The DOMON domain protein LmKnk contributes to correct chitin content, pore canal formation and lipid deposition in the cuticle of Locusta migratoria during moulting

Insects prevent uncontrolled penetration of water and xenobiotics by producing an impermeable cuticle. The major component of the cuticle is chitin that adopts a crystalline structure thereby contributing to cuticle stability. Our understanding of the contribution of chitin to the cuticle barrier function is limited. Here, we studied the role of the DOMON domain protein Knickkopf (LmKnk) that is involved in chitin organization and cuticle permeability in the migratory locust Locusta migratoria. We show that LmKnk localizes to the chitin layer in the newly produced cuticle. Injection of double-stranded RNA targeting LmKnk (dsLmKnk) in locust nymphs caused failure of moulting to the next stage. Histological experiments revealed that apolysis, i.e., the detachment of the old cuticle from the body surface, was normal; however, the newly synthesized cuticle was thinner than the cuticle of the control insects. Indeed, chitin content dropped after suppression of LmKnk expression. As seen by transmission electron microscopy, crystalline chitin organization was lost in dsLmKnk-treated insects. In addition, the structure of pore canals, which are lipid transporting routes in the cuticle, was abnormal. Consistently, their content was reduced and, probably by consequence, lipid deposition on the cuticle was decreased after injection of dsLmKnk. Suppression of LmKnk transcript levels rendered L. migratoria more susceptible to each of four selected insecticides including malathion, chlorpyrifos, carbaryl and deltamethrin. Overall, our data show that LmKnk is needed for correct chitin amounts and organization, and their changes ultimately affect cuticular permeability in L. migratoria.

Chitin synthase 1 and five cuticle protein genes are involved in serosal cuticle formation during early embryogenesis to enhance eggshells in Nilaparvata lugens

Many holo- and hemimetabolous insects enhance their eggshells during embryogenesis by forming a serosal cuticle (SC). To date, scholarly understanding of the SC composition and SC-related gene functions has been limited, especially for hemimetabolous insects. In this study, we initially performed transmission electron microscopic (TEM) observation and chitin staining of the SC in Nilaparvata lugens, a hemimetabolous rice pest known as the brown planthopper (BPH). We confirmed that the SC was a chitin-rich lamellar structure deposited gradually during the early embryogenesis. Parental RNA interference (RNAi) against Nilaparvata lugens chitin synthase 1 (NlCHS1) in newly emerged and matured females resulted in decreases of egg hatchability by 100% and 76%, respectively. Ultrastructural analyses revealed loss of the lamellar structure of the SC in dsNlCHS1-treated eggs. According to temporal expression profiles, five cuticle protein coding genes, NlugCpr1/2/3/8/90, were specifically or highly expressed during the SC formation period, and NlugCpr1/2/3/90 were further detected in 72 h eggshells extract by ultra-performance liquid chromatography-tandem mass spectrometry/mass spectrometry. NlugCpr2/3/90 were likely three SC-specific cuticle proteins. TEM observations of the SC following parental RNAi against NlugCpr1/2/3/8/90 demonstrated that NlugCpr3/8/90 were essential for SC formation. The study provided an understanding of the SC formation process and SC-related cuticle proteins in BPHs, which offer potential targets for pest control in the egg stage as well.

Autophagy-mediated plasma membrane removal promotes the formation of epithelial syncytia

Epithelial wound healing in Drosophila involves the formation of multinucleate cells surrounding the wound. We show that autophagy, a cellular degradation process often deployed in stress responses, is required for the formation of a multinucleated syncytium during wound healing, and that autophagosomes that appear near the wound edge acquire plasma membrane markers. In addition, uncontrolled autophagy in the unwounded epidermis leads to the degradation of endo‐membranes and the lateral plasma membrane, while apical and basal membranes and epithelial barrier function remain intact. Proper functioning of TORC1 is needed to prevent destruction of the larval epidermis by autophagy, in a process that depends on phagophore initiation and expansion but does not require autophagosomes fusion with lysosomes. Autophagy induction can also affect other sub‐cellular membranes, as shown by its suppression of experimentally induced laminopathy‐like nuclear defects. Our findings reveal a function for TORC1‐mediated regulation of autophagy in maintaining membrane integrity and homeostasis in the epidermis and during wound healing.

