Transient transcription of a putative RNase containing BEN domain encoded in Cotesia plutellae bracovirus induces an immunosuppression of the diamondback moth, Plutella xylostella

Spätzle processing enzyme is required to activate dorsal switch protein 1 induced Toll immune signalling pathway in Tenebrio molitor

Dorsal switch protein 1 (DSP1) acts as a damage-associated molecular pattern (DAMP) molecule to activate immune responses in Tenebrio molitor. From a previous study in Spodoptera exigua, we found that DSP1 activates Toll immune signalling pathway to induce immune responses by melanisation, PLA2 activity and AMP synthesis. However, the target site of DSP1 in this pathway remains unknown. The objective of this study was to determine the role of spätzle processing enzyme in the DSP1 induced toll immune signalling pathway. To address this, we analyzed spätzle processing enzyme (Tm-SPE) of the three-step serine protease cascade of T. molitor Toll pathway. Tm-SPE expressed in all developmental stages and larval tissues. Upon immune challenge, its expression levels were upregulated but significantly reduced after RNA interference (RNAi). In addition, the induction of immune responses upon immune challenge or recombinant DSP1 injection was significantly increased. Loss of function using RNA interference revealed that the Tm-SPE is involved in connecting DSP1 induced immune responses like hemocyte nodule formation, phenoloxidase (PO) activity, phospholipase A2 (PLA2) activity and antimicrobial peptide (AMP) synthesis. These suggest that Tm-SPE controls the DSP1 induced activation of Toll immune signalling pathway required for both cellular and humoral immune responses. However, to confirm the target molecule of DSP1 in three-step proteolytic cascade, we have to check other upstream serine proteases like Spatzle activating enzyme (SAE) or modular serine protease (MSP).

A novel physiological function of pheromone biosynthesis-activating neuropeptide in production of aggregation pheromone

The western flower thrips, Frankliniella occidentalis, is an insect pest, and its aggregation pheromone (AP) plays a crucial role in the recruitment of both sexes. A novel pheromone biosynthesis-activating neuropeptide (PBAN)-like gene is encoded in F. occidentalis genome, but its physiological function has yet to be elucidated. This study hypothesized the physiological role played by PBAN in mediating AP production. AP has been known to be produced only by male adults in F. occidentalis. Surprisingly, our extraction of headspace volatiles contained two AP components in females as well as in males with similar composition. PBAN injection elevated the AP production whereas RNA interference (RNAi) of the gene expression suppressed the AP production in both sexes. A biosynthetic pathway to produce AP components were predicted and the enzymes catalyzing the main steps were confirmed in their expressions. Individual RNAi treatments of these genes significantly suppressed AP production. RNAi of PBAN gene downregulated the expressions of these biosynthesis-associated genes in both sexes. These results suggest that the novel neuropeptide acts as PBAN mediating AP production through stimulating its biosynthetic machinery in F. occidentalis.

Greenhouse test of spraying dsRNA to control the western flower thrips, Frankliniella occidentalis, infesting hot peppers

BACKGROUND

The western flower thrips Frankliniella occidentalis is an insect pest that damages various crops, including hot peppers. It is a vector of a plant pathogen, tomato spotted wilt virus. To control this pest, chemical insecticides have been used in the past, but the control efficacy is unsatisfactory owing to rapid resistance development by F. occidentalis.

METHODOLOGY

This study reports a novel control technology against this insect pest using RNA interference (RNAi) of the vacuolar-type ATPase (vATPase) expression. Eight subunit genes (vATPase-A ∼ vATPase-H) of vATPase were obtained from the F. occidentalis genome and confirmed for their expressions at all developmental stages.

RESULTS

Double-stranded RNAs (dsRNAs) specific to the eight subunit genes were fed to larvae and adults, which significantly suppressed the corresponding gene expressions after 24-h feeding treatment. These RNAi treatments resulted in significant mortalities, in which the dsRNA treatments at ∼2,000 ppm specific to vATPase-A or vATPase-B allowed complete control efficacy near 100% mortality in 7 days after treatment. To prevent dsRNA degradation by the digestive proteases during oral feeding, dsRNAs were formulated in a liposome and led to an enhanced mortality of the larvae and adults of F. occidentalis. The dsRNAs were then sprayed at 2,000 ppm on F. occidentalis infesting hot peppers in a greenhouse, which resulted in 53.5-55.9% control efficacy in 7 days after treatment. Even though the vATPases are conserved in different organisms, the dsRNA treatment was relatively safe for non-target insects owing to the presence of mismatch sequences compared to the dsRNA region of F. occidentalis.

CONCLUSION

These results demonstrate the practical feasibility of spraying dsRNA to control F. occidentalis infesting crops.

HMGB1-like dorsal switch protein 1 of the mealworm, Tenebrio molitor, acts as a damage-associated molecular pattern

High-mobility group box 1 (HMGB1) is a nuclear protein highly conserved in eukaryotes and ubiquitously expressed to regulate transcription and chromatin remodeling. Dorsal switch protein 1 (DSP1) is its insect homolog. A lepidopteran DSP1 acts as a damage-associated molecular pattern (DAMP) in response to immune challenge. The objective of this study was to determine the role of DAMP in the mealworm beetle, Tenebrio molitor, a coleopteran insect. DSP1 of T. molitor (Tm-DSP1) encodes 536 amino acids and shares sequence similarities with Homo sapiens HMGB1 (56.3%) and Spodoptera exigua DSP1 (59.2%). An antisera raised against S. exigua DSP1 was cross-reactive to Tm-DSP1. Like other insect DSPs, Tm-DSP1 has a relatively long N-terminal extension in addition to two conserved HMG box domains. It was expressed in all developmental stages of T. molitor and different larval tissues. Upon immune challenge, its expression level was upregulated. Its RNA interference (RNAi) treatment resulted in a significant reduction in immune responses measured by hemocyte nodule formation against bacterial infection. In addition, the induction of some antimicrobial peptide genes to the immune challenge was suppressed by its RNAi treatment. Interestingly, phospholipase A₂ associated with eicosanoid biosynthesis was significantly suppressed in its catalytic activity by the RNAi treatment specific to Tm-DSP1 expression. Without any pathogen infection, injection of a lepidopteran DSP1 induced both cellular and humoral immune responses. These results suggest that Tm-DSP1 in T. molitor can act as a DAMP molecule and mediate immune responses upon immune challenge.

Immune mediation of HMG-like DSP1 via Toll-Spätzle pathway and its specific inhibition by salicylic acid analogs

Xenorhabdus hominickii, an entomopathogenic bacterium, inhibits eicosanoid biosynthesis of target insects to suppress their immune responses by inhibiting phospholipase A₂ (PLA₂) through binding to a damage-associated molecular pattern (DAMP) molecule called dorsal switch protein 1 (DSP1) from Spodoptera exigua, a lepidopteran insect. However, the signalling pathway between DSP1 and PLA₂ remains unknown. The objective of this study was to determine whether DSP1 could activate Toll immune signalling pathway to activate PLA₂ activation and whether X. hominickii metabolites could inhibit DSP1 to shutdown eicosanoid biosynthesis. Toll-Spätzle (Spz) signalling pathway includes two Spz (SeSpz1 and SeSpz2) and 10 Toll receptors (SeToll1-10) in S. exigua. Loss-of-function approach using RNA interference showed that SeSpz1 and SeToll9 played crucial roles in connecting DSP1 mediation to activate PLA₂. Furthermore, a deletion mutant against SeToll9 using CRISPR/Cas9 abolished DSP1 mediation and induced significant immunosuppression. Organic extracts of X. hominickii culture broth could bind to DSP1 at a low micromolar range. Subsequent sequential fractionations along with binding assays led to the identification of seven potent compounds including 3-ethoxy-4-methoxyphenol (EMP). EMP could bind to DSP1 and prevent its translocation to plasma in response to bacterial challenge and suppress the up-regulation of PLA₂ activity. These results suggest that X. hominickii inhibits DSP1 and prevents its DAMP role in activating Toll immune signalling pathway including PLA₂ activation, leading to significant immunosuppression of target insects.

Immune responses of insects are highly effective in defending various entomopathogens. Xenorhabdus hominickii is an entomopathogenic bacterium that uses a pathogenic strategy of suppressing host insect immunity by inhibiting phospholipase A₂ (PLA₂) which catalyzes the committed step for eicosanoid biosynthesis. Eicosanoids mediate both cellular and humoral immune responses in insects. This study discovers an upstream signalling pathway to activate PLA₂ in response to bacterial challenge. Se-DSP1 is an insect homolog of vertebrate HMGB1 that acts as a damage-associated molecular pattern. Upon bacterial infection, Se-DSP1 is released to the circulatory system to activate Spätzle, an insect cytokine that can bind to Toll receptor. Toll immune signalling pathway can activate antimicrobial peptide gene expression and PLA₂. A deletion mutant against a Toll gene abolished immune responses mediated by Se-DSP1. Indeed, X. hominickii can produce and secrete secondary metabolites including salicylic acid analogs that can strongly bind to Se-DSP1. These bacterial metabolites prevented the release of Se-DSP1, which impaired the activation of PLA₂ and resulted in a significant immunosuppression of target insects against bacterial infection.

