Enhanced expression of stress-responsive cytokine-like gene retards insect larval growth

Drosophila cytokine GBP2 exerts immune responses and regulates GBP1 expression through GPCR receptor Mthl10

Growth-blocking peptide (GBP), an insect cytokine, was first found in armyworm Mythimna separata. A functional analogue of GBP, stress-responsive peptide (SRP), was also identified in the same species. SRP gene expression has been demonstrated to be enhanced by GBP, indicating that both cytokines are organized within a hierarchical regulatory network. Although GBP1 (CG15917) and GBP2 (CG11395) have been identified in Drosophila melanogaster, immunological functions have only been characterized for GBP1. It is expected that the biological responses of two structurally similar peptides should be coordinated, but there is little information on this topic. Here, we demonstrate that GBP2 replicates the GBP1-mediated cellular immune response from Drosophila S2 cells. Moreover, the GBP2-induced response was silenced by pre-treatment with dsRNA targeting the GBP receptor gene, Mthl10. Furthermore, treatment of S2 cells with GBP2 enhanced GBP1 expression levels, but GBP1 did not affect GBP2 expression. GBP2 derived enhancement of GBP1 expression was not observed in the presence of GBP1, indicating that GBP2 is an upstream expressional regulator of a GBP1/GBP2 cytokine network. GBP2-induced enhancement of GBP1 expression was not observed in Mthl10 knockdown cells. Enhancement of GBP2 expression was observed in both Drosophila larvae and S2 cells under heat stress conditions; expressional enhancement of both GBP1 and GBP2 was eliminated in Mthl10 knockdown cells and larvae. Finally, Ca2+ mobilization assay in GCaMP3-expressing S2 cells demonstrated that GBP2 mobilizes Ca2+ upstream of Mthl10. Our finding revealed that Drosophila GBP1 and GBP2 control immune responses as well as their own expression levels through a hierarchical cytokine network, indicating that Drosophila GBP1/GBP2 system can be a simple model that is useful to investigate the detailed regulatory mechanism of related cytokine complexes.

Chromosome-Scale Genome Assembly of the Solitary Parasitoid Wasp Microplitis manilae Ashmead, 1904 (Braconidae: Microgastrinae)

Parasitoid wasps are invaluable natural enemies extensively used to control coleopteran, dipteran, and lepidopteran pests in agriculture and forestry owing to their killing and reproductive actions on hosts. The important larval endoparasitoid wasp Microplitis manilae, which belongs to the Microgastrinae subfamily, parasitizes the larval stages of Spodoptera spp., such as Spodoptera litura and Spodoptera frugiperda. The absence of a genomic resource for M. manilae has impeded studies on chemosensory- and detoxification-related genes. This study presents a chromosome-level genome assembly of M. manilae with a genome size of 293.18 Mb, which includes 222 contigs (N50 size, 7.58 Mb) and 134 scaffolds (N50 size, 27.33 Mb). A major proportion of the genome (284.76 Mb; 97.13%) was anchored to 11 pseudochromosomes with a single-copy BUSCO score of 98.4%. Furthermore, 14,316 protein-coding genes, 165.14 Mb (57.99%) repetitive elements, and 871 noncoding RNAs were annotated and identified. Additionally, a manual annotation of 399 genes associated with chemosensation and 168 genes involved in detoxification was conducted. This study provides a valuable and high-quality genomic resource to facilitate further functional genomics research on parasitoid wasps.