Xenobiotic responses in insects

Insects have evolved a powerful detoxification system to protect themselves against environmental and anthropogenic xenobiotics including pesticides and nanoparticles. The resulting tolerance to insecticides is an immense problem in agriculture. In this study, we summarize advances in our understanding of insect xenobiotic responses: the detoxification strategies and the regulation mechanisms against xenobiotics including nanoparticles, the problem of response specificity and the potential usefulness of this study field for an elaborate pest management. In particular, we highlight that versatility of the detoxification system relies on the relatively unspecific recognition of a broad range of potential toxic substances that trigger either of various canonical xenobiotic responses signaling pathways, including CncC/Keap1, HR96, AHR/ARNT, GPCR, and MAPK/CREB. However, it has emerged that the actual response to an inducer may nevertheless be specific. There are two nonexclusive possibilities that may explain response specificity: (1) differential cross-talk between the known pathways and (2) additional, yet unidentified regulators and pathways of detoxification. Hence, a deeper and broader understanding of the regulation mechanisms of xenobiotic response in insects in the future might facilitate the development and application of highly efficient and environmentally friendly pest control methods, allowing us to face the challenge of the world population growth.

Musca domestica (Diptera: Muscidae) as a biological model for the assessment of magnetite nanoparticles toxicity

The use of nanoparticles (NPs) is rapidly expanding; there is a critical need for efficient assays to first determine the potential toxicity of NPs before their use in human applications. Magnetite nanoparticles (Fe₃O₄ NPs) have tremendous applications which include cell separation, arsenic removal from water and DNA separation. Spherically shaped Fe₃O₄ NPs with sizes ranging from 23 to 30 nm were used in this study. The housefly, Musca domestica is the most common fly species. It is present worldwide and considered to be an important medical insect which can carry and transmit over 100 human pathogens and zoonotic agents. It has been used in this study to assess Fe₃O₄NPs toxicity and give us an overview of their impact. The larvicidal activity of Fe₃O₄NPs was tested against the third instar larvae of M. domestica. We investigated the effects of six varying concentrations (15, 30, 45, 60, 75 and 90 μg/mL) used under laboratory conditions in two differential application assays: contact and feeding. The LC₅₀ value for Fe₃O₄ NPs was 60 and 75 μg/mL by feeding and contact, respectively. To investigate the toxicity effects of Fe₃O₄ NPs on houseflies, morphological and histoarchitectural changes in larvae, pupae and adult flies were analyzed. NP exposure caused morphological abnormalities of larvae and pupae as well as larval pupal intermediates, and deformed adult with crumpled wings. Also, some adults couldn't emerge and remained in their puparia. The histological examinations showed that Fe₃O₄ NPs caused severe tissue damage especially in the cuticle and the digestive system. Thus, besides affecting the organ of first contact (digestive system), remote organs such as the integument are also targeted by Fe₃O₄ NPs suggesting a systemic impact on fly development and physiology.

Identification and Functional Characterization of Argonaute (Ago) Proteins in Insect Genomes

RNA processing is a vital process in all organisms. In eukaryotes, the RNA induced silencing complex (RISC) mediates this function during development and physiological processes and, at least in arthropods, during RNA-viral infections. Argonaute-like RNA-binding proteins are central components of this complex. RNA-based insecticides are gaining more and more a central role in pest control. Understanding of the underlying molecular mechanisms including Ago-like proteins is crucial in designing powerful, species-specific and environmental-friendly insecticides. This chapter describes a protocol for identification and genetic functional analyses of insect Ago-like proteins in the fruit fly Drosophila melanogaster that serves as a living test tube.

