Cloning, characterisation and expression profiling of the cDNA encoding the ryanodine receptor in diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae)

Establishment and evaluation of targeted molecular screening model for the ryanodine receptor or sarco/endoplasmic reticulum calcium ATPase

BACKGROUD

Endoplasmic reticulum/sarcoplasmic reticulum (ER/SR) is crucial for maintaining intracellular calcium homeostasis due to the calcium-signaling-related proteins on its membrane. While ryanodine receptors (RyR) on insect ER/SR membranes are well-known as targets for diamide insecticides, little is known about other calcium channels. Given the resistance of diamide insecticides, the establishment of molecular screening models targeting RyR or sarco/endoplasmic reticulum calcium ATPase (SERCA) is conducive to the discovery of new insecticidal molecules.

RESULTS

The morphological features of Mythimna separata SR have closed vesicles with integrity and high density. The 282 proteins in the SR component contained RyR and SERCA. A measurement model for the release and uptake of calcium was successfully established by detecting calcium ions outside the SR membrane using a fluorescence spectrophotometer. In vitro testing systems using SR vesicles found that diamide insecticides could activate dose-dependently RyR, with EC50 values of 0.14 μM (Chlorantraniliprole), 0.21 μM (Flubendiamide), and 0.57 μM (Cyantraniliprole), respectively. However, dantrolene inhibited RyR-mediated calcium release with an IC50 value of 353.9 μM, suggesting that dantrolene can weakly antagonize RyR. Moreover, cyclopiazonic acid significantly reduced the enzyme activity and calcium uptake capacity of SERCA. On the contrary, CDN1163 markedly activated the enzyme activity and improved the calcium transport capacity of SERCA.

CONCLUSIONS

SR vesicles can be used to study the function of unknown proteins on the SR membranes, as well as for high-throughput screening of highly active compounds targeting RyR or SERCA. © 2024 Society of Chemical Industry.

Molecular characterization and expression patterns of a ryanodine receptor in soybean looper, Chrysodeixis includens

Diamide insecticides, such as chlorantraniliprole, have been widely used to control insect pests by targeting the insect ryanodine receptor (RyR). Due to the efficacious insecticidal activity of diamides, as well as an increasing number of resistance cases, the molecular structure of RyR has been studied in many economically important insects. However, no research has been conducted on diamide resistance and RyR in the soybean looper, Chrysodeixis includens, a significant crop pest. In this study, we found moderate resistance to chlorantraniliprole in a field population from Puerto Rico and sequenced the full-length cDNA of the C. includens RyR gene, which encodes a 5124 amino acid-long protein. Genomic analysis revealed that the CincRyR gene consists of 113 exons, one of the largest exon numbers reported for RyR. Alternative splicing sites were detected in the cytosolic region. The protein sequence showed high similarity to other noctuid RyRs. Conserved structural features included the selectivity filter motif critical for ryanodine binding and ion conduction, as well as various domains involved in ion transport. Two mutation sites associated with diamide resistance in other insects were screened but not found in the Puerto Rico field populations or in the susceptible lab strain. Gene expression analysis indicated high expression of RyR in the third instar larval stage, particularly in muscle-containing tissues. Furthermore, exposure to a sublethal dose of chlorantraniliprole reduced RyR expression levels after 96 h. This study provides a molecular basis for understanding RyR structure and sheds light on potential mechanisms of diamide resistance in C. includens.

Functional Characterization of the Ryanodine Receptor Gene in Diaphorina citri

The Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae) is a major citrus pest spread around the world. It is also a vector of the bacterium 'Candidatus Liberibacter asiaticus', considered the cause of the fatal citrus disease huanglongbing (HLB). Insect ryanodine receptors (RyRs) are the primary target sites of diamide insecticides. In this study, full-length RyR cDNA from D. citri (named DcRyR) was isolated and identified. The 15,393 bp long open reading frame of DcRyR encoded a 5130 amino acid protein with a calculated molecular weight of 580,830 kDa. This protein had a high sequence identity (76-79%) with other insect homologs and a low sequence identity (43-46%) with mammals. An MIR domain, two RIH domains, three SPRY domains, four RyR repeat domains, an RIH-associated domain at the N-terminus, two consensus calcium-binding EF-hands, and six transmembrane domains were among the characteristics that DcRyR shared with insect and vertebrate RyRs. In expression analysis, the DcRyR gene displayed transcript abundance in all tissues and developmental stages as well as gene-differential and stage-specific patterns. In addition, diagnostic PCR experiments revealed that DcRyR had three potential alternative splice variants and that splicing events might have contributed to the various functions of DcRyR. However, diamide resistance-related amino acid residue mutations I4790M/K and G4946E were not found in DcRyR. These results can serve as the basis for further investigation into the target-based diamide pesticide resistance of D. citri.