Transcriptional regulation of host insulin signaling pathway genes controlling larval development by Microplitis manilae parasitization

Idiobiont parasitoids using other insects as hosts sabotage the host growth and development to ensure their offspring survival. Numerous studies have discovered that insect development is subtly regulated by the conserved insulin signaling pathway. However, little is known about how wasp parasitization disrupts host development controlled by the insulin signaling pathway. Here we address this study to determine the effect of wasp parasitism on host Spodoptera frugiperda development using the idiobiont parasitoid Microplitis manilae as a model. Upon M. manilae parasitization, the body weight, body length, and food consumption of host insect were dramatically reduced compared to the unparasitized S. frugiperda. We next identified the core genes involved in host insulin signaling pathway and further analyzed the domain organizations of these genes. Phylogenetic reconstruction based on the insulin receptors clustered S. frugiperda together with other noctuidae insects. In the latter study, we profiled the expression patterns of host insulin signaling pathway genes in response to M. manilae parasitization at 2, 24, and 48 h, significant decreases in mRNA levels were recorded in S. frugiperda larvae upon 24 and 48 h parasitization. These current findings substantially add to our understanding of the physiological interaction between parasitoid and host insects, thus contributing to revealing the molecular mechanism of parasitic wasps regulating host development.

Host Transcriptome Analysis of Spodoptera frugiperda Larvae Parasitized by Microplitis manilae

It has been extensively found that parasitoids manipulate host physiology to benefit the survival and development of their offspring. However, the underlying regulatory mechanisms have not received much attention. To reveal the effects of parasitization of the larval solitary endoparasitoid Microplitis manilae (Hymenoptera: Braconidae) on host Spodoptera frugiperda (Lepidoptera: Noctuidae), one of the most destructive agricultural pests in China, deep-sequencing-based transcriptome analysis was conducted to compare the host gene expression levels after 2 h, 24 h, and 48 h parasitization. A total of 1861, 962, and 108 differentially expressed genes (DEGs) were obtained from the S. frugiperda larvae at 2 h, 24 h, and 48 h post-parasitization, respectively, compared with unparasitized controls. The changes in host gene expressions were most likely caused by the injection of wasp parasitic factors, including PDVs, that were injected along with the eggs during oviposition. Based on the functional annotations in GO and KEGG databases, we revealed that most DEGs were implicated in host metabolism and immunity. Further analysis of the common DEGs in three comparisons between the unparasitized and parasitized groups identified four genes, including one unknown and three prophenoloxidase (PPO) genes. Moreover, 46 and 7 common DEGs involved in host metabolism and immunity were identified at two or three time points after parasitization, respectively. Among these, most DEGs showed increased expressions at 2 h post-wasp parasitization while exhibiting significantly decreased expression levels at 24 h post-parasitization, demonstrating the expression regulations of M. manilae parasitization on host metabolism and immune-related genes. Further qPCR verification in 20 randomly selected DEGs confirmed the accuracy and reproducibility of the gene expression profiles generated from RNA-seq. This study reveals the molecular regulatory network about how host insects respond to wasp parasitism, laying a solid foundation for revealing the physiological manipulation of wasp parasitization on host insects, which facilitates the development of biological control practices for parasitoids.

Effect of Pupal Cold Storage on Reproductive Performance of Microplitis manilae (Hymenoptera: Braconidae), a Larval Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae)

Simple Summary

Parasitoids are one of the most important biological control agents, and there are increasing requirements for long-term breeding. It is critical to figure out the parasitoid biological properties and disclose the effects of cold storage on them to extend their longevity. In this study, we investigated the field parasitism rate and clarified the biological parameters of Microplitis manilae, a dominant larval parasitoid of Spodoptera frugiperda. Further analysis revealed that the pupal cold storage, including different storage temperatures, storage period and storage time, significantly affected the emergence rate, parasitism rate and longevity of wasp adults, and the optimal storage condition was middle-aged pupae stored at 10 °C for 5–10 d. These results provide a novel insight into the mass-rearing of M. manilae and contribute to the biological control using M. manilae against S. frugiperda.

