RNA interference in Lepidoptera: an overview of successful and unsuccessful studies and implications for experimental design

Overexpression of small heat shock protein 21 protects the Chinese oak silkworm Antheraea pernyi against thermal stress

Small heat shock proteins (sHSPs) usually act as molecular chaperones to prevent proteins from being denatured in extreme conditions. We first report the sHSP21 gene, named as Ap-sHSP21, in the Chinese oak silkworm Antheraea pernyi (Lepidoptera: Saturniidae). The full-length cDNA of Ap-sHSP21 is 976 bp, including a 5'-untranslated region (UTR) of 99 bp, a 3'-UTR of 316 bp and an open reading frame (ORF) of 561 bp encoding a polypeptide of 186 amino acids. The deduced A. pernyi sHSP21 protein sequence reveals the percent identity is 82-93% in comparison to other sHSPs from insects. Real-time quantitative reverse transcription-PCR (qRT-PCR) analysis shows that Ap-sHSP21 expression is higher in testis than that in other examined tissues and significantly up-regulated after heat shock. In addition, prokaryotic expression and purification of the Ap-sHSP21 protein were performed. SDS-PAGE and Western blot analysis demonstrated that a 25 kDa recombinant protein was successfully expressed in Escherichia coli cells and the purified recombinant protein was also confirmed to protect restriction enzymes from thermal inactivation. The expression of Ap-sHSP21 was significantly down-regulated after RNA interference, which was confirmed by qRT-PCR and Western blot analysis. All together, these results suggest that Ap-sHSP21 play a key role in thermal tolerance.

Modularity of a leaf moth-wing pattern and a versatile characteristic of the wing-pattern ground plan

BACKGROUND

One of the most intriguing questions in evolutionary developmental biology is how an insect acquires a mimicry pattern within its body parts. A striking example of pattern mimicry is found in the pattern diversity of moth and butterfly wings, which is thought to evolve from preexisting elements illustrated by the nymphalid ground plan (NGP). Previous studies demonstrated that individuality of the NGP facilitates the decoupling of associated common elements, leading to divergence. In contrast, recent studies on the concept of modularity have argued the importance of a combination of coupling and decoupling of the constituent elements. Here, we examine the modularity of a mimicry wing pattern in a moth and explore an evolvable characteristic of the NGP.

RESULTS

This study examined the wings of the noctuid moth Oraesia excavata, which closely resemble leaves with a leaf venation pattern. Based on a comparative morphological procedure, we found that this leaf pattern was formed by the NGP common elements. Using geometric morphometrics combined with network analysis, we found that each of the modules in the leaf pattern integrates the constituent components of the leaf venation pattern (i.e., the main and lateral veins). Moreover, the detected modules were established by coupling different common elements and decoupling even a single element into different modules. The modules of the O. excavata wing pattern were associated with leaf mimicry, not with the individuality of the NGP common elements. For comparison, we also investigated the modularity of a nonmimetic pattern in the noctuid moth Thyas juno. Quantitative analysis demonstrated that the modules of the T. juno wing pattern regularly corresponded to the individuality of the NGP common elements, unlike those in the O. excavata wing pattern.

CONCLUSIONS

This study provides the first evidence for modularity in a leaf mimicry pattern. The results suggest that the evolution of this pattern involves coupling and decoupling processes to originate these modules, free from the individuality of the NGP system. We propose that this evolution has been facilitated by a versatile characteristic of the NGP, allowing the association of freely modifiable subordinate common elements to make modules.

RNAi silencing of the HaHMG-CoA reductase gene inhibits oviposition in the Helicoverpa armigera cotton bollworm

RNA interference (RNAi) has considerable promise for developing novel pest control techniques, especially because of the threat of the development of resistance against current strategies. For this purpose, the key is to select pest control genes with the greatest potential for developing effective pest control treatments. The present study demonstrated that the 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoA reductase; HMGR) gene is a potential target for insect control using RNAi. HMGR is a key enzyme in the mevalonate pathway in insects. A complete cDNA encoding full length HMGR (encoding an 837-aa protein) was cloned from Helicoverpa armigera (Lepidoptera: Noctuidae). The HaHMGR (H. armigera HMGR) knockdown using systemic RNAi in vivo inhibited the fecundity of the females, effectively inhibited ovipostion, and significantly reduced vitellogenin (Vg) mRNA levels. Moreover, the oviposition rate of the female moths was reduced by 98% by silencing HaHMGR compared to the control groups. One-pair experiments showed that both the proportions of valid mating and fecundity were zero. Furthermore, the HaHMGR-silenced females failed to lay eggs (approximate 99% decrease in oviposition) in the semi-field cage performance. The present study demonstrated the potential implications for developing novel pest management strategies using HaHMGR RNAi in the control of H. armigera and other insect pests.

Comparison of the RNA interference effects triggered by dsRNA and siRNA in Tribolium castaneum

BACKGROUND

Even though both dsRNA and siRNA have been widely used in laboratory studies to knock down target genes, it has been found that siRNA rarely triggers phenotypic changes for intact insects. In this study, Tribolium castaneum was used as a model species to compare dsRNA and siRNA and select the more efficient of these for insect RNAi.

RESULTS

Four dsRNAs and ten siRNAs targeting the same three genes were tested by checking the suppression of target genes and the deformation of treated insects. The results showed that dsRNAs could reduce the expression of target genes by 55-75% for more than 7 days and deformed 78.8-87.0% of the larvae tested. Injection of their corresponding siRNAs targeting the same gene did yield about 45% silencing of target genes, but only in a short time period (from day 2 to day 4), and it did not trigger any phenotypic changes.

CONCLUSION

SiRNAs were less efficient than their corresponding dsRNAs. Most of them could hardly trigger any phenotypic changes owing to the temporary silencing effect on target genes. Meanwhile, the efficiency of different siRNAs varied with their matching sequence, and careful selection was necessary for proper use.

Transcriptome Analysis and Screening for Potential Target Genes for RNAi-Mediated Pest Control of the Beet Armyworm, Spodoptera exigua

The beet armyworm, Spodoptera exigua (Hübner), is a serious pest worldwide that causes significant losses in crops. Unfortunately, genetic resources for the beet armyworm is extremely scarce. To improve these resources we sequenced the transcriptome of S. exigua representing all stages including eggs, 1^st to 5^th instar larvae, pupae, male and female adults using the Illumina Solexa platform. We assembled the transcriptome with Trinity that yielded 31,414 contigs. Of these contigs, 18,592 were annotated as protein coding genes by Blast searches against the NCBI nr database. It has been shown that knockdown of important insect genes by dsRNAs or siRNAs is a feasible mechanism to control insect pests. The first key step towards developing an efficient RNAi-mediated pest control technique is to find suitable target genes. To screen for effective target genes in the beet armyworm, we selected nine candidate genes. The sequences of these genes were amplified using the RACE strategy. Then, siRNAs were designed and chemically synthesized. We injected 2 µl siRNA (2 µg/µl) into the 4^th instar larvae to knock down the respective target genes. The mRNA abundance of target genes decreased to different levels (∼20–94.3%) after injection of siRNAs. Knockdown of eight genes including chitinase7, PGCP, chitinase1, ATPase, tubulin1, arf2, tubulin2 and arf1 caused a significantly high level of mortality compared to the negative control (P<0.05). About 80% of the surviving insects in the siRNA-treated group of five genes (PGCP, chitinase1, tubulin1, tubulin2 and helicase) showed retarded development. In chitinase1-siRNA and chitinase7-siRNA administered groups, 12.5% survivors exhibited “half-ecdysis”. In arf1-siRNA and arf2-siRNA groups, the body color of 15% became black 48 h after injections. In summary, the transcriptome could be a valuable genetic resource for identification of genes in S. exigua and this study provided putative targets for RNAi pest control.