Roles of LmCDA1 and LmCDA2 in cuticle formation in the foregut and hindgut of Locusta migratoria

Chitin deacetylases (CDAs, including CDA1 and CDA2) are considered key enzymes for body cuticle formation and tracheal morphogenesis in various insect species. However, their functions in the formation of the cuticular intima of the foregut and hindgut are unclear. Here, we investigated the roles of their respective genes LmCDA1 and LmCDA2 in this process, in the hemimetabolous insect Locusta migratoria. Transcripts of LmCDA1 and LmCDA2 were highly expressed both before and after molting in the foregut. In the hindgut, their expression was high only before molting. In both the foregut and hindgut, LmCDA1 protein was localized in the basal half of the chitin matrix (procuticle), whereas LmCDA2 was detected in the upper half of the procuticle. Knockdown of LmCDA1 by RNA interference (RNAi) in 5th-instar nymphs caused no visible defects of the hindgut cuticle. By contrast, the chitinous lamellae of the cuticular intima in the foregut of knockdown animals were less compact than in control animals. RNAi against LmCDA2 led to thickening of both the foregut and hindgut cuticles, with a greater number of thinner laminae than in the respective control cuticles. Taken together, our results show that LmCDA1 and LmCDA2 have distinct, but overlapping, functions in chitin organization in the foregut cuticle. However, in the hindgut, this process seems independent of LmCDA1 activity but requires LmCDA2 function. Thus, the CDAs reflect tissue-specific differences in cuticular organization and function, which need further detailed molecular and histological analyses for full comprehension.

Ratio between Lactobacillus plantarum and Acetobacter pomorum on the surface of Drosophila melanogaster adult flies depends on cuticle melanisation

Objectives

As in most organisms, the surface of the fruit fly Drosophila melanogaster is associated with bacteria. To examine whether this association depends on cuticle quality, we isolated and quantified surface bacteria in normal and melanized flies applying a new and simple protocol.

Results

On wild flies maintained in the laboratory, we identified two persistently culturable species as Lactobacillus plantarum and Acetobacter pomorum by 16S rDNA sequencing. For quantification, we showered single flies for DNA extraction avoiding the rectum to prevent contamination from the gut. In quantitative PCR analyses, we determined the relative abundance of these two species in surface wash samples. On average, we found 17-times more A. pomorum than L. plantarum. To tentatively study the importance of the cuticle for the interaction of the surface with these bacteria, applying Crispr/Cas9 gene editing in the initial wild flies, we generated flies mutant for the ebony gene needed for cuticle melanisation and determined the L. plantarum to A. pomorum ratio on these flies. We found that the ratio between the two bacterial species reversed on ebony flies. We hypothesize that the cuticle chemistry is crucial for surface bacteria composition. This finding may inspire future studies on cuticle-microbiome interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13104-021-05766-7.

Group I CDAs are responsible for a selective CHC-independent cuticular barrier in Locusta migratoria

Chitin deacetylases including CDA1 and CDA2, containing a chitin deacetylase domain and an LDL domain, have been reported to be essential for cuticle structure differentiation in different insect species. However, it is yet unexplored whether CDA1 and CDA2 activity is needed for the function of the cuticle as a barrier against pathogen and xenobiotics penetration. In this study, we studied the efficiency of fungal infection in the migratory locust Locusta migratoria in dependence of LmCDA1 and LmCDA2 function. Second instar nymphs injected with dsRNA against LmCDA1 and LmCDA2 transcripts were less resistant against the infection by the fungus Metarhizium anisopliae than control nymphs. At the same time, permeability to organophosphorus pesticides was increased in these nymphs. Interestingly, the CHC amounts at the cuticle surface were unaffected upon LmCDA1 and LmCDA2 reduction. These results suggest that the barrier function of the locust cuticle not only depends on surface CHCs, but also on an intact procuticle.

A lateral oviduct secreted protein plays a vital role for egg movement through the female reproductive tract in the brown planthopper