Abstract

As a major invasive pest in China, Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae) has caused great damage to crops. Hymenopteran parasitoids, especially the braconid wasps, play crucial roles in depressing pest populations. However, there was little information about the ideal storage of parasitoids to achieve their mass-rearing. Here, we identified a dominant parasitoid of S. frugiperda, Microplitis manilae (Ashmead) (Hymenoptera: Braconidae), in the Hainan province of China with a field parasitism rate of 5.66–19.10%. The investigation of biological parameters revealed that the parasitism rate of M. manilae significantly decreased with an increase in both wasp adult longevity and host age, and the wasp of 1–3 d post eclosion performed best on the first instar of host larvae, showing the highest parasitism rate. We also discovered that the decreased temperature from 30 to 20 °C greatly extended the longevity of wasp adults, and a similar result was observed after feeding on 10% sucrose water compared with sterile water. Then, the effects of different pupal cold storage temperatures (4 and 10 °C), storage period (prepupa, middle-aged pupa, late-aged pupa) and storage time (5, 10 or 20 d) on the emergence rate, parasitism rate, female proportion and longevity of M. manilae were investigated. The results demonstrated that the middle-aged wasp pupae stored at 10 °C for 5–10 d possessed a stronger parasitic ability and longer longevity. These findings may promote the flexibility and efficacy of large-scale production of M. manilae, thus contributing to its biological field control against S. frugiperda.

Protein and phytohormone profiles of Mahanarva spectabilis salivary glands infesting different forages

Given the importance of pastures for feeding cattle, the study of factors that affect their productivity is essential to get plant material of higher nutritional quality. Thus, the study of insect-plant interaction is important for the development of control strategies. Pasture spittlebugs affect forage grasses causing severe damage. We tested hormone and protein profiles differentially expressed in the salivary glands of Mahanarva spectabilis when fed with different pasture genotypes. The LC/MS approaches combined with bioinformatics tools were used to identify the mains biological processes in the salivary glands. The grouping revealed a greater number of proteins involved in biological processes of metabolic synthesis, biotic/abiotic stress, and ion transport across the membrane. The proteomic profiles were altered when insects were fed with different grasses. We also detected phytohormones in the salivary glands involved in the modulation of defense responses in host plants. These results allowed the analysis of important biological processes such as cell homeostasis, stress proteins, nucleic acid metabolism, regulation of muscle contraction, and transport and export of biomolecules. This represents an important advance in the understanding of the plant-pest interaction and can contribute to the choice of target elicitors, which allow effective strategies in the control of pasture spittlebugs.

Biocontrol of mosquito vectors through herbal-derived silver nanoparticles: prospects and challenges

Mosquitoes spread several life-threatening diseases such as malaria, filaria, dengue, Japanese encephalitis, West Nile fever, chikungunya, and yellow fever and are associated with millions of deaths every year across the world. However, insecticides of synthetic origin are conventionally used for controlling various vector-borne diseases but they have various associated drawbacks like impact on non-targeted species, negative effects on the environment, and development of resistance in vector species by alteration of the target site. Plant extracts, phytochemicals, and their nanoformulations can serve as ovipositional attractants, insect growth regulators, larvicides, and repellents with least effects on the environment. Such plant-derived products exhibit broad-spectrum resistance against various mosquito species and are relatively cheaper, environmentally safer, biodegradable, easily accessible, and are non-toxic to non-targeted organisms. Therefore, in this review article, the current knowledge of phytochemical sources exhibiting larvicidal activity and their variations in response to solvents used for their extraction is underlined. Also, different methods such as physical, chemical, and biological for silver nanoparticle (AgNPs) synthesis, their mechanism of synthesis using plant extract, their potent larvicidal activity, and the possible mechanism by which these particles kill mosquito larvae are discussed. In addition, constraints related to commercialization of nanoherbal products at government and academic or research level and barriers from laboratory experiments to field trial have also been discussed. This comprehensive information can be gainfully employed for the development of herbal larvicidal formulations and nanopesticides against insecticide-resistant vector species in the near future. Graphical abstract.

