Short term transcriptional responses of P450s to phytochemicals in insects and mites

A relaxin receptor gene RpGPCR41 is involved in the resistance of Rhopalosiphum padi to pyrethroids

Rhopalosiphum padi is a global pest that poses a significant threat to wheat crops and has developed resistance to various insecticides. G protein-coupled receptors (GPCRs), known for their crucial role in signaling and biological processes across insect species, have recently gained attention as a potential target for insecticides. GPCR has the potential to contribute to insect resistance through the regulation of P450 gene expression. However, GPCRs in R. padi remained unexplored until this study. We identified a total of 102 GPCRs in R. padi, including 81 receptors from family A, 10 receptors from family B, 8 receptors from family C, and 3 receptors from family D. Among these GPCR genes, 16 were up-regulated in both lambda-cyhalothrin and bifenthrin-resistant strains of R. padi (LC-R and BIF-R). A relaxin receptor gene, RpGPCR41, showed the highest up-regulated expression in both the resistant strains, with a significant increase of 14.3-fold and 22.7-fold compared to the susceptible strain (SS). RNA interference (RNAi) experiments targeting the relaxin receptor significantly increase the mortality of R. padi when exposed to the LC50 concentration of lambda-cyhalothrin and bifenthrin. The expression levels of five P450 genes (RpCYP6CY8, RpCYP6DC1, RpCYP380B1, RpCYP4CH2, and RpCYP4C1) were significantly down-regulated following knockdown of RpGPCR41 in LC-R and BIF-R strains. Our results highlight the involvement of GPCR gene overexpression in the resistance of R. padi to pyrethroids, providing valuable insights into the mechanisms underlying aphid resistance and a potential target for aphid control.

Secondary metabolites in host pears defense against two fruit borers and cytochrome-P450-mediated counter-defense

Herbivorous insects have evolved metabolic strategies to survive the challenges posed by plant secondary metabolites (SMs). This study reports an exploration of SMs present in pears, which serve as a defense against invasive Cydia pomonella and native Grapholita molesta and their counter-defense response. The feeding preferences of fruit borers are influenced by the softening of two pear varieties as they ripen. The content of SMs, such as quercetin and rutin, increases due to feeding by fruit borers. Notably, quercetin levels only increase after C. pomonella feeding. The consumption of SMs affects the growth of fruit borer population differently, potentially due to the activation of P450 genes by SMs. These two fruit borers are equipped with specific P450 enzymes that specialize in metabolizing quercetin and rutin, enabling them to adapt to these SMs in their host fruits. These findings provide valuable insights into the coevolution of plants and herbivorous insects.

CncC-Keap1-P450s pathway is involved in the detoxification of emamectin benzoate in the spongy moth Lymantria dispar

The detoxification of insecticides in insects is dependent on the expression and activity of multiple detoxification enzymes. As an important modulator of detoxification enzymes, the CncC-Keap1 pathway was involved in the detoxification of various pesticides. However, whether the CncC-Keap1 pathway is involved in the detoxification of emamectin benzoate (EMB) is unclear. In this study, we cloned the LdCncC and LdKeap1 from spongy moths (Lymantria dispar). Our results showed that EMB exposure induced oxidative stress, and activated the CncC-Keap1 pathway at mRNA and protein levels. Removing ROS by N-acetylcysteine remarkably decreased H2O2 levels and restored the expression of LdCncC and LdKeap1. The silencing LdCncC, not LdKeap1, by dsRNA significantly decreased the cytochrome P450 activities, and increased the sensitivity of larvae to EMB. Besides, the expression of CYP6B7v1, CYP321A7 and CYP4S4v1 were significantly decreased after silencing LdCncC. Notably, the knockdown of CYP6B7v1, CYP321A7 or CYP4S4v1 significantly increased the mortality induced by EMB exposure. Therefore, we proposed that activation of CncC-Keap1 pathway induced by ROS increased the detoxification of EMB in spongy moths by regulating the expression of CYP6B7v1, CYP321A7 and CYP4S4v1. Our study strengthened the understanding of the detoxification of EMB from the perspective of CncC-Keap1-P450s pathway.