The oviduct serves as a delivery tube for mature eggs ovulated from ovaries to egg-laying sites. Oviduct secreted components play important roles in ovulation and fertilization in mammals, however, no oviduct secreted protein has been characterized in an insect to date. Here, we identified a gene highly expressed in the lateral oviduct of the adult females in the brown planthopper (BPH), Nilaparvata lugens, the most destructive rice insect pest. Western blotting and immunofluorescence analyses revealed that the gene encodes a protein that is specifically expressed in the lateral oviduct as a component of the gel-like material secreted by the oviduct epithelial cells into the lumen of the swollen part of the lateral oviducts. The protein was tentatively named N. lugens oviduct secreted protein (Nlodsp). RNA interference (RNAi) against NlOdsp transcripts caused a failure of the lateral oviducts to deliver oocytes to the common oviduct that was, by consequence, plugged by 1-2 oocytes. Moreover, although oocytes in the Nlodsp-deficient ovariole were not released to the oviduct, they continued to develop, finally resulting in the presence of several matured oocytes in an ovariole. These defects evidently declined female fecundity. Together, our results demonstrate that NlOdsp plays an essential role in egg transport through the oviduct during ovulation. This work deepens our understanding of insect reproductive system and provides a potential target gene for RNAi-based insect pest control.

Eco-genetics of desiccation resistance in Drosophila

Climate change globally perturbs water circulation thereby influencing ecosystems including cultivated land. Both harmful and beneficial species of insects are likely to be vulnerable to such changes in climate. As small animals with a disadvantageous surface area to body mass ratio, they face a risk of desiccation. A number of behavioural, physiological and genetic strategies are deployed to solve these problems during adaptation in various Drosophila species. Over 100 desiccation-related genes have been identified in laboratory and wild populations of the cosmopolitan fruit fly Drosophila melanogaster and its sister species in large-scale and single-gene approaches. These genes are involved in water sensing and homeostasis, and barrier formation and function via the production and composition of surface lipids and via pigmentation. Interestingly, the genetic strategy implemented in a given population appears to be unpredictable. In part, this may be due to different experimental approaches in different studies. The observed variability may also reflect a rich standing genetic variation in Drosophila allowing a quasi-random choice of response strategies through soft-sweep events, although further studies are needed to unravel any underlying principles. These findings underline that D. melanogaster is a robust species well adapted to resist climate change-related desiccation. The rich data obtained in Drosophila research provide a framework to address and understand desiccation resistance in other insects. Through the application of powerful genetic tools in the model organism D. melanogaster, the functions of desiccation-related genes revealed by correlative studies can be tested and the underlying molecular mechanisms of desiccation tolerance understood. The combination of the wealth of available data and its genetic accessibility makes Drosophila an ideal bioindicator. Accumulation of data on desiccation resistance in Drosophila may allow us to create a world map of genetic evolution in response to climate change in an insect genome. Ultimately these efforts may provide guidelines for dealing with the effects of climate-related perturbations on insect population dynamics in the future.

Three-dimensional reconstruction of a whole insect reveals its phloem sap-sucking mechanism at nano-resolution

Using serial block-face scanning electron microscopy, we report on the internal 3D structures of the brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae) at nanometer resolution for the first time. Within the reconstructed organs and tissues, we found many novel and fascinating internal structures in the planthopper such as naturally occurring three four-way rings connecting adjacent spiracles to facilitate efficient gas exchange, and fungal endosymbionts in a single huge insect cell occupying 22% of the abdomen volume to enable the insect to live on plant sap. To understand the muscle and stylet movement during phloem sap-sucking, the cephalic skeleton and muscles were reconstructed in feeding nymphs. The results revealed an unexpected contraction of the protractors of the stylets and suggested a novel feeding model for the phloem sap-sucking.

Since the 19^th century, scientists have been investigating how the organs of insects are shaped and arranged. However, classic microscopy methods have struggled to image these small, delicate structures. Understanding how the organs of insects are configured could help to identify new methods for controlling pests, such as chemicals that target the mouthparts that some insects use to feed on plants.

Most insects that feed on the sap of plants suck out the nutrient via their stylet bundle – a thin, straw-like structure surrounded by a sheath called the labium. As well as drying out the plant and damaging its tissues, the stylet bundle also allows the insect to transmit viruses that cause further harm. To investigate these mouthparts in more detail, Wang, Guo et al. used a method called SBF-SEM to determine the three-dimensional structure of one of the most destructive pests of rice crops, the brown planthopper.