Male-biased Adult Production of the Striped Fruit Fly, Zeugodacus scutellata, by Feeding dsRNA Specific to Transformer-2

Sterile insect release technique (SIT) is effective for eradicating quarantine insects including various tephritid fruit flies. When SIT is used for fruit flies, it is challenging to remove females from sterile males due to oviposition-associated piercing damage. This study developed a sex transition technique by feeding double-stranded RNA (dsRNA) specific to a sex-determining gene, Transformer-2 (Zs-Tra2) of the striped fruit fly, Zeugodacus scutellata. Zs-Tra2 is homologous to other fruit fly orthologs. It is highly expressed in female adults. RNA interference (RNAi) of Zs-Tra2 by injecting or feeding its specific dsRNA to larvae significantly increased male ratio. Recombinant Escherichia coli cells expressing dsRNA specific to Zs-Tra2 were prepared and used to feed larvae to suppress Zs-Tra2 gene expression levels. When these recombinant bacteria were fed to larvae during the entire feeding stage, the test population was significantly male-biased. Some females treated with such recombinant E. coli exhibited mosaic morphological characters such as the presence of male-specific abdominal setae in females. This study proposes a novel technique by feeding dsRNA specific to Transformer-2 to reduce female production during mass-rearing of tephritid males for SIT.

Alteration of insulin signaling to control insect pest by using transformed bacteria expressing dsRNA

BACKGROUND

Insulin/insulin-like growth factor signaling (IIS) is known to mediate larval growth and adult reproduction in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae). Four IIS components (InR, FOXO, Akt, and TOR) play crucial roles in the IIS pathway.

RESULTS

RNA interference (RNAi) against any of these four IIS component genes was effective in suppressing each target mRNA level by either hemocoelic injection or oral administration using gene-specific double-stranded RNAs (dsRNAs). These RNAi treatments interfered with larval growth, leading to small pupae or significant larval mortality. For massive production of dsRNA, transformed bacteria expressing dsRNAs of these four IIS components were prepared with L4440 expression vector and HT115 strain of Escherichia coli. The transformed bacteria killed the larvae in a dose-dependent manner by feeding administration. An ultra-sonication pretreatment was performed to impair bacterial membrane and increase dsRNA release from the bacteria in insect intestine. This pretreatment increased the insecticidal activity of these recombinant bacteria. To further increase dsRNA toxicity, its mixture with Bacillus thuringiensis (Bt) was prepared and showed significant increase of Bt insecticidal activity in the laboratory. The bacterial mixture also showed a high control efficacy (83.3%) in an adzuki bean (Vigna angularis) field infested by M. vitrata. Furthermore, such a dsRNA effect was specific for M. vitrata, but not for non-target insects.

CONCLUSION

The bacteria expressing dsRNA specific to IIS components can be used to develop dsRNA insecticide. © 2019 Society of Chemical Industry.

Inhibition of prostaglandin biosynthesis leads to suppressed ovarian development in Spodoptera exigua

Prostaglandins (PGs) are a group of eicosanoids that are C20 oxygenated polyunsaturated fatty acids. PGs can mediate various physiological processes such as immunity, salivary secretion, excretion, and reproduction in insects. The objective of this study was to determine the effect of PG on oocyte development in Spodoptera exigua, a lepidopteran insect known to biosynthesize PGs. Polytrophic ovarioles of S. exigua females exhibited follicle development in germarium, in which oocytes were distinct from nurse cells. During vitellogenesis, nurse cells degenerated by losing cytoplasm called "nurse cell dumping" while oocytes showed increase in cell volume. When PG biosynthesis inhibitors such as ibuprofen or aspirin were applied, nurse cell dumping was not complete and no chorion was formed, thus preventing egg formation. However, addition of PGE₂ significantly rescued such inhibition and resumed oocyte development and choriogenesis. To support the observation with genetic factor, RNA interference (RNAi) specific to peroxynectins (Pxts: Se-Pxt1 and Se-Pxt2) known to act as insect cyclooxygenase was performed to suppress PG biosynthesis. Both Se-Pxt1 and Se-Pxt2 were highly expressed in the ovary of control female. RNAi treatment against Se-Pxt1 or Se-Pxt2 specifically suppressed target genes and inhibited oocyte development. Addition of PGE₂ to adults treated with RNAi rescued the suppressed development of oocytes. Results of this study suggest that PGs can stimulate oocyte development as autocrine/paracrine mediators of vitellogenesis and choriogenesis in insects.

Novel Factors of Viral Origin Inhibit TOR Pathway Gene Expression

Polydnaviruses (PDVs) are obligate symbionts of endoparasitoid wasps, which exclusively attack the larval stages of their lepidopteran hosts. The Polydnavirus is injected by the parasitoid female during oviposition to selectively infect host tissues by the expression of viral genes without undergoing replication. Toxoneuron nigriceps bracovirus (TnBV) is associated with Toxoneuron nigriceps (Hymenoptera: Braconidae) wasp, an endoparasitoid of the tobacco budworm larval stages, Heliothis virescens (Lepidoptera: Noctuidae). Previous studies showed that TnBV is responsible for alterations in host physiology. The arrest of ecdysteroidogenesis is the main alteration which occurs in last (fifth) instar larvae and, as a consequence, prevents pupation. TnBV induces the functional inactivation of H. virescens prothoracic glands (PGs), resulting in decreased protein synthesis and phosphorylation. Previous work showed the involvement of the PI3K/Akt/TOR pathway in H. virescens PG ecdysteroidogenesis. Here, we demonstrate that this cellular signaling is one of the targets of TnBV infection. Western blot analysis and enzyme immunoassay (EIA) showed that parasitism inhibits ecdysteroidogenesis and the phosphorylation of the two targets of TOR (4E-BP and S6K), despite the stimulation of PTTH contained in the brain extract. Using a transcriptomic approach, we identified viral genes selectively expressed in last instar H. virescens PGs, 48 h after parasitization, and evaluated expression levels of PI3K/Akt/TOR pathway genes in these tissues. The relative expression of selected genes belonging to the TOR pathway (tor, 4e-bp, and s6k) in PGs of parasitized larvae was further confirmed by qRT-PCR. The down-regulation of these genes in PGs of parasitized larvae supports the hypothesis of TnBV involvement in blocking ecdysteroidogenesis, through alterations of the PI3K/Akt/TOR pathway at the transcriptional level.

An Insect Prostaglandin E2 Synthase Acts in Immunity and Reproduction

Eicosanoids, oxygenated metabolites of C20 polyunsaturated fatty acids (PUFAs), mediate fundamental physiological processes, including immune reactions and reproduction, in insects. Prostaglandins (PGs) make up one group of eicosanoids, of which PGE₂ is a relatively well-known mediator in various insect taxa. While PG biosynthesis has been reported, the specific biosynthetic pathway for PGE₂ is not known in insects. Here, we posed the hypothesis that Se-mPGES2 mediates biosynthesis of physiologically active PGE₂ through its cognate protein. To test this hypothesis, we interrogated a transcriptome of the lepidopteran insect, Spodoptera exigua, to identify a candidate PGE₂ synthase (Se-mPGES2) and analyzed its sequence and expression. Its predicted amino acid sequence contains a consensus thioredoxin homology sequence (Cys-x-x-Cys) responsible for catalytic activity along with an N-terminal membrane-associated hydrophobic domain and C-terminal cytosolic domain. It also shares sequence homology (36.5%) and shares almost overlapping three dimensional structures with a membrane-bound human PGES2 (mPGES2). Se-mPGES2 was expressed in all developmental stages with high peaks during the late larval instar and adult stages. Immune challenge significantly up-regulated its expression levels in hemocytes and fat body. Injecting double-stranded RNA (dsRNA) specific to Se-mPGES2 significantly impaired two cellular immune responses, hemocyte-spreading behavior and nodule formation following bacterial challenge. Humoral immunity was also significantly suppressed, registered as reduced phenoloxidase activity and antimicrobial peptide expression levels. The suppressed immune responses were reversed following PGE₂, but not arachidonic acid (AA), treatments. RNAi treatments also reduced the egg-laying behavior of females. Control females mated with the RNAi-treated males led to substantially reduced egg-laying behavior, which was also reversed following PGE₂ injections into females. These results strongly bolster our hypothesis that Se-mPGES2 acts in the biosynthesis of PGE₂, a crucial biochemical signal mediating immune and reproductive physiology of S. exigua.