Role of miR-276-3p in the cyantraniliprole resistance mechanism of Bemisia tabaci via CYP6CX3 targeting

The sweet potato whitefly, Bemisia tabaci, is an important insect pest that transmits over 200 different plant viruses and causes serious damage to the production of cotton and Solanaceae vegetables. Cyantraniliprole is the first diamide insecticide, showing toxicity against B. tabaci. However, B. tabaci has developed resistance to this insecticide by upregulating the expressions of cytochrome P450 genes such as CYP6CX3, while there is limited information on the regulatory mechanism mediated by miRNA. In the present study, ten miRNAs were predicted to target CYP6CX3, in which miR-276-3p showed an inverse expression pattern with CYP6CX3 in two cyantraniliprole resistant strains and under cyantraniliprole exposure. A luciferase assay demonstrated that miR-276-3p suppressed CYP6CX3 expression by pairing with residues 1445-1453. Overexpression or knockdown of miR-276-3p directly impacted B. tabaci resistance to cyantraniliprole. In addition, exposure to cyantraniliprole led to a significant reduction in the expressions of five genes (drosha, dicer1, dicer2, Ago1, and Ago2A) associated with miRNA biogenesis. Suppressing genes such as drosha, dicer1, and Ago2A reduced the expression of miR-276-3p, increased CYP6CX3 expression, and decreased B. tabaci resistance to cyantraniliprole. These results improve our understanding of the role of miRNAs in P450 regulation and cyantraniliprole resistance in B. tabaci.

Impacts of cultivar and management practices on the diversity and population dynamics of mites in soybean crops

The objective of this study was to evaluate the diversity and population dynamics of mites in soybean crops with different cultivars and management practices. The study was conducted in two soybean production fields in the municipalities of Mato Queimado (L1) and Três de Maio (L2), Rio Grande do Sul state, Brazil. Two transgenic cultivars were used, and insecticide applications varied among treatments. Sampling began at the V2 stage, with 60 leaves/area/collection that were sorted, mites were collected and identified using dichotomous keys. A total of 18,100 mites belonging to 12 species were found. Among the species, Tetranychus urticae Koch, Mononychellus planki (McGregor), and Tetranychus ludeni Zacher (Tetranychidae) were the most abundant, whereas the most abundant predatory mites were the phytoseiids Neoseiulus californicus McGregor and Neoseiulus idaeus Denmark and Muma, with N. idaeus being more abundant and present in all areas. The acarofauna was influenced by environmental conditions and management practices. Neoseiulus idaeus was commonly associated with populations of M. planki, T. ludeni, and T. urticae. Neoseiulus californicus tolerated pesticide use but was affected by severe water stress, whereas N. idaeus tolerated periods of low relative humidity and high temperatures.

Overexpression of the F116V allele of CYP9A186 in transgenic Helicoverpa armigera confers high-level resistance to emamectin benzoate

Insect cytochrome P450s play important roles in the detoxification of xenobiotics and the metabolic resistance to insecticides. However, the approach for in vivo validation of the contribution of specific candidate P450s to resistance is still limited in most non-model insect species. Previous studies with heterologous expression and in vitro functional assays have confirmed that a natural substitution (F116V) in the substrate recognition site 1 (SRS1) of the CYP9A186 of Spodoptera exigua is a gain-of-function mutation, which results in detoxification capability of and thus high-level resistance to both emamectin benzoate (EB) and abamectin. In this study, we established an effective piggyBac-based transformation system in the serious agricultural pest Helicoverpa armigera and overexpressed in vivo a resistance P450 allele, CYP9A186-F116V, from another lepidopteran pest Spodoptera exigua. Bioassays showed that transgenic H. armigera larvae expressing CYP9A186-F116V obtained 358-fold and 38.6-fold resistance to EB and abamectin, respectively. In contrast, a transgenic line of Drosophila melanogaster overexpressing this P450 variant only confers ∼20-fold resistance to the two insecticides. This bias towards the resistance level revealed that closely related species might provide a more appropriate cellular environment for gene expression and subsequent toxicokinetics of insecticides. These results not only present an alternative method for in vivo functional characterization of P450s in H. armigera and other phylogenetically close species but also provide a valuable genetic engineering toolkit for the genetic manipulation of H. armigera.