Novel host immune evasion strategy of the endoparasitoid Drino inconspicuoides

The tachinid fly Drino inconspicuoides (Diptera: Tachinidae) is an ovolarviparous endoparasitoid whose larvae develop in the host haemocoel and avoids the host immune system. In this study, we investigated the immune evasion mechanisms of this species during infestation in the host Mythimna separata (Lepidoptera: Noctuidae). We discovered a unique 'cloak' that surrounded D. inconspicuoides larvae that penetrated into the host and determined through genomic polymerase chain reaction analysis that this structure originated from the host rather than the tachinid. The 'cloak' contained both haemocytes and fat body cells from the host, with the haemocytes assembling around the larvae first and the fat body cells then covering the haemocyte layer, following which the two mixed. Living D. inconspicuoides larvae that were wrapped in the 'cloak' were not melanized whereas encapsulated dead larvae were melanized, suggesting that this structure contributes to the avoidance of host immune reactions.

Functional Multiplicity of an Insect Cytokine Family Assists Defense Against Environmental Stress

The widespread distribution of insects over many ecological niches owes much to evolution of multiple mechanisms to defend against environmental stress, especially because their ectothermic nature and small body size render them particularly susceptible to extremes in temperature and water availability. In this review, we will summarize the latest information describing a single, multifunctional cytokine family that is deployed by six orders of insect species to combat a diverse variety of environmental stresses. The originating member of this peptide family was identified in Mythimna (formerly called Pseudaletia) separata armyworm; the cytokine was named growth-blocking peptide (GBP), reflecting its actions in combating parasitic invasion. The peptide’s name has been retained, though the list of its regulatory activities has greatly expanded. All members of this family are small peptides, 19–25 amino acid residues, whose major source is fat body. They are now known to regulate embryonic morphogenesis, larval growth rates, feeding activities, immune responses, nutrition, and aging. In this review, we will describe recent developments in our understanding of the mechanisms of action of the GBP family, but we will also highlight remaining gaps in our knowledge.

Prolonged Bat Call Exposure Induces a Broad Transcriptional Response in the Male Fall Armyworm (Spodoptera frugiperda; Lepidoptera: Noctuidae) Brain

Predation risk induces broad behavioral and physiological responses that have traditionally been considered acute and transitory. However, prolonged or frequent exposure to predators and the sensory cues of their presence they broadcast to the environment impact long-term prey physiology and demographics. Though several studies have assessed acute and chronic stress responses in varied taxa, these attempts have often involved a priori expectations of the molecular pathways involved in physiological responses, such as glucocorticoid pathways and neurohormone production in vertebrates. While relatively little is known about physiological and molecular predator-induced stress in insects, many dramatic insect defensive behaviors have evolved to combat selection by predators. For instance, several moth families, such as Noctuidae, include members equipped with tympanic organs that allow the perception of ultrasonic bat calls and facilitate predation avoidance by eliciting evasive aerial flight maneuvers. In this study, we exposed adult male fall armyworm (Spodoptera frugiperda) moths to recorded ultrasonic bat foraging and attack calls for a prolonged period and constructed a de novo transcriptome based on brain tissue from predator cue-exposed relative to control moths kept in silence. Differential expression analysis revealed that 290 transcripts were highly up- or down-regulated among treatment tissues, with many annotating to noteworthy proteins, including a heat shock protein and an antioxidant enzyme involved in cellular stress. Though nearly 50% of differentially expressed transcripts were unannotated, those that were are implied in a broad range of cellular functions within the insect brain, including neurotransmitter metabolism, ionotropic receptor expression, mitochondrial metabolism, heat shock protein activity, antioxidant enzyme activity, actin cytoskeleton dynamics, chromatin binding, methylation, axonal guidance, cilia development, and several signaling pathways. The five most significantly overrepresented Gene Ontology terms included chromatin binding, macromolecular complex binding, glutamate synthase activity, glutamate metabolic process, and glutamate biosynthetic process. As a first assessment of transcriptional responses to ecologically relevant auditory predator cues in the brain of moth prey, this study lays the foundation for examining the influence of these differentially expressed transcripts on insect behavior, physiology, and life history within the framework of predation risk, as observed in ultrasound-sensitive Lepidoptera and other 'eared' insects.