Toll immune signal activates cellular immune response via eicosanoids

Upon immune challenge, insects recognize nonself. The recognition signal will propagate to nearby immune effectors. It is well-known that Toll signal pathway induces antimicrobial peptide (AMP) gene expression. Eicosanoids play crucial roles in mediating the recognition signal to immune effectors by enhancing humoral immune response through activation of AMP synthesis as well as cellular immune responses, suggesting a functional cross-talk between Toll and eicosanoid signals. This study tested a cross-talk between these two signals. Two signal transducing factors (MyD88 and Pelle) of Toll immune pathway were identified in Spodoptera exigua. RNA interference (RNAi) of either SeMyD88 or SePelle expression interfered with the expression of AMP genes under Toll signal pathway. Bacterial challenge induced PLA₂ enzyme activity. However, RNAi of these two immune factors significantly suppressed the induction of PLA₂ enzyme activity. Furthermore, RNAi treatment prevented gene expression of cellular PLA₂. Inhibition of PLA₂ activity reduced phenoloxidase activity and subsequent suppression in cellular immune response measured by hemocyte nodule formation. However, immunosuppression induced by RNAi of Toll signal molecules was significantly reversed by addition of arachidonic acid (AA), a catalytic product of PLA₂. The addition also significantly reduced the enhanced fungal susceptibility of S. exigua treated by RNAi against two Toll signal molecules. These results indicate that there is a cross-talk between Toll and eicosanoid signals in insect immunity.

A novel calcium-independent phospholipase A2 and its physiological roles in development and immunity of a lepidopteran insect, Spodoptera exigua

Phospholipase A₂ (PLA₂) catalyzes hydrolysis of ester linkage at sn-2 position of phospholipids. At least 15 groups (I-XV) of PLA₂ gene superfamily are associated with various physiological processes such as digestion, secretion, immunity, and maintenance of membrane integrity. This study suggests that various insects encode putative Group VI PLA₂s representing intracellular and calcium-independent PLA₂s (iPLA₂). These insect iPLA₂s are separated into at least two subgroups: iPLA₂A (Group VIA-like) and iPLA₂B (non-Group VIA). Most insects encode genes of iPLA₂B type, although their biological functions are currently unknown. This study predicted a novel iPLA₂ from Spodoptera exigua (a lepidopteran insect) (SeiPLA₂B) and analyzed its physiological functions by RNA interference (RNAi). SeiPLA₂B encodes 336 amino acid sequence with a predicted size of about 36.6 kDa and an isoelectric point at pH 8.61. It possesses a lipase catalytic site, but does not have ankyrin repeats in the amino terminal region. Phylogenetic analysis indicated that SeiPLA₂B was clustered with other Group VI iPLA₂s, in which SeiPLA₂B was closely associated with Group VIF gene while SeiPLA₂A was closely related to Group VIA gene. SeiPLA₂B was expressed in all developmental stages of S. exigua. In larval stage, SeiPLA₂B was expressed in fat body, hemocyte, and epidermis, but not in digestive tract. SeiPLA₂B RNAi significantly reduced PLA₂ enzyme activities and resulted in developmental retardation and immunosuppression. Though RNAi treatment did not significantly change fatty acid composition in fat body lipids, it significantly increased lipid peroxidation. Taken together, our results suggest that SeiPLA₂B plays important roles in the development and immunity of S. exigua.

Chlorine dioxide enhances lipid peroxidation through inhibiting calcium-independent cellular PLA2 in larvae of the Indianmeal moth, Plodia interpunctella

Polyunsaturated fatty acids usually undergo lipid peroxidation induced by reactive oxygen species (ROS). Calcium-independent cellular phospholipase A₂ (iPLA₂) can maintain fatty acid compositions in phospholipids depending on physiological conditions. An insect iPLA₂ (Pi-iPLA₂) was predicted from the transciptome of the Indianmeal moth, Plodia interpunctella. It encodes 835 amino acids. It possesses five ankyrin repeats in the N terminal and patatin lipase domain in the C terminal. Pi-iPLA₂ was expressed in all developmental stages of the Indianmeal moth. In the larval stage, it was expressed in all tissues tested. RNA interference (RNAi) specific to Pi-iPLA₂ was performed using specific double-stranded RNA (dsRNA). It resulted in almost 70% of reduction in gene expression. Under such RNAi condition, P. interpunctella exhibited significant accumulation of lipid peroxidation based on the amount of malondialdehyde. RNAi of Pi-PLA₂ expression also impaired cellular immune response of P. interpunctella. Chlorine dioxide (ClO₂), an insecticidal agent by generating ROS, increased lipid peroxidation in a dose-dependent manner. However, the addition of vitamin E (an antioxidant) reduced the formation of lipid peroxidation. ClO₂ treatment significantly reduced expression of Pi-iPLA₂ but increased lipid peroxidation in larval fat body of P. interpunctella. Furthermore, larvae treated with dsRNA specific to Pi-iPLA₂ were significantly susceptible to ClO₂ treatment. These results suggest that Pi-iPLA₂ plays a crucial role in repairing damaged fatty acids from phospholipids. Our results also suggest that ClO₂ can elevate lipid peroxidation through inhibiting Pi-iPLA₂ expression in addition to direct ROS production.

Nitric Oxide Mediates Insect Cellular Immunity via Phospholipase A2 Activation

After infection or invasion is recognized, biochemical mediators act in signaling insect immune functions. These include biogenic amines, insect cytokines, eicosanoids, and nitric oxide (NO). Treating insects or isolated hemocyte populations with different mediators often leads to similar results. Separate treatments with an insect cytokine, 2 biogenic amines, and an eicosanoid lead to a single result, hemocyte spreading, understood in terms of intracellular cross-talk among these signaling systems. This study focuses on the cross-talk between NO and eicosanoid signaling in our model insect, Spodoptera exigua. Bacterial injection increased NO concentrations in the larval hemocytes and fat body, and RNA interference (RNAi) of the S. exigua NO synthase (NOS) gene suppressed NO concentrations. RNAi treatment also led to a significant reduction in hemocyte nodulation following bacterial injection. Similar RNAi treatments led to significantly reduced PLA2 activities in the hemocytes and fat body compared to control larvae. Injection of L-NAME also prevented the induction of PLA2 activity following bacterial challenge. An injected NO donor, S-nitroso-N-acetyl-DL-penicillamine, increased PLA2 activity in a dose-dependent manner. However, eicosanoids did not influence NO concentrations in immune-challenged larvae. We infer that NO and eicosanoid signaling operate via cross-talk mechanisms in which the elevated NO concentrations activate PLA2 and eicosanoid biosynthesis, which finally mediates various immune responses.

Heat Tolerance Induction of the Indian Meal Moth (Lepidoptera: Pyralidae) Is Accompanied by Upregulation of Heat Shock Proteins and Polyols

The Indian meal moth, Plodia interpunctella, causes massive damage to stored grains and processed foods. Heat treatment has been widely used to control insect pests infesting stored grains. However, heat treatment may result in unsatisfactory control owing to heat tolerance of target insects. This study quantified the heat tolerance and analyzed its induction in P. interpunctella. Susceptibility of P. interpunctella to different high temperatures was assessed in all developmental stages. Heat treatment at 44 °C for 1 h caused significant mortalities to all developmental stages, with late-instar larvae exhibiting the highest tolerance. However, the survivorship to heat treatment was significantly increased by pre-exposure to 37 °C for 30 min. The induction of heat tolerance was accompanied by upregulation of two heat shock proteins of Hsc70 and Hsp90. Trehalose and glycerol concentrations in the hemolymph also increased after pre-exposure to 37 °C for 30 min. RNA interference (RNAi) by specific double-stranded RNAs effectively suppressed the inducible expressions of both Hsc70 and Hsp90 in response to 37 °C for 30 min. Either RNAi of Hsc70 or Hsp90 significantly impaired the heat tolerance induction of P. interpunctella. These results suggest that the induction of heat tolerance in P. interpunctella involves the upregulation of these heat shock proteins and hemolymph polyol levels.

