RNA interference knockdown of aminopeptidase N genes decrease the susceptibility of Chilo suppressalis larvae to Cry1Ab/Cry1Ac and Cry1Ca-expressing transgenic rice

Strategies for mitigation of pesticides from the environment through alternative approaches: A review of recent developments and future prospects

Chemical-based peticides are having negative impacts on both the healths of human beings and plants as well. The World Health Organisation (WHO), reported that each year, >25 million individuals in poor nations are having acute pesticide poisoning cases along with 20,000 fatal injuries at global level. Normally, only ∼0.1% of the pesticide reaches to the intended targets, and rest amount is expected to come into the food chain/environment for a longer period of time. Therefore, it is crucial to reduce the amounts of pesticides present in the soil. Physical or chemical treatments are either expensive or incapable to do so. Hence, pesticide detoxification can be achieved through bioremediation/biotechnologies, including nano-based methodologies, integrated approaches etc. These are relatively affordable, efficient and environmentally sound methods. Therefore, alternate strategies like as advanced biotechnological tools like as CRISPR Cas system, RNAi and genetic engineering for development of insects and pest resistant plants which are directly involved in the development of disease- and pest-resistant plants and indirectly reduce the use of pesticides. Omics tools and multi omics approaches like metagenomics, genomics, transcriptomics, proteomics, and metabolomics for the efficient functional gene mining and their validation for bioremediation of pesticides also discussed from the literatures. Overall, the review focuses on the most recent advancements in bioremediation methods to lessen the effects of pesticides along with the role of microorganisms in pesticides elimination. Further, pesticide detection is also a big challenge which can be done by using HPLC, GC, SERS, and LSPR ELISA etc. which have also been described in this review.

The CsmiR1579-CsKr-h1 module mediates rice stem borer development and reproduction: An effective target for transgenic insect-resistant rice

The rice stem borer (RSB, Chilo suppressalis) is a significant agricultural pest that mainly depends on chemical control. However, it has grown to varied degrees of pesticide resistance, which poses a severe threat to rice production and emphasizes the need for safer, more efficient alternative pest management strategies. Here, in vitro and in vivo experiments analyses reveal miR-1579 binds to the critical transcription factor Krüppel homologue 1 (Kr-h1) and negatively regulates its expression. Overexpression of miR-1579 in larvae with significantly lower levels of Kr-h1 was associated with a decline in larval growth and survival. Furthermore, in female pupae, miR-1579 overexpression led to abnormalities in ovarian development, suggesting that targeting miR-1579 could be a potential management strategy against C. suppressalis. Therefore, we generated transgenic rice expressing miR-1579 and screened three lines that had a single copy of highly abundant mature miR-1579 transcripts. Expectedly, fed with transgenic miR-1579 rice lines were significantly lower survival rates in larvae and high levels of resistance to damage caused by C. suppressalis infestation. These findings suggest that miRNA-mediated RNAi could provide an effective and species-specific strategy for C. suppressalis control.

Analysis of the Effect of Plutella xylostella Polycalin and ABCC2 Transporter on Cry1Ac Susceptibility by CRISPR/Cas9-Mediated Knockout

Many insects, including the Plutella xylostella (L.), have developed varying degrees of resistance to many insecticides, including Bacillus thuringiensis (Bt) toxins, the bioinsecticides derived from Bt. The polycalin protein is one of the potential receptors for Bt toxins, and previous studies have confirmed that the Cry1Ac toxin can bind to the polycalin protein of P. xylostella, but whether polycalin is associated with the resistance of Bt toxins remains controversial. In this study, we compared the midgut of larvae from Cry1Ac-susceptible and -resistant strains, and found that the expression of the Pxpolycalin gene was largely reduced in the midgut of the resistant strains. Moreover, the spatial and temporal expression patterns of Pxpolycalin showed that it was mainly expressed in the larval stage and midgut tissue. However, genetic linkage experiments showed that the Pxpolycalin gene and its transcript level were not linked to Cry1Ac resistance, whereas both the PxABCC2 gene and its transcript levels were linked to Cry1Ac resistance. The larvae fed on a diet containing the Cry1Ac toxin showed no significant change in the expression of the Pxpolycalin gene in a short term. Furthermore, the knockout of polycalin and ATP-binding cassette transporter subfamily C2 (ABCC2) genes separately by CRISPR/Cas9 technology resulted in resistance to decreased susceptibility to Cry1Ac toxin. Our results provide new insights into the potential role of polycalin and ABCC2 proteins in Cry1Ac resistance and the mechanism underlying the resistance of insects to Bt toxins.