Characterization of the spectrum of insecticidal activity of a double-stranded RNA with targeted activity against Western Corn Rootworm (Diabrotica virgifera virgifera LeConte)

The sequence specificity of the endogenous RNA interference pathway allows targeted suppression of genes essential for insect survival and enables the development of durable and efficacious insecticidal products having a low likelihood to adversely impact non-target organisms. The spectrum of insecticidal activity of a 240 nucleotide (nt) dsRNA targeting the Snf7 ortholog in Western Corn Rootworm (WCR; Diabrotica virgifera virgifera) was characterized by selecting and testing insects based upon their phylogenetic relatedness to WCR. Insect species, representing 10 families and 4 Orders, were evaluated in subchronic or chronic diet bioassays that measured potential lethal and sublethal effects. When a specific species could not be tested in diet bioassays, the ortholog to the WCR Snf7 gene (DvSnf7) was cloned and corresponding dsRNAs were tested against WCR and Colorado potato beetle (Leptinotarsa decemlineata); model systems known to be sensitive to ingested dsRNA. Bioassay results demonstrate that the spectrum of activity for DvSnf7 is narrow and activity is only evident in a subset of beetles within the Galerucinae subfamily of Chrysomelidae (>90% identity with WCR Snf7 240 nt). This approach allowed for evaluating the relationship between minimum shared nt sequence length and activity. A shared sequence length of ≥ 21 nt was required for efficacy against WCR (containing 221 potential 21-nt matches) and all active orthologs contained at least three 21 nt matches. These results also suggest that WCR resistance to DvSnf7 dsRNA due to single nucleotide polymorphisms in the target sequence of 240 nt is highly unlikely.

Effects of period RNAi on V-ATPase expression and rhythmic pH changes in the vas deferens of Spodoptera littoralis (Lepidoptera: Noctuidae)

Circadian clocks (oscillators) regulate multiple aspects of insect behaviour and physiology. The circadian system located in the male reproductive tract of Lepidoptera orchestrates rhythmic sperm release from testis and sperm maturation in the upper vas deferens (UVD). Our previous research on the cotton leafworm, Spodoptera littoralis, suggested rhythmic changes in the V-ATPase levels in the UVD epithelium, which correlated with rhythmic pH fluctuations in the UVD lumen. However, it was not known whether UVD cells contain clock mechanism that generates these daily fluctuations. In the current paper, we show circadian rhythm in the expression of clock gene period at the mRNA and protein level in the UVD epithelium. To determine the role of PER in V-ATPase and pH regulation, testes-UVD complexes were treated in vitro with double-stranded fragments of per mRNA (dsRNA). This treatment, which transiently lowered per mRNA and protein in the UVD, altered expression of V-ATPase c subunit. In addition, per RNAi caused a significant delay in the UVD lumen acidification. These data demonstrate that the UVD molecular oscillator involving the period gene plays an essential role in the regulation of rhythmic V-ATPase activity and periodic acidification of the UVD lumen.

Transcriptional response of BmToll9-1 and RNAi machinery genes to exogenous dsRNA in the midgut of Bombyx mori

Injection of dsRNA is widely applied to silence endogenous genes and study gene function in insects. However, it is not yet clear to what extent it can also exert non-specific effects, for instance by interference with the innate immune response. In this study, we report on the transcriptional response of BmToll9-1 to lipopolysaccharide (LPS) and dsRNA in the silkmoth, Bombyx mori. BmToll9-1 encodes a Toll receptor highly expressed in midgut tissue and that shows limited similarity to the mammalian TLR3 endolysosome receptor for dsRNA; while Dcr2 and Ago2 encode two key components of the RNAi machinery. An expression pattern study of all 14 Toll receptors in B. mori showed that BmToll9-1 was expressed in different larval and pupal tissues with the highest expression level detected in the midgut, indicating a possible function in immunity against pathogens taken up by the food. In order to investigate the response of BmToll9-1, different ways to deliver dsRNA, specific for GFP (dsGFP), and LPS were applied in Bombyx 5th instar larvae. The feeding experiments suggested that dsGFP did not suppress the expression of BmToll9-1 significantly, while LPS could suppress the expression of BmToll9-1 after 3h of feeding. On the other hand, the injection experiments showed that dsGFP, as well as LPS, could significantly inhibit the expression of BmToll9-1 in 3h. Bacteria that constantly expressed dsGFP could also down-regulate the expression of BmToll9-1 to a greater extent than bacteria that do not express dsGFP. The failure of dsGFP by feeding to affect the expression of BmToll9-1 was correlated with the rapid degradation of dsGFP by dsRNase in the midgut juice. Expression of the RNAi machinery genes Dcr2 and Ago2, as well as dsRNase, was also affected by injection of dsRNA and not by feeding, but in these cases an induction was observed instead of a down-regulation. Because LPS is a well-known pathogen-associated molecular pattern (PAMP), it suggested that the decrease in BmToll9-1 expression is a consequence of the activation of the innate immune response by LPS. The similar response of BmToll9-1 between the two triggers, LPS and dsRNA, suggests that dsRNA can also act as a PAMP in the midgut of Bombyx. Furthermore, induction of the genes Dcr2, Ago2 and dsRNase may also constitute a defense mechanism against invading dsRNA.

Silencing abnormal wing disc gene of the Asian citrus psyllid, Diaphorina citri disrupts adult wing development and increases nymph mortality

Huanglongbing (HLB) causes considerable economic losses to citrus industries worldwide. Its management depends on controlling of the Asian citrus Psyllid (ACP), the vector of the bacterium, Candidatus Liberibacter asiaticus (CLas), the causal agent of HLB. Silencing genes by RNA interference (RNAi) is a promising tool to explore gene functions as well as control pests. In the current study, abnormal wing disc (awd) gene associated with wing development in insects is used to interfere with the flight of psyllids. Our study showed that transcription of awd is development-dependent and the highest level was found in the last instar (5(th)) of the nymphal stage. Micro-application (topical application) of dsRNA to 5(th) instar of nymphs caused significant nymphal mortality and adult wing-malformation. These adverse effects in ACP were positively correlated with the amounts of dsRNA used. A qRT-PCR analysis confirmed the dsRNA-mediated transcriptional down-regulation of the awd gene. Significant down-regulation was required to induce a wing-malformed phenotype. No effect was found when dsRNA-gfp was used, indicating the specific effect of dsRNA-awd. Our findings suggest a role for awd in ACP wing development and metamorphosis. awd could serve as a potential target for insect management either via direct application of dsRNA or by producing transgenic plants expressing dsRNA-awd. These strategies will help to mitigate HLB by controlling ACP.

Mutualistic polydnaviruses share essential replication gene functions with pathogenic ancestors

Viruses are usually thought to form parasitic associations with hosts, but all members of the family Polydnaviridae are obligate mutualists of insects called parasitoid wasps. Phylogenetic data founded on sequence comparisons of viral genes indicate that polydnaviruses in the genus Bracovirus (BV) are closely related to pathogenic nudiviruses and baculoviruses. However, pronounced differences in the biology of BVs and baculoviruses together with high divergence of many shared genes make it unclear whether BV homologs still retain baculovirus-like functions. Here we report that virions from Microplitis demolitor bracovirus (MdBV) contain multiple baculovirus-like and nudivirus-like conserved gene products. We further show that RNA interference effectively and specifically knocks down MdBV gene expression. Coupling RNAi knockdown methods with functional assays, we examined the activity of six genes in the MdBV conserved gene set that are known to have essential roles in transcription (lef-4, lef-9), capsid assembly (vp39, vlf-1), and envelope formation (p74, pif-1) during baculovirus replication. Our results indicated that MdBV produces a baculovirus-like RNA polymerase that transcribes virus structural genes. Our results also supported a conserved role for vp39, vlf-1, p74, and pif-1 as structural components of MdBV virions. Additional experiments suggested that vlf-1 together with the nudivirus-like gene int-1 also have novel functions in regulating excision of MdBV proviral DNAs for packaging into virions. Overall, these data provide the first experimental insights into the function of BV genes in virion formation.

Microorganisms form symbiotic associations with animals and plants that range from parasitic (pathogens) to beneficial (mutualists). Although numerous examples of obligate, mutualistic bacteria, fungi, and protozoans exist, viruses are almost always considered to be pathogens. An exception is the family Polydnaviridae, which consists of large DNA viruses that are obligate mutualists of insects called parasitoid wasps. Prior studies show that polydnaviruses in the genus Bracovirus evolved approximately 100 million years ago from a group of viruses called nudiviruses, which are themselves closely related to a large family of insect pathogens called baculoviruses. Polydnaviruses are thus of fundamental interest for understanding the processes by which viruses can evolve into mutualists. In this study we characterized the composition of virus particles from Microplitis demolitor bracovirus (MdBV) and conducted functional experiments to assess whether BV genes share similar functions with related essential baculovirus replication genes. Our results indicate that several genes in MdBV retain ancestral functions, but select other genes have novel functions unknown from baculoviruses. Our results also provide the first experimental data on the function of polydnavirus replication genes and enhance understanding of the similarities between these viruses and their pathogenic ancestors.

