Functional characterization of Pol III U6 promoters for gene knockdown and knockout in Plutella xylostella

Functional characteristics of Dicer genes in Plutella xylostella

BACKGROUND

Dicer is an endonuclease that belongs to the RNase III family and can specifically recognize and cleave double-stranded RNA (dsRNA). In most insects, there are two Dicer genes, Dicer-1 (Dcr-1) and Dicer-2 (Dcr-2), which are involved in the micro-RNA and small-interfering RNA pathways in many species, respectively. The function of Dicer in Plutella xylostella remains unknown.

RESULTS

The full-length open reading frames of P. xylostella Dicer-1 (PxDcr-1) and Dicer-2 (PxDcr-2) were cloned and sequenced. Dcr-1 and Dcr-2 proteins shared similar structural domains with the Dicer-Partner Binding Domain (Dicer-PBD) and the double-strand RNA binding domain (dsRBD) present only in Dcr-1. The phylogenetic trees showed that lepidopteran Dcr-1s or Dcr-2s clustered in one branch, with PxDcr-1 in the basal position and PxDcr-2 closest to Plodia interpunctella Dicer. Two homozygous knockout lines, ΔPxDcr-1 and ΔPxDcr-2, were obtained by using the CRISPR-Cas9 technique. The ΔPxDcr-1 strain exhibited a high mortality rate, a low eclosion rate, a low egg-laying rate, a low hatching rate, and a shriveled ovariole without mature eggs. The ΔPxDcr-2 strain showed no significant difference from the wild-type in terms of survival, development and reproduction, but the RNA interference (RNAi) efficiency caused by dsRNA was significantly reduced.

CONCLUSION

These findings demonstrate the involvement of PxDcr-1 in the development and reproduction of P. xylostella, specifically in the formation of ovarioles and eggs, and PxDcr-2 is indispensable for RNAi. These findings shed light on the function of Dcr-1 and Dcr-2 in Lepidoptera. © 2023 Society of Chemical Industry.

Exploring the role of the ovary-serine protease gene in the female fertility of the diamondback moth using CRISPR/Cas9

BACKGROUND

Oogenesis is a complex pathway necessary for proper female reproduction in insects. Ovary-serine protease (Osp) is a homologous gene of serine protease Nudel (SpNudel) and plays an essential role in the oogenesis and ovary development of Drosophila melanogaster. However, the function of Osp is not determined in Plutella xylostella, a highly destructive pest of cruciferous crops.

RESULTS

The PxOsp gene comprises a 5883-bp open-reading frame that encodes a protein consisting of 1994 amino acids, which contain four conserved domains. PxOsp exhibited a high relative expression in adult females with a specific expression in the ovary. Through the utilization of CRISPR/Cas9 technology, homozygous mutants of PxOsp were generated. These homozygous mutant females produced fewer eggs (average of 56 eggs/female) than wild-type (WT) females (average of 97 eggs/female) when crossed with WT males, and these eggs failed to hatch. Conversely, mutant males produced normal progeny when crossed with WT females. The ovarioles in homozygous mutant females were significantly shorter (5.02 mm in length) and contained fewer eggs (average of 3 eggs/ovariole) than WT ovarioles (8.09 mm in length with an average of 8 eggs/ovariole). Moreover, eggs laid by homozygous mutant females were fragile, with irregular shapes, and were unable to maintain structural integrity due to eggshell ruptures. However, no significant differences were observed between WT and mutant individuals regarding developmental duration, pupal weight, and mating behavior.

CONCLUSION

Our study suggesteds that PxOsp plays a vital role in female reproduction, particularly in ovary and egg development. Disrupting PxOsp results in recessive female sterility while leaving the male reproductive capability unaffected. This report represents the first study of a haplosufficient gene responsible for female fertility in lepidopteran insects. Additionally, these findings emphasize PxOsp as a potential target for genetically-based pest management of P. xylostella. © 2024 Society of Chemical Industry.