Characterization of the ryanodine receptor gene in Encarsia formosa (Gahan) and its expression profile in response to diamide insecticides

Ryanodine receptors (RyRs) are the targets of diamide insecticides, which have been identified and characterized in a dozen insect pests of Lepidoptera, Hemiptera, Diptera and Coleoptera, but limited attention has been paid to the RyR in parasitoid natural enemies. Without this knowledge, it will hinder our effective and efficient application using both parasitoid natural enemies and diamide insecticides simultaneously in the integrated pest management (IPM). In this study, the full-length cDNA of RyR was cloned from Encarsia formosa (EfRyR), a parasitic wasp used worldwide for the biological control of whitefly. Its expression profile was examined in various tissues of E. formosa adults. The toxicities of four diamide insecticides to E. formosa were measured, and then the expression profile of EfRyR after 12 h and 24 h exposure to the LC₅₀ dosages of diamide insecticides was investigated. The results showed that the full-length cDNA of EfRyR was 16, 778 bp including a 15, 345 bp open reading frame, and two alternative splice (AS) sites. Comparing to its expression in the abdomen, EfRyR was highly expressed in the head (11.9-fold) and the thorax (3.7-fold). The toxicities of four dimide insecticides against E. formosa from low to high were chlorantraniliprole (LC₅₀ = 367.84 mg L^-1), cyantraniliprole (221.72 mg L^-1), cyclaniliprole (51.77 mg L^-1), and tetrachlorantraniliprole (8.35 mg L^-1). The expressions of EfRyR and its variants with AS were significantly increased after E. formosa adults were exposed to different diamide insecticides. This study improves our understanding of the RyR in parasitoid wasps and provides useful information on IPM by using E. formosa.

A highly selective and sensitive fluorescence probe for a specific binding site on insect ryanodine receptors

There is a significant need to study the binding of active compounds to the specific sites on insect ryanodine receptors (RyRs) that are the targets of two novel classes of diamide insecticides to which insects are becoming increasingly resistant. Here, we describe a rapid assay to study the action of potential compounds on the flubendiamide (Flu) binding site of insect RyRs that uses a fluorescence polarization assay with the fluorescence probe Flu-R-L that we synthesized. The IC₅₀ of Flu for inhibiting probe binding on insect RyR was 18.82 ng/mL. The binding of 86 novel phthalic diamide derivatives on insect RyRs was studied using this newly established assay, and the compounds that exhibited high-affinity binding in the assay also possessed in vivo insecticidal activity against Plutella xylostella. Thus, Flu-R-L is a highly selective and sensitive fluorescence probe for studying the binding affinity of novel compounds to the Flu binding site of insect RyRs. The assay based on Flu-R-L is a rapid, accurate, and sensitive method for the screening of potentially bioactive molecules that bind specifically to insect RyRs.

Multiple target-site mutations occurring in lepidopterans confer resistance to diamide insecticides

Diamide resistant phenotypes have evolved in the field and the resistance has been attributed to target-site mutations in some lepidopteran pests. In this study, we documented the resistance status of Chilo suppressalis to chlorantraniliprole during 2016-2018 in seven provinces of China. To investigate the possible role of target-site mutations as known from lepidopterans, we sequenced respective domains of the RyR gene of C. suppressalis with different levels of diamide resistance. The results revealed that I4758M (corresponding to I4790M in P. xylostella), Y4667D/C (numbered according to C. suppressalis), G4915E (corresponding to G4946E in P. xylostella), and one novel Y4891F (numbered according to C. suppressalis) RyR target-site mutations were present. The contribution of these mutations was further investigated by diamide toxicity bioassays with eight genome modified Drosophila melanogaster lines. The study showed that genome modified flies bearing the Y4667D mutation (corresponding to the Y4667D and I4758M simultaneous mutation in C. suppressalis) exhibited high resistance ratios to chlorantraniliprole (1542.8-fold), cyantraniliprole (487.9-fold) and tetrachlorantraniliprole (290.1-fold). The M4758I and G4915E simultaneous mutations (corresponding to single G4915E mutation in C. suppressalis) showed high resistance ratios to chlorantraniliprole (153.1-fold) and cyantraniliprole (323.5-fold), and relatively low resistance to flubendiamide (28.9-fold) and tetrachlorantraniliprole (25.2-fold). These findings suggest that multiple point mutations in RyR confer diamide resistance of C. suppressalis. The results contribute to a better understanding of insect diamide resistance mechanisms and provide insights on the impact of RyR target-site mutations in insects.

Characterization of the Expression and Functions of Two Odorant-Binding Proteins of Sitophilus zeamais Motschulsky (Coleoptera: Curculionoidea)

Odorant-binding proteins (OBPs) are important in insect chemical communication. The objective of this research was to identify the functions of two OBPs in Sitophilus zeamais. qRT-PCR and western blot (WB) were performed to investigate the expression profiles at the transcript and protein levels, respectively. Fluorescence competitive binding assays were used to measure the ability of the OBPs to bind to host volatiles, and a Y-tube olfactometer was used to verify the results (attraction/no response) via behavioral experiments. The RNAi was used to verify the function by knocking down the ability of proteins to bind odorants. qRT-PCR showed the highest expression SzeaOBP1 and SzeaOBP28 at the low-instar larva (LL) and eclosion adult (EA) stages, respectively. WB showed that both SzeaOBP1 and SzeaOBP28 were highly expressed in the EA stage. Fluorescence competitive binding assays indicated that SzeaOBP1 exhibited extremely high binding affinity with cetanol. SzeaOBP28 exhibited a pronounced binding affinity for 4-hydroxy-3-methoxybenzaldehyde. The behavioral experiment showed that the adult S. zeamais responded strongly to 4-hydroxy-3-methoxybenzaldehyde and valeraldehyde from Sorghum bicolor. The RNAi knockdown individuals displayed behavioral differences between normal insects and dsRNA (SzeaOBP1)-treated insects. We infer that they both have functions in perception and recognition of host volatiles, whereas SzeaOBP28 may also have other functions.

