IDENTIFICATION AND HORMONE INDUCTION OF PUTATIVE CHITIN SYNTHASE GENES AND SPLICE VARIANTS IN Leptinotarsa decemlineata (SAY)

Buprofezin delayed the molting of Pardosa pseudoannulata, a predatory enemy for insect pests, by suppressing chitin synthase 1 expression

Spiders, the major predatory enemies of insect pests in fields, are vulnerable to insecticides. In this study, we observed that the recommended dose of buprofezin delayed the molting of the pond wolf spider Pardosa pseudoannulata, although it had no lethal effect on the spiders. Since buprofezin is an insect chitin biosynthesis inhibitor, we identified two chitin synthase genes (PpCHS1 and PpCHS2) in P. pseudoannulata. Tissue-specific expression profiling showed that PpCHS1 was most highly expressed in cuticle. In contrast, PpCHS2 showed highest mRNA levels in the midgut and fat body. RNAi knockdown of PpCHS1 significantly delayed the molting of 12-days old spiderlings, whereas no significant effect on the molting was observed in the PpCHS2-silencing spiderlings. The expression of PpCHS1 was significantly suppressed in the spiderlings treated with buprofezin, but rescued by exogenous ecdysteroid ponasterone A (PA). Consistent with this result, the molting delay caused by buprofezin was also rescued by PA. The results revealed that buprofezin delayed the molting of spiders by suppressing PpCHS1 expression, which will benefit the protection of P. pseudoannulate and related spider species.

Regulation of juvenile hormone and ecdysteroid analogues on the development of the predatory spider, Pardosa pseudoannulata, and its regulatory mechanisms

Insecticides harm the beneficial organisms, such as predatory spiders, through direct killing or regulation of the development and reproduction. In this study, the bioassay showed that the treatment of juvenile hormone (JH) analogue fenoxycarb delayed the moulting of Pardosa pseudoannulata, a dominant predatory spider in paddy fields. In order to figure out the regulatory mechanism of fenoxycarb on the spider development, we systematically analyzed JH biosynthesis in P. pseudoannulata. All genes involved in JH biosynthesis pathway were retrieved from the genome of P. pseudoannulata, except for CYP15A1. The absence of CYP15A1 was in agreement with the identification of methyl farnesoate (MF) rather than JH III in the spider. The delayed moulting and decreased expression of JH biosynthesis-related genes in the MF-applied spiderlings supported that MF was an active JH. Fenoxycarb treatment significantly upregulated the transcriptional level of JH biosynthesis-related genes and consequently delayed the spiderling moulting. In the spider development, ecdysteroid played the opposite role, in contrast to MF, to accelerate the development, as our previous study. Here we found that the treatment of ecdysteroid analogue tebufenozide accelerated P. pseudoannulata spiderling moulting, which resulted from the expressional suppression of ecdysteroid biosynthesis-related genes. In total, the JH and ecdysteroid analogues affected the development of P. pseudoannulata by the expressional regulation of biosynthesis-related genes, which would be helpful for the evaluation of hormone analogue insecticides in environmental safety, and useful for the protection and application of P. pseudoannulate and related spider species.

RNAi for chitin synthase 1 rather than 2 causes growth delay and molting defect in Henosepilachna vigintioctopunctata

Chitin synthase (CHS) plays a critical role in chitin synthesis and excretion. In most insects, CHSs have been segregated into 1 and 2 classes. CHS1 is responsible for chitin production in the ectodermally-derived epidermal cells. CHS2 is dedicated to chitin biosynthesis in the midgut peritrophic matrix (PM). Henosepilachna vigintioctopunctata is a serious pest of Solanaceae and Cucurbitaceae plants. In this study, we identified HvCHS1 and HvCHS2. We found that HvCHS1 was abundantly transcribed in the larval tracheae and epidermis, whereas HvCHS2 was mainly expressed in the guts. Escherichia coli HT115 expressed double stranded RNAs targeting HvCHS1 and HvCHS2 (dsCHS1 and dsCHS2) were used to immerse potato foliage and the treated leaves were provided to the newly-molted fourth- and third-instar larvae. Ingestion of dsCHS1 by the fourth-instar larvae significantly diminished the target mRNA level and had slight influence on the expression of HvCHS2. In contrast, consumption of dsCHS2 significantly lowered the target mRNA level but triggered the transcription of HvCHS1. Knockdown of HvCHS1, rather than HvCHS2, arrested larval development and impaired larva-pupa-adult transition. A large proportion of HvCHS1 hypomorphs became stunting prepupae, deformed pupae or misshapen adults. Moreover, knockdown of HvCHS1 damaged gut integrity, decreased cuticle thickness, and delayed the formation of newly-generated cuticle layer during ecdysis. Furthermore, depletion of HvCHS1 inhibited the development of trachea system and thinned tracheal taenidia. Ingestion of dsCHS1 at the third-instar stage caused similar but severe negative effects. Our results demonstrated that HvCHS1 is responsible for chitin biosynthesis during ecdysis. Moreover, HvCHS1 is a potential amenable target gene and young larvae are more susceptible to dsRNA.