Identification and functional analysis of Cochliomyia hominivorax U6 gene promoters

The New World screwworm, Cochliomyia hominivorax, is an obligate parasite, which is a major pest of livestock. While the sterile insect technique was used very successfully to eradicate C. hominivorax from North and Central America, more cost-effective genetic methods will likely be needed in South America. The recent development of CRISPR/Cas9-based genetic approaches, such as homing gene drive, could provide a very efficient means for the suppression of C. hominivorax populations. One component of a drive system is the guide RNA(s) driven by a U6 gene promoter. Here, we have developed an in vivo assay to evaluate the activity of the promoters from seven C. hominivorax U6 genes. Embryos from the related blowfly Lucilia cuprina were injected with plasmid DNA containing a U6-promoter-guide RNA construct and a source of Cas9, either protein or plasmid DNA. Activity was assessed by the number of site-specific mutations in the targeted gene in hatched larvae. One promoter, Chom U6_b, showed the highest activity. These U6 gene promoters could be used to build CRISPR/Cas9-based genetic systems for the control of C. hominivorax.

Glucose Oxidase of a Crucifer-Specialized Insect: A Potential Role in Suppressing Plant Defense via Modulating Antagonistic Plant Hormones

Glucose oxidase (GOX) is a representative compound found in most insect saliva that can suppress plant-defensive responses. However, little is known about the origin and role of GOX in the crucifer-specialized pest Plutella xylostella. In this study, we showed obvious regurgitation from the larval gut of P. xylostella and identified abundant peptides highly similar to known GOX. Three PxGOX genes were verified with PxGOX2 preferentially expressed in the gut. The heterologously expressed PxGOX2 confirmed its function to be a GOX, and it was detected in plant wounds together with the gut regurgitant. Further experiments revealed that PxGOX2 functioned as an effector and may suppress defensive responses in plant through the production of H2O2, which modulates levels of antagonistic salicylic acid and jasmonic acid. However, excessive H2O2 in the host plant may be neutralized by peroxidase, thus forming defensive feedback. Our findings provided new insights into understanding the GOX-mediated insect-plant interactions.

Establishment of highly efficient transgenic system for black soldier fly (Hermetia illucens)

The black soldier fly (BSF), Hermetia illucens, is a promising insect for mitigating solid waste problems as its larvae are able to bioconvert organic waste into valuable biomass. We recently reported a high-quality genome assembly of the BSF; analysis of this genome sequence will further the understanding of insect biology and identify genes that can be manipulated to improve efficiency of bioconversion. To enable genetic manipulation of the BSF, we have established the first transgenic methods for this economically important insect. We cloned and identified the ubiquitous actin5C promoter (Hiactin5C-p3k) and 3 endogenous U6 promoters (HiU6:1, HiU6:2, and HiU6:3). The Hiactin5C promoter was used to drive expression of a hyperactive variant of the piggyBac transposase, which exhibited up to 6-fold improvement in transformation rate when compared to the wild-type transposase. Furthermore, we evaluated the 3 HiU6 promoters using this transgenic system. HiU6:1 and HiU6:2 promoters provided the highest knockdown efficiency with RNAi and are thus promising candidates for future Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) development. Overall, our findings provide valuable genetic engineering toolkits for basic research and genetic manipulation of the BSF.

New Insights into the Plutella xylostella Detoxifying Enzymes: Sequence Evolution, Structural Similarity, Functional Diversity, and Application Prospects of Glucosinolate Sulfatases