Molecular characterization and expression profiling of ryanodine receptor gene in the pink stem borer, Sesamia inferens (Walker)

The susceptibilities of three field populations of pink stem borer (PSB), Sesamia inferens (walker) to diamide insecticides, chlorantraniliprole and flubendiamide, were evaluated in this study. The results showed that these PSB field populations were still sensitive to the two diamide insecticides after many years of exposure. To further understand PSB and diamide insecticide, the full-length ryanodine receptor (RyR) cDNA (named as SiRyR), the molecular target of diamide insecticides was cloned from PSB and characterized. The SiRyR gene contains an open reading frame of 15,420 nucleotides, encoding 5140 amino acid residues, which shares 77% to 98% sequence identity with RyR homologous of other insects. All hallmarks of RyR proteins are conserved in the SiRyR protein, including the conserved C-terminal domain with the consensus calcium-biding EF-hands (calcium-binding motif), the six transmembrane domains, as well as mannosyltransferase, IP3R and RyR (pfam02815) (MIR) domains. Real-time qPCR analysis revealed that the highest mRNA expression levels of SiRyR were observed in pupa and adults, especially in males. SiRyR was expressed at the highest level in thorax, and the lowest level in wing. The full genetic characterization of SiRyR could provide useful information for future functional expression studies and for discovery of new insecticides with selective insecticidal activity.

Monitoring and mechanisms of insecticide resistance in Chilo suppressalis (Lepidoptera: Crambidae), with special reference to diamides

BACKGROUND

The rice stem borer, Chilo suppressalis Walker (Lepidoptera: Crambidae), is one of the most economically important pests of rice in Asia. Chemical control remains the most efficient primary means for controlling this pest.

RESULTS

Significant variations among field populations in their resistance to seven insecticides were observed. The populations exhibited LC₅₀ values that ranged between 0.605 and 108.088 mg AI L^-1 for chlorantraniliprole and between 0.046 and 3.919 mg AI L^-1 for flubendiamide. The YY14 population collected from Yuyao in Zhejiang Province in 2014 showed a moderate resistance level to the two diamides, i.e. up to 77.6-fold and 42.6-fold for chlorantraniliprole and flubendiamide respectively. Synergism tests and biochemical assays showed no obvious correlations between diamide resistance and three detoxifying enzymes. Sequence comparison of the ryanodine receptor gene between the YY14 resistant population and susceptible population revealed that a glycine to glutamic acid substitution (G4910E) was presented in the YY14 population.

CONCLUSION

G4910E mutation might be involved in the resistance evolution of C. suppressalis to the diamides. An appropriate insecticide resistance management programme should be established to maintain the effectiveness of the insecticides and to ensure sustainable management. © 2016 Society of Chemical Industry.

De novo transcriptome and expression profile analyses of the Asian corn borer (Ostrinia furnacalis) reveals relevant flubendiamide response genes

Background

The Asian corn borer (ACB), Ostrinia furnacalis (Guenée), has become the most damaging insect pest of corn in Asia. However, the lack of genome or transcriptome information heavily hinders our further understanding of ACB in every aspect at a molecular level and on a genome-wide scale. Here, we used the Ion Torrent Personal Genome Machine (PGM) Sequencer to explore the ACB transcriptome and to identify relevant genes in response to flubendiamide, showing high selective activity against ACB.

Results

We obtained 35,430 unigenes, with an average length of 716 bp, representing a dramatic expansion of existing cDNA sequences available for ACB. These sequences were annotated with Non-redundant Protein (Nr), Gene Ontology (GO), Clusters of Orthologous Groups (COG) and Kyoto Encyclopedia of Genes and Genomes (KEGG) to better understand their functions. A total of 31 cytochrome P450 monooxygenases (P450s), 27 carboxyl/cholinesterases (CCEs) and 19 glutathione S-transferases (GSTs) were manually curated to construct phylogenetic trees, and 25 unigenes encoding target proteins (acetylcholinesterase, nicotinic acetylcholine receptor, gamma-aminobutyric acid receptor, glutamate-gated chloride channel, voltage-gated sodium channel and ryanodine receptor) were identified. In addition, we compared and validated the differentially expressed unigenes upon flubendiamide treatment, revealing that the genes for detoxification enzymes (P450s and esterase), calcium signaling pathways and muscle control pathways (twitchin and tropomyosin), immunoglobulin (hemolin), chemosensory protein and heat shock protein 70 were significantly overexpressed in response to flubendiamide, while the genes for cuticular protein, protease and oxidoreductase showed much lower expression levels.

Conclusion

The obtained transcriptome information provides large genomic resources available for further studies of ACB. The differentially expressed gene data will elucidate the molecular mechanisms of ACB in response to the novel diamide insecticide, flubendiamide. In particular, these findings will facilitate the identification of the genes involved in insecticide resistance and the development of new compounds to control the ACB.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3431-6) contains supplementary material, which is available to authorized users.

