clades of RR-1 and RR-2 genes, suggesting that CP gene duplication might occur independently among insect taxa, while we also found that some other CPs (such as CPAP1 and CPAP3) had a closer relationship based on their conserved domain architecture. Using available RNAseq libraries, the expression profiles of the CPs were analyzed over the four developmental stages of the insect (i.e., egg, larva, pupa, and adult), revealing stage-specific expression patterns for the CPs. In a chlorpyrifos resistant strain, 18 CP genes were found to be more than two-fold upregulated compared to the susceptible control strain, and qRT-PCR analysis showed that these CP genes were overexpressed after exposure to chlorpyrifos, suggesting a potential role in the molecular mechanism of insecticide resistance in P. xylostella. This study provides the tools and molecular basis to study the role of CPs in the post-embryonal development and the mechanisms of insecticide resistance of P. xylostella.

Transcriptomic, proteomic and ultrastructural studies on salinity-tolerant Aedes aegypti in the context of rising sea levels and arboviral disease epidemiology

BACKGROUND

Aedes aegypti mosquito, the principal global vector of arboviral diseases, lays eggs and undergoes larval and pupal development to become adult mosquitoes in fresh water (FW). It has recently been observed to develop in coastal brackish water (BW) habitats of up to 50% sea water, and such salinity tolerance shown to be an inheritable trait. Genomics of salinity tolerance in Ae. aegypti has not been previously studied, but it is of fundamental biological interest and important for controlling arboviral diseases in the context of rising sea levels increasing coastal ground water salinity.

RESULTS

BW- and FW-Ae. aegypti were compared by RNA-seq analysis on the gut, anal papillae and rest of the carcass in fourth instar larvae (L4), proteomics of cuticles shed when L4 metamorphose into pupae, and transmission electron microscopy of cuticles in L4 and adults. Genes for specific cuticle proteins, signalling proteins, moulting hormone-related proteins, membrane transporters, enzymes involved in cuticle metabolism, and cytochrome P450 showed different mRNA levels in BW and FW L4 tissues. The salinity-tolerant Ae. aegypti were also characterized by altered L4 cuticle proteomics and changes in cuticle ultrastructure of L4 and adults.

CONCLUSIONS

The findings provide new information on molecular and ultrastructural changes associated with salinity adaptation in FW mosquitoes. Changes in cuticles of larvae and adults of salinity-tolerant Ae. aegypti are expected to reduce the efficacy of insecticides used for controlling arboviral diseases. Expansion of coastal BW habitats and their neglect for control measures facilitates the spread of salinity-tolerant Ae. aegypti and genes for salinity tolerance. The transmission of arboviral diseases can therefore be amplified in multiple ways by salinity-tolerant Ae. aegypti and requires appropriate mitigating measures. The findings in Ae. aegypti have attendant implications for the development of salinity tolerance in other fresh water mosquito vectors and the diseases they transmit.

Chitinase 10 controls chitin amounts and organization in the wing cuticle of Drosophila

Wings are essential for insect fitness. A number of proteins and enzymes have been identified to be involved in wing terminal differentiation, which is characterized by the formation of the wing cuticle. Here, we addressed the question whether chitinase 10 (Cht10) may play an important role in chitin organization in the wings of the fruit fly Drosophila melanogaster. Initially, we first found that Cht10 expression coincides with the expression of the chitin synthase coding gene kkv. This suggests that the respective proteins may cooperate during wing differentiation. In tissue-specific RNA interference experiments, we demonstrate that suppression of Cht10 causes an excess in chitin amounts in the wing cuticle. Chitin organization is severely disrupted in these wings. Based on these data, we hypothesize that Cht10 restricts chitin amounts produced by Kkv in order to ensure normal chitin organization and wing cuticle formation. In addition, we found by scanning electron microscopy that Cht10 suppression also affects the cuticle surface. In turn, cuticle inward permeability is enhanced in Cht10-less wings. Moreover, flies with reduced Cht10 function are unable to fly. In conclusion, Cht10 is essential for wing terminal differentiation and function.