Identification of a hypertrehalosemic factor in Spodoptera exigua

Trehalose is a major blood sugar in insects with a range of physiological functions, including an energy source and a cryoprotectant. Hemolymph trehalose concentrations are tightly regulated according to physiological conditions. An insulin-like peptide, SeILP1, downregulates hemolymph trehalose concentrations in Spodoptera exigua. Here, we identified a factor that upregulates hemolymph trehalose concentration in S. exigua. Hemolymph trehalose concentrations were significantly increased after immune challenge or under starvation in a time-dependent manner. To determine endocrine factors responsible for the upregulation, stress-associated mediators, such as octopamine, serotonin, or eicosanoids were injected, but they did not upregulate hemolymph trehalose. On the other hand, injection with Schistocerca gregaria adipokinetic hormone (AKH) significantly increased hemolymph trehalose concentration in S. exigua. During upregulation of hemolymph trehalose by AKH injection, trehalose degradation appeared to be inhibited because expression of trehalase and SeILP1 were significantly suppressed while that of trehalose phosphate synthase was not significantly changed. Interrogation of a Spodoptera genome database identified an S. exigua AKH-like gene and its expression was confirmed. During starvation, its expression concentrations were increased, although RNA interference specific to the AKH-like hypertrehalosemic factor (SeHTF) gene significantly prevented the upregulation of hemolymph trehalose concentrations during starvation. A synthetic peptide of SeHTF was prepared and injected into S. exigua larvae. At nanomolar concentration, the synthetic SeHTF peptide effectively upregulated hemolymph trehalose concentrations. Here we report a novel hypertrehalosemic factor in S. exigua (SeHTF).

An endoparasitoid wasp influences host DNA methylation

Parasitism by endoparasitoid wasps changes the expression of various host genes, and alters host immune and developmental processes. However, it is not clearly understood how parasitism changes host gene expression in a whole genome scale. This study focused on an epigenetic control of Cotesia plutellae, an endoparasitoid wasp, against its host, Plutella xylostella. Two DNA methyltransferases (DNMT-1 and DNMT-2) are encoded in the genome of P. xylostella. In addition, methyl-binding domain proteins (MBDs) and DNA demethylation factor, ten-eleven translation protein (TET) are encoded. DNA methylation of P. xylostella genomic DNA was confirmed by restriction digestion with Gla I specific to 5-methylcytosine. DNA methylation intensity in parasitized (P) larvae was decreased compared to that in nonparasitized (NP) larvae, especially at late parasitic stage, at which expression levels of both DNMT-1 and DNMT-2 were also decreased. DNA demethylation of P. xylostella was confirmed in both NP and P larvae by restriction digestion with PvuRts1I recognizing 5-hydroxymethyl cytosine. Parasitism also suppressed expression levels of TET and MBDs. Treatment of 5-aza-2′-deoxycytidine (AZA) reduced DNA methylation intensity of NP larvae, causing suppression of hemocyte-spreading behavior and delay of immature development. RNA interference of DNMT-1 or DNMT-2 mimicked the adverse effects of AZA.

Down-regulation of a chitin synthase a gene by RNA interference enhances pathogenicity of Beauveria bassiana ANU1 against Spodoptera exigua (HÜBNER)

Chitin synthase (CHS) is an important enzymatic component, which is required for chitin formation in the cuticles and cuticular linings of other tissues in insects. CHSs have been divided into two classes, classes A and B, based on their amino acid sequence similarities and functions. Class A CHS (CHS-A) is specifically expressed in the epidermis and related ectodermal cells such as tracheal cells, while class B CHS (CHS-B) is expressed in gut epithelial cells that produce peritrophic matrices. In this study, we cloned the CHS-A gene from the beet armyworm, Spodoptera exigua (SeCHS-A). The SeCHS-A contains an open reading frame of 4,698 nucleotides, encoding a protein of 1,565 amino acids with a predicted molecular mass of approximately 177.8 kDa. The SeCHS-A mRNA was expressed in all developmental stages and specifically in the epidermis and tracheae tissue by quantitative real-time-PCR analysis. Expression of SeCHS-A gene was suppressed by feeding double-stranded RNA (dsCHS-A, 400 ng/larva) in the third instar larvae of S. exigua. Suppression of the SeCHS-A gene expression significantly increased 35% of mortality on pupation of S. exigua. Also, the third instar larvae fed with dsCHS-A significantly increased susceptibility to entomopathogenic fungi, Beauveria bassiana ANU1 at 3 days after treatment. These results suggest that the SeCHS-A gene plays an important role in development of S. exigua and RNA interference may apply to effective pest control with B. bassiana.

A viral histone H4 suppresses insect insulin signal and delays host development

Parasitization by an endoparasitoid wasp, Cotesia plutellae, alters host development of Plutella xylostella by extending larval period and preventing metamorphosis. Insulin signal plays a crucial role in mediating insect development and controlling blood sugar level in insects. In this study, three insulin-like peptide genes (PxILP1-3) were predicted from the genome of P. xylostella. However, only PxILP1 was confirmed to be expressed in P. xylostella. Starvation suppressed the expression level of PxILP1 and up-regulated plasma trehalose level. RNA interference against PxILP1 mimicked starvation effect and extended the larval period of P. xylostella. Parasitized larvae exhibited significantly lower levels of PxILP1 expression compared to nonparasitized larvae. Injection of wasp-symbiotic polydnavirus C. plutellae bracovirus (CpBV) also suppressed PxILP1 expression and extended the larval period. Injection of a viral segment (CpBV-S30) containing a viral histone H4 (CpBV-H4) also suppressed PxILP1 expression. Co-injection of CpBV-S30 and double-stranded RNA (dsCpBV-H4) specific to CpBV-H4 rescued the suppression of PxILP1 expression. Injection of CpBV-S30 significantly extended larval development. Co-injection of CpBV-S30 with dsCpBV-H4 rescued the delay of larval development. Injection of a bovine insulin to parasitized larvae prevented parasitoid development. These results indicate that parasitism of C. plutellae can down-regulate host insulin signaling with the help of parasitic factor CpBV-H4.

Suppressive activity of a viral histone H4 against two host chromatin remodelling factors: lysine demethylase and SWI/SNF

Histone H4, a nucleosome subunit in eukaryotes, plays crucial roles in DNA package and regulation of gene expression through covalent modification. A viral histone H4 encoded in Cotesia plutellae bracovirus (CpBV), a polydnavirus, is called CpBV-H4. It is highly homologous to other histone H4 proteins excepting 38 extra amino acid residues in the N terminus. CpBV-H4 can form octamer with other histone subunits and alter host gene expression. In this study, CpBV-H4 was transiently expressed in a natural host (Plutella xylostella) and its suppressive activity on host gene expression was evaluated by the suppressive subtractive hybridization (SSH) technique. The SSH targets down-regulated by CpBV-H4 were read with the 454 pyrosequencing platform and annotated using the genome of P. xylostella. The down-regulated genes (610 contigs) were annotated in most functional categories based on gene ontology. Among these SSH targets, 115 genes were functionally distinct, including two chromatin remodelling factors: a lysine-specific demethylase (Px-KDM) and a chromatin remodelling complex [Px-SWI/SNF (SWItch/Sucrose Non-Fermentable)]. Px-KDM was highly expressed in all tested tissues during the entire larval period. Suppression of Px-KDM expression by specific RNA interference (RNAi) significantly (P<0.05) reduced haemocyte nodule formation in response to immune challenge and impaired both larval and pupal development. Px-SWI/SNF was expressed in all developmental stages. Suppression of Px-SWI/SNF expression by RNAi reduced cellular immune response and interfered with adult metamorphosis. These results suggest that CpBV-H4 can alter host gene expression by interfering with chromatin modification and remodelling factors in addition to its direct epigenetic control activity.

Glyceraldehyde-3-phosphate dehydrogenase is a mediator of hemocyte-spreading behavior and molecular target of immunosuppressive factor CrV1

Cellular immunity is accompanied by hemocyte-spreading behavior, which undergoes cytoskeletal rearrangement. Polydnaviral factor CpBV-CrV1 can inhibit the hemocyte-spreading behavior and suppress host immune response of Plutella xylostella. However, host target molecule of CpBV-CrV1 that inhibits the hemocyte behavior has not been identified yet. This study used a pull-down approach to identify the target molecule of CpBV-CrV1. A protein bound to CpBV-CrV1 was co-precipitated and identified to be glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by LC-MS/MS analysis. RNA interference (RNAi) specific to GAPDH of P. xylostella was found to be able to inhibit the hemocyte-spreading behavior, while RNAi treatments with other glycolytic genes had no effect on the spreading behavior. An addition of recombinant CpBV-CrV1 on hemocyte monolayer interrupted the association between GAPDH and α-tubulin in the cytoplasm. Overlay of mutant proteins (Y492A or Y501A with tyrosine to alanine at putative GAPDH-binding site) of CpBV-CrV1 on hemocyte monolayer revealed that they could enter hemocytes unlike a mutant in the N-terminal coiled-coil domain. However, they failed to inhibit the hemocyte-spreading behavior without any binding affinity to GAPDH. These results suggest that GAPDH plays a critical role in hemocyte-spreading behavior during immune challenge as a molecular target of viral factor CpBV-CrV1.