Functional characterization of developmentally critical genes in the white-backed planthopper: Efficacy of nanoparticle-based dsRNA sprays for pest control

BACKGROUND

Epidermal growth factor receptor (EGFR), zinc finger homeodomain-2 (zfh-2), Abdominal-A (Abd-A), and Abdominal-B (Abd-B) regulate the growth and development of the insect abdomen. However, their potential roles in pest control have not been fully assessed. The development of insecticide resistance to multiple chemistries in the white-backed planthopper (WBPH), a major pest of rice, has prompted interest in novel pest control approaches that are ecologically friendly. Although pest management approaches based on double-stranded RNA (dsRNA)-mediated RNA interference (RNAi) have potential, their susceptibility to degradation limits large-scale field applications. These limitations, however, can be overcome with nanoparticle-dsRNA complexes that have greater environmental stability and improved cellular uptake.

RESULTS

In this study, at 5 days post-injection, transcripts for the four gene targets were reduced relative to controls and all of the experimental groups exhibited significant phenotypic defects and increased mortality. To evaluate the potential of these gene targets for field applications, a nanocarrier-dsRNA spray delivery system was assessed for RNAi efficacy. At 11 days post-spray, significant phenotypic defects and increased mortality were observed in all experimental groups.

CONCLUSION

Taken together, the results confirm the suitability of the target genes (SfEGFR, Sfzfh-2, SfAbd-A, and SfAbd-B) for pest management and demonstrate the efficacy of the nanocarrier spray system for inducing RNAi-mediated knockdown. As such, the study lays the foundation for the further development and optimization of this technology for large-scale field applications. © 2022 Society of Chemical Industry.

FAR knockout significantly inhibits Chilo suppressalis survival and transgene expression of double-stranded FAR in rice exhibits strong pest resistance

Chilo suppressalis is one of the most prevalent and damaging rice pests, causing significant economic losses each year. Chemical control is currently the primary method of controlling C. suppressalis. However, the indiscriminate use of chemical insecticides increases pest resistance, pollutes the environment and poses a significant health threat to humans and livestock, highlighting the need to find safer, more pest-specific and more effective alternatives to pest control. Plant-mediated RNA interference (RNAi) is a promising agricultural pest control method that is highly pest-specific and has less of an impact on the environment. Using multi-sgRNAs/Cas9 technology to delete Fatty acyl-CoA reductase (FAR) of C. suppressalis in the G0 generation, we show that downregulating FAR transcription may significantly increase the mortality rate and darken the epidermis of C. suppressalis compared with the control. Subsequently, we developed dsFAR transgenic rice lines using Agrobacterium-mediated genetic transformation and then screened three strains expressing dsFAR at high levels using transcriptional level analysis. Using transgenic rice stems, a laboratory feeding bioassay indicated that at least one line (L#10) displayed a particularly high level of insect resistance, with an insect mortality rate of more than 80%. In the field trials, dsFAR transgenic rice displayed high levels of resistance to C. suppressalis damage. Collectively, these results suggest the potential of a new environment-friendly, species-specific strategy for rice pest management.

RNA interference knockdown of insulin receptor inhibits ovarian development in Chilo suppressalis

BACKGROUND

The nutritional signaling pathway regulates an insect's size, development, and lifespan, as well as playing a vital role in reproduction. The insulin/insulin-like growth factor signaling (IIS) pathway plays a key role in the nutrition signaling pathway. As an integral component of the IIS pathway, insulin receptor (InR), a receptor tyrosine kinase, plays a role in the insulin pathway by controlling reproduction in many insect species. However, the precise molecular function of InR in non-model insect reproduction is poorly understood.