Reverse and forward engineering of Drosophila corneal nanocoatings

Insect eyes have an anti-reflective coating, owing to nanostructures on the corneal surface creating a gradient of refractive index between that of air and that of the lens material^1,2. These nanocoatings have also been shown to provide anti-adhesive functionality³. The morphology of corneal nanocoatings are very diverse in arthropods, with nipple-like structures that can be organized into arrays or fused into ridge-like structures⁴. This diversity can be attributed to a reaction-diffusion mechanism⁴ and patterning principles developed by Alan Turing⁵, which have applications in numerous biological settings⁶. The nanocoatings on insect corneas are one example of such Turing patterns, and the first known example of nanoscale Turing patterns⁴. Here we demonstrate a clear link between the morphology and function of the nanocoatings on Drosophila corneas. We find that nanocoatings that consist of individual protrusions have better anti-reflective properties, whereas partially merged structures have better anti-adhesion properties. We use biochemical analysis and genetic modification techniques to reverse engineer the protein Retinin and corneal waxes as the building blocks of the nanostructures. In the context of Turing patterns, these building blocks fulfil the roles of activator and inhibitor, respectively. We then establish low-cost production of Retinin, and mix this synthetic protein with waxes to forward engineer various artificial nanocoatings with insect-like morphology and anti-adhesive or anti-reflective function. Our combined reverse- and forward-engineering approach thus provides a way to economically produce functional nanostructured coatings from biodegradable materials.

Effect of RNAi-mediated silencing of two Knickkopf family genes (LmKnk2 and LmKnk3) on cuticle formation and insecticide susceptibility in Locusta migratoria

BACKGROUND

Knickkopf (Knk) proteins play crucial roles in the formation of insect cuticle. Recent studies in the holometabolous insect red flour beetle (Tribolium castaneum) have shown that three Knk genes encoding TcKnk, TcKnk2 and TcKnk3 play different but essential roles at different developmental stages and in different tissues. However, the functions of Knk genes had not been fully explored in hemimetabolous insects such as the migratory locust Locusta migratoria.

RESULTS

We identified three transcripts of LmKnk-like genes LmKnk2 and LmKnk3 with the full-length cDNA sequences, which were named as LmKnk2, LmKnk3-FL and LmKnk3-5'. These three transcripts were highly expressed before molting and mainly expressed in the integument. Among these genes, silencing only LmKnk3-5' by RNA interference (RNAi) caused molting defects and high mortality of the locusts. Injection of dsLmKnk3-5' dramatically decreased chitin content, but did not affect cuticle laminar ultra-structures in the integument. After the knockdown of LmKnk3-5' transcript, lipid deposition on the cuticle surface was impeded, and locusts exhibited increased susceptibility to each of four insecticides in three different classes. However, no visible phenotypic changes were observed after LmKnk2 or LmKnk3-FL was silenced by RNAi.

CONCLUSION

We demonstrate that LmKnk3-5' is essential for cuticle formation in L. migratoria. This contrasts the findings that the cognate protein in T. castaneum TcKnk3-5' is dispensable for cuticle formation and survival. Hence, we provide some evidence that the function of Knk-type proteins may be species-specific. We therefore think that LmKnk3-5' may be a good target for the application of RNAi-based technologies for species-specific insect pest management. © 2020 Society of Chemical Industry.

The interaction between cuticle free fatty acids (FFAs) of the cockroaches Blattella germanica and Blatta orientalis and hydrolases produced by the entomopathogenic fungus Conidiobolus coronatus

The interactions between entomopathogenic fungi and insects serve a classic example of a co-evolutionary arms race between pathogens and their target host. The cuticle, site of the first contact between insects and entomopathogenic fungus, is an important defensive barrier against pathogens. It is covered by a layer of lipids that appears to play a key role in these processes and cuticular free fatty acid (FFA) profiles are consider as a determinant of susceptibility, or resistance, to fungal infections. These profiles are species-specific. The cockroaches Blattella germanica (Blattodea: Blattidae) and Blatta orientalis (Blattodea: Ectobiidae) are unsusceptible to the soil fungus Conidiobolus coronatus (Entomophthorales: Ancylistaceae) infection, therefore we studied the profiles of FFAs in order to understand the defensive capabilities of the cockroaches. The fungus was cultivated for three weeks in minimal medium. Cell-free filtrate was obtained, assayed for elastase, N-acetylglucosaminidase, chitobiosidase and lipase activity, and then used for in vitro hydrolysis of the cuticle from wings and thoraces of adults and oothecae. The amounts of amino acids, N-glucosamine and FFAs released from the hydrolysed cuticle samples were measured after eight hours of incubation. The FFA profiles of the cuticle of adults, and the wings, thoraces and oothecae of both species were established using GC-MS and the results were correlated with the effectiveness of fungal proteases, chitinases and lipases in the hydrolyzation of cuticle samples. Positive correlations would suggest the existence of compounds used by the fungus as nutrients, whereas negative correlations may indicate that these compounds could be engaged in insect defence.