A single type of cadherin is involved in Bacillus thuringiensis toxicity in Plutella xylostella

Cadherins have been described as one the main functional receptors for the toxins of the entomopathogenic bacterium, Bacillus thuringiensis (Bt). With the availability of the whole genome of Plutella xylostella, different types of cadherins have been annotated. In this study we focused on determining those members of the cadherin-related proteins that potentially play a role in the mode of action of Bt toxins. For this, we mined the genome of P. xylostella to identify these putative cadherins. The genome screening revealed 52 genes that were annotated as cadherin or cadherin-like genes. Further analysis revealed that six of these putative cadherins had three motifs common to all Bt-related cadherins: a signal peptide, cadherin repeats and a transmembrane domain. From the six selected cadherins, only P. xylostella cadherin 1 (PxCad1) was expressed in the larval midgut and only the silencing of this gene by RNA interference (double-stranded RNA feeding) reduce toxicity and binding to the midgut of the Cry1Ac type toxin from Bt. These results indicate that from the whole set of cadherin-related genes identified in P. xylostella, only PxCad1 is associated with the Cry1Ac mode of action.

A novel calcium-independent cellular PLA2 acts in insect immunity and larval growth

Phospholipase A2 (PLA2) catalyzes the position-specific hydrolysis of fatty acids linked to the sn-2 position of phospholipids (PLs). PLA2s make up a very large superfamily, with more than known 15 groups, classified into secretory PLA2 (sPLA2), Ca(2+)-dependent cellular PLA2 (sPLA2) and Ca(2+)-independent cellular PLA2 (iPLA2). Only a few insect sPLA2s, expressed in venom glands and immune tissues, have been characterized at the molecular level. This study aimed to test our hypothesis that insects express iPLA2, using the beet armyworm, Spodoptera exigua, our model insect. Substantial PLA2 activities under calcium-free condition were recorded in several larval tissue preparations. The PLA2 activity was significantly reduced in reactions conducted in the presence of a specific iPLA2 inhibitor, bromoenol lactone (BEL). Analysis of a S. exigua hemocyte transcriptome identified a candidate iPLA2 gene (SeiPLA2-A). The open reading frame encoded 816 amino acid residues with a predicted molecular weight of 90.5 kDa and 6.15 pI value. Our phylogenetic analysis clustered SeiPLA2-A with the other vertebrate iPLA2s. SeiPLA2-A was expressed in all tissues we examined, including hemocytes, fat body, midgut, salivary glands, Malpighian tubules and epidermis. Heterologous expression in Sf9 cells indicated that SeiPLA2-A was localized in cytoplasm and exhibited significant PLA2 activity, which was independent of Ca(2+) and inhibited by BEL. RNA interference (RNAi) of SeiPLA2-A using its specific dsRNA in the fifth instar larvae significantly suppressed iPLA2 expression and enzyme activity. dsSeiPLA2-A-treated larvae exhibited significant loss of cellular immune response, measured as nodule formation in response to bacterial challenge, and extended larval-to-pupal developmental time. These results support our hypothesis, showing that SeiPLA2-A predicted from the transcriptome analysis catalyzes hydrolysis of fatty acids from cellular PLs and plays crucial physiological roles in insect immunity and larval growth.

Oxidative stress induced by chlorine dioxide as an insecticidal factor to the Indian meal moth, Plodia interpunctella

A novel fumigant, chlorine dioxide (ClO2) is a commercial bleaching and disinfection agent. Recent study indicates its insecticidal activity. However, its mode of action to kill insects is yet to be understood. This study set up a hypothesis that an oxidative stress induced by ClO2 is a main factor to kill insects. The Indian meal moth, Plodia interpunctella, is a lepidopteran insect pest infesting various stored grains. Larvae of P. interpunctella were highly susceptible to ClO2 gas, which exhibited an acute toxicity. Physiological damages by ClO2 were observed in hemocytes. At high doses, the larvae of P. interpunctella suffered significant reduction of total hemocytes. At low doses, ClO2 impaired hemocyte behaviors. The cytotoxicity of ClO2 was further analyzed using two insect cell lines, where Sf9 cells were more susceptible to ClO2 than High Five cells. The cells treated with ClO2 produced reactive oxygen species (ROS). The produced ROS amounts increased with an increase of the treated ClO2 amount. However, the addition of an antioxidant, vitamin E, significantly attenuated the cytotoxicity of ClO2 in a dose-dependent manner. To support the oxidative stress induced by ClO2, two antioxidant genes (superoxide dismutase (SOD) and thioredoxin-peroxidase (Tpx)) were identified from P. interpunctella EST library using ortholog sequences of Bombyx mori. Both SOD and Tpx were expressed in larvae of P. interpunctella especially under oxidative stress induced by bacterial challenge. Exposure to ClO2 gas significantly induced the gene expression of both SOD and Tpx. RNA interference of SOD or Tpx using specific double stranded RNAs significantly enhanced the lethality of P. interpunctella to ClO2 gas treatment as well as to the bacterial challenge. These results suggest that ClO2 induces the production of insecticidal ROS, which results in a fatal oxidative stress in P. interpunctella.

Recurrent Domestication by Lepidoptera of Genes from Their Parasites Mediated by Bracoviruses

Bracoviruses are symbiotic viruses associated with tens of thousands of species of parasitic wasps that develop within the body of lepidopteran hosts and that collectively parasitize caterpillars of virtually every lepidopteran species. Viral particles are produced in the wasp ovaries and injected into host larvae with the wasp eggs. Once in the host body, the viral DNA circles enclosed in the particles integrate into lepidopteran host cell DNA. Here we show that bracovirus DNA sequences have been inserted repeatedly into lepidopteran genomes, indicating this viral DNA can also enter germline cells. The original mode of Horizontal Gene Transfer (HGT) unveiled here is based on the integrative properties of an endogenous virus that has evolved as a gene transfer agent within parasitic wasp genomes for ≈100 million years. Among the bracovirus genes thus transferred, a phylogenetic analysis indicated that those encoding C-type-lectins most likely originated from the wasp gene set, showing that a bracovirus-mediated gene flux exists between the 2 insect orders Hymenoptera and Lepidoptera. Furthermore, the acquisition of bracovirus sequences that can be expressed by Lepidoptera has resulted in the domestication of several genes that could result in adaptive advantages for the host. Indeed, functional analyses suggest that two of the acquired genes could have a protective role against a common pathogen in the field, baculovirus. From these results, we hypothesize that bracovirus-mediated HGT has played an important role in the evolutionary arms race between Lepidoptera and their pathogens.

Eukaryotes are generally thought to evolve mainly through the modification of existing genetic information. However, evidence of horizontal gene transfer (HGT) in eukaryotes-the accidental acquisition of a novel gene from another species, allowing acquisition of novel traits—is now recognized as an important factor in their evolution. We show here that in several lineages, lepidopteran genomes have acquired genes from a bracovirus that is symbiotically used by parasitic wasps to inhibit caterpillar host immune defences. Integration of parts of the viral genome into host caterpillar DNA strongly suggests that integration can sporadically occur in the germline, leading to the production of lepidopteran lineages that harbor bracovirus sequences. Moreover, some of the transferred bracovirus genes reported here originate from the wasp genome, demonstrating that a gene flux exists between the two insect orders Hymenoptera and Lepidoptera that diverged ≈300 MYA. As bracovirus gene organisation has evolved to allow expression in Lepidoptera, these transferred genes can be readily domesticated. Additionally, we present functional analyses suggesting that some of the acquired genes confer to caterpillars a protection toward baculovirus, a very common pathogen in the field. This phenomenon may have implications for understanding how caterpillars acquire resistance against baculoviruses used in biological control.

A whole genome screening and RNA interference identify a juvenile hormone esterase-like gene of the diamondback moth, Plutella xylostella

Juvenile hormone (JH) plays a crucial role in preventing precocious metamorphosis and stimulating reproduction. Thus, its hemolymph titer should be under a tight control. As a negative controller, juvenile hormone esterase (JHE) performs a rapid breakdown of residual JH in the hemolymph during last instar to induce a larval-to-pupal metamorphosis. A whole genome of the diamondback moth (DBM), Plutella xylostella, has been annotated and proposed 11 JHE candidates. Sequence analysis using conserved motifs commonly found in other JHEs proposed a putative JHE (Px004817). Px004817 (64.61 kDa, pI=5.28) exhibited a characteristic JHE expression pattern by showing high peak at the early last instar, at which JHE enzyme activity was also at a maximal level. RNA interference of Px004817 reduced JHE activity and interrupted pupal development with a significant increase of larval period. This study identifies Px004817 as a JHE-like gene of P. xylostella.