Testing insecticidal activity of novel chemically synthesized siRNA against Plutella xylostella under laboratory and field conditions

BACKGROUND

Over the last 60 years, synthetic chemical pesticides have served as a main tactic in the field of crop protection, but their availability is now declining as a result of the development of insect resistance. Therefore, alternative pest management agents are needed. However, the demonstration of RNAi gene silencing in insects and its successful usage in disrupting the expression of vital genes opened a door to the development of a variety of novel, environmentally sound approaches for insect pest management.

METHODOLOGY/PRINCIPAL FINDINGS

Six small interfering RNAs (siRNAs) were chemically synthesized and modified according to the cDNA sequence of P. xylostella acetylcholine esterase genes AChE1 and AChE2. All of them were formulated and used in insecticide activity screening against P. xylostella. Bioassay data suggested that Si-ace1_003 and Si-ace2_001 at a concentration of 3 µg cm(-2) displayed the best insecticidal activity with 73.7% and 89.0%, mortality, respectively. Additional bioassays were used to obtain the acute lethal concentrations of LC50 and LC90 for Si-ace2_001, which were 53.66 µg/ml and 759.71 µg/ml, respectively. Quantitative Real-time PCR was used to confirm silencing and detected that the transcript levels of P. xylostella AChE2 (PxAChE2) were reduced by 5.7-fold compared to the control group. Consequently, AChE activity was also reduced by 1.7-fold. Finally, effects of the siRNAs on treated plants of Brassica oleracea and Brassica alboglabra were investigated with different siRNA doses. Our results showed that Si-ace2_001 had no negative effects on plant morphology, color and growth of vein under our experimental conditions.

CONCLUSIONS

The most important finding of this study is the discovery that chemically synthesized and modified siRNA corresponding to P. xylostella AChE genes cause significant mortality of the insect both under laboratory and field conditions, which provides a novel strategy to control P. xylostella and to develop bio-pesticides based on the RNA interference technology.

Silencing the HaHR3 gene by transgenic plant-mediated RNAi to disrupt Helicoverpa armigera development

RNA interference (RNAi) caused by exogenous double-stranded RNA (dsRNA) has developed into a powerful technique in functional genomics, and to date it is widely used to down-regulate crucial physiology-related genes to control pest insects. A molt-regulating transcription factor gene, HaHR3, of cotton bollworm (Helicoverpa armigera) was selected as the target gene. Four different fragments covering the coding sequence (CDS) of HaHR3 were cloned into vector L4440 to express dsRNAs in Escherichia coli. The most effective silencing fragment was then cloned into a plant over-expression vector to express a hairpin RNA (hpRNA) in transgenic tobacco (Nicotiana tabacum). When H. armigera larvae were fed the E. coli or transgenic plants, the HaHR3 mRNA and protein levels dramatically decreased, resulting developmental deformity and larval lethality. The results demonstrate that both recombinant bacteria and transgenic plants could induce HaHR3 silence to disrupt H. armigera development, transgenic plant-mediated RNAi is emerging as a powerful approach for controlling insect pests.

Long-term effect of systemic RNA interference on circadian clock genes in hemimetabolous insects

RNA interference (RNAi) strategy, which enables gene-specific knock-down of transcripts, has been spread across a wide area of insect studies for investigating gene function without regard to model and non-model insects. This technique is of particular benefit to promote molecular studies on non-model insects. However, the optimal conditions for RNAi are still not well understood because of its variable efficiency depending on the species, target genes, and experimental conditions. To apply RNAi technique to long-running experiments such as chronobiological studies, the effects of RNAi have to persist throughout the experiment. In this study, we attempted to determine the optimal concentration of double-stranded RNA (dsRNA) for systemic RNAi and its effective period in two different insect species, the cricket Gryllus bimaculatus and the firebrat Thermobia domestica. In both species, higher concentrations of dsRNA principally yielded a more efficient knock-down of mRNA levels of tested clock genes, although the effect depended on the gene and the species. Surprisingly, the effect of the RNAi reached its maximum effect 1-2 weeks and 1 month after the injection of dsRNA in the crickets and the firebrats, respectively, suggesting a slow but long-term effect of RNAi. Our study provides fundamental information for utilizing RNAi technique in any long-running experiment.

A survey of the effectiveness of non-cell autonomous RNAi throughout development in the sawfly, Athalia rosae (Hymenoptera)

RNA interference (RNAi) is a powerful and convenient tool not only for functional analysis of specific genes, but also for large-scale screening of gene function in insects; however, reports on its efficiency throughout development in a single species are limited. We demonstrate here that non-cell autonomous RNAi by injection of double-stranded RNA (dsRNA) knocks down targeting genes in most developmental stages in the sawfly, Athalia rosae. Injection of dsRNA targeting the green fluorescence protein (gfp) gene into eggs of a transgenic strain carrying the constitutively expressing gfp gene resulted in the absence of GFP fluorescence during embryogenesis, while a portion of the gfp dsRNA-injected embryos began exhibiting GFP fluorescence at late embryogenesis. When gfp dsRNA was injected into parental female pupae, the RNAi effect was carried over to all embryos of the next generation and the effect lasted until mid-larval stages. Parental injection of dsRNA was more efficient than embryonic injection in terms of penetrance of the effect and the survival rate. After injection of gfp dsRNA into last instar larvae, the RNAi effect was sustained during prepupal and pupal stages and in adults. The gfp gene transcript markedly decreased in these knockdown phenotypes. It was revealed by employing fluorescence-labeled dsRNA that injected dsRNA was taken up in internal organs. Knockdown of an endogenous gene, Distal-less (Dll), resulted in typical phenotypes represented by the lack and malformation of Dll-expressing organs, such as distal parts of the appendages and wing edges without showing off-target effects. In contrast, RNAi by dsRNA injection seems to be hardly effective in mid- to late-larval stages.

Soaking RNAi-mediated modification of Sf9 cells for baculovirus expression system by ectopic expression of Caenorhabditis elegans SID-1

RNA interference (RNAi) is a biological phenomenon that silences the expression of genes of interest. Passive double-stranded RNA (dsRNA) uptake has been uniquely observed in Caenorhabditis elegans due to the expression of systemic RNAi defective-1 (SID-1). We report that ectopic expression of CeSID-1 endows the Sf9 cells with a capacity for soaking RNAi. Soaking the Sf9-SID1 with dsRNA corresponding to either exogenous or endogenous target genes induced a significant decrease in the amount of mRNA or protein. These results enabled us to modify the target proteins of baculovirus expression vector system in both quantities and posttranslational modifications. The current low-cost and high-efficiency RNAi system is useful for high-throughput gene function analysis and mass production of recombinant protein.

A genome-wide survey of small interfering RNA and microRNA pathway genes in a galling insect

Deployment of resistance (R) genes is the most effective control for Hessian fly, Mayetiola destructor (Say); however, deployment of R genes results in an increased frequency of pest genotypes that display virulence to them. RNA interference (RNAi) is a useful reverse genetics tool for studying such insect virulence pathways, but requires a systemic phenotype, which is not found in all species. In an effort to correlate our observed weak RNAi phenotype in M. destructor with a genetic basis, we have aggregated and compared RNAi related genes across M. destructor, three other insect species, and the nematode Caenorhabditis elegans. We report here the annotation of the core genes in the small interfering RNA (siRNA) and microRNA (miRNA) pathways in M. destructor. While most of the miRNA pathway genes were highly conserved across the species studied, the siRNA pathway genes showed increased relative variability in comparison to the miRNA pathway. In particular, the Piwi/Argonaute/Zwille (PAZ) domain of Dicer-2 (DCR-2) had the least amount of sequence similarity of any domain among species surveyed, with a trend of increased conservation in those species with amenable systemic RNAi. A homolog of the systemic interference defective-1 (Sid-1) gene of C. elegans was also not annotated in the M. destructor genome. Indeed, it is of interest that a Sid-1 homolog has not been detected in any dipteran species to date. We hypothesize the sequence architecture of the PAZ domain in the M. destructor DCR-2 protein is related to reduced efficacy of this enzyme and this taken together with the lack of a Sid-1 homolog may account for the weak RNAi response observed to date in this species as well as other dipteran species.