Crustacean cardioactive peptide as a stimulator of feeding and a regulator of ecdysis in Leptinotarsa decemlineata

Crustacean cardioactive peptide (CCAP), a highly conserved amidated neuropeptide, stimulates feeding in Drosophila melanogaster and Periplaneta americana, and regulates pupa-adult transition in Tribolium castaneum and Manduca sexta. In the present paper, we intended to address whether CCAP plays the dual roles in the Colorado potato beetle Leptinotarsa decemlineata. We found that the levels of Ldccap were high in the dissected samples of brain-corpora cardiaca-corpora allata complex and ventral nerve cord, midgut and hindgut in the final (fourth)-instar larvae. A pulse of 20-hydroxyecdysone triggered the expression of Ldccap in the central nervous system but decreased the transcription in the midgut. In contrast, juvenile hormone intensified the expression of Ldccap in the midgut. RNA interference (RNAi)-aided knockdown of Ldccap at the penultimate instar stage inhibited foliage consumption, reduced the contents of trehalose and chitin, and lowered the mRNA levels of two chitin biosynthesis genes (LdUAP1 and LdChSAb). Moreover, around 70% of the Ldccap RNAi larvae remained as prepupae, completely wrapped in the old larval exuviae, and finally died. The remaining RNAi beetles continually developed to severely-deformed adults: most having wrinkled and smaller elytra and hindwings, and shortened legs. Therefore, CCAP plays three distinct roles, stimulating feeding in foraging larval stage, regulating ecdysis, and facilitating wing expansion and appendage elongation in a coleopteran. In addition, Ldccap can be used as a potential target gene for developing novel management strategies against this coleopteran pest.

Molecular Characterization of Chitin Synthase Gene in Tetranychus cinnabarinus (Boisduval) and Its Response to Sublethal Concentrations of an Insecticide

The carmine spider mite, Tetranychus cinnabarinus (Boisduval), is one of the most important acarine pest species. At present, its control remains primarily dependent on using various chemical insecticides/acaricides in agricultural crops worldwide. To clarify the mechanism whereby T. cinnabarinus responds to insecticide exposure, we identified the chitin synthase 1 gene (TcCHS1) and then explored the gene expression levels of TcCHS1 at different developmental stages of T. cinnabarinus. We also investigated the effects of sublethal concentrations of diflubenzuron on the toxicities and survivals of T. cinnabarinus eggs and larvae as well as TcCHS1 expression levels. The full-length cDNA sequence contains an open reading frame (ORF) of 4881 nucleotides that encoded for a 1474 amino acid residues protein. The predicted TcCHS1 protein had a molecular mass of 168.35 kDa and an isoelectric point of 6.26, and its amino acid sequence contained all the signature motifs (EDR, QRRRW and TWGTR) of chitin synthases. The results of phylogenetic analyses demonstrated that the putative CHS1 amino acid sequence of T. cinnabarinus revealed high similarities with chitin synthases in other insects and mites. Additionally, at the molecular level, transcriptional analysis by real-time quantitative PCR in different developmental stages of T. cinnabarinus revealed that TcCHS1 mRNA was expressed in all stages, and highest in eggs and female adults, but lowest in deutonymphs. Furthermore, the results of toxicity bioassays indicated that diflubenzuron treatment resulted in high mortality rates in eggs and larvae of T. cinnabarinus. The mRNA expression levels of TcCHS1 from the eggs and larvae of T. cinnabarinus were up-regulated in response to sublethal concentrations of diflubenzuron exposures. Together, all these results demonstrate that diflubenzuron has ovicidal and larvicidal effects and TcCHS1 may play an important role in the growth and development of T. cinnabarinus and may disrupt the chitin biosynthesis, thereby controlling T. cinnabarinus populations.