Genome-wide identification and analysis of genes encoding cuticular proteins in the endoparasitoid wasp Pteromalus puparum (Hymenoptera: Pteromalidae)

The multifunctional insect cuticle serves as the exoskeleton, determines body shape, restricts water loss, provides attachment sites for muscles and internal organs and is a formidable barrier to invaders. It is morphologically divided into three layers, including envelope, epicuticle, and procuticle and is composed of chitin and cuticular proteins (CPs). Annotation of CPs and their cognate genes may help understand the structure and functions of insect cuticles. In this paper, we interrogated the genome of Pteromalus puparum, an endoparasitoid wasp that parasitizes Pieris rapae and Papilio xuthus pupae, and identified 82 genes encoding CPs belonging to six CP families, including 62 in the CPR family, 8 in CPAP3, 5 in CPF/CPFL, 2 low complexity proteins, 2 in TWDL, and 3 in Apidermin. We used six RNA-seq libraries to determine CP gene expression profiles through development and compared the cuticle hydrophobicity between the P. puparum and the ectoparasitoid Nasonia vitripennis based on GRAVY values of CPR sequences. In the Nasonia-Pteromalus comparison, we found in both N. vitripennis and P. puparum, the peak of their CPR hydrophobicity displayed at their pupal stage, whereas their adult stage showed the lowest level. Except at the adult stage, the CPR hydrophobicity in N. vitripennis is always higher than P. puparum. Finally, we identified three novel Apidermin genes, a family found solely in Hymenoptera and revealed a new sequence feature of this family. This new information contributes to a broader understanding of insect CPs generally.

Comparative Proteomics on Deep-Sea Amphipods after in Situ Copper Exposure

The interest in deep-sea mining increased along with the environmental concerns of these activities to the deep-sea fauna. The discovery of optimal biomarkers of deep-sea mining activities in deep-sea species is a crucial step toward the supply of important ecological information for environmental impact assessment. In this study, an in situ copper exposure experiment was performed on deep-sea scavenging amphipods. Abyssorchomene distinctus individuals were selected among all the exposed amphipods for molecular characterization. Copper concentration within the gut was assessed, followed by a tandem mass tag-based coupled with two-dimensional liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) applied to identify and quantify the protein expression changes after 48 h of exposure. 2937 proteins were identified and annotated, and 1918 proteins among all identified proteins were assigned by at least two nonambiguous peptides. The screening process was performed based on the differences in protein abundance and the specific correlation between the proteins and copper in previous studies. These differentially produced proteins include Na⁺/K⁺ ATPase, cuticle, chitinase, and proteins with unknown function. Their abundances showed correlation with copper and had high sensitivity to indicate the copper level, being here proposed as biomarker candidates for deep-sea mining activities in the future. This is a key step in the development of environmental impact assessment of deep-sea mining activities integrating ecotoxicological data.

Comparative transcriptomic analysis and endocuticular protein gene expression of alate adults, workers and soldiers of the termite Reticulitermes aculabialis

BACKGROUND

The insect cuticle is mainly composed of exocuticle and endocuticle layers that consist of a large number of structural proteins. The thickness and synthesis of the exocuticle depend on different castes that perform various functions in alates, workers and soldiers. However, it is not clear whether the soft endocuticle is involved in the division of labour in termite colonies. To reveal the structural characteristics of the endocuticle in different castes, we investigated the thickness of endocuticle layers in alates, workers and soldiers of the termite Reticulitermes aculabialis, and then we sequenced their transcriptome and detected the endocuticle protein genes. The differential expression levels of the endocuticular protein genes were confirmed in the three castes.