Nanoformulations and their mode of action in insects: a review of biological interactions

While nanoparticles (NPs) can be used as insecticides by themselves, they can also be carriers for insecticidal chemicals. Existing literature suggests that the smaller the NP size, the greater the toxicity and penetration into the insect's body. Nonetheless, there is a lack of literature pertaining to the mode of action within insects. This review article summarizes the currently available entomological studies on the mechanisms of NP-insect interactions. Externally, NPs affect pigmentation and integrity of the cuticle, while internally they induce immune responses and alter gene expression leading to altered protein, lipid, and carbohydrate metabolism along with cellular toxicity that impairs development and reproduction of the insect. Consequently, insects are incapacitated due to the disruption of the nutrient intake, production of reactive oxygen species and altered biochemical activity while some NPs can promote growth and development as well as diminish the effects of nontarget toxicity.

Identification of a cytokine combination that protects insects from stress

Growth-blocking peptide (GBP) and stress-responsive peptide (SRP) are insect cytokines whose expression levels are elevated by various stressful conditions such as parasitization and high or low temperatures. Both GBP and SRP are synthesized as precursors and released into the hemolymph, where they are enzymatically processed to active peptides. Injection of active GBP or SRP into early last instar larvae elicits a reduction in feeding and consequent growth retardation in the armyworm Mythimna separata. Although such functions are thought to benefit insects under stressful conditions by affecting their physiologies and behaviors, the relationship between GBP and SRP remains elusive. Here we show that heat stress-induced reactive oxygen species (ROS) elevated hemolymph GBP, which activated SRP transcription and increased the SRP concentration in the hemolymph. Injection of both GBP and SRP elevated hemolymph antioxidant levels. We found that simultaneous increases in both active cytokines occurred in the larval hemolymph from 2 to 3 h after heat stress or H₂O₂ injection, suggesting a synergic action of the two factors. This speculation was confirmed by demonstrating that co-injection of GBP and SRP caused a more severe reduction in appetite and growth retardation than injection of an individual peptide alone. However, injection of GBP together with SRP did not elevate SRP expression at all, indicating the effect of negative feedback regulation. Furthermore, SRP RNAi larvae showed higher body weights compared to controls, and GBP-induced growth retardation was partially abrogated in SRP RNAi larvae. These results led us to conclude that GBP is an upstream cytokine in the regulation of SRP expression and that these cytokines synergistically retard larval growth by repressing feeding activities when insects are exposed to stress conditions.

A gene-driven recovery mechanism: Drosophila larvae increase feeding activity for post-stress weight recovery

Recovery from weight loss after stress is important for all organisms, although the recovery mechanisms are not fully understood. We are working to clarify these mechanisms. Here, we recorded enhanced feeding activity of Drosophila melanogaster larvae from 2 to 4 h after heat stress at 35°C for 1 h. During the post-stress period, expression levels of sweet taste gustatory receptor genes (Grs), Gr5a, Gr43a, Gr64a, and Gr64f, were elevated, whereas bitter taste Grs, Gr66a, and Gr33a, were decreased in expression and expression of a non-typical taste receptor Gr, Gr68a, was unchanged. Similar upregulation of Gr5a and downregulation of Gr66a was recorded after cold stress at 4°C. Expression levels of tropomyosin and ATP synthase ß subunit were significantly increased in larval mouth parts around 3 to 5 h after the heat stress. We infer that up-regulation of post-stress larval feeding activity, and weight recovery, is mediated by increasing capacity for mouth part muscular movements and changes in taste sensing physiology. We propose that Drosophila larvae, and likely insects generally, express an efficient mechanism to recover from weight loss during post-stress periods.