Ryanodine receptor genes of the rice stem borer, Chilo suppressalis: Molecular cloning, alternative splicing and expression profiling

The ryanodine receptor (RyR) of the calcium release channel is the main target of anthranilic and phthalic diamide insecticides which have high selective insecticidal activity relative to mammalian toxicity. In this study, the full-length cDNA of Chilo suppressalis RyR (CsRyR) was isolated and characterized. The CsRyR mRNA has an open reading frame (ORF) of 15,387bp nucleotides, which encodes 5128 amino acids with GenBank ID: KR088972. Comparison of protein sequences showed that CsRyR shared high identities with other insects of 77-96% and lower identity to mammals and nematodes with only 42-45%. One alternative splicing site (KENLG) unique to Lepidoptera was found and two exclusive exons of CsRyR (I /II) were revealed. Spatial and temporal expression of CsRyR mRNA was at the highest relative level in 3rd instar larvae and head (including brain and muscle), and at the lowest expression level in egg and fat body. The expression levels of whole body CsRyR mRNA were increased remarkably after injection of 4th instar larvae with chlorantraniliprole at 0.004 to 0.4μg/g. This structural and functional information on CsRyR provides the basis for further understanding the selective action of chlorantraniliprole and possibly other diamide insecticides.

Rapid selection for resistance to diamide insecticides in Plutella xylostella via specific amino acid polymorphisms in the ryanodine receptor

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Baseline susceptibility of Plutella xylostella to diamide insecticides collated.

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Instances of diamide resistance in P. xylostella summarized.

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Diamide insecticide specificity for the insect ryanodine receptor highlighted.

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Efforts to isolate and characterize the P. xylostella ryanodine receptor described.

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Molecular mechanisms of diamide resistance in P. xylostella discussed.

Diamide insecticides, such as flubendiamide and chlorantraniliprole, are a new class of insecticide with a novel mode of action, selectively activating the insect ryanodine receptor (RyR). They are particularly active against lepidopteran pests of cruciferous vegetable crops, including the diamondback moth, Plutella xylostella. However, within a relatively short period following their commercialisation, a comparatively large number of control failures have been reported in the field. In this review we summarise the current body of knowledge regarding the molecular mechanisms of diamide resistance in P. xylostella. Resistant phenotypes collected from different countries can often be linked to specific target-site mutation(s) in the ryanodine receptors’ transmembrane domain. Metabolic mechanisms of resistance have also been proposed. Rapid resistance development is probably a consequence of over-reliance on this one class of chemistry for diamondback moth control.

Characterization of the Ryanodine Receptor Gene With a Unique 3'-UTR and Alternative Splice Site From the Oriental Fruit Moth

The ryanodine receptor (RyR), the largest calcium channel protein, has been studied because of its key roles in calcium signaling in cells. Insect RyRs are molecular targets for novel diamide insecticides. The target has been focused widely because of the diamides with high activity against lepidopterous pests and safety for nontarget organisms. To study our understanding of effects of diamides on RyR, we cloned the RyR gene from the oriental fruit moth, Grapholita molesta, which is the most serious pest of stone and pome tree fruits throughout the world, to investigate the modulation of diamide insecticides on RyR mRNA expression in G. molesta (GmRyR). The full-length cDNAs of GmRyR contain a unique 3'-UTR with 625 bp and an open reading frame of 15,402 bp with a predicted protein consisting of 5,133 amino acids. GmRyR possessed a high level of overall amino acid homology with insect and vertebrate isoforms, with 77-92% and 45-47% identity, respectively. Furthermore, five alternative splice sites were identified in GmRyR. Diagnostic PCR showed that the inclusion frequency of one optional exon (f) differed between developmental stages, a finding only found in GmRyR. The lowest expression level of GmRyR mRNA was in larvae, the highest was in male pupae, and the relative expression level in male pupae was 25.67 times higher than that of in larvae. The expression level of GmRyR in the male pupae was 8.70 times higher than in female pupae, and that in male adults was 5.70 times higher than female adults.

Affinity-based fluorescence polarization assay for screening molecules acting on insect ryanodine receptors

A fluorescence polarization assay was developed for studying affinity binding of active molecules to specific binding site on insect ryanodine receptor.

Stable expression and functional characterisation of the diamondback moth ryanodine receptor G4946E variant conferring resistance to diamide insecticides

Diamides, such as flubendiamide and chlorantraniliprole, belong to a new chemical class of insecticides that act as conformation-sensitive activators of insect ryanodine receptors (RyRs). Both compounds are registered for use against lepidopteran species such as the diamondback moth, Plutella xylostella, a notorious global pest of cruciferous crops. Recently acquired resistance to diamide insecticides in this species is thought to be due to a target-site mutation conferring an amino acid substitution (G4946E), located within the trans-membrane domain of the RyR, though the exact role of this mutation has not yet been fully determined. To address this we have cloned a full-length cDNA encoding the P. xylostella RyR and established clonal Sf9 cell lines stably expressing either the wildtype RyR or the G4946E variant, in order to test the sensitivity to flubendiamide and chlorantraniliprole on the recombinant receptor. We report that the efficacy of both diamides was dramatically reduced in clonal Sf9 cells stably expressing the G4946E modified RyR, providing clear functional evidence that the G4946E RyR mutation impairs diamide insecticide binding.