Brassica plants have glucosinolate (GLs)-myrosinase defense mechanisms to deter herbivores. However, Plutella xylostella specifically feeds on Brassica vegetables. The larvae possess three glucosinolate sulfatases (PxGSS1-3) that compete with plant myrosinase for shared GLs substrates and produce nontoxic desulfo-GLs (deGLs). Although PxGSSs are considered potential targets for pest control, the lack of a comprehensive review has hindered the development of PxGSSs-targeted pest control methods. Recent advances in integrative multi-omics analysis, substrate-enzyme kinetics, and molecular biological techniques have elucidated the evolutionary origin and functional diversity of these three PxGSSs. This review summarizes research progress on PxGSSs over the past 20 years, covering sequence properties, evolution, protein modification, enzyme activity, structural variation, substrate specificity, and interaction scenarios based on functional diversity. Finally, we discussed the potential applications of PxGSSs-targeted pest control technologies driven by artificial intelligence, including CRISPR/Cas9-mediated gene drive, transgenic plant-mediated RNAi, small-molecule inhibitors, and peptide inhibitors. These technologies have the potential to overcome current management challenges and promote the development and field application of PxGSSs-targeted pest control.

RNA Pol III promoters-key players in precisely targeted plant genome editing

The clustered regularly interspaced short palindrome repeat (CRISPR)/CRISPR-associated protein Cas) system is a powerful and highly precise gene-editing tool in basic and applied research for crop improvement programs. CRISPR/Cas tool is being extensively used in plants to improve crop yield, quality, and nutritional value and make them tolerant to environmental stresses. CRISPR/Cas system consists of a Cas protein with DNA endonuclease activity and one CRISPR RNA transcript that is processed to form one or several short guide RNAs that direct Cas9 to the target DNA sequence. The expression levels of Cas proteins and gRNAs significantly influence the editing efficiency of CRISPR/Cas-mediated genome editing. This review focuses on insights into RNA Pol III promoters and their types that govern the expression levels of sgRNA in the CRISPR/Cas system. We discussed Pol III promoters structural and functional characteristics and their comparison with Pol II promoters. Further, the use of synthetic promoters to increase the targeting efficiency and overcome the structural, functional, and expressional limitations of RNA Pol III promoters has been discussed. Our review reports various studies that illustrate the use of endogenous U6/U3 promoters for improving editing efficiency in plants and the applicative approach of species-specific RNA pol III promoters for genome editing in model crops like Arabidopsis and tobacco, cereals, legumes, oilseed, and horticultural crops. We further highlight the significance of optimizing these species-specific promoters' systematic identification and validation for crop improvement and biotic and abiotic stress tolerance through CRISPR/Cas mediated genome editing.

Optimization and Application of CRISPR/Cas9 Genome Editing in a Cosmopolitan Pest, Diamondback Moth

The CRISPR/Cas9 system is an efficient tool for reverse genetics validation, and the application of this system in the cell lines provides a new perspective on target gene analysis for the development of biotechnology tools. However, in the cell lines of diamondback moth, Plutella xylostella, the integrity of the CRISPR/Cas9 system and the utilization of this cell lines still need to be improved to ensure the application of the system. Here, we stabilize the transfection efficiency of the P. xylostella cell lines at different passages at about 60% by trying different transfection reagents and adjusting the transfection method. For Cas9 expression in the CRIPSPR/Cas9 system, we identified a strong endogenous promoter: the 217–2 promoter. The dual-luciferase and EGFP reporter assay demonstrated that it has a driving efficiency close to that of the IE1 promoter. We constructed pB-Cas9-Neo plasmid and pU6-sgRNA plasmid for CRISPR/Cas9 system and subsequent cell screening. The feasibility of the CRISPR/Cas9 system in P. xylostella cell lines was verified by knocking out endogenous and exogenous genes. Finally, we generated a transgenic Cas9 cell line of P. xylostella that would benefit future exploitation, such as knock-in and multi-threaded editing. Our works provides the validity of the CRISPR/Cas9 system in the P. xylostella cell lines and lays the foundation for further genetic and molecular studies on insects, particularly favoring gene function analysis.