METHODS

In our study, Chilo suppressalis, a well-known rice pest, was used as a molecular system to determine the role of InR in insect reproduction. Sequencing the InR gene of C. suppressalis, comparing the amino acid sequence-specific structure, and constructing a phylogenetic tree revealed that this gene has four main domains: ligand binding L domain, Furin-like region, fibronectin type III domains, and Tyrosine kinase catalytic domain, which were all highly conserved in insects.

RESULTS

By characterizing the spatiotemporal expression profile of InR in different developmental stages and tissues, we found that InR gene expression was highest on the 3-day old in female pupae, 6th instar larvae, and fat body on the 6-day old in female pupae. InR gene expression may promote the molting and pupation of larvae and play a role in reproduction in the fat body. Furthermore, the RNA interference knockdown of InR dramatically reduced yolk deposition and blocked oocyte maturation. After suppression of InR, the expression of several other genes fluctuated to varying degrees.

CONCLUSION

In conclusion, InR is vital to reproduction and is expected to become a new target for pest management.

Molecular characterization and functional analysis of Cry toxin receptor-like genes from the model insect Galleria mellonella

Crystal (Cry) toxins produced from the soil bacterium, Bacillus thuringiensis (Bt), have gained worldwide attention for long due to their insecticidal potential. A number of receptor proteins located on the epithelial cells of the larval midgut were shown to be crucial for Cry intoxication in different insect pests belonging to order Lepidoptera, Diptera and Coleoptera. A beehive pest, Galleria mellonella, serves as an excellent insect model for biochemical research. However, information on the Cry receptor-like genes in G. mellonella is limited. In the present study, the full-length sequences of four putative Cry receptor genes (ABC transporter, alkaline phosphatase, aminopeptidase N and cadherin) were cloned from G. mellonella. All these receptor genes were substantially upregulated in the midgut tissue of fourth-instar G. mellonella larvae upon early exposure (6 h) to a sub-lethal dose of Cry1AcF toxin. Oral and independent delivery of bacterially-expressed dsRNAs corresponding to four receptor genes in G. mellonella suppressed the transcription of target receptors which in turn significantly reduced the larval sensitivity to Cry1AcF toxin. As the laboratory populations of G. mellonella develop Bt resistance in a relatively short time, molecular characterization of Cry receptor genes in G. mellonella performed in the present study may provide some useful information for future research related to the genetic basis of Bt resistance in the model insect.

Biotechnological Approaches for Host Plant Resistance to Insect Pests

Annually, the cost of insect pest control in agriculture crosses billions of dollars around the world. Until recently, broad-spectrum synthetic pesticides were considered as the most effective means of pest control in agriculture. However, over the years, the overreliance on pesticides has caused adverse effects on beneficial insects, human health and the environment, and has led to the development of pesticide resistant insects. There is a critical need for the development of alternative pest management strategies aiming for minimum use of pesticides and conservation of natural enemies for maintaining the ecological balance of the environment. Host plant resistance plays a vital role in integrated pest management but the development of insect-resistant varieties through conventional ways of host plant resistance takes time, and is challenging as it involves many quantitative traits positioned at various loci. Biotechnological approaches such as gene editing, gene transformation, marker-assisted selection etc. in this direction have recently opened up a new era of insect control options. These could contribute towards about exploring a much wider array of novel insecticidal genes that would otherwise be beyond the scope of conventional breeding. Biotechnological interventions can alter the gene expression level and pattern as well as the development of transgenic varieties with insecticidal genes and can improve pest management by providing access to novel molecules. This review will discuss the emerging biotechnological tools available to develop insect-resistant engineered crop genotypes with a better ability to resist the attack of insect pests.