RESULTS

We found that there was a great difference in the thickness of the endocuticle among the alates, soldiers and workers. The thickest endocuticle layers were found in the heads of the workers 7.88 ± 1.67 μm. The endocuticle layer in the head of the workers was approximately three-fold and nine-fold thicker than that in the heads of soldiers and alates, respectively. The thinnest endocuticle layers occurred in the head, thorax and abdomen of alates, which were 0.86 ± 0.15, 0.76 ± 0.24 and 0.52 ± 0.17 μm thick, respectively, and had no significant differences. A total of 43,531,650 clean sequencing reads was obtained, and 89,475 unigenes were assembled. Of the 70 identified cuticular protein genes, 10 endocuticular genes that belong to the RR-1 family were selected. qRT-PCR analysis of the five endocuticular genes (SgAbd-2, SgAbd-9, Abd-5, SgAbd-2-like and Abd-4-like) revealed that the endocuticle genes were more highly expressed in workers than in soldiers and alates.

CONCLUSION

These results suggest that SgAbd and Abd are the key components of the endocuticle. We infer that the thicker endocuticle in workers is helpful for them to perform their functions against environmental stress.

Genome-wide annotation and comparative analysis of cuticular protein genes in the noctuid pest Spodoptera litura

Insect cuticle is considered an adaptable and versatile building material with roles in the construction and function of exoskeleton. Its physical properties are varied, as the biological requirements differ among diverse structures and change during the life cycle of the insect. Although the bulk of cuticle consists basically of cuticular proteins (CPs) associated with chitin, the degree of cuticular sclerotization is an important factor in determining its physical properties. Spodoptera litura, the tobacco cutworm, is an important agricultural pest in Asia. Compared to the domestic silkworm, Bombyx mori, another lepidopteran whose CP genes have been well annotated, S. litura has a shorter life cycle, hides in soil during daytime beginning in the 5th instar and is exposed to soil in the pupal stage without the protection of a cocoon. In order to understand how the CP genes may have been adapted to support the characteristic life style of S. litura, we searched its genome and found 287 putative cuticular proteins that can be classified into 9 CP families (CPR with three groups (RR-1, RR-2, RR-3), CPAP1, CPAP3, CPF, CPFL, CPT, CPG, CPCFC and CPLCA), and a collection of unclassified CPs named CPH. There were also 112 cuticular proteins enriched in Histidine residues with content varying from 6% to 30%, comprising many more His-rich cuticular proteins than B. mori. A phylogenetic analysis between S. litura, M. sexta and B. mori uncovered large expansions of RR-1 and RR-2 CPs, forming large gene clusters in different regions of S. litura chromosome 9. We used RNA-seq analysis to document the expression profiles of CPs in different developmental stages and tissues of S. litura. The comparative genomic analysis of CPs between S. litura and B. mori integrated with the unique behavior and life cycle of the two species offers new insights into their contrasting ecological adaptations.

LmCDA1 organizes the cuticle by chitin deacetylation in Locusta migratoria

Cells produce an extracellular matrix (ECM) with a stereotypic organization that is important for tissue function. The insect cuticle is a layered ECM that mainly consists of the polysaccharide chitin and associated proteins adopting a quasi-crystalline structure. Our understanding of the molecular mechanisms deployed during construction of the highly ordered protein-chitin ECM so far is limited. In this study, we report on the role of the chitin deacetylase 1 (LmCDA1) in the organization of the protein-chitin ECM in the migratory locust Locusta migratoria, and LmCDA1 localizes predominantly to the apical tier of the protein-chitin ECM, but it is also found in lower regions. Reduction of LmCDA1 function correlates with lower amounts of chitin and impedes conversion of chitin to chitosan by deacetylation. Establishment of the quasi-crystalline architecture of the protein-chitin ECM is, however, independent of LmCDA1 activity, but it is dependent on another chitin deacetylase, LmCDA2, which has no detectable effects on chitin deacetylation and, as shown previously, no influence on chitin content. Our data reveal that LmCDA1 and LmCDA2 act in parallel and independently from each other in defining the dimensions of the cuticle. Both enzymes are non-uniformly distributed within the protein-chitin matrix, suggesting a site-autonomous function.