An inducible gene from glycoside hydrolase one family of Plutella xylostella decreases larval survival when feeding on host plant

Glycoside hydrolase family 1 (GH1) members exhibit a broad substrate spectrum and play important roles in insect-plant interactions, such as the defensive β-glucosidase and β-thioglucosidase (so-called myrosinase). However, knowledge about the expression profiling and function of glycoside hydrolase family 1 members in a specialist pest of crucifers Plutella xylostella is still limited. In this study, 13 putative glycoside hydrolase family 1 members of P. xylostella were identified based on the sequence characteristics, while no myrosinase activity was detectable in P. xylostella using gas chromatography-mass spectrometry (GC-MS). Expression profiling of these glycoside hydrolase family 1 members identified the midgut-specific gene Px008848 that is induced by host plant. Further experiments revealed that the in vitro expressed Px008848 protein had β-glucosidase activity and the survival rate of the larvae feeding on wounded Arabidopsis thaliana leaves declined when leaves were treated with purified Px008848 protein. When CRISPR/Cas9-based homozygous mutant larvae of Px008848 and wild-type larvae were respectively transferred onto the A. thaliana, the larval survival rate of the mutant larvae was significantly higher than that of the wild-type individuals. Our work showed that certain insect glycoside hydrolase family 1 gene may have negative effect on the development of larvae feeding on the host plant, which broadened our understandings on the evolutionary function of this gene family in the insect-plant interaction.

Functional Characterization of Two RNA Methyltransferase Genes METTL3 and METTL14 Uncovers the Roles of m6A in Mediating Adaptation of Plutella xylostella to Host Plants

N⁶-methyladenosine (m⁶A) is one of the major epigenetic modifications in eukaryotes. Although increasing functions of m⁶A have been identified in insects, its role in Plutella xylostella L. for host plant adaptation remains unclear. In the current study, we show that the m⁶A content of P. xylostella was relatively low in different developmental stages and tissues, with no significant differences. Two RNA methyltransferase genes, PxMETTL3 (methyltransferase-like 3) and PxMETTL14 (methyltransferase-like 14), were identified and characterized. PxMETTL3 could be transcribed into two transcripts, and PxMETTL14 had only one transcript; both of these genes were highly expressed in egg and adult stages and reproductive tissues. The CRISPR/Cas9-mediated knockout of PxMETTL3 (ΔPxMETTL3-2) or PxMETTL14 (ΔPxMETTL14-14) confirmed their function in m⁶A installation into RNA. Furthermore, upon transfer from an artificial diet to the host plant, the mutant strains were affected in terms of larval and pupal weight or adult emergence rate, while the wildtype (WT) strain did not exhibit any difference. In addition, the fecundity and egg hatching rate of the WT strain decreased significantly, whereas only the ΔPxMETTL14-14 mutant strain displayed significantly decreased fecundity. There seemed to be a tradeoff between the stress adaptation and reproduction in P. xylostella mediated by m⁶A modification. During host transfer, the expression of PxMETTL14 was consistent with the change in m⁶A content, which implied that PxMETTL14 could respond to host plant defense effectively, and may regulate m⁶A content. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the differentially expressed transcripts with changes in m⁶A levels revealed that the potential functions of m⁶A-related genes may be involved in steroid biosynthesis for larval performance and metabolic pathways for adult reproduction. Overall, our work reveals an epigenetic regulation mechanism for the rapid adaptation of P. xylostella to variations in the host environment.

Development of CRISPR/Cas9-Mediated Gene-Drive Construct Targeting the Phenotypic Gene in Plutella xylostella

The gene-drive system can ensure that desirable traits are transmitted to the progeny more than the normal Mendelian segregation. The clustered regularly interspersed palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) mediated gene-drive system has been demonstrated in dipteran insect species, including Drosophila and Anopheles, not yet in other insect species. Here, we have developed a single CRISPR/Cas9-mediated gene-drive construct for Plutella xylostella, a highly-destructive lepidopteran pest of cruciferous crops. The gene-drive construct was developed containing a Cas9 gene, a marker gene (EGFP) and a gRNA sequence targeting the phenotypic marker gene (Pxyellow) and site-specifically inserted into the P. xylostella genome. This homing-based gene-drive copied ∼12 kb of a fragment containing Cas9 gene, gRNA, and EGFP gene along with their promoters to the target site. Overall, 6.67%–12.59% gene-drive efficiency due to homology-directed repair (HDR), and 80.93%–86.77% resistant-allele formation due to non-homologous-end joining (NHEJ) were observed. Furthermore, the transgenic progeny derived from male parents showed a higher gene-drive efficiency compared with transgenic progeny derived from female parents. This study demonstrates the feasibility of the CRISPR/Cas9-mediated gene-drive construct in P. xylostella that inherits the desired traits to the progeny. The finding of this study provides a foundation to develop an effective CRISPR/Cas9-mediated gene-drive system for pest control.