Downregulation of the CsABCC2 gene is associated with Cry1C resistance in the striped stem borer Chilo suppressalis

Chilo suppressalis is a major target pest of transgenic rice expressing the Bacillus thuringiensis (Bt) Cry1C toxin in China. The evolution of resistance of this pest is a major threat to Bt rice. Since Bt functions by binding to receptors in the midgut (MG) of target insects, identification of Bt functional receptors in C. suppressalis is crucial for evaluating potential resistance mechanisms and developing effective management strategies. ATP-binding cassette (ABC) transporters have been vastly reported to interact with Cry1A toxins, as receptors and their mutations cause insect Bt resistance. However, the role of ABC transporters in Cry1C resistance to C. suppressalis remains unknown. Here, we measured CsABCC2 expression in C. suppressalis Cry1C-resistant (Cry1C-R) and Cry1C-susceptible strains (selected in the laboratory) via quantitative real-time PCR (qRT-PCR); the transcript level of CsABCC2 in the Cry1C-R strain was significantly lower than that in the Cry1C-susceptible strain. Furthermore, silencing CsABCC2 in C. suppressalis via RNA interference (RNAi) significantly decreased Cry1C susceptibility. Overall, CsABCC2 participates in Cry1C mode of action, and reduced expression of CsABCC2 is functionally associated with Cry1C resistance in C. suppressalis.

A Novel Reference for Bt-Resistance Mechanism in Plutella xylostella Based on Analysis of the Midgut Transcriptomes

Simple Summary

Plutella xylostella is a very serious pest to cruciferous vegetables. At present, the control methods used are mainly traditional insecticides and the cultivation of Bt crops. However, with the long-term and large-scale use of insecticides, the diamondback moth has developed strong resistance to many kinds of insecticides and Bt crops. The Cry1S1000 strain of P. xylostella used here is a strain with more than 8000 times resistance to Bt Cry1Ac protoxin. In this paper, we used transcriptome sequencing to determine the midgut transcriptome of the G88-susceptible strain, Cry1S1000-resistant strain and its corresponding toxin-induced strains to find more genes related to Bt resistance. Our results will provide a reference for optimizing the control strategy of diamondback moth resistance and improving the control efficiency of biopesticides and Bt crops.

Abstract

The diamondback moth, Plutella xylostella, is a lepidopteran insect that mainly harms cruciferous vegetables, with strong resistance to a variety of agrochemicals, including Bacillus thuringiensis (Bt) toxins. This study intended to screen genes associated with Bt resistance in P. xylostella by comparing the midgut transcriptome of Cry1Ac-susceptible and -resistant strains together with two toxin-treated strains 24 h before sampling. A total of 12 samples were analyzed by BGISEQ-500, and each sample obtained an average of 6.35 Gb data. Additionally, 3284 differentially expressed genes (DEGs) were identified in susceptible and resistant strains. Among them, five DEGs for cadherin, 14 for aminopeptidase, zero for alkaline phosphatase, 14 for ATP binding cassette transport, and five heat shock proteins were potentially involved in resistance to Cry1Ac in P. xylostella. Furthermore, DEGs associated with “binding”, “catalytic activity”, “cellular process”, “metabolic process”, and “cellular anatomical entity” were more likely to be responsible for resistance to Bt toxin. Thus, together with other omics data, our results will offer prospective genes for the development of Bt resistance, thereby providing a brand new reference for revealing the resistance mechanism to Bt of P. xylostella.

Cloning and RNAi-mediated three lethal genes that can be potentially used for Chilo suppressalis (Lepidoptera: Crambidae) management

RNA interference (RNAi) has gained attention in recent years as a viable pest control strategy. Here, RNAi assays were performed to screen the potential functionality of genes in Chilo suppressalis, a serious pest of rice, and to determine their potential for developing a highly targeted molecular control approach. Potential homologs of NADH dehydrogenase (ND), glycerol 3-phosphate dehydrogenase (GPDH) and male specific lethal 3 (MSL3) were cloned from C. suppressalis, and their spatiotemporal gene expression evaluated. The expression of all three genes was higher in the pupal and adult stages than the larval stages and largely higher in the larval head compared to other tissues. Newly hatched larvae exhibited high mortalities and suppressed growth when fed bacteria producing double-stranded RNAs (dsRNAs) corresponding to the three target genes. This study provides insights into the function of ND, GPDH and MSL3 during C. suppressalis larval development and suggests that all may be candidate gene targets for C. suppressalis pest management.