Transcription factors, cap 'n' collar isoform C regulates the expression of CYP450 genes involving in insecticides susceptibility in Locusta migratoria

BACKGROUND

The cap 'n' collar (Cnc) belongs to the Basic Leucine Zipper (bZIP) transcription factor super family. Cap 'n' collar isoform C (CncC) is highly conserved in the animal kingdom. CncC contributes to the regulation of growth, development, and aging and takes part in the maintenance of homeostasis and the defense against endogenous and environmental stress. Insect CncC participates in the regulation of various kinds of stress-responsive genes and is involved in the development of insecticide resistance.

RESULTS

In this study, one full-length CncC sequence of Locusta migratoria was identified and characterized. Upon RNAi silencing of LmCncC, insecticide bioassays showed that LmCncC played an essential role in deltamethrin and imidacloprid susceptibility. To fully investigate the downstream genes regulated by LmCncC and further identify the LmCncC-regulated genes involved in deltamethrin and imidacloprid susceptibility, a comparative transcriptome was constructed. Thirty-five up-regulated genes and 73 down-regulated genes were screened from dsLmCncC-knockdown individuals. We selected 22 LmCncC-regulated genes and verified their gene expression levels using RT-qPCR. Finally, six LmCYP450 genes belonging to the CYP6 family were selected as candidate detoxification genes, and LmCYP6FD1 and LmCYP6FE1 were further validated as detoxification genes of insecticides via RNAi, insecticide bioassays, and metabolite identification.

CONCLUSIONS

Our data suggest that the locust CncC gene is associated with deltamethrin and imidacloprid susceptibility via the regulation of LmCYP6FD1 and LmCYP6FE1, respectively.

The host plant strongly modulates acaricide resistance levels to mitochondrial complex II inhibitors in a multi-resistant field population of Tetranychus urticae

The two-spotted spider mite Tetranychus urticae is a polyphagous pest with an extraordinary ability to develop acaricide resistance. Here, we characterize the resistance mechanisms in a T. urticae population (VR-BE) collected from a Belgian tomato greenhouse, where the grower was unsuccessful in chemically controlling the mite population resulting in crop loss. Upon arrival in the laboratory, the VR-BE population was established both on bean and tomato plants as hosts. Toxicity bioassays on both populations confirmed that the population was highly multi-resistant, recording resistance to 12 out of 13 compounds tested from various mode of action groups. DNA sequencing revealed the presence of multiple target-site resistance mutations, but these could not explain resistance to all compounds. In addition, striking differences in toxicity for six acaricides were observed between the populations on bean and tomato. The highest difference was recorded for the complex II inhibitors cyenopyrafen and cyflumetofen, which were 4.4 and 3.3-fold less toxic for VR-BE mites on tomato versus bean. PBO synergism bioassays suggested increased P450 based detoxification contribute to the host-dependent toxicity. Given the involvement of increased detoxification, we subsequently determined genome-wide gene expression levels of VR-BE on both hosts, in comparison to a reference susceptible population, revealing overexpression of a large set of detoxification genes in VR-BE on both hosts compared to the reference. In addition, a number of mainly detoxification genes with higher expression in VR-BE on tomato compared to bean was identified, including several cytochrome P450s. Together, our work suggests that multi-resistant field populations can accumulate a striking number of target-site resistance mutations. We also show that the host plant can have a profound effect on the P450-associated resistance levels to cyenopyrafen and cyflumetofen.

The genome sequence of a spider mite, Tetranychus truncatus, provides insights into interspecific host range variation and the genetic basis of adaptation to a low-quality host plant