Toward a CRISPR-Cas9-based Gene Drive in the Diamondback Moth Plutella xylostella

Promising to provide powerful genetic control tools, gene drives have been constructed in multiple dipteran insects, yeast, and mice for the purposes of population elimination or modification. However, it remains unclear whether these techniques can be applied to lepidopterans. Here, we used endogenous regulatory elements to drive Cas9 and single guide RNA (sgRNA) expression in the diamondback moth (DBM), Plutella xylostella, and test the first split gene drive system in a lepidopteran. The DBM is an economically important global agriculture pest of cruciferous crops and has developed severe resistance to various insecticides, making it a prime candidate for such novel control strategy development. A very high level of somatic editing was observed in Cas9/sgRNA transheterozygotes, although no significant homing was revealed in the subsequent generation. Although heritable Cas9-medated germline cleavage as well as maternal and paternal Cas9 deposition were observed, rates were far lower than for somatic cleavage events, indicating robust somatic but limited germline activity of Cas9/sgRNA under the control of selected regulatory elements. Our results provide valuable experience, paving the way for future construction of gene drives or other Cas9-based genetic control strategies in DBM and other lepidopterans.

Identification of Helicoverpa armigera promoters for biotechnological applications

Helicoverpa armigera and Helicoverpa zea are highly polyphagous major agricultural pests with a global distribution. Their control is based on insecticides, however, new, effective, and environmentally friendly control tools are required to be developed and validated. In an effort to facilitate the development of advanced biotechnological tools in these species that will take advantage of new powerful molecular biology techniques like CRISPR/Cas9, we used available transcriptomic data and literature resources, in order to identify RNA polymerase II and III promoters active in RP-HzGUT-AW1(MG), a midgut derived cell line from Helicoverpa zea. Following functional analysis in insect cell lines, four RNA polymerase II promoters from the genes HaLabial, HaTsp-2A, HaPtx-I and HaCaudal were found to exhibit high transcriptional activity in vitro. The HaTsp-2A promoter did not exhibit any activity in the non-midgut derived cell lines Sf-9 and Hi-5 despite high sequence conservation among Lepidoptera, suggesting that it may function in a gut specific manner. Furthermore, considering the utility of RNA polymerase III U6 promoters in methodologies such as RNAi and CRISPR/Cas9, we identified and evaluated four different U6 promoters of H. armigera. In vitro experiments based on luciferase and GFP reporter assays, as well as in vivo experiments targeting an essential gene of Helicoverpa, indicate that these U6 promoters are functional and can be used to experimentally silence or knockout target genes through the expression of shRNAs and sgRNAs respectively. Taking our findings together, we provide a set of promoters useful for the genetic manipulation of Helicoverpa species, that can be used in various applications in the context of agricultural biotechnology.

The Combination of Bacillus Thuringiensis and Its Engineered Strain Expressing dsRNA Increases the Toxicity against Plutella Xylostella