Differences in midgut transcriptomes between resistant and susceptible strains of Chilo suppressalis to Cry1C toxin

Background

Chilo suppressalis is a widespread rice pest that poses a major threat to food security in China. This pest can develop resistance to Cry toxins from Bacillus thuringiensis (Bt), threatening the sustainable use of insect-resistant transgenic Bt rice. However, the molecular basis for the resistance mechanisms of C. suppressalis to Cry1C toxin remains unknown. This study aimed to identify genes associated with the mechanism of Cry1C resistance in C. suppressalis by comparing the midgut transcriptomic responses of resistant and susceptible C. suppressalis strains to Cry1C toxin and to provide information for insect resistance management.

Results

A C. suppressalis midgut transcriptome of 139,206 unigenes was de novo assembled from 373 million Illumina HiSeq and Roche 454 clean reads. Comparative analysis identified 5328 significantly differentially expressed unigenes (DEGs) between C. suppressalis Cry1C-resistant and -susceptible strains. DEGs encoding Bt Cry toxin receptors, aminopeptidase-P like protein, the ABC subfamily and alkaline phosphatase were downregulated, suggesting an association with C. suppressalis Cry1C resistance. Additionally, Cry1C resistance in C. suppressalis may be related to changes in the transcription levels of enzymes involved in hydrolysis, digestive, catalytic and detoxification processes.

Conclusion

Our study identified genes potentially involved in Cry1C resistance in C. suppressalis by comparative transcriptome analysis. The assembled and annotated transcriptome data provide valuable genomic resources for further study of the molecular mechanisms of C. suppressalis resistance to Cry toxins.

Knockout of three aminopeptidase N genes does not affect susceptibility of Helicoverpa armigera larvae to Bacillus thuringiensis Cry1A and Cry2A toxins

Bacillus thuringiensis (Bt) insecticidal toxins have been globally utilized for control of agricultural insects through spraying or transgenic crops. Binding of Bt toxins to special receptors on midgut epithelial cells of target insects is a key step in the mode of action. Previous studies suggested aminopeptidase N1 (APN1) as a receptor or putative receptor in several lepidopteran insects including Helicoverpa armigera through evidence from RNA interefence-based gene silencing approaches. In the current study we tested the role of APNs in the mode of action of Bt toxins using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated gene knockout. Three APN genes (HaAPN1, HaAPN2 and HaAPN5) were individually knocked out in a susceptible strain (SCD) of H. armigera to establish three homozygous knockout strains. Qualitative in vitro binding studies indicated binding of Cry1Ac or Cry2Ab to midgut brush border membrane vesicles was not obviously affected by APN knockout. Bioassay results showed that none of the three knockouts had significant changes in susceptibility to Cry1A or Cry2A toxins when compared with the SCD strain. This suggests that the three HaAPN genes we tested may not be critical in the mode of action of Cry1A or Cry2A toxins in H. armigera.

RNA interference-mediated knockdown of the transcription factor Krüppel homologue 1 suppresses vitellogenesis in Chilo suppressalis

Vitellogenesis in holometabolous insects involves the production and secretion of vitellogenin (Vg) and other yolk protein precursors in developing oocyte by the fat body, all of which is predominantly orchestrated by juvenile hormone (JH). Krüppel homologue 1 (Kr-h1) is a zinc finger transcription factor that has been demonstrated to be a JH-early inducible gene and to contribute to reproduction. However, the exact molecular function of Kr-h1 in insect reproduction is poorly understood. In the current study, we used the notorious pest Chilo suppressalis as a model system to investigate the role of Kr-h1 in female reproduction. Cloning and sequencing C. suppressalis Kr-h1 revealed that it shares high identity with its homologues from other lepidopteran insects. Moreover, RNA interference-mediated knockdown of CsKr-h1 substantially reduced the transcription of Vg in the fat body, dramatically decreased yolk protein deposition and also impaired oocyte maturation and ovarian development, indicating that Kr-h1 is indispensable for normal vitellogenesis in C. suppressalis. Based on these results, we conclude that Kr-h1 is crucial to reproduction in insects and that targeting this gene could potentially be a new way to suppress rice pests.