The phytophagous mite Tetranychus truncatus is a serious pest in East Asia but has a relatively narrower host range than the pest mite Tetranychus urticae, which can feed on over 1200 plant species. Here, we generated a high-quality chromosomal level genome of T. truncatus and compared it with that of T. urticae, with an emphasis on the genes related to detoxification and chemoreception, to explore the genomic basis underlying the evolution of host range. We also conducted population genetics analyses (in 86 females from 10 populations) and host transfer experiments (in 4 populations) to investigate transcription changes following transfer to a low-quality host (Solanum melongena, eggplant), and we established possible connections between fitness on eggplant and genes related to detoxification and chemoreception. We found that T. truncatus has fewer genes related to detoxification, transport, and chemoreception than T. urticae, with a particularly strong reduction in gustatory receptor (GR) genes. We also found widespread transcriptional variation among T. truncatus populations, which varied in fitness on eggplant. We characterized selection on detoxification-related genes through ω values and found a negative correlation between expression levels and ω values. Based on the transcription results, as well as the fitness and genetic differences among populations, we identified genes potentially involved in adaptation to eggplant in T. truncatus. Our work provides a genomic resource for this pest mite and new insights into mechanisms underlying the adaptation of herbivorous mites to host plants.

Genome-Wide Scanning Loci and Differentially Expressed Gene Analysis Unveils the Molecular Mechanism of Chlorantraniliprole Resistance in Spodoptera frugiperda

Chlorantraniliprole has been widely used to controlSpodoptera frugiperda, but it has led to the development of chlorantraniliprole resistance. Multiomics analysis of strains with two extreme traits helps to elucidate the complex mechanisms involved. Herein, following genome resequencing and application of the Euclidean distance algorithm, 550 genes within a 16.20-Mb-linked region were identified from chlorantraniliprole-resistant (Ch-R) and chlorantraniliprole-susceptible (Ch-Sus) strains. Using transcriptome sequencing, 2066 differentially expressed genes were identified between Ch-R and Ch-Sus strains. Through association analysis, three glutathione S-transferase family genes and four trehalose transporter genes were selected for functional verification. Notably, SfGSTD1 had the strongest binding ability with chlorantraniliprole and is responsible for chlorantraniliprole tolerance. The Ch-R strain also increased the intracellular trehalose content by upregulating the transcription of SfTret1, thereby contributing to chlorantraniliprole resistance. These findings provide a new perspective to reveal the mechanism of resistance of agricultural pests to insecticides.

Divergent amplifications of CYP9A cytochrome P450 genes provide two noctuid pests with differential protection against xenobiotics

Here, we provide mechanistic support for the involvement of the CYP9A subfamily of cytochrome P450 monooxygenases in the detoxification of host plant defense compounds and chemical insecticides in Spodoptera exigua and Spodoptera frugiperda. Our comparative genomics shows that a large cluster of CYP9A genes occurs in the two species but with significant differences in its contents, including several species-specific duplicates and substantial sequence divergence, both between orthologs and between duplicates. Bioassays of CRISPR-Cas9 knockouts of the clusters show that, collectively, the CYP9As can detoxify two furanocoumarin plant defense compounds (imperatorin and xanthotoxin) and insecticides representing three different chemotypes (pyrethroids, avermectins, and oxadiazines). However, in vitro metabolic assays of heterologously expressed products of individual genes show several differences between the species in the particular CYP9As with activities against these compounds. We also find that the clusters show tight genetic linkage with high levels of pyrethroid resistance in field strains of the two species. We propose that their divergent amplifications of the CYP9A subfamily have not only contributed to the development of the broad host ranges of these species over long evolutionary timeframes but also supplied them with diverse genetic options for evolving resistance to chemical insecticides in the very recent past.

A nuclear receptor HR96-related gene underlies large trans-driven differences in detoxification gene expression in a generalist herbivore

The role, magnitude, and molecular nature of trans-driven expression variation underlying the upregulation of detoxification genes in pesticide resistant arthropod populations has remained enigmatic. In this study, we performed expression quantitative trait locus (eQTL) mapping (n = 458) between a pesticide resistant and a susceptible strain of the generalist herbivore and crop pest Tetranychus urticae. We found that a single trans eQTL hotspot controlled large differences in the expression of a subset of genes in different detoxification gene families, as well as other genes associated with host plant use. As established by additional genetic approaches including RNAi gene knockdown, a duplicated gene with a nuclear hormone receptor HR96-related ligand-binding domain was identified as causal for the expression differences between strains. The presence of a large family of HR96-related genes in T. urticae may enable modular control of detoxification and host plant use genes, facilitating this species' known and rapid evolution to diverse pesticides and host plants.