Molecular characterization of a ryanodine receptor gene from Spodoptera exigua and its upregulation by chlorantraniliprole

Chlorantraniliprole is a novel diamide insecticide that targets the insect ryanodine receptor, a Ca(2+) release channel. Spodoptera exigua is a significant insect pest, and chlorantraniliprole is the most popular diamide insecticide used against this pest. To better understand the effects of diamides on RyR expression and [Ca(2+)], we isolated the SeRyR cDNA and investigated changes in SeRyR expression as a result of the application of chlorantraniliprole. The full-length cDNAs of SeRyR contain an open reading frame (ORF) of 15,357 bp with a predicted protein consisting of 5118 amino acids. SeRyR shares 77-92% identity with other insect RyR isoforms and 45-47% identity with vertebrate RyR isoforms. Furthermore, the relative expression abundances of RyR mRNA extracted from S. exigua fat body cells after 24 h of culture in 0.1, 1, 10, 100 nM, 1 µM and 100 µM of chlorantraniliprole changed 1.04-, 0.89-, 1.83-, 2.58-, 4.03- and 3.12-fold compared to blank control, respectively. The regression equation for the relative expression levels of SeRyR after 24 h as a function of the chlorantraniliprole concentration was Y = 0.6455 + 0.8188LgX, R(2) = 0.97093 for the cell line IOZCAS-Spex-II. These results outline the effects of chlorantraniliprole on the expression of SeRyR and provide a basis for the discovery of a compound that may exhibit selective insect activity.

Geographic spread, genetics and functional characteristics of ryanodine receptor based target-site resistance to diamide insecticides in diamondback moth, Plutella xylostella

Anthranilic diamides and flubendiamide belong to a new chemical class of insecticides acting as conformation sensitive activators of the insect ryanodine receptor (RyR). These compounds control a diverse range of different herbivorous insects including diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), a notorious global pest on cruciferous crops, which recently developed resistance due to target-site mutations located in the trans-membrane domain of the Plutella RyR. In the present study we further investigated the genetics and functional implications of a RyR G4946E target-site mutation we recently identified in a Philippine diamondback moth strain (Sudlon). Strain Sudlon is homozygous for the G4946E mutation and has been maintained under laboratory conditions without selection pressure for almost four years, and still exhibit stable resistance ratios of >2000-fold to all commercial diamides. Its F1 progeny resulting from reciprocal crosses with a susceptible strain (BCS-S) revealed no maternal effects and a diamide susceptible phenotype, suggesting an autosomally almost recessive mode of inheritance. Subsequent back-crosses indicate a near monogenic nature of the diamide resistance in strain Sudlon. Radioligand binding studies with Plutella thoracic microsomal membrane preparations provided direct evidence for the dramatic functional implications of the RyR G4946E mutation on both diamide specific binding and its concentration dependent modulation of [(3)H]ryanodine binding. Computational modelling based on a cryo-EM structure of rabbit RyR1 suggests that Plutella G4946E is located in trans-membrane helix S4 close to S4-S5 linker domain supposed to be involved in the modulation of the voltage sensor, and another recently described mutation, I4790M in helix S2 approx. 13 Å opposite of G4946E. Genotyping by pyrosequencing revealed the presence of the RyR G4946E mutation in larvae collected in 2013/14 in regions of ten different countries where diamide insecticides largely failed to control diamondback moth populations. Thus, our study highlights the global importance of the G4946E RyR target-site mutation, which as a mechanism on its own, confers high-level resistance to diamide insecticides in diamondback moth.

Knockdown of the ionotropic γ-aminobutyric acid receptor (GABAR) RDL gene decreases fipronil susceptibility of the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae)

Insect γ-aminobutyric acid receptors (GABARs) are important molecular targets of cyclodiene and phenylpyrazole insecticides. Previously GABARs encoding rdl (resistant to dieldrin) genes responsible for dieldrin and fipronil resistance were identified in various economically important insect pests. In this study, we cloned the open reading frame cDNA sequence of rdl gene from fipronil-susceptible and fipronil-resistant strains of Laodelphax striatellus (Lsrdl). Sequence analysis confirmed the presence of a previously identified resistance-conferring mutation. Different alternative splicing variants of Lsrdl were noted. Injection of dsLsrdl reduced the mRNA abundance of Lsrdl by 27-82%, and greatly decreased fipronil-induced mortality of individuals from both susceptible and resistant strains. These data indicate that Lsrdl encodes a functional RDL subunit that mediates susceptibility to fipronil. Additionally, temporal and spatial expression analysis showed that Lsrdl was expressed at higher levels in eggs, fifth-instar nymphs, and female adults than in third-instar and fourth-instar nymphs. Lsrdl was predominantly expressed in the heads of 2-day-old female adults. All these results provide useful background knowledge for better understanding of fipronil resistance related ionotropic GABA receptor rdl gene expressed variants and potential functional differences in insects.

Molecular cloning, characterisation and mRNA expression of the ryanodine receptor from the peach-potato aphid, Myzus persicae

The peach potato aphid, Myzus persicae, is one of the most important agricultural pests of temperate climates. It is mainly controlled through the judicious application of insecticides; however, over time, aphids have developed resistance to many insecticidal classes. The recent introduction of synthetic diamide insecticides, with a novel mode of action, potentially offers new tools to control aphid populations. These diamides act on the ryanodine receptor (RyR), a large endoplasmic calcium release channel. In this study we have cloned cDNAs encoding the complete open reading frame of the RyR from M. persicae. The open reading frame is 15,306 base pairs long and encodes a protein of 5101 amino acids. The aphid RyR shares many of the features of other insect and vertebrate RyRs, including a highly conserved transmembrane region. However, unlike the other RyRs characterised to date, the M. persicae channel does not display alternative splicing at any stage of its developmental cycle, so it cannot generate functional variants of the channel.