In this technique, a picture of the planthopper was taken every time a thin slice of its body was removed. This continuous slicing and re-imaging generated thousands of images that were compiled into a three-dimensional model of the brown planthopper’s whole body and internal organs. Previously unknown features emerged from the reconstruction, including a huge cell in the planthopper’s abdomen which is full of fungi that provide the nutrients absent in plants.

Next, Wang, Guo et al. used this technique to see how the muscles in the labium and surrounding the stylet move by imaging planthoppers that were frozen at different stages of the feeding process. This revealed that when brown planthoppers bow their heads to eat, the labium compresses and pushes out the stylet, allowing it to pierce deeper into the plant.

This is the first time that the body of such a small insect has been reconstructed three-dimensionally using SBF-SEM. Furthermore, these findings help explain how brown planthoppers and other sap-feeding insects insert their stylet and damage plants, potentially providing a stepping stone towards identifying new strategies to stop these pests from destroying millions of crops.

Drosophila, Chitin and Insect Pest Management

Insects are a great menace in agriculture and vectors of human diseases. Hence, controlling insect populations is an important issue worldwide. A common strategy to control insects is the application of insecticides. However, insecticides entail three major problems. First, insecticides are chemicals that stress ecosystems and may even be harmful to humans. Second, insecticides are often unspecific and also eradicate beneficial insect species like the honeybee. Third, insects are able to develop resistance to insecticides. Therefore, the efficient generation of new potent insecticides and their intelligent delivery are the major tasks in agriculture. In addition, acceptance or refusal in society is a major issue that has to be considered in the application of a pest management strategy. In this paper, we unify two issues: 1) we illustrate that our molecular knowledge of the chitin synthesis and organization pathways may offer new opportunities to design novel insecticides that are environmentally harmless at the same time being specific to a pest species; and 2) we advocate that the fruit fly Drosophila melanogaster may serve as an excellent model of insect to study the effects of insecticides at the genetic, molecular and histology level in order to better understand their mode of action and to optimize their impact. Especially, chitin synthesis and organization proteins and enzymes are excellently dissected in the fruit fly, providing a rich source for new insecticide targets. Thus, D. melanogaster offers a cheap, efficient and fast assay system to address agricultural questions, as has been demonstrated to be the case in bio-medical research areas.

Toxicity of Dithiothreitol (DTT) to Drosophila melanogaster

The thiol-containing compound Dithiothreitol (DTT) has been shown to be toxic to cultured cells by inducing the generation of reactive oxygen species that ultimately cause cell death. However, its effects on multicellular organisms and the environment have not been investigated yet in detail. In this work, we tested the toxicity of DTT to the model insect Drosophila melanogaster. We show that DTT is lethal to D. melanogaster by topical application but not through feeding. DTT treatment triggers the transcription of the canonical apoptosis regulators grim, hid and rpr at low amounts. The amplitude of this induction declines with elevating DTT amounts. By live microscopy, we observe apoptotic cells especially in the gut of DTT treated flies. In parallel, low DTT amounts also activate the expression of the cuticle barrier component gene snsl. This indicates that a physical defence response is launched upon DTT contact. This combined measure is seemingly successful in preventing fly death. The expression of a number of known detoxification genes including cyp6a2, cyp6a8, cyp12d1 and GstD2 is also enhanced through DTT contact. The degree of upregulation of these genes is proportional to the applied DTT amounts. Despite this effort, flies exposed to high amounts of DTT eventually die. Together, D. melanogaster is able to sense DTT toxicity and adjust the defence response successfully at least at low concentrations. This is the first time to analyse the molecular consequences of DTT exposure in a multicellular organism. Our work provides a new model to discuss the physiological response of animals against thiol toxins and to resurvey the effect of redox agents on the environment.

Resilin matrix distribution, variability and function in Drosophila

Background

Elasticity prevents fatigue of tissues that are extensively and repeatedly deformed. Resilin is a resilient and elastic extracellular protein matrix in joints and hinges of insects. For its mechanical properties, Resilin is extensively analysed and applied in biomaterial and biomedical sciences. However, there is only indirect evidence for Resilin distribution and function in an insect. Commonly, the presence of dityrosines that covalently link Resilin protein monomers (Pro-Resilin), which are responsible for its mechanical properties and fluoresce upon UV excitation, has been considered to reflect Resilin incidence.