Eicosanoids up-regulate production of reactive oxygen species by NADPH-dependent oxidase in Spodoptera exigua phagocytic hemocytes

Eicosanoids mediate cellular immune responses in insects, including phagocytosis of invading microbes. Phagocytosis entails two major steps, the internalization of microbes and the subsequent killing of them via formation of reactive oxygen species (ROS). Here, we posed the hypothesis that eicosanoids mediate ROS production by activating NADPH-dependent oxidase (NOX) and tested the idea in the model insect, Spodoptera exigua. A NOX gene (we named SeNOX4) was identified and cloned, yielding a full open reading frame encoding 547 amino acid residues with a predicted molecular weight of 63,410Da and an isoelectric point at 9.28. A transmembrane domain and a large intracellular domain containing NADPH and FAD-binding sites were predicted. Phylogenetic analysis indicated SeNOX4 clusters with other NOX4 genes. SeNOX4 was expressed in all life stages except eggs, and exclusively in hemocytes. Bacterial challenge and, separately, arachidonic acid (AA, a precursor of eicosanoid biosynthesis) injection increased its expression. The internalization step was assessed by counting hemocytes engulfing fluorescence-labeled bacteria. The phagocytic behavior was inhibited by dsRNA suppression of SeNOX4 expression and, separately by dexamethasone (DEX, a specific inhibitor of eicosanoid biosynthesis) treatments. However, injecting AA to dsSeNOX4-treated larvae did not rescue the phagocytic activity. Hemocytic ROS production increased following bacterial challenge, which was sharply reduced in dsSeNOX4-treated, and separately, in DEX-treated larvae. AA partially reversed the suppressed ROS production in dsSeNOX4-treated larvae. Treating larvae with either the ROS-suppressing dsSeNOX4 construct or DEX rendered experimental larvae unable to inhibit bacterial proliferation in their hemocoels. We infer that eicosanoids mediate ROS production during phagocytosis by inducing expression of SeNOX4.

Integrin β subunit and its RNA interference in immune and developmental processes of the Oriental tobacco budworm, Helicoverpa assulta

Integrins are cell surface heterodimeric proteins interacting with the extracellular matrix and mediating environmental signals through cell membranes. A full-length cDNA sequence of the integrin β1 subunit gene (HaITGb1) was cloned from the Oriental tobacco budworm, Helicoverpa assulta, and analyzed for its physiological role in both immune response and development. HaITGb1 was expressed in all developmental stages from egg to adult and in all tested larval tissues of hemocytes, fat body, gut, and epidermis. Utilizing an RNA interference (RNAi) approach, injection of a specific double-stranded RNA (dsRNA) in larvae suppressed HaITGb1 transcript levels and significantly impaired hemocytes in their extracellular matrix adherence properties. Furthermore, the RNAi treatment significantly suppressed hemocyte nodule formation in response to bacterial challenge, which resulted in significantly enhanced susceptibility to both pathogenic and non-pathogenic bacteria. The RNAi treatment also interfered with H. assulta larval and pupal development. These results suggest that the extensive and constitutive expression of HaITGb1 is necessary for H. assulta to perform an efficient immune response against microbial pathogens and undergo normal immature development.

Antiviral activity of the inducible humoral immunity and its suppression by eleven BEN family members encoded in Cotesia plutellae bracovirus

Upon parasitization by some endoparasitoids, polydnaviruses (PDVs) play a crucial role in inducing host immunosuppression. This study reports a novel immunosuppressive activity against humoral immune responses by BEN family genes encoded in Cotesia plutellae bracovirus (CpBV). A total of 11 BEN family members are encoded in 10 different CpBV DNA segments. When the CpBV segments were individually injected, specific BEN genes were expressed and suppressed the expression of antimicrobial peptide (AMP) and prophenoloxidase genes following bacterial challenge. The suppressive activities of the BEN genes were reversed by injection of the double-stranded RNA (dsRNA) specific to each BEN gene. The suppression of the AMP gene expressions by the BEN genes was also confirmed using an inhibition zone assay against Gram-positive and Gram-negative bacterial growth. The significance of the suppressive activity of BEN genes against humoral immune responses was analyzed in terms of suppression of antiviral activity by the host humoral immunity. When CpBV was incubated with the plasma obtained from the larvae challenged with bacteria, the immunized plasma severely impaired the expression activity of the viral genes. However, an expression of BEN gene significantly rescued the viral gene expression by suppressing humoral immune response. These results suggest that BEN family genes of CpBV play a crucial role in defending the antiviral response of the parasitized Plutella xylostella by inhibiting humoral immune responses.

Phylogenetic analyses and characterization of RNase X25 from Drosophila melanogaster suggest a conserved housekeeping role and additional functions for RNase T2 enzymes in protostomes

Ribonucleases belonging to the RNase T2 family are enzymes associated with the secretory pathway that are almost absolutely conserved in all eukaryotes. Studies in plants and vertebrates suggest they have an important housekeeping function in rRNA recycling. However, little is known about this family of enzymes in protostomes. We characterized RNase X25, the only RNase T2 enzyme in Drosophila melanogaster. We found that RNase X25 is the major contributor of ribonuclease activity in flies as detected by in gel assays, and has an acidic pH preference. Gene expression analyses showed that the RNase X25 transcript is present in all adult tissues and developmental stages. RNase X25 expression is elevated in response to nutritional stresses; consistent with the hypothesis that this enzyme has a housekeeping role in recycling RNA. A correlation between induction of RNase X25 expression and autophagy was observed. Moreover, induction of gene expression was triggered by oxidative stress suggesting that RNase X25 may have additional roles in stress responses. Phylogenetic analyses of this family in protostomes showed that RNase T2 genes have undergone duplication events followed by divergence in several phyla, including the loss of catalytic residues, and suggest that RNase T2 proteins have acquired novel functions. Among those, it is likely that a role in host immunosuppression evolved independently in several groups, including parasitic Platyhelminthes and parasitoid wasps. The presence of only one RNase T2 gene in the D. melanogaster genome, without any other evident secretory RNase activity detected, makes this organism an ideal system to study the cellular functions of RNase T2 proteins associated with RNA recycling and maintenance of cellular homeostasis. On the other hand, the discovery of gene duplications in several protostome genomes also presents interesting new avenues to study additional biological functions of this ancient family of proteins.

A specific glycerol kinase induces rapid cold hardening of the diamondback moth, Plutella xylostella

Insects in temperate zones survive low temperatures by migrating or tolerating the cold. The diamondback moth, Plutella xylostella, is a serious insect pest on cabbage and other cruciferous crops worldwide. We showed that P. xylostella became cold-tolerant by expressing rapid cold hardiness (RCH) in response to a brief exposure to moderately low temperature (4°C) for 7h along with glycerol accumulation in hemolymph. Glycerol played a crucial role in the cold-hardening process because exogenously supplying glycerol significantly increased the cold tolerance of P. xylostella larvae without cold acclimation. To determine the genetic factor(s) responsible for RCH and the increase of glycerol, four glycerol kinases (GKs), and glycerol-3-phosphate dehydrogenase (PxGPDH) were predicted from the whole P. xylostella genome and analyzed for their function associated with glycerol biosynthesis. All predicted genes were expressed, but differed in their expression during different developmental stages and in different tissues. Expression of the predicted genes was individually suppressed by RNA interference (RNAi) using double-stranded RNAs specific to target genes. RNAi of PxGPDH expression significantly suppressed RCH and glycerol accumulation. Only PxGK1 among the four GKs was responsible for RCH and glycerol accumulation. Furthermore, PxGK1 expression was significantly enhanced during RCH. These results indicate that a specific GK, the terminal enzyme to produce glycerol, is specifically inducible during RCH to accumulate the main cryoprotectant.

ABCC transporters mediate insect resistance to multiple Bt toxins revealed by bulk segregant analysis

BACKGROUND

Relatively recent evidence indicates that ABCC2 transporters play a main role in the mode of action of Bacillus thuringiensis (Bt) Cry1A-type proteins. Mapping of major Cry1A resistance genes has linked resistance to the ABCC2 locus in Heliothis virescens, Plutella xylostella, Trichoplusia ni and Bombyx mori, and mutations in this gene have been found in three of these Bt-resistant strains.