Functional analysis of a point mutation in the ryanodine receptor of Plutella xylostella (L.) associated with resistance to chlorantraniliprole

BACKGROUND

The diamondback moth, Plutella xylostella (L.) has developed extremely high resistance to chlorantraniliprole and other diamide insecticides in the field. A glycine to glutamic acid substitution (G4946E) in the P. xylostella ryanodine receptor (PxRyR) has been found in two resistant populations collected in Thailand and Philippines and was considered associated with the diamide insecticides resistance but no experimental evidence was provided. The present study aimed to clarify the function of the reported mutation in chlorantraniliprole resistance in P. xylostella.

RESULTS

We identified the same mutation (G4946E) in PxRyR from four field collected chlorantraniliprole resistant populations of Plutella xylostella in China. Most importantly, we found that the frequency of the G4946E mutation is significantly correlated to the chlorantraniliprole resistance ratios in P. xylostella (R(2)  = 0.82, P = 0.0003). Ligand binding assays showed that the binding affinities of the PxRyR to the chlorantraniliprole in three field resistant populations were 2.41-, 2.54- and 2.60-times lower than that in the susceptible one.

CONCLUSION

For the first time we experimentally proved that the G4946E mutation in PxRyR confers resistance to chlorantraniliprole in Plutella xylostella. These findings pave the way for the complete understanding of the mechanisms of diamide insecticides resistance in insects.

Molecular cloning and characterization of a ryanodine receptor gene in brown planthopper (BPH), Nilaparvata lugens (Stål)

BACKGROUND

Ryanodine receptors (RyRs) are a distinct class of intracellular calcium (Ca(2+)) release channel. The recent discovery of diamide insecticides has prompted studies on insect RyRs. However, information about the structure and function of insect RyRs is still limited. In this study, we isolated and characterized a full-length RyR cDNA (named NlRyR) from the brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae), a serious rice pest throughout Asia.

RESULTS

The composite NlRyR gene contains an open reading frame of 15 423 bp encoding a protein of 5140 amino acid residues, which shares high sequence identity (78-81%) with other insect homologues, except for two regions (IDR1: 4379-4732; IDR2: 1307-1529) with markedly low identity (44-48 and 38-41%, respectively). All hallmarks of the RyR proteins are conserved in the NlRyR protein, including the RyR domain as well as mannosyltransferase, IP3 R and RyR (pfam02815) (MIR) and RyR and IP3 R homology (pfam01365) (RIH) domains. Expression analysis of NlRyR revealed significant differences in mRNA expression levels among N. lugens developmental stages. Furthermore, three alternative splicing sites were identified in NlRyR, one of which forms the mutually exclusive exons A/B and is conserved in various insect species. Diagnostic PCR assays showed that the splice variant containing exon A was predominantly detected in all developmental stages.

CONCLUSION

NlRyR may play an important role in the control of developmental processes of N. lugens. Alternative splicing may generate the functional diversity of NlRyR. The results provided the basis for further structural and functional characterization of NlRyR.

RNAi suppression of the ryanodine receptor gene results in decreased susceptibility to chlorantraniliprole in Colorado potato beetle Leptinotarsa decemlineata

Leptinotarsadecemlineata is the most important pest in potato and causes serious yield loss each year. Chlorantraniliprole acts on insect ryanodine receptors (RyRs) and is among the most active compounds against L. decemlineata. Here we cloned and characterized a 15,792-bp full-length LdRyR cDNA that encoded a 5128-amino acid protein. LdRyR shares 85-92% amino acid similarities with other insect RyR homologues, and 59-61% similarities with those from Caenorhabditis elegans and Homo sapiens. All hallmarks of the RyR proteins are conserved in LdRyR. LdRyR has a MIR domain, two RIH domains, three SPRY domains, four copies of RyR domain and a RIH-associated domain in the N-terminus, and it possesses two consensus calcium ion-binding EF-hand motifs and six predicted transmembrane helices in the C-terminus. Temporal, spatial and tissue-specific expression patterns of LdRyR were evaluated. LdRyR expression level was increased constantly from egg to wandering stages, dropped in pupal stage and was increased again in the adult stage. It was widely expressed in the head, thorax and abdomen of day 3 fourth-instar larvae. Moreover, it was ubiquitously expressed in all inspected tissues including epidermis, foregut, midgut, ileum, rectum, fat body, ventral ganglia and Malpighian tubules in day 3 fourth-instar larvae. Dietary introduction of double-stranded RNA of LdRyR significantly reduced the mRNA levels of the target gene in the larvae and adults, respectively, and significantly decreased chlorantraniliprole-induced mortalities. Thus, our results suggested that LdRyR encoded a functional ryanodine receptor in L. decemlineata.