Results

Using a GFP-tagged Resilin version, we directly identify Resilin in pliable regions of the Drosophila body, some of which were not described before. Interestingly, the amounts of dityrosines are not proportional to the amounts of Resilin in different areas of the fly body, arguing that the mechanical properties of Resilin matrices vary according to their need. For a functional analysis of Resilin matrices, applying the RNA interference and Crispr/Cas9 techniques, we generated flies with reduced or eliminated Resilin function, respectively. We find that these flies are flightless but capable of locomotion and viable suggesting that other proteins may partially compensate for Resilin function. Indeed, localizations of the potentially elastic protein Cpr56F and Resilin occasionally coincide.

Conclusions

Thus, Resilin-matrices are composite in the way that varying amounts of different elastic proteins and dityrosinylation define material properties. Understanding the biology of Resilin will have an impact on Resilin-based biomaterial and biomedical sciences.

Tweedle proteins form extracellular two-dimensional structures defining body and cell shape in Drosophila melanogaster

Tissue function and shape rely on the organization of the extracellular matrix (ECM) produced by the respective cells. Our understanding of the underlying molecular mechanisms is limited. Here, we show that extracellular Tweedle (Twdl) proteins in the fruit fly Drosophila melanogaster form two adjacent two-dimensional sheets underneath the cuticle surface and above a distinct layer of dityrosinylated and probably elastic proteins enwrapping the whole body. Dominant mutations in twdl genes cause ectopic spherical aggregation of Twdl proteins that recruit dityrosinylated proteins at their periphery within lower cuticle regions. These aggregates perturb parallel ridges at the surface of epidermal cells that have been demonstrated to be crucial for body shaping. In one scenario, hence, this disorientation of epidermal ridges may explain the squatty phenotype of Twdl mutant larvae. In an alternative scenario, this phenotype may be due to the depletion of the dityrosinylated and elastic layer, and the consequent weakening of cuticle resistance against the internal hydrostatic pressure. According to Barlow's formula describing the distribution of internal pressure forces in pipes in dependence of pipe wall material properties, it follows that this reduction in turn causes lateral expansion at the expense of the antero-posterior elongation of the body.

Two fatty acid synthase genes from the integument contribute to cuticular hydrocarbon biosynthesis and cuticle permeability in Locusta migratoria

Lipids of the insect cuticle have important roles in resistance against the arid environment and invasion of foreign substances. Fatty acid synthase (FAS) is an important enzyme of the insect lipid synthesis pathway. In the present study, we identified three FAS genes from transcriptome data of the migratory locust, Locusta migratoria, based on bioinformatics analyses. Among them, two FAS genes (LmFAS1 and LmFAS3) are highly expressed in the integument of fifth instar nymphs. Suppression of LmFAS1 and LmFAS3 by RNA interference caused lethality during ecdysis or shortly after moulting. The weight of the locusts and the content of lipid droplets were reduced compared with those of the control. The results of gas chromatography-mass spectrometry analysis showed that knockdown of LmFAS3 led to a decrease of both cuticular hydrocarbons and inner hydrocarbons (CHCs and IHCs) contents, especially the content of methyl branched hydrocarbons. By contrast, knockdown of LmFAS1 only resulted in a decrease in the IHC content, but not that of CHCs. By consequence, in LmFAS1- and LmFAS3-suppressed locusts, hydrocarbon deficiency reduced desiccation resistance and enhanced cuticle permeability and sensitivity to insecticides. These results indicate that LmFAS1 and LmFAS3 are essential for hydrocarbon production and cuticle permeability, which play influential roles in waterproofing the insect cuticle.

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.

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.