The fatty acid elongase gene family in the brown planthopper, Nilaparvata lugens

The cuticular hydrocarbon (CHC) biosynthetic pathways branches off from the synthesis of fatty acids. Fatty acid elongases (ELOs) are enzymes catalyzing the synthesis of long-chain fatty acids and thereby contribute to the diversification of CHCs. Based on bioinformatics analyses we identified 20 ELO genes in the brown planthopper, Nilaparvata lugens. RNA interference against these genes demonstrated that 9 NlELO genes were essential for the survival of N. lugens nymphs and adults. Indeed, knockdown of NlELOs 1, 3, 4, 7, 8, 9, 10, 12 and 18 caused lethal phenotypes with a thin and wizened body and reduced lipids in the fat body. Surface analysis by scanning electron microscopy and CHC quantification indicated that knockdown of NlELOs 2, 3, 8 and 16 additionally resulted in a smooth body surface and a decrease in CHC amounts. Therefore, we speculate that long-chain CHCs are needed for CHC attachment to the cuticle surface. CHC deficiency, in turn, resulted in increased adhesion of water droplets and secreted honeydew to the animal surface and the inability of N. lugens to survive in paddy fields with varying humidity. Our present study provides an initial comprehensive analysis of ELO gene functions in an insect, and may serve to better understand the biology of CHCs.

The putative C-type lectin Schlaff ensures epidermal barrier compactness in Drosophila

The stability of extracellular matrices is in general ensured by cross-linking of its components. Previously, we had shown that the integrity of the layered Drosophila cuticle relies on the presence of a covalent cuticular dityrosine network. Production and composition of this structure remained unstudied. In this work, we present our analyses of the schlaff (slf) gene coding for a putative C-type lectin that is needed for the adhesion between the horizontal cuticle layers. The Slf protein mainly localizes between the two layers called epicuticle and procuticle that separate from each other when the function of Slf is reduced or eliminated paralleling the phenotype of a cuticle with reduced extracellular dityrosine. Localisation of the dityrosinylated protein Resilin to the epicuticle-procuticle interface suggests that the dityrosine network mediates the adhesion of the epicuticle to the procuticle. Ultimately, compromised Slf function is associated with massive water loss. In summary, we propose that Slf is implied in the stabilisation of a dityrosine layer especially between the epicuticle and the procuticle that in turn constitutes an outward barrier against uncontrolled water flow.

Chitin in Arthropods: Biosynthesis, Modification, and Metabolism

Chitin is a structural constituent of extracellular matrices including the cuticle of the exoskeleton and the peritrophic matrix (PM) of the midgut in arthropods. Chitin chains are synthesized through multiple biochemical reactions, organized in several hierarchical levels and associated with various proteins that give their unique physicochemical characteristics of the cuticle and PM. Because, arthropod growth and morphogenesis are dependent on the capability of remodeling chitin-containing structures, chitin biosynthesis and degradation are highly regulated, allowing ecdysis and regeneration of the cuticle and PM. Over the past 20 years, much progress has been made in understanding the physiological functions of chitinous matrices. In this chapter, we mainly discussed the biochemical processes of chitin biosynthesis, modification and degradation, and various enzymes involved in these processes. We also discussed cuticular proteins and PM proteins, which largely determine the physicochemical properties of the cuticle and PM. Although rapid advances in genomics, proteomics, RNA interference, and other technologies have considerably facilitated our research in chitin biosynthesis, modification, and metabolism in recent years, many aspects of these processes are still partially understood. Further research is needed in understanding how the structural organization of chitin synthase in plasma membrane accommodate chitin biosynthesis, transport of chitin chain across the plasma membrane, and release of the chitin chain from the enzyme. Other research is also needed in elucidating the roles of chitin deacetylases in chitin organization and the mechanism controlling the formation of different types of chitin in arthropods.

An ungrouped cuticular protein is essential for normal endocuticle formation in the brown planthopper

Using transcriptome analysis of tissues of the brown planthopper (BPH), Nilaparvata lugens, we identified a gene tentatively designated NlCP21.92 that was expressed at high levels in the integument. Spatiotemporal expression profiling with quantitative PCR and Western blotting verified its integument-specific expression and showed periodic expression during molting. The open reading frame was GC-rich and encoded a hydrophobic polypeptide. The polypeptide contained AAPA/V repeat motifs and other sequence features similar to several reported cuticular proteins but lacked an R&R consensus and other chitin-binding domains. Double-stranded RNA-mediated RNA interference of the NlCP21.92 resulted in abnormal and lethal morphological phenotypes, and transmission electron microscopy revealed the corresponding ultrastructural defects. Immunohistochemical staining demonstrated that the NlCP21.92 protein was primarily located in the procuticle. Our results suggest that NlCP21.92 is a novel ungrouped cuticular protein essential for normal endocuticle formation.