Knockdown of the aminopeptidase N genes decreases susceptibility of Chilo suppressalis larvae to Cry1Ab/Cry1Ac and Cry1Ca

Bacillus thuringiensis (Bt) insecticide is currently the most widely used bioinsecticide. Bt expressing cry genes are some of the most successful foreign-genome-inserting genes used in transgenic insect-resistant crop development. Cry toxins are resistant to lepidopteran pests, such as Chilo suppressalis, a major insect pest of rice worldwide. Since Cry toxins exert their activity by binding to specific receptors in the midgut of target insects, identification of functional Cry toxin receptors in the midgut of C. suppressalis larvae is crucial to evaluate potential resistance mechanisms and develop effective strategies for inhibiting insect resistance. In this study, we isolated two aminopeptidase N genes (APN6 and APN8) from C. suppressalis and determined that they were expressed in the foregut. APN6 was highly expressed at the fourth instar, and APN8 was highly expressed in adult and pupa. Knockdown of CsAPN6 and CsAPN8 by RNA interference resulted in significantly decreased susceptibility of larvae to Bt rice varieties TT51 (expressing cry1Ac/cry1Ab fusion genes) and T1C-19 (expressing cry1Ca), but not T2A-1 (expressing cry2Aa). These findings suggest that both APN6 and APN8 are involved in the toxicity of Cry1Ac/Cry1Ab and Cry1Ca toxins.

The midgut V-ATPase subunit A gene is associated with toxicity to crystal 2Aa and crystal 1Ca-expressing transgenic rice in Chilo suppressalis

Insecticidal crystal (Cry) proteins produced by the bacterium Bacillus thuringiensis (Bt) are toxic to a diverse range of insects. Transgenic rice expressing Cry1A, Cry2A and Cry1C toxins have been developed that are lethal to Chilo suppressalis, a devastating insect pest of rice in China. Identifying the mechanisms underlying the interactions of Cry toxins with susceptible hosts will improve both our understanding of Cry protein toxicology and long-term efficacy of Bt crops. In this study, we tested the hypothesis that V-ATPase subunit A contributes to the action of Cry1Ab/1Ac, Cry2Aa and Cry1Ca toxins in C. suppressalis. The full-length V-ATPase subunit A transcript was initially cloned from the C. suppressalis larval midgut and then used to generate double-stranded RNA (dsRNA)-producing bacteria. Toxicity assays using transgenic rice lines TT51 (Cry1Ab and Cry1Ac fusion genes), T2A-1 (Cry2Aa), and T1C-19 (Cry1Ca) in conjunction with V-ATPase subunit A dsRNA-treated C. suppressalis larvae revealed significantly reduced larval susceptibility to T2A-1 and T1C-19 transgenic rice, but not to TT51 rice. These results suggest that the V-ATPase subunit A plays a crucial role in mediating Cry2Aa and Cry1Ca toxicity in C. suppressalis. These findings will have significant implications on the development of future resistance management tools.