The Developmental Transcriptome of Bagworm, Metisa plana (Lepidoptera: Psychidae) and Insights into Chitin Biosynthesis Genes

Bagworm, Metisa plana (Lepidoptera: Psychidae) is a ubiquitous insect pest in the oil palm plantations. M. plana infestation could reduce the oil palm productivity by 40% if it remains untreated over two consecutive years. Despite the urgency to tackle this issue, the genome and transcriptome of M. plana have not yet been fully elucidated. Here, we report a comprehensive transcriptome dataset from four different developmental stages of M. plana, comprising of egg, third instar larva, pupa and female adult. The de novo transcriptome assembly of the raw data had produced a total of 193,686 transcripts, which were then annotated against UniProt, NCBI non-redundant (NR) database, Gene Ontology, Cluster of Orthologous Group, and Kyoto Encyclopedia of Genes and Genomes databases. From this, 46,534 transcripts were annotated and mapped to 146 known metabolic or signalling KEGG pathways. The paper further identified 41 differentially expressed transcripts encoding seven genes in the chitin biosynthesis pathways, and their expressions across each developmental stage were further analysed. The genetic diversity of M. plana was profiled whereby there were 21,516 microsatellite sequences and 379,895 SNPs loci found in the transcriptome of M. plana. These datasets add valuable transcriptomic resources for further study of developmental gene expression, transcriptional regulations and functional gene activities involved in the development of M. plana. Identification of regulatory genes in the chitin biosynthesis pathway may also help in developing an RNAi-mediated pest control management by targeting certain pathways, and functional studies of the genes in M. plana.

Role of Chitin Deacetylase 1 in the Molting and Metamorphosis of the Cigarette Beetle Lasioderma serricorne

Chitin deacetylases (CDAs) are chitin-modifying enzymes known to play vital roles in insect metamorphosis and development. In this study, we identified and characterized a chitin deacetylase1 gene (LsCDA1) from the cigarette beetle Lasioderma serricorne. LsCDA1 contains a 1614 bp open reading frame encoding a protein of 537 amino acids that includes domain structures typical of CDAs. LsCDA1 was mainly expressed in the late larval and late pupal stages. In larval tissues, the highest level of LsCDA1 was detected in the integument. The expression of LsCDA1 was induced by 20-hydroxyecdysone (20E) in vivo, and it was significantly suppressed by knocking down the expression of ecdysteroidogenesis genes and 20E signaling genes. RNA interference (RNAi)-aided silencing of LsCDA1 in fifth-instar larvae prevented the larval–pupal molt and caused 75% larval mortality. In the late pupal stage, depletion of LsCDA1 resulted in the inhibition of pupal growth and wing abnormalities, and the expression levels of four wing development-related genes (LsDY, LsWG, LsVG, and LsAP) were dramatically decreased. Meanwhile, the chitin contents of LsCDA1 RNAi beetles were significantly reduced, and expressions of three chitin synthesis pathway genes (LsTRE1, LsUAP1, and LsCHS1) were greatly decreased. The results suggest that LsCDA1 is indispensable for larval–pupal and pupal–adult molts, and that it is a potential target for the RNAi-based control of L. serricorne.

Silencing Taiman impairs larval development in Leptinotarsa decemlineata

An exploration of novel control strategies for Leptinotarsa decemlineata is becoming more pressing given rapid evolution of insecticide resistance and rise of production loss of potato. Dietary delivery of bacterially expressed double-stranded RNA (dsRNA) is a promising alternative for management. An important first step is to uncover possible RNA-interference (RNAi)-target genes effective against both young and old larvae. Taiman (Tai) is a basic-helix-loop-helix/Per-Arnt-Sim transcription factor that is involved in the mediation of both juvenile hormone (JH) and 20-hydroxyecdysone (20E) signaling. In the present paper, we found that continuous ingestion of dsTai for three days by third (penultimate)-instar larvae caused approximately 20% larval mortality and 80% pupation failure. The larval lethality resulted from failed cuticle and tracheae shedding, which subsequently reduced foliage consumption and nutrient absorption, and depleted lipid stores. In contrast, pupation failure derived from disturbed JH and 20E signals, and disordered nutrient homeostasis including, among others, inhibition of trehalose metabolism and reduction of chitin content. Knockdown of LdTai caused similar larval lethality and pupation impairment in second and fourth (final) larval instars. Therefore, LdTai is among the most attractive candidate genes for RNAi to control L. decemlineata larvae.