Beyond aerodynamics: The critical roles of the circulatory and tracheal systems in maintaining insect wing functionality

Insect wings consist almost entirely of lifeless cuticle; yet their veins host a complex multimodal sensory apparatus and other tissues that require a continuous supply of water, nutrients and oxygen. This review provides a survey of the various living components in insect wings, as well as the specific contribution of the circulatory and tracheal systems to provide all essential substances. In most insects, hemolymph circulates through the veinal network in a loop flow caused by the contraction of accessory pulsatile organs in the thorax. In other insects, hemolymph oscillates into and out of the wings due to the complex interaction of several factors, such as heartbeat reversal, intermittent pumping of the accessory pulsatile organs in the thorax, and the elasticity of the wall of a special type of tracheae. A practically unexplored subject is the need for continuous hydration of the wing cuticle to retain its flexibility and toughness, including the associated problem of water loss due to evaporation. Also, widely neglected is the influence of the hemolymph mass and the circulating flow in the veins on the aerodynamic properties of insect wings during flight. Ventilation of the extraordinarily long wing tracheae is probably accomplished by intricate interactions with the circulatory system, and by the exchange of oxygen via cutaneous respiration.

A comprehensive omics analysis and functional survey of cuticular proteins in the brown planthopper

The cuticle, mainly composed of chitin and cuticular proteins (CPs), is a multifunctional structure of arthropods. CPs usually account for >1% of the total insect proteins encoded in the genome. Why does an insect need so many different CPs? In this study, we use comprehensive large-scale technologies to study the full complement of CPs and their functions in the brown planthopper (BPH). A total of 32 of the 140 BPH CP genes are found to be essential for nymph/adult development, egg production, or embryo development; in addition, redundant and complementary functions of CPs are revealed.

Cuticle, mainly composed of chitin and cuticular proteins (CPs), is a multifunctional structure of arthropods. CPs usually account for >1% of the total insect proteins. Why does an insect encode so many different CP genes in the genome? In this study, we use comprehensive large-scale technologies to study the full complement of CPs (i.e., the CP-ome) of the brown planthopper (BPH), Nilaparvata lugens, a major rice plant pest. Eight CP families (CPR, CPF, TWDL, CPLCP, CPG, CPAP1, CPAP3, and CPAPn) including 140 proteins in BPH, in which CPAPn is a CP family that we discovered. The CPG family that was considered to be restricted to the Lepidoptera has also been identified in BPH. As reported here, CPLCP family members are characterized by three conserved sequence motifs. In addition, we identified a testis protein family with a peritrophin A domain that we named TPAP. We authenticated the real existence of 106 proteins among the 140 CPs. RNA interference (RNAi) experiments were conducted against 135 CP genes in early- and late-instar nymphs and newly emerged female adults, demonstrating that 32 CPs were essential for BPH normal development or egg production. Combined RNAi experiments suggested redundant and complementary functions of the large number of CPs. Transcriptomic data revealed that the CP genes were expressed in a tissue-specific manner, and there were four clusters of developmental expression patterns. This study gives a comprehensive understanding of the roles of CPs in an insect cuticle.

Revisiting the developmental and cellular role of the pigmentation gene yellow in Drosophila using a tagged allele

Pigmentation is a diverse and ecologically relevant trait in insects. Pigment formation has been studied extensively at the genetic and biochemical levels. The temporality of pigment formation during animal development, however, is more elusive. Here, we examine this temporality, focusing on yellow, a gene involved in the formation of black melanin. We generated a protein-tagged yellow allele in the fruit fly Drosophila melanogaster, which allowed us to precisely describe Yellow expression pattern at the tissue and cellular levels throughout development. We found Yellow expressed in the pupal epidermis in patterns prefiguring black pigmentation. We also found Yellow expressed in a few central neurons from the second larval instar to adult stages, including a subset of neurons adjacent to the clock neurons marked by the gene Pdf. We then specifically examined the dynamics of Yellow expression domain and subcellular localization in relationship to pigment formation. In particular, we showed how a late step of re-internalization is regulated by the large low-density lipoprotein receptor-related protein Megalin. Finally we suggest a new function for Yellow in the establishment of sharp pigmentation pattern boundaries, whereby this protein may assume a structural role, anchoring pigment deposits or pigmentation enzymes in the cuticle.