Metal ions control product specificity of isoprenyl diphosphate synthases in the insect terpenoid pathway

Isoprenyl diphosphate synthases (IDSs) produce the ubiquitous branched-chain diphosphates of different lengths that are precursors of all major classes of terpenes. Typically, individual short-chain IDSs (scIDSs) make the C10, C15, and C20 isoprenyl diphosphates separately. Here, we report that the product length synthesized by a single scIDS shifts depending on the divalent metal cofactor present. This previously undescribed mechanism of carbon chain-length determination was discovered for a scIDS from juvenile horseradish leaf beetles, Phaedon cochleariae. The recombinant enzyme P. cochleariae isoprenyl diphosphate synthase 1 (PcIDS1) yields 96% C10-geranyl diphosphate (GDP) and only 4% C15-farnesyl diphosphate (FDP) in the presence of Co(2+) or Mn(2+) as a cofactor, whereas it yields only 18% C10 GDP but 82% C15 FDP in the presence of Mg(2+). In reaction with Co(2+), PcIDS1 has a Km of 11.6 μM for dimethylallyl diphosphate as a cosubstrate and 24.3 μM for GDP. However, with Mg(2+), PcIDS1 has a Km of 1.18 μM for GDP, suggesting that this substrate is favored by the enzyme under such conditions. RNAi targeting PcIDS1 revealed the participation of this enzyme in the de novo synthesis of defensive monoterpenoids in the beetle larvae. As an FDP synthase, PcIDS1 could be associated with the formation of sesquiterpenes, such as juvenile hormones. Detection of Co(2+), Mn(2+), or Mg(2+) in the beetle larvae suggests flux control into C10 vs. C15 isoprenoids could be accomplished by these ions in vivo. The dependence of product chain length of scIDSs on metal cofactor identity introduces an additional regulation for these branch point enzymes of terpene metabolism.

Feasibility, limitation and possible solutions of RNAi-based technology for insect pest control

Numerous studies indicate that target gene silencing by RNA interference (RNAi) could lead to insect death. This phenomenon has been considered as a potential strategy for insect pest control, and it is termed RNAi-mediated crop protection. However, there are many limitations using RNAi-based technology for pest control, with the effectiveness target gene selection and reliable double-strand RNA (dsRNA) delivery being two of the major challenges. With respect to target gene selection, at present, the use of homologous genes and genome-scale high-throughput screening are the main strategies adopted by researchers. Once the target gene is identified, dsRNA can be delivered by micro-injection or by feeding as a dietary component. However, micro-injection, which is the most common method, can only be used in laboratory experiments. Expression of dsRNAs directed against insect genes in transgenic plants and spraying dsRNA reagents have been shown to induce RNAi effects on target insects. Hence, RNAi-mediated crop protection has been considered as a potential new-generation technology for pest control, or as a complementary method of existing pest control strategies; however, further development to improve the efficacy of protection and range of species affected is necessary. In this review, we have summarized current research on RNAi-based technology for pest insect management. Current progress has proven that RNAi technology has the potential to be a tool for designing a new generation of insect control measures. To accelerate its practical application in crop protection, further study on dsRNA uptake mechanisms based on the knowledge of insect physiology and biochemistry is needed.

Advances in the use of the RNA interference technique in Hemiptera

RNA interference (RNAi) suppresses the expression of target genes by post-transcriptional regulation. Because double-stranded RNA (dsRNA) mediated gene silencing is a conserved mechanism in many eukaryotes, RNAi has become a valuable tool for unveiling gene function in many model insects. Recent research has also shown that RNAi can also be effective in the downregulation of target genes in Hemiptera. In this review, we discuss the use of the RNAi technique in gene functional analysis in hemipterans, highlighting the methods of dsRNA uptake by these insects and discuss the knock-down efficiency of these techniques. Although the RNAi technique has disadvantages, our primary goal here is to determine whether it can be exploited further in the discovery of new gene functions, and as a pest control strategy, in some important Hemipteran pests.

Persistence of double-stranded RNA in insect hemolymph as a potential determiner of RNA interference success: evidence from Manduca sexta and Blattella germanica

RNA interference (RNAi) is a specific gene silencing mechanism mediated by double-stranded RNA (dsRNA), which has been harnessed as a useful reverse genetics tool in insects. Unfortunately, however, this technology has been limited by the variable sensitivity of insect species to RNAi. We propose that rapid degradation of dsRNA in insect hemolymph could impede gene silencing by RNAi and experimentally investigate the dynamics of dsRNA persistence in two insects, the tobacco hornworm, Manduca sexta, a species in which experimental difficulty has been experienced with RNAi protocols and the German cockroach, Blattella germanica, which is known to be highly susceptible to experimental RNAi. An ex vivo assay revealed that dsRNA was rapidly degraded by an enzyme in M. sexta hemolymph plasma, whilst dsRNA persisted much longer in B. germanica plasma. A quantitative reverse transcription PCR-based assay revealed that dsRNA, accordingly, disappeared rapidly from M. sexta hemolymph in vivo. The M. sexta dsRNAse is inactivated by exposure to high temperature and is inhibited by EDTA. These findings lead us to propose that the rate of persistence of dsRNA in insect hemolymph (mediated by the action of one or more nucleases) could be an important factor in determining the susceptibility of insect species to RNAi.

Delivery of dsRNA for RNAi in insects: an overview and future directions

RNA interference (RNAi) refers to the process of exogenous double-stranded RNA (dsRNA) silencing the complementary endogenous messenger RNA. RNAi has been widely used in entomological research for functional genomics in a variety of insects and its potential for RNAi-based pest control has been increasingly emphasized mainly because of its high specificity. This review focuses on the approaches of introducing dsRNA into insect cells or insect bodies to induce effective RNAi. The three most common delivery methods, namely, microinjection, ingestion, and soaking, are illustrated in details and their advantages and limitations are summarized for purpose of feasible RNAi research. In this review, we also briefly introduce the two possible dsRNA uptake machineries, other dsRNA delivery methods and the history of RNAi in entomology. Factors that influence the specificity and efficiency of RNAi such as transfection reagents, selection of dsRNA region, length, and stability of dsRNA in RNAi research are discussed for further studies.

Application of RNA interference in triatomine (Hemiptera: Reduviidae) studies

Triatomines (Hemiptera: Reduviidae) are obligate hematophagous insects. They are of medical importance because they are vectors of Trypanosoma cruzi, the causative agent of Chagas disease in the Americas. In recent years, the RNA interference (RNAi) technology has emerged as a practical and useful alternative means of studying gene function in insects, including triatomine bugs. RNAi research in triatomines is still in its early stages, several issues still need to be elucidated, including the description of the molecules involved in the RNAi machinery and aspects related to phenotype evaluation and persistence of the knockdown in different tissues and organs. This review considers recent applications of RNAi to triatomine research, describing the major methods that have been applied during the knockdown process such as the double-stranded RNA delivery mechanism (injection, microinjection, or ingestion) and the phenotype characterization (mRNA and target protein levels) in studies conducted with the intent to provide greater insights into the biology of these insects. In addition to the characterization of insect biomolecules, some with biopharmacological potential, RNAi may provide a new view of the interaction between triatomine and trypanosomatids, enabling the development of new measures for vector control and transmission of the parasite.

In vitro and in vivo application of RNA interference for targeting genes involved in peritrophic matrix synthesis in a lepidopteran system

The midgut of most insects is lined with a semipermeable acellular tube, the peritrophic matrix (PM), composed of chitin and proteins. Although various genes encoding PM proteins have been characterized, our understanding of their roles in PM structure and function is very limited. One promising approach for obtaining functional information is RNA interference, which has been used to reduce the levels of specific mRNAs using double-stranded RNAs administered to larvae by either injection or feeding. Although this method is well documented in dipterans and coleopterans, reports of its success in lepidopterans are varied. In the current study, the silencing midgut genes encoding PM proteins (insect intestinal mucin 1, insect intestinal mucin 4, PM protein 1) and the chitin biosynthetic or modifying enzymes (chitin synthase-B and chitin deacetylase 1) in a noctuid lepidopteran, Mamestra configurata, was examined in vitro and in vivo. In vitro studies in primary midgut epithelial cell preparations revealed an acute and rapid silencing (by 24 h) for the gene encoding chitin deacetylase 1 and a slower rate of silencing (by 72 h) for the gene encoding PM protein 1. Genes encoding insect intestinal mucins were slightly silenced by 72 h, whereas no silencing was detected for the gene encoding chitin synthase-B. In vivo experiments focused on chitin deacetylase 1, as the gene was silenced to the greatest extent in vitro. Continuous feeding of neonates and fourth instar larvae with double-stranded RNA resulted in silencing of chitin deacetylase 1 by 24 and 36 h, respectively. Feeding a single dose to neonates also resulted in silencing by 24 h. The current study demonstrates that genes encoding PM proteins can be silenced and outlines conditions for RNA interference by per os feeding in lepidopterans.