CYP6CX2 and CYP6CX3 mediate thiamethoxam resistance in field whitefly, Bemisia tabaci (Hemiptera:Aleyrodidae)

Cytochrome P450 monooxygenases (P450s) are well-known for their crucial roles in the detoxification of xenobiotics. However, whether CYP6CX2 and CYP6CX3, 2 genes from our Bemisia tabaci (B. tabaci) MED/Q genome data were associated with detoxification metabolism and confer resistance to thiamethoxam is unclear. In this study, we investigated the role of CYP6CX2 and CYP6CX3 in mediating whitefly thiamethoxam resistance. Our results showed that mRNA levels of CYP6CX2 and CYP6CX3 were up-regulated after exposure to thiamethoxam. Transcriptional levels of 2 genes were overexpressed in laboratory and field thiamethoxam resistant strains by RT-qPCR. These results indicate that the enhanced expression of CYP6CX2 and CYP6CX3 appears to confer thiamethoxam resistance in B. tabaci. Moreover, linear regression analysis showed that the expression levels of CYP6CX2 and CYP6CX3 were positively correlated with thiamethoxam resistance levels among populations. The susceptibility of whitefly adults was markedly increased after silencing 2 genes by RNA interference (RNAi) which further confirming their major role in thiamethoxam resistance. Our findings provide information to better understand the roles of P450s in resistance to neonicotinoids and suggest that these genes may be applied to develop target genes for sustainable management tactic of agricultural pests such as B. tabaci.

Increased metabolism in combination with the novel cytochrome b target-site mutation L258F confers cross-resistance between the Qo inhibitors acequinocyl and bifenazate in Tetranychus urticae

Acequinocyl and bifenazate are potent acaricides acting at the Q_o site of complex III of the electron transport chain, but frequent applications of these acaricides have led to the development of resistance in spider mites. Target-site resistance caused by mutations in the conserved cd1- and ef-helices of the Q_o pocket of cytochrome b has been elucidated as the main resistance mechanism. We therefore monitored Q_o pocket mutations in European field populations of Tetranychus urticae and uncovered a new mutation, L258F. The role of this mutation was validated by revealing patterns of maternal inheritance and by the independently replicated introgression in an unrelated susceptible genetic background. However, the parental strain exhibited higher resistance levels than conferred by the mutation alone in isogenic lines, especially for acequinocyl, implying the involvement of strong additional resistance mechanisms. This was confirmed by revealing a polygenic inheritance pattern with classical genetic crosses and via synergism experiments. Therefore, a genome-wide expression analysis was conducted that identified a number of highly overexpressed detoxification genes, including many P450s. Functional expression revealed that the P450 CYP392A11 can metabolize bifenazate by hydroxylation of the ring structure. In conclusion, the novel cytochrome b target-site mutation L258F was uncovered in a recently collected field strain and its role in acequinocyl and bifenazate resistance was validated. However, the high level of resistance in this strain is most likely caused by a combination of target-site resistance and P450-based increased detoxification, potentially acting in synergism.

Metabolic functional redundancy of the CYP9A subfamily members leads to P450-mediated lambda-cyhalothrin resistance in Cydia pomonella

BACKGROUND

The evolution of insect resistance to pesticides poses a continuing threat to sustainable pest management. While much is known about the molecular mechanisms that confer resistance in model insects and few agricultural pests, far less is known about fruit pests. Field-evolved resistance to synthetic insecticides such as lambda-cyhalothrin has been widely documented in Cydia pomonella, a major invasive pest of pome fruit worldwide, and the increased production of cytochrome P450 monooxygenases (P450s) has been linked to resistance in field-evolved resistant populations. However, the underlying molecular mechanisms of P450-mediated insecticide resistance remain largely unknown.