Gene expression and molecular characterization of a novel C-type lectin, encapsulation promoting lectin (EPL), in the rice armyworm, Mythimna separata

Insect cellular immune reactions differ depending on the target species. Phagocytosis is activated to scavenge microorganisms such as bacteria and fungi. On the other hand, larger invaders such as parasitoid wasps are eliminated by activation of encapsulation. In this study, we hypothesized that novel determinants regulate cellular immunities independent of surface molecular pattern recognition involving pattern recognition receptors (PRRs). Immune-related genes differentially expressed depending on the treated material size were screened in larval hemocytes of the rice armyworm, Mythimna separata. Consequently, we identified a novel C-type lectin gene up-regulated by injection of large beads but not small beads of identical material. Examination of in vitro effect of the recombinant protein on the immune reactions clarified that the protein activated encapsulation reaction, while it suppressed phagocytosis. These results suggest that this novel C-type lectin designated "encapsulation promoting lectin (EPL)" regulates cellular immunity by a novel immune target size-recognition mechanism.

Solution Structure and Expression Profile of an Insect Cytokine: Manduca sexta Stress Response Peptide-2

Manduca sexta stress response peptide-2 (SRP2) is predicted to be a 25-residue peptide (FGVKDGKCPSGRVRRLGICVPDDDY), which may function as an insect cytokine to regulate immune responses. Produced as an inactive precursor, endogenous proSRP2 is probably converted to active SRP2 by limited proteolysis in response to invading pathogens, along with prophenoloxidase and pro-Spätzle activation. In addition to immunity, SRP2 may control head morphogenesis or other developmental processes in the lepidopteran insect. We have examined the profiles of SRP2 gene expression in terms of immune induction capacity, tissue specificity, and developmental changes. To gain insights into its functions, we chemically synthesized SRP2, injected the peptide solution into naïve larvae, and detected significant up-regulation of several antimicrobial peptide genes. We determined the 3D molecular structure in solution of SRP2 by two-dimensional 1H-1H NMR spectroscopy. SRP2 has an ordered structure, which is composed of two short β-strands at regions R12 - R15 and I18 - V20, one type-I' β-turn at region R15 - I18, and a half turn at region C8 - S10 in its welldefined core stabilized by a covalent disulfide bond between C8 and C19. The secondary and tertiary structures are further stabilized by hydrogen bonds. Possible relationships between the structure and function are also discussed.

Stress responses sculpt the insect immune system, optimizing defense in an ever-changing world

A whole organism, network approach can help explain the adaptive purpose of stress-induced changes in immune function. In insects, mediators of the stress response (e.g. stress hormones) divert molecular resources away from immune function and towards tissues necessary for fight-or-flight behaviours. For example, molecules such as lipid transport proteins are involved in both the stress and immune responses, leading to a reduction in disease resistance when these proteins are shifted towards being part of the stress response system. Stress responses also alter immune system strategies (i.e. reconfiguration) to compensate for resource losses that occur during fight-or flight events. In addition, stress responses optimize immune function for different physiological conditions. In insects, the stress response induces a pro-inflammatory state that probably enhances early immune responses.

The parasitic wasp Cotesia congregata uses multiple mechanisms to control host (Manduca sexta) behaviour

Some parasites alter the behaviour of their hosts. The larvae of the parasitic wasp Cotesia congregata develop within the body of the caterpillar Manduca sexta During the initial phase of wasp development, the host's behaviour remains unchanged. However, once the wasps begin to scrape their way out of the caterpillar, the caterpillar host stops feeding and moving spontaneously. We found that the caterpillar also temporarily lost sensation around the exit hole created by each emerging wasp. However, the caterpillars regained responsiveness to nociception in those areas within 1 day. The temporary reduction in skin sensitivity is probably important for wasp survival because it prevents the caterpillar from attacking the emerging wasp larvae with a defensive strike. We also found that expression of plasmatocyte spreading peptide (PSP) and spätzle genes increased in the fat body of the host during wasp emergence. This result supports the hypothesis that the exiting wasps induce a cytokine storm in their host. Injections of PSP suppressed feeding, suggesting that an augmented immune response may play a role in the suppression of host feeding. Injection of wasp larvae culture media into non-parasitized caterpillars reduced feeding, suggesting that substances secreted by the wasp larvae may help alter host behaviour.