Advances in RNA interference technology and its impact on nutritional improvement, disease and insect control in plants

This review highlights the advances in the knowledge of RNA interference (RNAi) and discusses recent progress on the functionality of different components RNAi machinery operating in the organisms. The silencing of genes by RNA interference has become the technology of choice for investigation of gene functions in different organisms. The refinement in the knowledge of the endogenous RNAi pathways in plants along with the development of new strategies and applications for the improvement of nutritional value of important agricultural crops through suppression of genes in different plants have opened new vistas for nutritional security. The improvement in the nutritional status of the plants and reduction in the level of toxins or antinutrients was desired for long, but the available technology was not completely successful in achieving the tissue specific regulation of some genes. In the recent years, a number of economically important crop plants have been tested successfully for improving plant nutritional value through metabolic engineering using RNAi. The implications of this technology for crop improvement programs, including nutritional enrichment, reduction of antinutrients, disease, and insect control have been successfully tested in variety of crops with commercial considerations. The enhancement of the nutraceutical traits for the desired health benefits in common crop plants through manipulation of gene expression has been elaborated in this article. The tremendous potential with RNAi technology is expected to revolutionize the modern agriculture for meeting the growing challenges is discussed.

Electroporation-mediated somatic transgenesis for rapid functional analysis in insects

Transgenesis is a powerful technique for determining gene function; however, it is time-consuming. It is virtually impossible to carry out in non-model insects in which egg manipulation and screening are difficult. We have established a rapid genetic functional analysis system for non-model insects using a low-cost electroporator (costing under US$200) designed for somatic transformation with the piggyBac transposon. Using this system, we successfully generated somatic transgenic cell clones in various target tissues (e.g. olfactory neurons, wing epidermis, larval epidermis, muscle, fat body and trachea) of the silkworm Bombyx mori during development. We also induced stable and transient RNA interference (RNAi) using short hairpin RNA (shRNA)-mediating DNA vectors and direct transfer of small interfering RNAs (siRNAs), respectively. We found that these electroporation-mediated approaches could also be applied to the swallowtail butterfly Papilio xuthus and the red flour beetle Tribolium castaneum. Thus, this method could be a powerful genetic tool for elucidating various developmental phenomena in non-model insects.

Non-Target Effects of Green Fluorescent Protein (GFP)-Derived Double-Stranded RNA (dsRNA-GFP) Used in Honey Bee RNA Interference (RNAi) Assays

RNA interference has been frequently applied to modulate gene function in organisms where the production and maintenance of mutants is challenging, as in our model of study, the honey bee, Apis mellifera. A green fluorescent protein (GFP)-derived double-stranded RNA (dsRNA-GFP) is currently commonly used as control in honey bee RNAi experiments, since its gene does not exist in the A. mellifera genome. Although dsRNA-GFP is not expected to trigger RNAi responses in treated bees, undesirable effects on gene expression, pigmentation or developmental timing are often observed. Here, we performed three independent experiments using microarrays to examine the effect of dsRNA-GFP treatment (introduced by feeding) on global gene expression patterns in developing worker bees. Our data revealed that the expression of nearly 1,400 genes was altered in response to dsRNA-GFP, representing around 10% of known honey bee genes. Expression changes appear to be the result of both direct off-target effects and indirect downstream secondary effects; indeed, there were several instances of sequence similarity between putative siRNAs generated from the dsRNA-GFP construct and genes whose expression levels were altered. In general, the affected genes are involved in important developmental and metabolic processes associated with RNA processing and transport, hormone metabolism, immunity, response to external stimulus and to stress. These results suggest that multiple dsRNA controls should be employed in RNAi studies in honey bees. Furthermore, any RNAi studies involving these genes affected by dsRNA-GFP in our studies should use a different dsRNA control.

Oral delivery of double-stranded RNA in larvae of the yellow fever mosquito, Aedes aegypti: implications for pest mosquito control

RNA interference has already proven itself to be a highly versatile molecular biology tool for understanding gene function in a limited number of insect species, but its widespread use in other species will be dependent on the development of easier methods of double-stranded RNA (dsRNA) delivery. This study demonstrates that RNA interference can be induced in the mosquito Aedes aegypti L. (Diptera: Culicidae) simply by soaking larvae in a solution of dsRNA for two hours. The mRNA transcripts for β-tubulin, chitin synthase-1 and -2, and heat shock protein 83 were reduced between 30 and 50% three days post-dsRNA treatment. The dsRNA was mixed with a visible dye to identify those individuals that fed on the dsRNA, and based on an absence of RNA interference in those individuals that contained no dye within their guts, the primary route of entry of dsRNA is likely through the gut epithelium. RNA interference was systemic in the insects, inducing measurable knock down of gene expression in tissues beyond the gut. Silencing of the β-tubulin and chitin synthase-1 genes resulted in reduced growth and/or mortality of the larvae, demonstrating the utility of dsRNA as a potential mosquito larvicide. Silencing of chitin synthase-2 did not induce mortality in the larvae, and silencing of heat shock protein 83 only induced mortality in the insects if they were subsequently subjected to a heat stress. Drosophila melanogaster Meigen (Diptera: Drosophilidae) larvae were also soaked in dsRNA designed to specifically target either their own β-tubulin gene, or that of A. aegypti, and significant mortality was only seen in larvae treated with dsRNA targeting their own gene, which suggests that dsRNA pesticides could be designed to be species-limited.

Study on ecdysteroid levels and gene expression of enzymes related to ecdysteroid biosynthesis in the larval testis of Spodoptera littoralis

We investigated here the ecdysteroid titers and the expression of six genes coding for known enzymes of the ecdysteroid biosynthesis in the testes of last instar larvae of the pest cotton leafworm, Spodoptera littoralis. We showed that the timing of the ecdysteroid profile was the same in testes and in hemolymph, with a small peak at day 2 and a large one at day 4 after ecdysis. Ecdysone and 20-hydroxyecdysone (20E) were detected in both tissues. 20E was the major ecdysteroid in testes and in hemolymph from day 4. Interestingly, the gene expression of the steroidogenetic enzymes, Neverland, and the five cytochrome P450 enzymes encoded by the Halloween genes was confirmed in the testes, and varied during the instar. However, from the data obtained so far, we cannot conclude that the measured ecdysteroids in the testes result from the activity of the genes under study. Indeed, it is suggested that the ecdysone produced centrally in the prothoracic glands, could have been transformed into 20E in the testes, where Sl-shade is well expressed.

Soaking RNAi in Bombyx mori BmN4-SID1 cells arrests cell cycle progression

RNA interference (RNAi) is an evolutionarily conserved mechanism for sequence-specific gene silencing. Previously, the BmN4-SID1 cell expressing Caenorhabditis ele gans SID-1 was established, in which soaking RNAi could induce effective gene silencing. To establish its utility, 6 cell cycle progression related cDNAs, CDK1, MYC, MYB, RNRS, CDT1, and GEMININ, were isolated from the silkworm, Bombyx mori L. (Lepidoptera: Bombycidae), and their expressions were further silenced by soaking RNAi in the BmN4-SID1 cells. The cell cycle progression analysis using flow cytometer demonstrated that the small amount of double stranded RNA was enough to arrest cell cycle progression at the specific cell phases. These data suggest that RNAi in the BmN4-SID1 cells can be used as a powerful tool for loss-of-function analysis of B. mori genes.