RESULTS

Here we found that functional redundancy and preference of metabolism by P450s genes in the CYP9A subfamily confer resistance to lambda-cyhalothrin in Cydia pomonella. A total of four CYP9A genes, including CYP9A61, CYP9A120, CYP9A121, and CYP9A122, were identified from Cydia pomonella. Among these, CYP9A120, CYP9A121, and CYP9A122 were predominantly expressed in the midgut of larvae. The expression levels of these P450 genes were significantly induced by a lethal dose that would kill 10% (LD₁₀ ) of lambda-cyhalothrin and were overexpressed in a field-evolved lambda-cyhalothrin resistant population. Knockdown of CYP9A120 and CYP9A121 by RNA-mediated interference (RNAi) increased the susceptibility of larvae to lambda-cyhalothrin. In vitro assays demonstrated that recombinant P450s expressed in Sf9 cells can metabolize lambda-cyhalothrin, but with functional redundancy and divergence through regioselectivity of metabolism. CYP9A121 preferred to convert lambda-cyhalothrin to 2'-hydroxy-lambda-cyhalothrin, whereas CYP9A122 only generated 4'-hydroxy metabolite of lambda-cyhalothrin. Although possesses a relatively low metabolic capability, CYP9A120 balanced catalytic competence to generate both 2'- and 4'-metabolites.

CONCLUSION

Collectively, these results reveal that metabolic functional redundancy of three members of the CYP9A subfamily leads to P450-mediated lambda-cyhalothrin resistance in Cydia pomonella, thus representing a potential adaptive evolutionary strategy during its worldwide expansion. © 2022 Society of Chemical Industry.

Tolerance to dietary linalool primarily involves co-expression of cytochrome P450s and cuticular proteins in Pagiophloeus tsushimanus (Coleoptera: Curculionidae) larvae using SMRT sequencing and RNA-seq

BACKGROUND

Pagiophloeus tsushimanus (Coleoptera: Curculionidae), an emerging forest pest exclusively infesting camphor trees, has recently caused severe ecological and economic damage in localized areas in China. Its population outbreak depends largely on the capacity to overcome the pressure of terpenoid-derived metabolites (e.g. linalool) from camphor trees. At present, the molecular basis of physiological adaptation of P. tsushimanus to dietary linalool is poorly understood, and there is no available reference genome or transcriptome.

RESULTS

Herein, we constructed the transcriptome profiling of P. tsushimanus larvae reared on linalool-infused diets using RNA sequencing and single-molecule real-time sequencing. A total of 20,325 high-quality full-length transcripts were identified as a reference transcriptome, of which 14,492 protein-coding transcripts including 130 transcription factors (TFs), and 5561 long non-coding RNAs (lncRNAs) were detected. Also, 30 alternative splicing events and 8049 simple sequence repeats were captured. Gene ontology enrichment of differential expressed transcripts revealed that overall up-regulation of both cytochrome P450s (CYP450s) and cuticular proteins (CPs), was the primary response characteristic against dietary linalool. Other physiological effects possibly caused by linalool exposure, such as increase in Reactive Oxygen Species (ROS) and hormetic stimulation, were compensated by a handful of induced genes encoding antioxidases, heat shock proteins (HSPs), juvenile hormone (JH) epoxide hydrolases, and digestive enzymes. Additionally, based on co-expression networks analysis, a diverse array of hub lncRNAs and TFs co-expressed with CYP450s and CPs were screened as the potential gene regulators. Temporal expression of candidate transcripts determined by quantitative real-time PCR also indicated a cooperative relationship between the inductions of CYP450s and CPs upon exposure to linalool.

CONCLUSIONS

Our present study provides an important transcriptome resource of P. tsushimanus, and lays a valuable foundation for understanding how this specialist pest copes with chemical challenges in its specific host environments.

A draft Diabrotica virgifera virgifera genome: insights into control and host plant adaption by a major maize pest insect

BACKGROUND

Adaptations by arthropod pests to host plant defenses of crops determine their impacts on agricultural production. The larval host range of western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), is restricted to maize and a few grasses. Resistance of D. v. virgifera to crop rotation practices and multiple insecticides contributes to its status as the most damaging pest of cultivated maize in North America and Europe. The extent to which adaptations by this pest contributes to host plant specialization remains unknown.