RESULTS

We have used a colony of Spodoptera exigua (Xen-R) highly resistant to a Bt commercial bioinsecticide to identify regions in the S. exigua genome containing loci for major resistance genes by using bulk segregant analysis (BSA). Results reveal a region containing three genes from the ABCC family (ABBC1, ABBC2 and ABBC3) and a mutation in one of them (ABBC2) as responsible for the resistance of S. exigua to the Bt commercial product and to its key Spodoptera-active ingredients, Cry1Ca. In contrast to all previously described mutations in ABCC2 genes that directly or indirectly affect the extracellular domains of the membrane protein, the ABCC2 mutation found in S. exigua affects an intracellular domain involved in ATP binding. Functional analyses of ABBC2 and ABBC3 support the role of both proteins in the mode of action of Bt toxins in S. exigua. Partial silencing of these genes with dsRNA decreased the susceptibility of wild type larvae to both Cry1Ac and Cry1Ca. In addition, reduction of ABBC2 and ABBC3 expression negatively affected some fitness components and induced up-regulation of arylphorin and repat5, genes that respond to Bt intoxication and that are found constitutively up-regulated in the Xen-R strain.

CONCLUSIONS

The current results show the involvement of different members of the ABCC family in the mode of action of B. thuringiensis proteins and expand the role of the ABCC2 transporter in B. thuringiensis resistance beyond the Cry1A family of proteins to include Cry1Ca.

Selectivity of a translation-inhibitory factor, CpBV15β, in host mRNAs and subsequent alterations in host development and immunity

An endoparasitoid wasp, Cotesia plutellae, parasitizes young larvae of the diamondback moth, Plutella xylostella. Its symbiotic virus, C. plutellae bracovirus (CpBV), has been shown to play a crucial role in inducing physiological changes in the parasitized host. A viral gene, CpBV15β, exhibits a specific translational control against host mRNAs by sequestering a eukaryotic translation initiation factor, eIF4A. Inhibitory target mRNAs have high thermal stability (>≈9 kcal/mol) of their secondary structures in 5'UTR. To determine the specificity of translational control in terms of 5'UTR complexity, this study screened target/nontarget mRNAs of CpBV15β using a proteomics approach through an in vivo transient expression technique. A proteomics analysis of host plasma proteins showed that 12.9% (23/178) spots disappeared along with the expression of CpBV15β. A total of ten spots were chosen, in which five spots ('target') were disappeared by expression of CpBV15β and the other five ('nontarget') were insensitive to expression of CpBV15β, and further analyzed by a tandem mass spectroscopy. The predicted genes of target spots had much greater complexity (-12.3 to -25.2 kcal/mol) of their 5'UTR in terms of thermal stability compared to those (-3.70 to -9.00 kcal/mol) of nontarget spots. 5'UTRs of one target gene (arginine kinase:Px-AK) and one nontarget gene (imaginal disc growth factor:Px-IDGF) were cloned and used for in vitro translation (IVT) assay using rabbit reticulocyte lysate. IVT assay clearly showed that mRNA of Px-IDGF was translated in the presence of CpBV15β, but mRNA of Px-AK was not. Physiological significance of these two genes was compared in immune and development processes of P. xylostella by specific RNA interference (RNAi). Under these RNAi conditions, suppression of Px-AK exhibited much more significant adverse effects on larval immunity and larva-to-pupa metamorphosis compared to the effect of suppression of Px-IDGF. These results support the hypothesis that 5'UTR complexity is a molecular motif to discriminate host mRNAs by CpBV15β for its host translational control and suggest that this discrimination would be required for altering host physiology to accomplish a successful parasitism of the wasp host, C. plutellae.

JH modulates a cellular immunity of Tribolium castaneum in a Met-independent manner

Juvenile hormone (JH) regulates diverse physiological processes in insects during entire developmental stages. Especially, the identification of Methoprene-tolerant (Met), a JH nuclear receptor, allows us to better understand molecular actions of JH to control gene expressions related with metamorphosis. However, several physiological processes including cellular immune response and some molecular actions of JH have been suspected to be mediated via its non-genomic actions. To prove its non-genomic action, JH nuclear signals were suppressed by RNA interference (RNAi) of Met or its downstream gene, Krüppel homolog 1 (Kr-h1), in the red flour beetle, Tribolium castaneum. These RNAi-treated larvae failed to undergo a normal development and suffered precocious metamorphosis. Hemocytes of T. castaneum exhibited their spreading behavior on extracellular matrix and nodule formation in response to bacterial challenge. When the larvae were treated with either RNAi of Met or Kr-h1, the hemocytes of the treated larvae were responsive to JH without any significant difference with those of control larvae. These results suggest that the response of hemocytes to JH is not mediated by its nuclear signal. On the other hand, the JH modulation of hemocyte behaviors of T. castaneum was significantly influenced by membrane and cytosolic protein activities, in which ethoxyzolamide (a specific inhibitor of carbonic anhydrase), calphostin C (a specific inhibitor of protein kinase C) or ouabain (a specific inhibitor of Na(+)-K(+) ATPase) significantly suppressed the responsiveness of hemocytes to JH.

Prostaglandin mediates down-regulation of phenoloxidase activation of Spodoptera exigua via plasmatocyte-spreading peptide-binding protein

Insect immunity is innate and highly efficient to defend against various pathogens. However, uncontrolled excessive immune responses would be highly detrimental and energy-consuming processes. An insect cytokine, plasmatocyte-spreading peptide (SePSP), induces hemocyte-spreading behavior as well as activates phenoloxidase (PO) in the beet armyworm, Spodoptera exigua. A hemocyte transcriptome of S. exigua contains a partial sequence of a putative PSP-binding protein (SePSP-BP1). SePSP-BP1 was expressed in most larval stages except in the last instar. However, a bacterial challenge induced SePSP-BP1 expression in the last instar especially in hemocytes and fat body. Injecting a double-stranded RNA specific to SePSP-BP1 (dsPSP-BP1) suppressed the induction of SePSP-BP1 expression in response to bacterial challenge. The larvae treated with dsPSP-BP1 suffered high mortality to infection of nonpathogenic bacteria due to uncontrolled high PO activity. SePSP significantly induced PO activity. The eicosanoid synthesis inhibitor, dexamethasone (DEX), inhibited SePSP-mediated PO activation. However, treatment with prostaglandin E2 (PGE2) induced a transient increase of PO activity under DEX treatment. Treatment of dsPSP decreased the duration of PO activation induced by PGE2, while treatment of dsPSP-BP1 increased the induced period. These results suggest that prostaglandin mediates PSP signals in both upregulation of PO activity and its subsequent downregulation via SePSP-BP1.

RNA interference of a heat shock protein, Hsp70, loses its protection role in indirect chilling injury to the beet armyworm, Spodoptera exigua

The beet armyworm, Spodoptera exigua, is freeze-susceptible, in which glycerol plays a crucial role in depressing supercooling point (SCP) to avoid the freezing injury. This study focused on a non-freezing injury classified into indirect chilling injury of S. exigua after a prolonged exposure to low temperatures much above SCPs. Exposure to 0 and 5°C for longer than 2weeks was lethal to all the immature stages. Among immature stages, eggs were the most susceptible to the low temperature treatments and pupae were the next susceptible. Among larvae, the third instar (L3) appeared to be more tolerant than the fifth instar (L5). The temperature treatment at 15°C allowed both L3 and L5 to exhibit a feeding behavior and induced little non-freezing injury, suggesting a minimal temperature threshold for optimal overwintering conditions of S. exigua. Three heat shock protein genes (Hsp70, Hsp74, Hsp83) were expressed in the larvae at the low temperature treatments. Only Hsp70 was inducible to the low temperatures in both L3 and L5 stages. RNA interference of Hsp70 expression led to significantly lose the survival rates of the treated larvae in the conditions inducing the non-freezing injury. These results suggest that Hsp70 plays a role in protecting S. exigua from the indirect chilling injury.

RNA interference of glycerol biosynthesis suppresses rapid cold hardening of the beet armyworm, Spodoptera exigua

The beet armyworm, Spodoptera exigua, is a freeze-susceptible species that overwinters in temperate zones without diapause. A rapid cold hardening (RCH) and supercooling capacity usually play crucial roles in survival during the overwintering period. This study identified a cryoprotectant as a RCH factor of S. exigua. Pre-exposure of S. exigua larvae to 4°C significantly increased survival at -10°C in all developmental stages from egg to adult. RCH was dependent on the duration of the pre-exposure period. RCH also significantly enhanced the supercooling capacity. Cryoprotectant analysis using HPLC showed that the pre-exposure treatment allowed the larvae to accumulate glycerol in the hemolymph. Two genes, glycerol-3-phosphate dehydrogenase (GPDH) and glycerol kinase (GK), were identified as being associated with glycerol biosynthesis, and were cloned from S. exigua larvae. Both GPDH and GK were expressed in all developmental stages of S. exigua. RNA interference (RNAi) of either GPDH or GK significantly inhibited glycerol accumulation in the hemolymph of S. exigua. Larvae treated with RNAi for GPDH or GK exhibited a significant decrease in RCH capacity. The glycerol accumulation in response to 4°C appeared to be under the control of a humoral signal, because a ligation experiment prevented glycerol accumulation in the other half of the body. This study indicates that glycerol is a RCH factor of S. exigua and its synthesis is in response to low temperature via humoral mediation.