RNAi targeting ecdysone receptor blocks the larva to adult development of Tetranychus cinnabarinus

RNA interference (RNAi) is a potentially useful pest control method because of its high specificity. Silencing the expression of important RNAi target genes of pests will block important biological processes and reduce pest damage. Ecdysone is a unique arthropod hormone and the ecdysone receptor (EcR) is a key factor in molting pathway. We investigated the possibility that dsRNA targeting of the EcR of Tetranychus cinnabarinus (TcEcR) could effectively block development from larvae to adults. The mRNA level of TcEcR was highest in the larva stage, and 73.1% of the mites failed to survive the larva stage when TcEcR expression was silenced. Only 11.7% of T. cinnabarinus ingesting dsRNA successfully developed into adults, while 86.7% in the control succeeded in molting across each stage. RNAi significantly increased the developmental intervals of T. cinnabarinus. Under the effects of dsRNA, development times for the larva and first nymph doubled. Phenotype of body size change and death were observed during the development of T. cinnabarinus ingesting dsRNA. These findings suggest that RNAi is a potential means for the control of T. cinnabarinus. Genes in hormone pathways such as EcR are possible RNAi targets.

Importance of Taiman in Larval-Pupal Transition in Leptinotarsa decemlineata

Insect Taiman (Tai) binds to methoprene-tolerant to form a heterodimeric complex, mediating juvenile hormone (JH) signaling to regulate larval development and to prevent premature metamorphosis. Tai also acts as a steroid receptor coactivator of 20-hydroxyecdysone (20E) receptor heterodimer, ecdysone receptor (EcR) and Ultraspiracle (USP), to control the differentiation of early germline cells and the migration of specific follicle cells and border cells in ovaries in several insect species. In holometabolous insects, however, whether Tai functions as the coactivator of EcR/USP to transduce 20E message during larval-pupal transition is unknown. In the present paper, we found that the LdTai mRNA levels were positively correlated with circulating JH and 20E titers in Leptinotarsa decemlineata; and ingestion of either JH or 20E stimulated the transcription of LdTai. Moreover, RNA interference (RNAi)-aided knockdown of LdTai at the fourth (final) instar stage repressed both JH and 20E signals, inhibited larval growth and shortened larval developing period. The knockdown caused 100% larval lethality due to failure of larval-pupal ecdysis. Under the apolysed larval cuticle, the LdTai RNAi prepupae possessed pupal thorax. In contrast, the process of tracheal ecdysis was uncompleted. Neither JH nor 20E rescued the aforementioned defectives in LdTai RNAi larvae. It appears that Tai mediates both JH and 20E signaling. Our results uncover a link between JH and 20E pathways during metamorphosis in L. decemlineata.

Genome-Wide Analysis and Hormone Regulation of Chitin Deacetylases in Silkworm

Chitin deacetylases (CDAs) are a group of enzymes involved in chitin metabolism in insects; they play a critical role in molting, pupation, and the modification of chitin. In this study, we identified several CDAs in the silkworm, Bombyx mori (BmCDA), and investigated the effect of various hormones on their expression in B. mori larvae and embryo cell lines (BmE). Eight genes encoding BmCDAs were identified in the silkworm genome. They showed different expression patterns in different tissues, and were classified into three types based on where they were expressed: the exoskeleton, digestive organs, and genital organs. Moreover, we found that some BmCDAs showed upregulated expression during the molting period, especially during the fourth molting period in larvae. We also verified that the expression of BmCDA1-6 was upregulated by treatment with 20-hydroxyecdysone not only in larvae, but also in BmE cells. Interestingly, juvenile hormone analog treatment also upregulated the expression of some BmCDAs. The overexpression of several transcription factors revealed that the POU transcription factor POUM2 may play a major role in the regulation of BmCDA expression. Finally, the silencing of BmCDA1 and BmCDA2 did not lead to abnormal phenotypes or death, but may have led to delays in silkworm pupation. These results provide important information about lepidopteran insects in terms of chitin deacetylases and the regulation of their expression.