Transgenic approaches to western corn rootworm control

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a significant corn pest throughout the United States corn belt. Rootworm larvae feed on corn roots causing yield losses and control expenditures that are estimated to exceed US$1 billion annually. Traditional management practices to control rootworms such as chemical insecticides or crop rotation have suffered reduced effectiveness due to the development of physiological and behavioral resistance. Transgenic maize expressing insecticidal proteins are very successful in protecting against rootworm damage and preserving corn yield potential. However, the high rate of grower adoption and early reliance on hybrids expressing a single mode of action and low-dose traits threatens the durability of commercialized transgenic rootworm technology for rootworm control. A summary of current transgenic approaches for rootworm control and the corresponding insect resistance management practices is included. An overview of potential new modes of action based on insecticidal proteins, and especially RNAi targeting mRNA coding for essential insect proteins is provided.

Efficient targeted mutagenesis in the monarch butterfly using zinc-finger nucleases

The development of reverse-genetic tools in "nonmodel" insect species with distinct biology is critical to establish them as viable model systems. The eastern North American monarch butterfly (Danaus plexippus), whose genome is sequenced, has emerged as a model to study animal clocks, navigational mechanisms, and the genetic basis of long-distance migration. Here, we developed a highly efficient gene-targeting approach in the monarch using zinc-finger nucleases (ZFNs), engineered nucleases that generate mutations at targeted genomic sequences. We focused our ZFN approach on targeting the type 2 vertebrate-like cryptochrome gene of the monarch (designated cry2), which encodes a putative transcriptional repressor of the monarch circadian clockwork. Co-injections of mRNAs encoding ZFNs targeting the second exon of monarch cry2 into "one nucleus" stage embryos led to high-frequency nonhomologous end-joining-mediated, mutagenic lesions in the germline (up to 50%). Heritable ZFN-induced lesions in two independent lines produced truncated, nonfunctional CRY2 proteins, resulting in the in vivo disruption of circadian behavior and the molecular clock mechanism. Our work genetically defines CRY2 as an essential transcriptional repressor of the monarch circadian clock and provides a proof of concept for the use of ZFNs for manipulating genes in the monarch butterfly genome. Importantly, this approach could be used in other lepidopterans and "nonmodel" insects, thus opening new avenues to decipher the molecular underpinnings of a variety of biological processes.

Differential involvement of Hedgehog signaling in butterfly wing and eyespot development

Butterfly eyespots may have evolved from the recruitment of pre-existent gene circuits or regulatory networks into novel locations on the wing. Gene expression data suggests one such circuit, the Hedgehog (Hh) signaling pathway and its target gene engrailed (en), was recruited from a role in patterning the anterior-posterior insect wing axis to a role patterning butterfly eyespots. However, while Junonia coenia expresses hh and en both in the posterior compartment of the wing and in eyespot centers, Bicyclus anynana lacks hh eyespot-specific expression. This suggests that Hh signaling may not be functioning in eyespot development in either species or that it functions in J. coenia but not in B. anynana. In order to test these hypotheses, we performed functional tests of Hh signaling in these species. We investigated the effects of Hh protein sequestration during the larval stage on en expression levels, and on wing size and eyespot size in adults. Hh sequestration led to significantly reduced en expression and to significantly smaller wings and eyespots in both species. But while eyespot size in B. anynana was reduced proportionately to wing size, in J. coenia, eyespots were reduced disproportionately, indicating an independent role of Hh signaling in eyespot development in J. coenia. We conclude that while Hh signaling retains a conserved role in promoting wing growth across nymphalid butterflies, it plays an additional role in eyespot development in some, but not all, lineages of nymphalid butterflies. We discuss our findings in the context of alternative evolutionary scenarios that led to the differential expression of hh and other Hh pathway signaling members across nymphalid species.

Tissue-dependence and sensitivity of the systemic RNA interference response in the desert locust, Schistocerca gregaria

We report on a comprehensive study of the systemic RNAi-response in the desert locust, Schistocerca gregaria. Upon intra-abdominal injection of dsRNA for two housekeeping genes (alpha-tubulin 1a and gapdh) in the range of pg amounts of dsRNA per mg tissue, a potent reduction of their corresponding mRNA was obtained. Moreover, the observed transcript knockdown significantly increased in at least a 10 days period and eventually resulted in high mortality upon silencing of the alpha-tubulin 1a gene. A more moderate RNAi-response was however observed in the reproductive systems. Analysis of the tissue-dependent transcript level profile of several putative RNAi-genes indicated reduced levels of two genes, namely sg-dicer-2 and sg-argonaute-2, in the reproductive systems. By silencing these components, we confirmed their importance in the RNAi-process and suggest that their expression levels are determinant for tissue-dependent differences in the potency of RNAi in the desert locust.

Engineering silkworms for resistance to baculovirus through multigene RNA interference

Bombyx mori nucleopolyhedrovirus (BmNPV) that infects the silkworm, B. mori, accounts for >50% of silk cocoon crop losses globally. We speculated that simultaneous targeting of several BmNPV essential genes in transgenic silkworm would elicit a stable defense against the virus. We introduced into the silkworm germline the vectors carrying short sequences of four essential BmNPV genes in tandem, either in sense or antisense or in inverted-repeat arrangement. The transgenic silkworms carrying the inverted repeat-containing transgene showed stable protection against high doses of baculovirus infection. Further, the antiviral trait was incorporated to a commercially productive silkworm strain highly susceptible to BmNPV. This led to combining the high-yielding cocoon and silk traits of the parental commercial strain and a very high level of refractoriness (>75% survival rate as compared to <15% in nontransgenic lines) to baculovirus infection conferred by the transgene. We also observed impaired infectivity of the occlusion bodies derived from the transgenic lines as compared to the wild-type ones. Currently, large-scale exploitation of these transgenic lines is underway to bring about economic transformation of sericulture.

Characterizing the mechanism of action of double-stranded RNA activity against western corn rootworm (Diabrotica virgifera virgifera LeConte)

RNA interference (RNAi) has previously been shown to be effective in western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) larvae via oral delivery of synthetic double-stranded RNA (dsRNA) in an artificial diet bioassay, as well as by ingestion of transgenic corn plant tissues engineered to express dsRNA. Although the RNAi machinery components appear to be conserved in Coleopteran insects, the key steps in this process have not been reported for WCR. Here we characterized the sequence of events that result in mortality after ingestion of a dsRNA designed against WCR larvae. We selected the Snf7 ortholog (DvSnf7) as the target mRNA, which encodes an essential protein involved in intracellular trafficking. Our results showed that dsRNAs greater than or equal to approximately 60 base-pairs (bp) are required for biological activity in artificial diet bioassays. Additionally, 240 bp dsRNAs containing a single 21 bp match to the target sequence were also efficacious, whereas 21 bp short interfering (si) RNAs matching the target sequence were not. This result was further investigated in WCR midgut tissues: uptake of 240 bp dsRNA was evident in WCR midgut cells while a 21 bp siRNA was not, supporting the size-activity relationship established in diet bioassays. DvSnf7 suppression was observed in a time-dependent manner with suppression at the mRNA level preceding suppression at the protein level when a 240 bp dsRNA was fed to WCR larvae. DvSnf7 suppression was shown to spread to tissues beyond the midgut within 24 h after dsRNA ingestion. These events (dsRNA uptake, target mRNA and protein suppression, systemic spreading, growth inhibition and eventual mortality) comprise the overall mechanism of action by which DvSnf7 dsRNA affects WCR via oral delivery and provides insights as to how targeted dsRNAs in general are active against insects.

Induction of RNA interference genes by double-stranded RNA; implications for susceptibility to RNA interference

Gene silencing by RNA interference (RNAi) can be a useful reverse genetics tool in eukaryotes. However, some species appear refractory to RNAi. To study the role of the differential expression of RNAi proteins in RNAi, we isolated partial dicer-2, argonaute-2 translin, vasa intronic gene (VIG) and tudor staphylococcus/micrococcal nuclease (TSN) genes from the tobacco hornworm, Manduca sexta, a well-studied insect model which we have found to be variably sensitive to RNAi. We found that the RNAi gene, translin, was expressed at minimal levels in M. sexta tissue and that there is a specific, dose-dependent upregulation of dicer-2 and argonaute-2 expression in response to injection with dsRNA, but no upregulation of the other genes tested. Upregulation of gene expression was rapid and transient. In order to prolong the upregulation we introduced multiple doses of dsRNA, resulting in multiple peaks of dicer-2 gene expression. Our results have implications for the design of RNAi experiments and may help to explain differences in the sensitivity of eukaryotic organisms to RNAi.