RNA interference (RNAi) has been developed and used as an emerging strategy for pest management. Here, an entomopathogen Bacillus thuringiensis (Bt) was used to express the dsRNA for the control of Plutella xylostella. A vector containing a 325-bp fragment of the conserved region of P. xylostella arginine kinase gene (PxAK) flanking in two ends with the promoter Pro3α was developed and transferred into Bt 8010 and BMB171, and consequently engineered Bt strains 8010AKi and BMB171AKi expressing dsRNA of PxAK were developed. The two engineered Bt strains were separately mixed with Bt 8010 in a series of ratios, and then fed to the P. xylostella larvae. We found that 8010:8010AKi of 9:1 and 8010:BMB171AKi of 7:3 caused a higher mortality than Bt 8010. PxAK expression levels in the individuals treated with the mixtures, 8010AKi and BMB171Aki, were lower than that in the control. The intrinsic rate of increase (r) and net reproductive rate (R₀) of the population treated with 8010:8010AKi of 9:1 were lower than those of the population treated with Bt 8010 or 8010AKi. We developed a Bt-mediated insect RNAi for the control of P. xylostella and demonstrated a practical approach to integrating the entomopathogen with RNAi technique for the pest management.

Double-Stranded RNA-Degrading Enzymes Reduce the Efficiency of RNA Interference in Plutella xylostella

DsRNA-degrading enzymes (dsRNases) have been recognized as important factors in reducing RNA interference (RNAi) efficiency in different insect species. However, dsRNases in Plutella xylostella are still unknown. We identified the full-length cDNAs of PxdsRNase1, PxdsRNase2, PxdsRNase3, and PxdsRNase4. Gene expression profile showed that PxdsRNase1 was mainly expressed in the hemolymph; and that PxdsRNase2 and PxdsRNase3 were mainly expressed in the intestinal tract. The expression of PxCht (Chitinase of P. xylostella) in P. xylostella larvae injected with the mixture of dsPxCht (dsRNA of PxCht) and dsPxdsRNase1 (dsRNA of PxdsRNase1), dsPxdsRNase2 (dsRNA of PxdsRNase2), or dsPxdsRNase3 (dsRNA of PxdsRNase3) was significantly higher than that in the larvae injected with the mixture of dsGFP (dsRNA of green fluorescent protein gene, GFP) and dsPxCht; the transcription level of PxCht in the larvae feeding on the mixture of dsPxCht and dsPxdsRNase1, dsPxdsRNase2, or dsPxdsRNase3 was significantly higher than that in the larvae feeding on the mixture of dsPxCht and dsGFP. The recombinant protein of PxdsRNase1 degraded dsRNA rapidly, PxdsRNase3 cleaved dsRNA without complete degradation, and PxdsRNase2 could not degrade dsRNA in vitro. These results suggested that PxdsRNases1, PxdsRNases2, and PxdsRNases3 were involved in the dsRNA degradation to reduce RNAi efficiency with different mechanisms.

Genome editing efficiency of four Drosophila suzukii endogenous U6 promoters

The invasive spotted-wing drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) has caused serious economic losses to the fruit industry. The conventional control methods have many limitations and genetic engineering technologies such as CRISPR/Cas9-mediated gene drive are promising approaches. In the CRISPR/Cas9 system, the transcriptional regulatory elements play an important role in the activities of gRNA. Thus, in order to improve the genome editing efficiency of the CRISPR/Cas9 system in D. suzukii, we cloned and tested four endogenous U6 promoters to drive mutagenesis of the white gene. Our results showed that all the four promoters could be used with variable efficiency. The promoter DsU6-3 had the highest genome editing efficiency among the four DsU6 promoters. Compared with the DsU6-3 promoter, the DmU6:3 promoter showed lower efficiency to drive mutagenesis in D. suzukii. These findings expand the range of promoters available to express gRNAs in D. suzukii, facilitating the basic and applied research on this important pest.

Mutation of P-element somatic inhibitor induces male sterility in the diamondback moth, Plutella xylostella

BACKGROUND

Genetic manipulation of sex determination pathways in insects provides the basis for a broad range of strategies to benefit agricultural security and human health. The P-element somatic inhibitor (PSI) protein, an exon splicing silencer that promotes male-specific splicing of dsx, plays a critical role in male sexual differentiation and development. The functions of PSI have been characterized in the lepidopteran model species Bombyx mori. However, the molecular mechanism and functions of PSI in Plutella xylostella, a worldwide agricultural pest and taxonomically basal species, are still unknown.