RNAi mediated knockdown of the ryanodine receptor gene decreases chlorantraniliprole susceptibility in Sogatella furcifera

The diamide insecticides activate ryanodine receptors (RyRs) to release and deplete intracellular calcium stores from the sarcoplasmic reticulum of muscles and the endoplasmic reticulum of many types of cells. They rapidly interrupt feeding of the target pest and eventually kill the pest due to starvation. However, information about the structure and function of insect RyRs is still limited. In this study, we isolated a 15,985bp full-length cDNA (named SfRyR) from Sogatella furcifera, a serious rice planthopper pest throughout Asia. SfRyR encodes a 5140-amino acid protein, which shares 78-97% sequence identities with other insect homologues, and less than 50% identities with Homo sapiens RyR1-3. All hallmarks of the RyR proteins are conserved in SfRyR. In the N-terminus, SfRyR has a MIR domain, two RIH domains, three SPRY domains, four copies of RyR repeated domain and a RIH-associated domain. In the C-terminus, SfRyR possesses two consensus calcium ion-binding EF-hand motifs, and six transmembrane helices. Temporal and spatial expression analysis showed that SfRyR was widely found in all development stages including egg, first through fifth instar nymphs, macropterous adult females and males. On day 2 fifth-instar nymphs, SfRyR was ubiquitously expressed in the head, thorax and abdomen. Dietary ingestion of dsSfRyR1 and dsSfRyR2 significantly reduced the mRNA level of SfRyR in the treated nymphs by 77.9% and 81.8% respectively, and greatly decreased chlorantraniliprole-induced mortality. Thus, our results suggested that SfRyR gene encoded a functional RyR that mediates chlorantraniliprole toxicity to S. furcifera.

De novo assembly, gene annotation, and marker discovery in stored-product pest Liposcelis entomophila (Enderlein) using transcriptome sequences

Background

As a major stored-product pest insect, Liposcelis entomophila has developed high levels of resistance to various insecticides in grain storage systems. However, the molecular mechanisms underlying resistance and environmental stress have not been characterized. To date, there is a lack of genomic information for this species. Therefore, studies aimed at profiling the L. entomophila transcriptome would provide a better understanding of the biological functions at the molecular levels.

Methodology/Principal Findings

We applied Illumina sequencing technology to sequence the transcriptome of L. entomophila. A total of 54,406,328 clean reads were obtained and that de novo assembled into 54,220 unigenes, with an average length of 571 bp. Through a similarity search, 33,404 (61.61%) unigenes were matched to known proteins in the NCBI non-redundant (Nr) protein database. These unigenes were further functionally annotated with gene ontology (GO), cluster of orthologous groups of proteins (COG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. A large number of genes potentially involved in insecticide resistance were manually curated, including 68 putative cytochrome P450 genes, 37 putative glutathione S-transferase (GST) genes, 19 putative carboxyl/cholinesterase (CCE) genes, and other 126 transcripts to contain target site sequences or encoding detoxification genes representing eight types of resistance enzymes. Furthermore, to gain insight into the molecular basis of the L. entomophila toward thermal stresses, 25 heat shock protein (Hsp) genes were identified. In addition, 1,100 SSRs and 57,757 SNPs were detected and 231 pairs of SSR primes were designed for investigating the genetic diversity in future.

Conclusions/Significance

We developed a comprehensive transcriptomic database for L. entomophila. These sequences and putative molecular markers would further promote our understanding of the molecular mechanisms underlying insecticide resistance or environmental stress, and will facilitate studies on population genetics for psocids, as well as providing useful information for functional genomic research in the future.

Molecular cloning and mRNA expression of a ryanodine receptor gene in the cotton bollworm, Helicoverpa armigera

Ryanodine receptors (RyRs) are the targets of novel diamide insecticides. The cotton bollworm, Helicoverpa armigera, is one of the most important cotton pests in the world. In this study, we report the full-length RyR cDNA sequence (named as HaRyR) of H. armigera. The 16,083-bp contiguous sequence encoded 5, 142 amino acid residues, which shares 80% and 78% overall identities with its homologues in Nilaparvata lugens (NlRyR) and Drosophila melanogaster (DmRyR), respectively. All hallmarks of RyR proteins are conserved in the HaRyR, including the GXRXGGGXGD motif conserved in the Ca(2+) release channels and four copies of RyR domain unique to RyR channels. The previously identified seven lepidopteran-specific RyR residues were also found in HaRyR (N(4977), N(4979), N(4990), L(5005), L(5036), N(5068) and T(5119)). An amino acid sequence alignment showed that the N-terminal region of HaRyR (residues 188-295) shared high sequence identity with NlRyR (94%) and DmRyR (92%), and moderate sequence identity (47-50%) with three rabbit RyR isoforms, while the short segment of the C-terminal transmembrane region of HaRyR (residues 4632-4676) exhibited moderate sequence identity with NlRyR (69%) and DmRyR (67%), and low sequence identity (19-28%) with three rabbit RyR isoforms. In addition, expression analysis of HaRyR revealed that the mRNA expression level in eggs was significantly lower than in third instar larvae, pupae and adults, and anatomical regulation of HaRyR expression was also observed with the highest expression level in head compared with thorax and abdomen. Our results lay a foundation for comprehensive structural and functional characterization of HaRyR and for understanding of the molecular mechanisms of toxicity selectivity of diamide insecticides among different species.