Silencing chitin deacetylase 2 impairs larval-pupal and pupal-adult molts in Leptinotarsa decemlineata

Insect chitin deacetylases (CDAs) are carbohydrate esterases that catalyze N-deacetylation of chitin to generate chitosan, a process essential for chitin organization and compactness during the formation of extracellular chitinous structure. Here we identified two CDA2 splice variants (LdCDA2a and LdCDA2b) in Leptinotarsa decemlineata. Both splices were abundantly expressed in larval foregut, rectum, and epidermis; their levels peaked immediately before ecdysis within each instar. In vivo results revealed that the two isoforms transcriptionally responded, positively and negatively respectively, to 20-hydroxyecdysone and juvenile hormone signaling pathways. RNA interference (RNAi)-aided knockdown of the two LdCDA2 variants (hereafter LdCDA2) or LdCDA2b, rather than LdCDA2a, resulted in three negative effects. First, foliage consumption was significantly reduced, larval developing period was lengthened, and larval growth was retarded. Second, chitin contents were reduced, whereas glucose, trehalose, and glycogen contents were increased in the LdCDA2 and LdCDA2b RNAi larvae. Third, approximately 20% of LdCDA2 and LdCDA2b RNAi larvae were trapped within the exuviae and finally died. About 60% of the abnormal pupae died as pharate adults. Around 20% of the RNAi pupae emerged as deformed adults, with small size and wrinkled wings. These adults eventually died within 1 week after molting. Our results reveal that knockdown of CDA2 affects chitin accumulation. Consequently, LdCDA2 may be a potential target for control of L. decemlineata larvae.

RNA interference of chitin synthase genes inhibits chitin biosynthesis and affects larval performance in Leptinotarsa decemlineata (Say)

Dietary introduction of bacterially expressed double-stranded RNA (dsRNA) has great potential for management of Leptinotarsa decemlineata. Identification of the most attractive candidate genes for RNA interference (RNAi) is the first step. In the present paper, three complete chitin synthase cDNA sequences (LdChSAa, LdChSAb and LdChSB) were cloned. LdChSAa and LdChSAb, two splicing variants of LdChSA gene, were highly expressed in ectodermally-derived epidermal cells forming epidermis, trachea, foregut and hindgut, whereas LdChSB was mainly transcribed in midgut cells. Feeding bacterially expressed dsChSA (derived from a common fragment of LdChSAa and LdChSAb), dsChSAa, dsChSAb and dsChSB in the second- and fourth-instar larvae specifically knocked down their target mRNAs. RNAi of LdChSAa+LdChSAb and LdChSAa lowered chitin contents in whole body and integument samples, and thinned tracheal taenidia. The resulting larvae failed to ecdyse, pupate, or emerge as adults. Comparably, knockdown of LdChSAb mainly affected pupal-adult molting. The LdChSAb RNAi pupae did not completely shed the old larval exuviae, which caused failure of adult emergence. In contrast, silencing of LdChSB significantly reduced foliage consumption, decreased chitin content in midgut sample, damaged midgut peritrophic matrix, and retarded larval growth. As a result, the development of the LdChSB RNAi hypomorphs was arrested. Our data reveal that these LdChSs are among the effective candidate genes for an RNAi-based control strategy against L. decemlineata.

Physiological roles of trehalose in Leptinotarsa larvae revealed by RNA interference of trehalose-6-phosphate synthase and trehalase genes

Trehalose is proposed to serve multiple physiological roles in insects. However, its importance remains largely unconfirmed. In the present paper, we knocked down either a trehalose biosynthesis gene (trehalose-6-phosphate synthase, LdTPS) or each of three degradation genes (soluble trehalases LdTRE1a, LdTRE1b or membrane-bound LdTRE2) in Leptinotarsa decemlineata by RNA interference (RNAi). Knockdown of LdTPS decreased trehalose content and caused larval and pupal lethality. The LdTPS RNAi survivors consumed a greater amount of foliage, obtained a heavier body mass, accumulated more glycogen, lipid and proline, and had a smaller amount of chitin compared with the controls. Ingestion of trehalose but not glucose rescued the food consumption increase and larval mass rise, increased survivorship, and recovered glycogen, lipid and chitin to the normal levels. In contrast, silencing of LdTRE1a increased trehalose content and resulted in larval and pupal lethality. The surviving LdTRE1a RNAi hypomorphs fed a smaller quantity of food, had a lighter body weight, depleted lipid and several glucogenic amino acids, and contained a smaller amount of chitin. Neither trehalose nor glucose ingestion rescued these LdTRE1a RNAi defects. Silencing of LdTRE1b caused little effects. Knockdown of LdTRE2 caused larval death, increased trehalose contents in several tissues and diminished glycogen in the brain-corpora cardiaca-corpora allata complex (BCC). Feeding glucose but not trehalose partially rescued the high mortality rate and recovered glycogen content in the BCC. It seems that trehalose is involved in feeding regulation, sugar absorption, brain energy supply and chitin biosynthesis in L. decemlineata larvae.