A chitinase with two catalytic domains is required for organization of the cuticular extracellular matrix of a beetle

Insect cuticle or exoskeleton is an extracellular matrix formed primarily from two different structural biopolymers, chitin and protein. During each molt cycle, a new cuticle is deposited simultaneously with degradation of the inner part of the chitinous procuticle of the overlying old exoskeleton by molting fluid enzymes including epidermal chitinases. In this study we report a novel role for an epidermal endochitinase containing two catalytic domains, TcCHT7, from the red flour beetle, Tribolium castaneum, in organizing chitin in the newly forming cuticle rather than in degrading chitin present in the prior one. Recombinant TcCHT7 expressed in insect cells is membrane-bound and capable of hydrolyzing an extracellular chitin substrate, whereas in vivo, this enzyme is also released from the plasma membrane and co-localizes with chitin in the entire procuticle. RNAi of TcCHT7 reveals that this enzyme is nonessential for any type of molt or degradation of the chitinous matrix in the old cuticle. In contrast, TcCHT7 is required for maintaining the integrity of the cuticle as a compact structure of alternating electron-dense and electron-lucent laminae. There is a reduction in thickness of elytral and leg cuticles after RNAi for TcCHT7. TcCHT7 is also required for formation of properly oriented long chitin fibers inside pore canals that are vertically oriented columnar structures, which contribute to the mechanical strength of a light-weight, yet rigid, adult cuticle. The conservation of CHT7-like proteins harboring such a unique domain configuration among many insect and other arthropod species indicates a critical role for the group III class of chitinases in the higher ordered organization of chitin fibers for development of the structural integrity of many invertebrate exoskeletons.

Insect cuticle or exoskeleton is an extracellular matrix consisting of three major morphologically distinct layers, the water-proofing envelope, the protein-rich epicuticle and the chitin/protein-rich procuticle. To accommodate growth, insects must periodically replace their cuticles in a process called “molting or ecdysis”. During each molt cycle a new cuticle is deposited simultaneously with degradation of the inner part of the chitinous procuticle of the old one by molting fluid enzymes including epidermal chitinases. We show that a chitinase, CHT7, from the red flour beetle, Tribolium castaneum, belonging to a subfamily (group III) of chitinases that have two catalytic domains, is necessary for organization of chitin-containing structures in nascent cuticle, which contributes to the rigidity of the extracellular matrix. This unexpected function is distinct from that of other groups of epidermal chitinases that catalyze the turnover of chitin in old cuticle during the molting process. Because group III chitinases are highly conserved among insect and other arthropod species, we propose that these enzymes have a novel function in processing nascent chitin chains during cuticle assembly and organization into higher order structures that include horizontally stacked laminae and vertically oriented pore canals of many invertebrate cuticular extracellular matrices.

Classification and infection mechanism of entomopathogenic fungi

ABSTRACT: Entomopathogenic fungi are important biological control agents throughout the world, have been the subject of intensive research for more than 100 years, and can occur at epizootic or enzootic levels in their host populations. Their mode of action against insects involves attaching a spore to the insect cuticle, followed by germination, penetration of the cuticle, and dissemination inside the insect. Strains of entomopathogenic fungi are concentrated in the following orders: Hypocreales (various genera), Onygenales (Ascosphaera genus), Entomophthorales, and Neozygitales (Entomophthoromycota).