Sickness behaviour in the cricket Gryllus texensis: Comparison with animals across phyla

Immune activation alters behaviour (i.e. sickness behaviour) in animals across phyla and is thought to aid recovery from infection. Hypotheses regarding the adaptive function of different sickness behaviours (e.g. decreased movement and appetite) include the energy conservation and predator avoidance hypotheses. These hypotheses were originally developed for mammals (e.g. Hart, 1988), however similar sickness behaviours are also observed in insects (e.g., crickets). We predicted that immune-challenged crickets (Gryllus texensis) would reduce feeding, grooming, and locomotion as well as increase shelter use, consistent with the energy conservation and predator avoidance hypotheses. We found evidence of illness-induced anorexia in adult and juvenile crickets, consistent with previous research (Adamo et al., 2010), but contrary to expectations, we found an increase in grooming, and no evidence that crickets decreased locomotion or increased shelter use in response to immune challenge. Therefore, our results do not support the energy conservation or predator avoidance hypotheses. The difference in sickness behaviour between insects and mammals is probably due, in part, to the lack of physiological fever in insects. We hypothesize that the lack of physiological fever reduces the need for energy conservation, decreasing the benefits of some sickness behaviours such as increased shelter use. These results, taken together with others in the literature, suggest that ectotherms and endotherms may differ significantly in the selective forces leading to the evolution of most sickness behaviours.

SRP gene is required for Helicoverpa armigera prophenoloxidase activation and nodulation response

SRP gene was first identified from the fall webworm, Hyphantria cunea as one of genes up-regulated after bacteria injection. A rent study in Spodoptera litura showed that stress-induced elevation of SRP expression highly correlates with reduced feeding activities and growth retardation of larvae. In this study, we identified a SRP gene from the cotton bollworm, Helicoverpa armigera, namely Ha-SRP, and studied its precise roles in insect immunity. Expressions of Ha-SRP were upregulated in H. armigera larval hemocytes after injection of Escherichia coli. When the expression of Ha-SRP in H. armigera larval hemocytes was inhibited by dsHa-SRP injection, the transcription of prophenoloxidase genes in hemocytes was repressed, phenoloxidase activity in bacteria-challenged larval hemolymph was significantly decreased, and nodule formation in bacteria-injected larvae was reduced. More importantly, RNAi-treated insects infected with E. coli showed higher bacterial growth in hemolymph compared with infected controls. These results suggest that Ha-SRP gene plays importance roles in H. armigera innate immunity, possibly by mediating prophenoloxidase activation and nodulation response.

Gain of long tonic immobility behavioral trait causes the red flour beetle to reduce anti-stress capacity

Tonic immobility (death-feigning) behavior of the red flour beetle, Tribolium castaneum, is a predator defense mechanism; it is a reflex elicited when a beetle is jarred with the substrate, often a result of the activities of a predator. We previously demonstrated that the frequency of predation by a jumping spider, Hasarius adansoni, was significantly lower among beetles with higher frequencies and longer durations of tonic immobility (L-type) than those with lower frequencies and shorter durations of tonic immobility (S-type). However, we found that the population of L-type beetles is much smaller than that of S-type beetles in their natural habitat. Here we demonstrated that L-type beetles are significantly more sensitive to environmental stressors such as mechanical vibration and high or low temperatures. We measured expression levels of stress-responsive genes such as heat shock proteins (Hsps) and antioxidant enzymes in both types of beetles. Among the genes we investigated, only catalase gene expression levels were significantly higher in S-type than in L-type beetles. Furthermore, a similar difference in the gene expression was observed in the T. castaneum ortholog of the insect cytokine growth-blocking peptide (GBP) gene. These results indicate the possibility that high expression of catalase and GBP in S-type beetles contributes to augmentation of their anti-stress capacity and expansion of their population in their natural habitat.