RESULTS

Here we identified PxPSI transcripts and analyzed their spatiotemporal expression pattern in P. xylostella. Multiple sequence alignment revealed that PxPSI contains four KH domains and is highly conserved in lepidopterans. We used the CRISPR-Cas9 system to generate mutations of the PxPSI genomic locus. Disruptions of PxPSI caused male-specific defects in internal and external genitals. In addition, we detected female-specific Pxdsx transcripts in PxPSI male mutants. Mutations also caused changes in expression of several sex-biased genes and induced male sterility.

CONCLUSION

Our study demonstrates that PxPSI plays a key role in male sex determination in P. xylostella and suggests a potential molecular target for genetic-based pest management in lepidopteran pests. © 2021 Society of Chemical Industry.

Ommochrome pathway genes kynurenine 3-hydroxylase and cardinal participate in eye pigmentation in Plutella xylostella

BACKGROUND

Eye pigmentation genes have been utilized as visible markers for constructing genetic control prototypes in several insect vectors of human disease. Here, orthologs of two ommochrome pathway genes, kynurenine 3-hydroxylase (kmo) and cardinal, were investigated in Plutella xylostella, a globally distributed, economically important pest of Brassica crops.

RESULTS

Both somatic mosaic and germline mutations were efficiently created using the CRISPR/Cas9 system, and null mutant strains of Pxkmo and Pxcardinal were obtained. A frame-shift mutation in Pxkmo caused yellow compound eyes at adult stage while an in-frame mutation lacking two amino acids resulted in a hypomorphic red eye phenotypes. In contrast, Pxcardinal-deficient moths with a frame-shift mutation exhibited yellow eye pigmentation in newly emerged adults which turned to red as the adults aged. Additionally, differences were observed in the coloration of larval ocelli, brains and testes in Pxkmo and Pxcardinal yellow-eye mutant lines.

CONCLUSIONS

Our work identifies the important roles of Pxkmo and Pxcardinal in P. xylostella eye pigmentation and provides tools for future genetic manipulation of this important crop pest.

Identification and characterization of the vasa gene in the diamondback moth, Plutella xylostella

Vasa is an ATP-dependent RNA helicase, participating in multiple biological processes. It has been widely used as a germ cell marker and its promoter has become a key component of several genetic pest control systems. Here we present the vasa gene structure and its promoter activity in Plutella xylostella, one of the most destructive pests of cruciferous crops. Full length Pxvasa cDNA sequences were obtained, revealing 14 exons and at least 30 alternatively spliced transcripts. Inferred amino acid sequences showed nine conserved DEAD-box family protein motifs with partial exclusion from some isoforms. Real-time quantitative PCR indicated the up-regulation of Pxvasa in both female and male adults compared with other developmental stages, and the expression levels of Pxvasa were found to be much higher in adult gonads, especially ovaries, than in other tissues. The putative promoter region of Pxvasa was sequenced and several ecdysone-induced transcription factor (TF) binding sites were predicted in silico. To further analyze the promoter region, two upstream regulatory fragments of different lengths were tested as putative promoters in transient cell and embryo expression assays, one of which was subsequently utilized to drive Cas9 expression in vivo. A transgenic line was recovered and the expression patterns of Cas9 and native Pxvasa were profiled in adult tissues and eggs with RT-PCR. This work provides the foundation for further studies on the gene functions of Pxvasa as well as the potential application of its promoter in genetic manipulation of P. xylostella.