Molecular cloning, characterization and expression profiling of a ryanodine receptor gene in Asian corn borer, Ostrinia furnacalis (Guenée)

Ryanodine receptor (RyR) Ca(2+) release channel is the target of diamide insecticides, which show selective insecticidal activity against lepidopterous insects. To study the molecular mechanisms underlying the species-specific action of diamide insecticides, we have cloned and characterized the entire cDNA sequence of RyR from Ostrinia furnacalis (named as OfRyR). The OfRyR mRNA has an Open Reading Frame of 15324 bp nucleotides and encodes a 5108 amino acid polypeptide that displays 79-97% identity with other insects RyR proteins and shows the greatest identity with Cnaphalocrocis medinalis RyR (97%). Quantitative real-time PCR showed that the OfRyR was expressed at the lowest level in egg and the highest level in adult. The relative expression level of OfRyR in first, third and fifth-instar larva were 1.28, 1.19 and 1.99 times of that in egg. Moreover, two alternative splicing sites were identified in the OfRyR gene. One pair of mutually exclusive exons (a/b) were present in the central part of the predicted SPRY domain, and an optional exon (c) was located between the third and fourth RyR domains. Diagnostic PCR demonstrated that exons a and b existed in all developmental stages of OfRyR cDNA, but exon c was not detected in the egg cDNA. And the usage frequencies of these exons showed a significant difference between different developmental stages. These results provided the crucial basis for the functional expression of OfRyR and for the discovery of compound with potentially selective insect activtity.

Molecular and cellular analyses of a ryanodine receptor from hemocytes of Pieris rapae

Ryanodine receptors (RyRs) are located in the sarcoplasmic/endoplasmic reticulum membrane and are a distinct class of ligand-gated calcium channels controlling the release of calcium from intracellular stores. Intracellular calcium level has a definite role in innate and adaptive immune signaling. However, very few information are accessible about calcium transients of invertebrate immunocytes, especially of insect hemocytes, the effector cells of insect immunity. In this study, we show that the RyR-stimulating agent flubendiamide inhibit hemocyte spreading and phagocytosis in the cabbage white butterfly, Pieris rapae. Furthermore, we cloned a cDNA encoding a ryanodine receptor (PrRyR) from the hemocytes of P. rapae. It encodes 5107 amino acids with a predicted molecular weight of 578.2 kDa. PrRyR shares a common feature with known RyRs: a well-conserved COOH-terminal domain with two consensus calcium-binding EF-hands and six transmembrane domains, and a large hydrophilic NH2-terminal domain. In the larval stage, PrRyR was highly expressed in epidermis tissue and also expressed in hemocytes at a moderate level. In the adult stage, PrRyR was expressed at high levels in thoraces and legs, while low levels in abdomens and antennae. Quantitative real-time PCR analysis showed that its expression did not display any significant change in response to bacterial challenge. Western blot analysis and immunohistochemistry assay displayed that PrRyR was detected and presented on hemocytes. We also showed that flubendiamide, a RyR-activating insecticide, induced Ca(2+) release and thereby confirmed functional expression of the PrRyR in the hemocytes of P. rapae.

Identification of a critical region in the Drosophila ryanodine receptor that confers sensitivity to diamide insecticides

Anthranilic diamides, which include the new commercial insecticide, chlorantraniliprole, are an exciting new class of chemistry that target insect ryanodine receptors. These receptors regulate release of stored intracellular calcium and play a critical role in muscle contraction. As with insects, nematodes express ryanodine receptors and are sensitive to the plant alkaloid, ryanodine. However the plant parasitic nematode, Meloidogyne incognita, is insensitive to anthranilic diamides. Expression of a full-length Drosophila melanogaster ryanodine receptor in an insect cell line confers sensitivity to the receptor agents, caffeine and ryanodine along with nanomolar sensitivity to anthranilic diamides. Replacement of a 46 amino acid segment in a highly divergent region of the Drosophila C-terminus with that from Meloidogyne results in a functional RyR which lack sensitivity to diamide insecticides. These findings indicate that this region is critical to diamide sensitivity in insect ryanodine receptors. Furthermore, this region may contribute to our understanding of the differential selectivity diamides exhibit for insect over mammalian ryanodine receptors.

Resistance to diamide insecticides in diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) is associated with a mutation in the membrane-spanning domain of the ryanodine receptor

Diamide insecticides such as chlorantraniliprole and flubendiamide are a new class of insecticide that selectively target insect ryanodine receptors (RyR), a distinct class of homo-tetrameric calcium release channels which play a pivotal role in calcium homeostasis in numerous cell types. Resistance to these insecticides has recently been reported in the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), a global lepidopteran pest of cruciferous crops. In the present study a region of the gene encoding the proposed diamide binding site of the RyR from P. xylostella collected from the Philippines and Thailand and found to be over 200-fold resistant to both chlorantraniliprole and flubendiamide compared to susceptible strains, were amplified by RT-PCR and sequenced. Comparison of the sequence with those from several susceptible reference strains revealed non-synonymous mutations in each of the resistant strains that in both cases lead to a glycine to glutamic acid substitution (G4946E) in the protein. The independent evolution of the same amino acid substitution within a highly conserved region of the proposed diamide binding site in two geographically separated resistant strains of P. xylostella strongly suggests a causal association with diamide resistance. Furthermore we designed a pyrosequencing-based diagnostic assay for resistance monitoring purposes that can be used to detect the G4946E mutation in field-collected samples of diamondback moth. The implications of the reported findings for resistance management strategies are discussed.