Development and ultrastructure of the rigid dorsal and flexible ventral cuticles of the elytron of the red flour beetle, Tribolium castaneum

Insect exoskeletons are composed of the cuticle, a biomaterial primarily formed from the linear and relatively rigid polysaccharide, chitin, and structural proteins. This extracellular material serves both as a skin and skeleton, protecting insects from environmental stresses and mechanical damage. Despite its rather limited compositional palette, cuticles in different anatomical regions or developmental stages exhibit remarkably diverse physicochemical and mechanical properties because of differences in chemical composition, molecular interactions and morphological architecture of the various layers and sublayers throughout the cuticle including the envelope, epicuticle and procuticle (exocuticle and endocuticle). Even though the ultrastructure of the arthropod cuticle has been studied rather extensively, its temporal developmental pattern, in particular, the synchronous development of the functional layers in different cuticles during a molt, is not well understood. The beetle elytron, which is a highly modified and sclerotized forewing, offers excellent advantages for such a study because it can be easily isolated at precise time points during development. In this study, we describe the morphogenesis of the dorsal and ventral cuticles of the elytron of the red flour beetle, Tribolium castaneum, during the period from the 0 d-old pupa to the 9 d-old adult. The deposition of exocuticle and mesocuticle is substantially different in the two cuticles. The dorsal cuticle is four-fold thicker than the ventral. Unlike the ventral cuticle, the dorsal contains a thicker exocuticle consisting of a large number of horizontal laminae and vertical pore canals with pore canal fibers and rib-like veins and bristles as well as a mesocuticle, lying right above the enodcuticle. The degree of sclerotization appears to be much greater in the dorsal cuticle. All of these differences result in a relatively thick and tanned rigid dorsal cuticle and a much thinner and less pigmented membrane-like ventral cuticle.

The ABC transporter ABCH-9C is needed for cuticle barrier construction in Locusta migratoria

ATP-binding cassette (ABC) transporters constitute a large superfamily of proteins that mediate transport of a diverse number of substrates including nutrients, lipids and xenobiotics across membranes serving a variety of developmental and physiological functions. Here, we report on the molecular properties and biological roles of the ABC transporter LmABCH-9C in the migratory locust Locusta migratoria. LmABCH-9C was expressed continuously during nymphal development in all tissues including the integument. Expression was highest just after molting. Suppression of LmABCH-9C transcript levels by RNA interference (RNAi) in nymphs provoked death during or soon after molting to the next stage. These nymphs lost weight within minutes after molting. Moreover, high humidity rescued the lethality of molted dsLmABCH-9C-injected nymphs. In histological experiments, we find that the amounts of inner-cuticular lipids are reduced in nymphs with suppressed LmABCH-9C expression. These data together indicate that LmABCH-9C is needed for lipid-dependent desiccation resistance, paralleling the function of ABCH-9C in Tribolium castaneum. Hence, the function of this ABC transporter seems to be conserved across insect species ranging from hemimetabolous (L. migratoria) to holometabolous (T. castaneum) species. In addition, we find that cuticle inward impermeability is compromised in nymphs with reduced LmABCH-9C function. In summary, consistent with the model that cuticular lipids are necessary to prevent desiccation and penetration of xenobiotics in insects, we hypothesize that LmABCH-9C is involved in the construction of a lipid-based barrier at the surface of the cuticle especially after molting to protect the animal against uncontrolled water loss and entry. Susceptibility of this ABC transporter to RNAi-mediated knockdown designates it as an excellent target for RNAi-based insect pest control.

Localization of RR-1 and RR-2 cuticular proteins within the cuticle of Anopheles gambiae

The largest arthropod cuticular protein family, CPR, has the Rebers and Riddiford (R&R) Consensus that in an extended form confers chitin-binding properties. Two forms of the Consensus, RR-1 and RR-2, have been recognized and initial data suggested that the RR-1 and RR-2 proteins were present in different regions within the cuticle itself. Thus, RR-2 proteins would contribute to exocuticle that becomes sclerotized, while RR-1s would be found in endocuticle that remains soft. An alternative, and more common, suggestion is that RR-1 proteins are used for soft, flexible cuticles such as intersegmental membranes, while RR-2s are associated with hard cuticle such as sclerites and head capsules. We used TEM immunogold detection to localize the position of several RR-1 and RR-2 proteins in the cuticle of Anopheles gambiae. RR-1s were localized in the procuticle of the soft intersegmental membrane except for one protein found in the endocuticle of hard cuticle. RR-2s were consistently found in hard cuticle and not in flexible cuticle. All RR-2 antibodies localized to the exocuticle and four out of six were also found in the endocuticle. Hence the location of RR-1s and RR-2s depends more on properties of individual proteins than on either hypothesis.