Rac1 mediates cytokine-stimulated hemocyte spreading via prostaglandin biosynthesis in the beet armyworm, Spodoptera exigua

Cell spreading is an integral component of insect hemocytic immune reactions to infections and invasions. Cell spreading is accomplished by cytoskeleton rearrangement, which is activated by three major immune mediators, biogenic monoamines, plasmatocyte-spreading peptide (PSP), and eicosanoids, particularly prostaglandin E2 (PGE2). However, little is known about how these immune mediators activate hemocyte spreading at the intra-cellular level. A small G protein, Rac1, acts in cytoskeleton arrangements in mammalian cells. Based on this information, we identified a Rac1 transcript (SeRac1) in hemocytes prepared from Spodoptera exigua. SeRac1 was expressed in most developmental stages and in the two main immunity-conferring tissues, hemocytes and fat body, in larvae. In response to bacterial challenge, its expression was up-regulated by >37-fold at 2h post-injection and returned to a basal level about 2h later. Silencing SeRac1 expression inhibited hemocyte spreading in response to three immune mediators, octopamine, 5-hydroxytryptamine, and PSP. Addition of PGE2 to SeRac1-silenced larvae rescued the influence of these three mediators on hemocyte spreading. These compounds also increased phospholipase A2 activity via SeRac1, which leads to prostaglandin biosynthesis. We infer that SeRac1 transduces OA, 5-HT, and PSP signaling via activating biosynthesis of prostaglandins and possibly other eicosanoids.

RNA interference of broad gene expression mimics antimetamorphic effect of pyriproxyfen on the beet armyworm, Spodoptera exigua

A larva-to-pupa metamorphosis is induced by a low or undetectable level of juvenile hormone (JH) during last instar in holometabolous insects. An exogenous application of JH agonist, pyriproxyfen (PYR), inhibited pupal metamorphosis of the beet armyworm, Spodoptera exigua. Last instar larvae of S. exigua exhibited increase of body size at first 3 days along with active feeding behavior. Also, at this period, prothoracic gland increased in size, while corpora allata remained little change. Storage proteins were accumulated in hemolymph plasma from penultimate to last instars, during which two storage protein genes (SeHex and SeSP1) were actively expressed. A Broad-Complex 1 (BRC1) gene of S. exigua (SeBRC1) was partially cloned and showed a specific expression at the last instar in all tested tissues including hemocytes, fat body, epidermis, gut, nerve, and salivary gland. Knockdown of SeBRC1 expression by its specific double-strand RNA mimicked the antimetamorphic effect induced by PYR treatment. PYR treatment at early last instar inhibited expression of SeBRC1, but did not that of other nuclear receptor, βFTZ-F1. These results indicate that a transcriptional factor, SeBRC1, plays a crucial role in pupal metamorphosis of S. exigua.

A novel polydnaviral gene family, BEN, and its immunosuppressive function in larvae of Plutella xylostella parasitized by Cotesia plutellae

A full genome sequence of the episomal form of Cotesia plutellae bracovirus (CpBV) suggests 11 BEN family genes. This study analyzed their expression and physiological function in the viral host, Plutella xylostella. All 11 BEN family genes were expressed during entire parasitization period of P. xylostella larvae. In addition, these BEN family genes were expressed in fat body, gut, epidermis, and hemocytes in final larval instar of parasitized P. xylostella. The 11 BEN family genes were transiently expressed in nonparasitized larvae by injection of each viral segment containing its corresponding BEN family gene. The transient expression of BEN family genes significantly suppressed hemocyte nodule formation in response to bacterial challenge. Subsequent injection of double-stranded RNA specific to each BEN family gene suppressed the expression of the BEN family gene and rescued the immunosuppression. These results indicate that 11 BEN family genes are expressed in larvae parasitized by C. plutellae and play crucial role in inducing immunosuppression. Homologous BEN family genes were found in other bracoviral genomes. We propose BEN domain-containing genes as a new functional gene family in polydnaviruses.

A target-specific feeding toxicity of β(1) integrin dsRNA against diamondback moth, Plutella xylostella

Integrin is a cell-surface protein consisting of α and β heterodimers. A predicted amino acid sequence of an integrin subunit of the diamondback moth, Plutella xylostella, was highly homologous to other lepidopteran β1 subunits and possessed essential functional domains. The β1 integrin of P. xylostella (βPx1) was expressed in all developmental stages of P. xylostella. It was also expressed in all tested tissues including hemocyte, fat body, gut, and epidermis of last instar. When βPx1 expression was suppressed by injection of dsRNA specific to βPx1 (dsRNA(βPx1)), the treated larvae exhibited significant suppression in immune response and also suffered significant larval mortality. When dsRNA(βPx1) was orally fed to young larvae, it suppressed the expression of âPx1 and resulted in a significant mortality. By contrast, a dsRNA specific to β1 subunit of Spodoptera exigua gave little adverse effects on βPx1 expression and larval development when it was treated by injection or oral administration, though these two genes showed 71% sequence homology. These results suggest a target-specific RNA interference of dsRNA(βPx1), which causes significant mortality to P. xylostella by feeding treatment.

RNA interference of β1 integrin subunit impairs development and immune responses of the beet armyworm, Spodoptera exigua

Integrin is a cell surface protein that is composed of α and β heterodimer and mediates cell interaction with extracellular matrix or other cells including microbial pathogens. A full length cDNA sequence (2862 bp) of a β1 subunit integrin (βSe1) was cloned from the beet armyworm, Spodoptera exigua. Phylogenetic analysis showed that βSe1 was clustered with other insect β integrin subunits with the highest amino acid sequence identity (98.3%) to β1 of Spodoptera litura. Structural analysis of the deduced amino acid sequence indicated that βSe1 possessed all functional domains known in other insect β1 integrins. RT-PCR analysis showed that βSe1 was expressed in all developmental stages and all tested tissues of S. exigua. Its expression was further upregulated in hemocytes by injections of various microbes from quantitative RT-PCR analysis. Injection of double-stranded βSe1 RNA (dsRNA(βSe1)) into late instar S. exigua suppressed βSe1 expression and resulted in significant reduction in pupal weight. The dsRNA(βSe1) injection significantly impaired hemocyte-spreading and nodule formation of S. exigua in response to bacterial challenge. Furthermore, oral ingestion of dsRNA(βSe1) induced reduction of βSe1 expression in midgut and resulted in significant mortality of S. exigua during immature development. These results suggest that βSe1 plays crucial roles in performing cellular immune responses as well as larval development in S. exigua.

Exogenous JH and ecdysteroid applications alter initiation of polydnaviral replication in an endoparasitoid wasp, Cotesia plutellae (Braconidae: Hymenoptera)

Polydnaviruses are a group of double-stranded DNA viruses and are symbiotically associated with some ichneumonoid wasps. As proviruses, the replication of polydnaviruses occurs in the female reproductive organ at the pupal stage. This study analyzed the effects of two developmental hormones, juvenile hormone (JH) and ecdysteroid, on the viral replication of Cotesia plutellae bracovirus (CpBV). All 23 CpBV segments identified contained a conserved excision/rejoining site ('AGCTTT') from their proviral segments. Using quantitative real-time PCR based on this excision/rejoining site marker, initiation of CpBV replication was determined to have occurred on day 4 on the pupal stage. Pyriproxyfen, a JH agonist, significantly inhibited adult emergence of C. plutellae, whereas RH5992, an ecdysteroid agonist, had no inhibitory effect. Although RH5992 had no effect dose on adult development, it significantly accelerated viral replication. The results of immunoblotting assays against viral coat proteins support the effects of the hormone agonists on viral replication.

Deep sequencing of Cotesia vestalis bracovirus reveals the complexity of a polydnavirus genome

Here we completed the whole genome sequence of Cotesia vestalis bracovirus (CvBV) by deep sequencing and compared the genome features of CvBV to those of other polydnaviruses (PDVs). The genome is 540,215 base pairs divided into 35 genomic segments that range from 2.6 to 39.2kb. Comparison of CvBV with other PDVs shows that more segments are found, including new segments that have no corresponding segments in other phylogenetically related PDVs, which suggests that there might be still more segments not being sequenced in the present known PDVs. We identified eight gene families and five genes in CvBV, including new genes which were first found in PDVs. Strikingly, we identified a putative helicase protein displaying similarity to human Pif1 helicase, which has never been reported for other PDVs. This finding will bring new insights in research of these special viruses.