Precise RNAi-mediated silencing of metabolically active proteins in the defence secretions of juvenile leaf beetles

Allomones are widely used by insects to impede predation. Frequently these chemical stimuli are released from specialized glands. The larvae of Chrysomelina leaf beetles produce allomones in gland reservoirs into which the required precursors and also the enzymes are secreted from attached gland cells. Hence, the reservoirs can be considered as closed bio-reactors for producing defensive secretions. We used RNA interference (RNAi) to analyse in vivo functions of proteins in biosynthetic pathways occurring in insect secretions. After a salicyl alcohol oxidase was silenced in juveniles of the poplar leaf beetles, Chrysomela populi, the precursor salicyl alcohol increased to 98 per cent, while salicyl aldehyde was reduced to 2 per cent within 5 days. By analogy, we have silenced a novel protein annotated as a member of the juvenile hormone-binding protein superfamily in the juvenile defensive glands of the related mustard leaf beetle, Phaedon cochleariae. The protein is associated with the cyclization of 8-oxogeranial to iridoids (methylcyclopentanoid monoterpenes) in the larval exudates made clear by the accumulation of the acylic precursor 5 days after RNAi triggering. A similar cyclization reaction produces the secologanin part of indole alkaloids in plants.

Bombyx mori DNA/RNA non-specific nuclease: expression of isoforms in insect culture cells, subcellular localization and functional assays

A DNA/RNA non-specific alkaline nuclease (BmdsRNase) was isolated from the digestive juice of Bombyx mori. While originally reported to be produced by the midgut only, in this project it was found that the mRNA of this enzyme was also expressed in the epidermis, fat body, gut, thoracic muscles, Malpighian tubules, brain, and silk glands of 5th instar larvae, indicating additional functions to its reported role in nucleic acid digestion in the midgut. In order to study the functional properties of BmdsRNase, three pEA-BmdsRNase expression constructs were generated, characterized by presence or absence of a signal peptide and a propeptide, and used for expression in lepidopteran Hi5 tissue culture cells. Western blot indicated that these different forms of BmdsRNase protein were not secreted into the growth medium, while they were detected in the pellets and supernatants of Hi5 cell extracts. Nucleic acids cleavage experiments indicated that full-length BmdsRNase could digest dsRNA and that the processed form (absence of signal peptide and propeptide) of BmdsRNase could degrade both DNA and dsRNA in Hi5 cell culture. Using a reporter assay targeted by transfected homologous dsRNA, it was shown that the digestive property of the processed form could interfere with the RNAi response. Immunostaining of processed BmdsRNase protein showed asymmetric localization in the cellular cytoplasm and co-localization with Flag-tagged Dicer-2 was also observed. In conclusion, our in vitro studies indicated that intracellular protein isoforms of BmdsRNase can be functional and involved in the regulation of nucleic acid metabolism in the cytoplasm. In particular, because of its propensity to degrade dsRNA, the enzyme might be involved in the innate immune response against invading nucleic acids such as RNA viruses.

RNAi: future in insect management

RNA interference is a post- transcriptional, gene regulation mechanism found in virtually all plants and animals including insects. The demonstration of RNAi in insects and its successful use as a tool in the study of functional genomics opened the door to the development of a variety of novel, environmentally sound approaches for insect pest management. Here the current understanding of the biogenesis of the two RNAi classes in insects is reviewed. These are microRNAs (miRNAs) and short interfering RNAs (siRNAs). Several other key approaches in RNAi -based for insect control, as well as for the prevention of diseases in insects are also reviewed. The problems and prospects for the future use of RNAi in insects are presented.

Improvement of pest resistance in transgenic tobacco plants expressing dsRNA of an insect-associated gene EcR

The adoption of pest-resistant transgenic plants to reduce yield loss and pesticide utilization has been successful in the past three decades. Recently, transgenic plant expressing double-stranded RNA (dsRNA) targeting pest genes emerges as a promising strategy for improving pest resistance in crops. The steroid hormone, 20-hydroxyecdysone (20E), predominately controls insect molting via its nuclear receptor complex, EcR-USP. Here we report that pest resistance is improved in transgenic tobacco plants expressing dsRNA of EcR from the cotton bollworm, Helicoverpa armigera, a serious lepidopteran pest for a variety of crops. When H. armigera larvae were fed with the whole transgenic tobacco plants expressing EcR dsRNA, resistance to H. armigera was significantly improved in transgenic plants. Meanwhile, when H. armigera larvae were fed with leaves of transgenic tobacco plants expressing EcR dsRNA, its EcR mRNA level was dramatically decreased causing molting defects and larval lethality. In addition, the transgenic tobacco plants expressing H. armigera EcR dsRNA were also resistant to another lepidopteran pest, the beet armyworm, Spodoptera exigua, due to the high similarity in the nucleotide sequences of their EcR genes. This study provides additional evidence that transgenic plant expressing dsRNA targeting insect-associated genes is able to improve pest resistance.

Toxins for transgenic resistance to hemipteran pests

The sap sucking insects (Hemiptera), which include aphids, whiteflies, plant bugs and stink bugs, have emerged as major agricultural pests. The Hemiptera cause direct damage by feeding on crops, and in some cases indirect damage by transmission of plant viruses. Current management relies almost exclusively on application of classical chemical insecticides. While the development of transgenic crops expressing toxins derived from the bacterium Bacillus thuringiensis (Bt) has provided effective plant protection against some insect pests, Bt toxins exhibit little toxicity against sap sucking insects. Indeed, the pest status of some Hemiptera on Bt-transgenic plants has increased in the absence of pesticide application. The increased pest status of numerous hemipteran species, combined with increased prevalence of resistance to chemical insecticides, provides impetus for the development of biologically based, alternative management strategies. Here, we provide an overview of approaches toward transgenic resistance to hemipteran pests.

Identification of chemosensory receptor genes in Manduca sexta and knockdown by RNA interference

BACKGROUND

Insects detect environmental chemicals via a large and rapidly evolving family of chemosensory receptor proteins. Although our understanding of the molecular genetic basis for Drosophila chemoreception has increased enormously in the last decade, similar understanding in other insects remains limited. The tobacco hornworm, Manduca sexta, has long been an important model for insect chemosensation, particularly from ecological, behavioral, and physiological standpoints. It is also a major agricultural pest on solanaceous crops. However, little sequence information and lack of genetic tools has prevented molecular genetic analysis in this species. The ability to connect molecular genetic mechanisms, including potential lineage-specific changes in chemosensory genes, to ecologically relevant behaviors and specializations in M. sexta would be greatly beneficial.

RESULTS

Here, we sequenced transcriptomes from adult and larval chemosensory tissues and identified chemosensory genes based on sequence homology. We also used dsRNA feeding as a method to induce RNA interference in larval chemosensory tissues.

CONCLUSIONS

We report identification of new chemosensory receptor genes including 17 novel odorant receptors and one novel gustatory receptor. Further, we demonstrate that systemic RNA interference can be used in larval olfactory neurons to reduce expression of chemosensory receptor transcripts. Together, our results further the development of M. sexta as a model for functional analysis of insect chemosensation.

Legionella pneumophila pathogenesis in the Galleria mellonella infection model

Legionella pneumophila is a facultative intracellular human pathogen and the etiological agent of severe pneumonia known as Legionnaires' disease. Its virulence depends on protein secretion systems, in particular, the Dot/Icm type IV secretion system (T4SS), which is essential to establish a replication-permissive vacuole in macrophages. The analysis of the role of these systems and their substrates for pathogenesis requires easy-to-use models which approximate human infection. We examined the effectiveness of the larvae of the wax moth Galleria mellonella as a new model for L. pneumophila infection. We found that the L. pneumophila strains 130b, Paris, and JR32 caused mortality of the G. mellonella larvae that was strain, infectious dose, growth phase, and T4SS dependent. Wild-type L. pneumophila persisted and replicated within the larvae, whereas T4SS mutants were rapidly cleared. L. pneumophila strain Lp02, which is attenuated in the absence of thymidine but has a functional T4SS, resisted clearance in G. mellonella up to 18 h postinfection without inducing mortality. Immunofluorescence and transmission electron microscopy revealed that L. pneumophila resided within insect hemocytes in a vacuole that ultrastructurally resembled the Legionella-containing vacuole (LCV) observed in macrophages. The vacuole was decorated with the T4SS effector and LCV marker SidC. Infection caused severe damage to the insect organs and triggered immune responses, including activation of the phenoloxidase cascade leading to melanization, nodule formation, and upregulation of antimicrobial peptides. Taken together, these results suggest that G. mellonella provides an effective model to investigate the interaction between L. pneumophila and the host.