Transcriptional Control of Quality Differences in the Lipid-Based Cuticle Barrier in Drosophila suzukii and Drosophila melanogaster

Cuticle barrier efficiency in insects depends largely on cuticular lipids. To learn about the evolution of cuticle barrier function, we compared the basic properties of the cuticle inward and outward barrier function in adults of the fruit flies Drosophila suzukii and Drosophila melanogaster that live on fruits sharing a similar habitat. At low air humidity, D. suzukii flies desiccate faster than D. melanogaster flies. We observed a general trend indicating that in this respect males are less robust than females in both species. Xenobiotics penetration occurs at lower temperatures in D. suzukii than in D. melanogaster. Likewise, D. suzukii flies are more susceptible to contact insecticides than D. melanogaster flies. Thus, both the inward and outward barriers of D. suzukii are less efficient. Consistently, D. suzukii flies have less cuticular hydrocarbons (CHC) that participate as key components of the cuticle barrier. Especially, the relative amounts of branched and desaturated CHCs, known to enhance desiccation resistance, show reduced levels in D. suzukii. Moreover, the expression of snustorr (snu) that encodes an ABC transporter involved in barrier construction and CHC externalization, is strongly suppressed in D. suzukii. Hence, species-specific genetic programs regulate the quality of the lipid-based cuticle barrier in these two Drosophilae. Together, we conclude that the weaker inward and outward barriers of D. suzukii may be partly explained by differences in CHC composition and by a reduced Snu-dependent transport rate of CHCs to the surface. In turn, this suggests that snu is an ecologically adjustable and therefore relevant gene in cuticle barrier efficiency.

Apolipophorin-II/I Contributes to Cuticular Hydrocarbon Transport and Cuticle Barrier Construction in Locusta migratoria

Apolipophorins are carrier proteins that bind lipids and mediate their transport from tissue to tissue in animals. Apolipophorin I and II (apoLp-II/I) are the major apolipophorins in insects. The implication of apoLp-II/I in cuticle lipid-barrier formation in insects has not been addressed to date. In the present study, we investigated the function of apoLp-II/I in the migratory locust Locusta migratoria (LmapoLp-II/I). During the development of fifth instar nymphs, LmapoLp-II/I transcript levels increased until mid-instar, and then decreased gradually until molting to the adult stage. We found that LmapoLp-II/I was predominately expressed in the fat body and the integument including oenocytes and epidermal cells. Immunodetection experiments revealed that LmapoLp-I mainly localized in the cytoplasm of oenocytes and epidermal cells. Silencing of LmapoLp-II/I caused molting defects in nymphs. Importantly, RNA interference against LmapoLp-II/I resulted in a significant decrease in the content of cuticle surface lipids including alkanes and methyl alkanes. Cuticular permeability was significantly enhanced in these nymphs in Eosin Y penetration assays. By consequence, desiccation resistance and insecticide tolerance of dsLmapoLp-II/I-treated locusts were reduced. Taken together, our results indicate that LmapoLp-II/I is involved in the transport and deposition of surface-cuticular lipids that are crucial for maintaining normal cuticle barrier function in L. migratoria.

Ten fatty acyl-CoA reductase family genes were essential for the survival of the destructive rice pest, Nilaparvata lugens

BACKGROUND

Fatty alcohols are the precursors of sex pheromone components, wax esters and hydrocarbons in insects. Fatty acyl-CoA reductases (FARs) are important enzymes required for the reduction of fatty alcohol and thereby contribute to the production of cuticular hydrocarbon (CHC).

RESULTS

Based on bioinformatics analyses we identified 17 FAR genes in the brown planthopper, Nilaparvata lugens. RNA interference against these genes demonstrated that ten NlFAR genes were essential for the survival of N. lugens. For instance, knockdown of NlFAR5, 6, 11 or 15 was lethal and caused a slender body shape, while the old cuticles of the respective animals remained attached to the abdomen or failed to split open from the nota. Knockdown of NlFAR9 resulted in a phenotype, with a smooth body surface and a decrease in CHC amounts. Similarly, CHC deficiency in N. lugens resulted in increased adhesion of water droplets and secreted honeydew to the insect surface and the inability of N. lugens to survive in paddy fields with varying humidity. Knockdown of NlFAR1, 4, 5, 6, 8, 9, 11 and 13 additionally resulted in female adult infertility.

CONCLUSION

The present study illustrates the structural and functional differences of FAR family genes and provides potential targets for RNA interference-based rice planthopper management. © 2020 Society of Chemical Industry.