RESULTS

A 2.42 Gb draft D. v. virgifera genome, Dvir_v2.0, was assembled from short shotgun reads and scaffolded using long-insert mate-pair, transcriptome and linked read data. K-mer analysis predicted a repeat content of ≥ 61.5%. Ortholog assignments for Dvir_2.0 RefSeq models predict a greater number of species-specific gene duplications, including expansions in ATP binding cassette transporter and chemosensory gene families, than in other Coleoptera. A majority of annotated D. v. virgifera cytochrome P450s belong to CYP4, 6, and 9 clades. A total of 5,404 transcripts were differentially-expressed between D. v. virgifera larvae fed maize roots compared to alternative host (Miscanthus), a marginal host (Panicum virgatum), a poor host (Sorghum bicolor) and starvation treatments; Among differentially-expressed transcripts, 1,908 were shared across treatments and the least number were between Miscanthus compared to maize. Differentially-expressed transcripts were enriched for putative spliceosome, proteosome, and intracellular transport functions. General stress pathway functions were unique and enriched among up-regulated transcripts in marginal host, poor host, and starvation responses compared to responses on primary (maize) and alternate hosts.

CONCLUSIONS

Manual annotation of D. v. virgifera Dvir_2.0 RefSeq models predicted expansion of paralogs with gene families putatively involved in insecticide resistance and chemosensory perception. Our study also suggests that adaptations of D. v. virgifera larvae to feeding on an alternate host plant invoke fewer transcriptional changes compared to marginal or poor hosts. The shared up-regulation of stress response pathways between marginal host and poor host, and starvation treatments may reflect nutrient deprivation. This study provides insight into transcriptomic responses of larval feeding on different host plants and resources for genomic research on this economically significant pest of maize.

Transcriptome analysis revealed detoxification gene expression changes in Tetranychus cinnabarinus challenged with ethyl oleate

Natural acaricides are potential biorational mite control alternatives to conventional chemical acaricides. However, little is known about the molecular mechanism of defense response to natural acaricides in mites. We previously reported significant acaricidal properties of ethyl oleate (EO) against Tetranychus cinnabarinus (here referred to as a sibling species of two-spotted spider mite, Tetranychus urticae), a highly polyphagous pest devastating crops in fields and greenhouses worldwide. In this study, we explored the molecular responses of T. cinnabarinus exposed to EO using RNA-Seq and differentially expressed gene (DEG) analysis. A total of 131, 185, and 154 DEGs were identified in T. cinnabarinus after 1, 6, and 24 h of EO treatment. In addition, 36 putative detoxification-related DEGs, including 10 cytochrome P450s (P450s), three glutathione S-transferases (GSTs), nine UDP-glycosyltransferases (UGTs), eight esterases (ESTs), and six ATP-binding cassette transporters (ABC transporters), were identified. Interestingly, the upregulation of these detoxification-related genes might be the main defense response of T. cinnabarinus exposed to EO. A quantitative real-time PCR analysis indicated that the expression profiles of 19 random DEGs were consistent with the RNA-Seq results. These findings serve as valuable information for a better understanding of the acaricide-mite interaction and molecular mechanisms involved in the defense response of T. cinnabarinus against EO.

Antennae-abundant expression of candidate cytochrome P450 genes associated with odorant degradation in the asian citrus psyllid, Diaphorina citri

The Asian citrus psyllid, Diaphorina citri, is a notorious pest that is an efficient vector for Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus huanglongbing (HLB). The olfactory system of insects is crucial for foraging and mating behavior. Antennae-abundant odorant degrading enzymes (ODEs), including cytochrome P450 (CYPs), are important in degrading redundant odorant molecules to recover the insect olfactory. In this study, to isolate the antennal CYP genes of D. citri, we generated four transcriptomes from female/male antennae and body through deep sequencing of RNA libraries. Seven DcCYP genes preferentially expressed in antennae were first identified by comparing the antennal and body transcriptomes. Phylogenetic analysis grouped four DcCYPs (DcCYP6a13, DcCYP6j1, DcCYP6k1, and DcCYP6a2) into the CYP3 class, whereas DcCYP4d2, DcCYP4c62, and DcCYP4d8 were clustered in the CYP4 clade. qRT-PCR analyses across developmental stages and tissues showed they were antennae-abundant in both genders and constantly expressed from the first instar nymph to the adult. The results presented here highlight the isolation and expression of CYP genes in D. citri antennae, providing valuable insights into their putative role in odorant degradation.