A comparison of growth and development of three major agricultural insect pests infected with Heliothis virescens ascovirus 3h (HvAV-3h)

Ascoviruses are double-stranded DNA viruses that are pathogenic to lepidopteran hosts, particularly noctuid larvae. Infection of a larva is characterized by retarded growth, reduced feeding and yellowish body color. In this paper, we reported the growth and development of three major agricultural noctuid insect pests, Helicoverpa armigera (Hübner), Spodoptera exigua (Hübner) and Spodoptera litura (Fabricius), infected with Heliothis virescens ascovirus 3h (HvAV-3h). Using 10-fold serial dilutions (0 to 7) of HvAV-3h-containing hemolymph to infect S. litura larvae, we found no significant difference in larval mortalities from 0 to 10(3)-fold dilutions; however, significant differences were observed at 10(4)-fold dilution and above. Using a 10-fold dilution of HvAV-3h-containing hemolymph to infect H. armigera, S. exigua and S. litura larvae, we found that the growth and development were significantly affected. All infected larvae could not pupate; the survival times of treated H. armigera, S. litura and S. exigua larvae were significantly longer than untreated control larvae. Body weight showed significant difference between treated and untreated control group from day 1 after inoculation in H. armigera and S. exigua, but day 2 in S. litura. Additionally, food intake also showed significant difference between treated and untreated control group from day 2 after inoculation in H. armigera and S. litura, but day 3 in S. exigua.

Characteristics common to a cytokine family spanning five orders of insects

Growth-blocking peptide (GBP) is a member of an insect cytokine family with diverse functions including growth and immunity controls. Members of this cytokine family have been reported in 15 species of Lepidoptera, and we have recently identified GBP-like peptides in Diptera such as Lucilia cuprina and Drosophila melanogaster, indicating that this peptide family is not specific to Lepidoptera. In order to extend our knowledge of this peptide family, we purified the same family peptide from one of the tenebrionids, Zophobas atratus,(1) isolated its cDNA, and sequenced it. The Z. atratus GBP sequence together with reported sequence data of peptides from the same family enabled us to perform BLAST searches against EST and genome databases of several insect species including Coleoptera, Diptera, Hymenoptera, and Hemiptera and identify homologous peptide genes. Here we report conserved structural features in these sequence data. They consist of 19-30 amino acid residues encoded at the C terminus of a 73-152 amino acid precursor and contain the motif C-x(2)-G-x(4,6)-G-x(1,2)-C-[KR], which shares a certain similarity with the motif in the mammalian EGF peptide family. These data indicate that these small cytokines belonging to one family are present in at least five insect orders.

Drosophila growth-blocking peptide-like factor mediates acute immune reactions during infectious and non-infectious stress

Antimicrobial peptides (AMPs), major innate immune effectors, are induced to protect hosts against invading microorganisms. AMPs are also induced under non-infectious stress; however, the signaling pathways of non-infectious stress-induced AMP expression are yet unclear. We demonstrated that growth-blocking peptide (GBP) is a potent cytokine that regulates stressor-induced AMP expression in insects. GBP overexpression in Drosophila elevated expression of AMPs. GBP-induced AMP expression did not require Toll and immune deficiency (Imd) pathway-related genes, but imd and basket were essential, indicating that GBP signaling in Drosophila did not use the orthodox Toll or Imd pathway but used the JNK pathway after association with the adaptor protein Imd. The enhancement of AMP expression by non-infectious physical or environmental stressors was apparent in controls but not in GBP-knockdown larvae. These results indicate that the Drosophila GBP signaling pathway mediates acute innate immune reactions under various stresses, regardless of whether they are infectious or non-infectious.