Molecular cloning, characterization, and expression profiling analysis of Cry toxin receptor genes from sugarcane shoot borer Chilo infuscatellus (Snellen)

The sugarcane shoot borer Chilo infuscatellus (Snellen) is known for causing severe damage to sugarcane yield in China. Methods have been developed to control this pest, including Cry toxin pesticide and transgenic Bt plants. In order to investigate the molecular mechanism of the Cry toxin binding process and provide a basis for understanding the insect's resistance mechanism, we used a high throughput sequencing platform to perform a de novo transcriptome assembly across different larval developmental stages and analyzed Cry toxin receptors based on our assembled transcripts. We cloned twelve Cry toxin receptor genes including 1 cadherin (Cad), 7 aminopeptidase-Ns (APNs), 3 alkaline phosphatases (ALPs), and 1 ATP-binding cassette transporter subfamily C2 (ABCC2), and three of them with full length. The sublethal dosage of Cry1Ac toxin was applied to sugarcane shoot borer and identified some Cry toxin receptor genes that were significantly induced after 48 h of exposure. Furthermore, quantitative RT-PCR was conducted to detect the expression profiles of these genes. Our transcriptome sequence data provided a valuable molecular resource for further study and the identified Cry toxin receptor data gave insights for improved research into the mechanism of Bt resistance.

Divergence in larval diapause induction between the rice and water-oat populations of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

Differences in diapause traits can result in the seasonal reproductive isolation of host plant-associated insect populations and thereby facilitate the population divergence. The striped stem borer, Chilo suppressalis, has two host plant-associated populations: rice population and water-oat population. Several studies have found evidence that seasonal reproductive isolation between these populations is at least partially due to interpopulation differences in diapause. However, there still lack unambiguous evidence comparing characteristics of diapause induction for both populations. We compared the photoperiodic response and the age of peak photoperiod sensitivity of these populations and used RNA-Seq to compare the molecular response of diapause induction between populations. The photoperiodic response of the two populations differed at 25 °C; the critical night length of larvae from the rice population was 11 h and 20 min, whereas no obvious critical night length was in those from the water-oat population. In rice population, larvae were most sensitive to photoperiod at 9-12 days of age, whereas in water-oat population, larvae were the most sensitive to photoperiod at 9-10 days of age. The RNA-Seq results indicated that there were several differences in the molecular response of diapause induction and small overlap in differentially expressed genes (DEGs) between populations. Furthermore, GO analysis indicated that both rice and water-oat population's DEGs were significantly enriched in heme and iron binding. Besides, water-oat population's DEGs were significantly enriched in metabolizing nutrients but rice population's DEGs do not. Thus, our results described differences in diapause induction between rice and water-oat populations of C. suppressalis which could affect the timing of diapause and thereby contribute to the seasonal reproductive isolation of these host plant-associated populations. In conclusion, this work suggests that difference in diapause induction could promote the population divergence in insects associated with different host plants.

Downregulation of Chilo suppressalis alkaline phosphatase genes associated with resistance to three transgenic Bacillus thuringiensis rice lines

Insecticidal crystal (Cry) proteins produced by the bacterium Bacillus thuringiensis are highly toxic to lepidopteran pests. Strains of transgenic rice expressing cry genes have been developed that are resistant to rice pests. Understanding the mode of action of Cry toxins in rice pests will improve our ability to use them effectively as insecticides. In this study, we tested the hypothesis that alkaline phosphatases (ALPs) are involved in Cry1A, Cry2Aa and Cry1Ca toxicity in Chilo suppressalis, an important insect pest of rice crops in China. We first cloned three novel C. suppressalis alps (Csalps) from the larval midgut of C. suppressalis. RNA interference knockdown of six different Csalp genes (Csalp1, Csalp2, Csalp3, Csalp4, Csalp5 and Csalp6) showed that knockdown of three of these, Csalp1, Csalp2 and Csalp4, reduced larval mortality to the transgenic rice strain TT51, which expresses a fusion protein of Cry1Ab and Cry1Ac, whereas suppression of Csalp1, Csalp2, Csalp3, Csalp4 and Csalp6 transcripts decreased the susceptibility of larvae to the transgenic rice strain T2A-1, which expresses cry2Aa. Moreover, downregulation of Csalp1, Csalp2, Csalp3, Csalp4 and Csalp5 transcripts conferred significant tolerance to the transgenic rice strain T1C-19, which expresses cry1Ca. These results suggest that these ALPs play a key role in the toxicity of Cry1A, Cry2A and Cry1C to C. suppressalis.