RNA interference in plant parasitic nematodes: a summary of the current status

SUMMARYRNA interference (RNAi) has emerged as an invaluable gene-silencing tool for functional analysis in a wide variety of organisms, particularly the free-living model nematode Caenorhabditis elegans. An increasing number of studies have now described its application to plant parasitic nematodes. Genes expressed in a range of cell types are silenced when nematodes take up double stranded RNA (dsRNA) or short interfering RNAs (siRNAs) that elicit a systemic RNAi response. Despite many successful reports, there is still poor understanding of the range of factors that influence optimal gene silencing. Recent in vitro studies have highlighted significant variations in the RNAi phenotype that can occur with different dsRNA concentrations, construct size and duration of soaking. Discrepancies in methodology thwart efforts to reliably compare the efficacy of RNAi between different nematodes or target tissues. Nevertheless, RNAi has become an established experimental tool for plant parasitic nematodes and also offers the prospect of being developed into a novel control strategy when delivered from transgenic plants.

Insect omics research coming of age1This review is part of a virtual symposium on recent advances in understanding a variety of complex regulatory processes in insect physiology and endocrinology, including development, metabolism, cold hardiness, food intake and digestion, and diuresis, through the use of omics technologies in the postgenomic era

As more and more insect genomes are fully sequenced and annotated, omics technologies, including transcriptomic, proteomic, peptidomics, and metobolomic profiling, as well as bioinformatics, can be used to exploit this huge amount of sequence information for the study of different biological aspects of insect model organisms. Omics experiments are an elegant way to deliver candidate genes, the function of which can be further explored by genetic tools for functional inactivation or overexpression of the genes of interest. Such tools include mainly RNA interference and are currently being developed in diverse insect species. In this manuscript, we have reviewed how omics technologies were integrated and applied in insect biology.

Saliva of Lygus lineolaris digests double stranded ribonucleic acids

The prospects for development of highly specific pesticides based on double stranded ribonucleic acid have been a recent focus of scientific research. Creative applications have been proposed and demonstrated. However, not all insects are sensitive to double stranded RNA (dsRNA) gene knockdown effects; applications in the order Lepidoptera, for example, have met with varied success. Gene knockdown has been demonstrated in several species in the order Hemiptera. In our laboratory, knockdown experiments relied on microinjection of dsRNA into the hemocoel of the tarnished plant bug, Lygus lineolaris. Subsequent experiments delivering dsRNA to insects by feeding were repeatedly unsuccessful in demonstrating knockdown, and a hypothesis was formulated that the dsRNA was digested and degraded by the insect prior to contact with the insect cells. Exposure of dsRNA to insect saliva, insect salivary glands, and insect hemolymph was compared with commercial RNAase III. The saliva of L. lineolaris was found to rapidly digest double stranded RNA. RNAase inhibitor did not affect the activity but heat treatment slowed enzymatic activity.

Beyond nutrients: food-derived microRNAs provide cross-kingdom regulation

Food turns out to be not only the nutrient supplier for our body but also a carrier of regulatory information. Interestingly, a recent study made the discovery that some plant/food-derived microRNAs (miRNAs) accumulate in the serum of humans or plant-feeding animals, and regulate mammalian gene expression in a sequence-specific manner. The authors provided striking evidence that miRNAs could function as active signaling molecules to transport information across distinct species or even kingdoms. Although the mechanism of how miRNAs are shuttled between different organisms is still not well characterized, initial results point to the involvement of microvesicles and specific RNA-transporter-like proteins. These findings raise both speculation about the potential impact that plants may have on animal physiology at the molecular level, and an appealing possibility that food-derived miRNAs may offer us another means to deliver necessary nutrients or therapeutics to our bodies.

SID-1 protein of Caenorhabditis elegans mediates uptake of dsRNA into Bombyx cells

RNA interference is one of the most revolutionary tools in the study of gene function, particularly in non-model systems. However, in Bombyx mori, as with many lepidopteran species, attempts at systemic RNAi have had mixed success. Gene identification and phylogenetic analyses suggest that Bombyx has the core RNAi machinery, which is necessary to undergo RNAi as a cellular response. We introduced sid genes from Caenorhabditis elegans into Bombyx BmN4 cells to enhance the uptake of dsRNA and revealed that the SID-1 protein, but not SID-2, has the ability to endow the RNAi effect with the addition of dsRNA to the medium. Observed RNAi effect was dependent on both the levels of sid-1 expression and the concentration of the dsRNA. These results suggest that SID-1 promotes the uptake of dsRNA from the medium into Bombyx cells. We generated transgenic animals that express sid-1 but have not detected significant enhancements of in vivo phenotype in response to the injection of the dsRNA into hemocoel.

Tobacco rattle virus vector: A rapid and transient means of silencing manduca sexta genes by plant mediated RNA interference

BACKGROUND

RNAi can be achieved in insect herbivores by feeding them host plants stably transformed to express double stranded RNA (dsRNA) of selected midgut-expressed genes. However, the development of stably transformed plants is a slow and laborious process and here we developed a rapid, reliable and transient method. We used viral vectors to produce dsRNA in the host plant Nicotiana attenuata to transiently silence midgut genes of the plant's lepidopteran specialist herbivore, Manduca sexta. To compare the efficacy of longer, undiced dsRNA for insect gene silencing, we silenced N. attenuata's dicer genes (NaDCL1- 4) in all combinations in a plant stably transformed to express dsRNA targeting an insect gene.

METHODOLOGY/PRINCIPAL FINDINGS

Stable transgenic N. attenuata plants harboring a 312 bp fragment of MsCYP6B46 in an inverted repeat orientation (ir-CYP6B46) were generated to produce CYP6B46 dsRNA. After consuming these plants, transcripts of CYP6B46 were significantly reduced in M. sexta larval midguts. The same 312 bp cDNA was cloned in an antisense orientation into a TRV vector and Agro-infiltrated into N. attenuata plants. When larvae ingested these plants, similar reductions in CYP6B46 transcripts were observed without reducing transcripts of the most closely related MsCYP6B45. We used this transient method to rapidly silence the expression of two additional midgut-expressed MsCYPs. CYP6B46 transcripts were further reduced in midguts, when the larvae fed on ir-CYP6B46 plants transiently silenced for two combinations of NaDCLs (DCL1/3/4 and DCL2/3/4) and contained higher concentrations of longer, undiced CYP6B46 dsRNA.

CONCLUSIONS

Both stable and transient expression of CYP6B46 dsRNA in host plants provides a specific and robust means of silencing this gene in M. sexta larvae, but the transient system is better suited for high throughput analyses. Transiently silencing NaDCLs in ir-CYP6B46 plants increased the silencing of MsCYP6B46, suggested that insect's RNAi machinery is more efficient with longer lengths of ingested dsRNA.

Functional validation of the carbon dioxide receptor genes in Aedes aegypti mosquitoes using RNA interference

Carbon dioxide (CO(2)) is an important long-range chemosensory cue used by blood-feeding female mosquitoes to find their hosts. The CO(2) receptor in Drosophila melanogaster was previously determined to be a heterodimer comprised of two gustatory receptor (Gr) proteins, DmGr21a and DmGr63a. In the mosquito Aedes aegypti, two putative orthologous genes, AaGr1 and AaGr3, were identified in the genome database, along with an apparent paralogue of AaGr1, AaGr2. In this study, RNA interference (RNAi)-mediated gene knockdown of either AaGr1 or AaGr3 resulted in a loss of CO(2) sensitivity in both male and female mosquitoes, suggesting that these two proteins, like the Drosophila orthologues, function as a heterodimer. RNAi-mediated knockdown of AaGr2 expression had no impact on CO(2) reception. All three Gr genes were expressed in the maxillary palps of both Ae. aegypti and the West Nile virus vector mosquito, Culex pipiens quinquefasciatus. Interestingly, expression of the two CO(2) receptor genes was not equivalent in the two sexes and the implications of differential sex expression of the CO(2) receptor in different species are discussed. The functional identification of the CO(2) receptor in a mosquito could prove invaluable in the strategic design of compounds that disrupt the mosquito's ability to find hosts.