Functions of duplicated glucosinolate sulfatases in the development and host adaptation of Plutella xylostella

Evolutionary adaptations of herbivorous insects are often dictated by the necessity to withstand a corresponding evolutionary innovation in host plant defense. Glucosinolate sulfatase (GSS) enzyme activity is considered a central adaptation strategy in Plutella xylostella against glucosinolates (GS)-myrosinase defense system in the Brassicales. The high functional versatility of sulfatases suggests that they may perform other vital roles in the process of growth and development. Here, we used a CRISPR/Cas9 system to generate stable homozygous single/double mutant lines of gss1 or/and gss2 with no predicted off-target effects, to analyze the functions of the pair of duplicated genes in the development and host adaptation of P. xylostella. The bioassays showed that, when fed on their usual artificial diet, significant reduction in egg hatching rate and final larval survival rate of the single mutant line of gss2 compared with the original strain or mutant lines of gss1, revealing unexpected functions of GSS2 in embryonic and larval development. When larvae of homozygous mutant lines were transferred onto a new food, Arabidopsis thaliana, no induced effect at protein level of GSS1/2 or gene expression level of gss1/gss2 was detected. The absence of GSS1 or GSS2 reduced the survival rate of larvae and prolonged the duration of the larval stage, indicating that both GSS1 and GSS2 played an important role in adaptation to host plants. The versatile functions of duplicated GSSs in this study provide a foundation for further research to understand potential functions of other sulfatase members and support evidence of adaptation in herbivorous insects.

Mutation of the seminal protease gene, serine protease 2, results in male sterility in diverse lepidopterans

Sterile insect technology (SIT) is an environmentally friendly method for pest control. As part of our efforts to develop a strategy that results in engineered male-sterile strains with minimum effects on viability and mating competition, we used CRISPR/Cas9 technology to disrupt Ser2, which encodes a seminal fluid protein, in the model lepidopteran insect, Bombyx mori, and an important agricultural pest, Plutella xylostella. Disruption of Ser2 resulted in dominant heritable male sterility. Wild-type females mated with Ser2-deficient males laid eggs normally, but the eggs did not hatch. We detected no differences in other reproductive behaviors in the mutant males. These results support the conclusion that Ser2 gene is necessary for male reproductive success in diverse lepidopterans. Targeting Ser2 gene has the potential to form the basis for a new strategy for pest control.

Genetic control of Plutella xylostella in omics era

Diamondback moth, Plutella xylostella (L.), is a specialist pest on cruciferous crops of economic importance. The large-scale use of chemical insecticides for the control of this insect pest has caused a number of challenges to agro-ecosystems. With the advent of the omics era, genetic pest management strategies are becoming increasingly feasible and show a powerful potential for pest control. Here, we review strategies for using transgenic plants and sterile insect techniques for genetic pest management and introduce the major advances in the control of P. xylostella using a female-specific RIDL (release of insects carrying a dominant lethal gene) strategy. Further, the advantages of gene drive developed in combination with sex determination and CRISPR/Cas9 systems are addressed, and the corresponding prospects and implementation issues are discussed. It is predictable that under the policy and regulation of professional committees, the genetic pest control strategy, especially for gene drive, will open a new avenue to sustainable pest management not only for P. xylostella but also for other insect pests.

Mutation of doublesex induces sex-specific sterility of the diamondback moth Plutella xylostella

DOUBLESEX (DSX): the downstream gene in the insect sex determination pathway, plays a critical role in sexual differentiation and development. The functions of dsx have been characterized in several model insect species. However, the molecular mechanism and functions of sex determination of dsx in Plutella xylostella, an agricultural pest, are still unknown. In present study, we identified a male-specific and three female-specific Pxdsx transcripts in P. xylostella. Phylogenetic analyses and multiple sequence alignment revealed that Pxdsx is highly conserved in lepidopterans. The CRISPR/Cas9 technology was used to induce mutations in the male-specific isoform, the female-specific isoform, and common regions of Pxdsx. Disruptions of Pxdsx sex-specific isoforms caused sex-specific defects in external genitals and partial sexual reversal. In addition, we found that female specific transcripts were detected in Pxdsx^M male mutants and male-specific transcripts were detected in Pxdsx^F female mutants. Mutations also caused changes in expression of several sex-biased genes and induced sex-specific sterility. This study demonstrates that Pxdsx plays a key role in sex determination of P. xylostella and suggests novel genetic control approaches for the management of P. xylostella.