Evolved transcriptional responses and their trade-offs after long-term adaptation of Bemisia tabaci to a marginally-suitable host

Although generalist insect herbivores can migrate and rapidly adapt to a broad range of host plants, they can face significant difficulties when accidentally migrating to novel and marginally-suitable hosts. What happens, both in performance and gene expression regulation, if these marginally-suitable hosts must be used for multiple generations before migration to a suitable host can take place, largely remains unknown. In this study, we established multigenerational colonies of the whitefly Bemisia tabaci, a generalist phloem-feeding species, adapted to a marginally-suitable host (habanero pepper) or an optimal host (cotton). We used reciprocal host tests to estimate the differences in performance of the populations on both hosts under optimal (30 oC) and mild-stressful (24 oC) temperature conditions, and documented the associated transcriptomic changes. The habanero pepper-adapted population greatly improved its performance on habanero pepper but did not reach its performance level on cotton, the original host. It also showed reduced performance on cotton, relative to the non-adapted population, and an antagonistic effect of the lower-temperature stressor. The transcriptomic data revealed that most of the expression changes, associated with long-term adaptation to habanero pepper, can be categorized as "evolved" with no initial plastic response. Three molecular functions dominated: enhanced formation of cuticle structural constituents, enhanced activity of oxidation-reduction processes involved in neutralization of phytotoxins and reduced production of proteins from the cathepsin B family. Taken together, these findings indicate that generalist insects can adapt to novel host plants by modifying the expression of a relatively small set of specific molecular functions.

Single amino acid variations drive functional divergence of cytochrome P450s in Helicoverpa species

Divergence of gene function is a hallmark of evolution, but assessing such divergence in one species or between species requires information on functional alterations of the alleles and homologs. Here, we explore the functional divergence of two paralogs, CYP6AE19 and CYP6AE20, from Helicoverpa armigera, and two close orthologs, CYP6B8 and CYP6B7, from two related species (Helicoverpa zea and H. armigera); although there is high sequence identity within each pair of enzymes, the latter P450 of each pair has lost metabolic competence towards the plant allelochemical xanthotoxin. Multiple chimeric and single/double site mutants were created by exchanging the diverse substrate recognition sites (SRSs) and amino acids within each pair of P450s. Heterologous expression in Sf9 cells and in vitro metabolism studies showed that the exchange of SRS4 swapped the activity of CYP6AE19 and CYP6AE20, and subsequent site-directed mutagenesis demonstrated that the CYP6AE20 V318M substitution causes a gain-of-function towards xanthotoxin. Meanwhile, a single amino acid substitution (L489P) in SRS6 was found to swap activity between the CYP6B orthologs. Sequence alignments of CYP6AE paralogs and all reported insect xanthotoxin-metabolizing P450s suggest M318 and P489 are essential for the catalytic activities of CYP6AE paralogs and CYP6B orthologs, respectively, but P450s in different subfamilies may have different mechanisms towards the same substrate. Our findings demonstrate that a single amino acid substitution can suffice to alter substrate metabolism and this functional divergence resulting from natural mutations will help to further our understanding of the process of natural selection of P450 genes and their role in insect-host plant interactions.

Biochemical and insecticidal effects of plant essential oils on insecticide resistant and susceptible populations of Musca domestica L. point to a potential cross-resistance risk

Essential oils (EOs) can provide important alternatives to chemical insecticides in the control of pests. In this study, 12 EOs of native plant species from Iran were evaluated for their adulticidal activity against the house fly. In addition, we examined the insecticidal activity of Zataria multiflora and Rosmarinus officinalis EOs on adult female house flies from pyrethroid and organophosphate resistant and susceptible populations, using both fumigant and topical bioassays. The involvement of detoxification enzymes in susceptibility was investigated with synergism experiments in vivo, while the inhibitory effects of R. officinalis and Zataria multiflora EOs on the activities of cytochrome P450-dependent monooxygenases (P450s), carboxylesterases (CarEs) and glutathione S-transferases (GSTs) were determined by enzymatic inhibition assays in vitro. The EOs of Z. multiflora, Mentha pulegium, R. officinalis and Thymus vulgaris were the most effective against adults in contact topical assays, while oils extracted from Eucalyptus cinerea, Z. multiflora, Citrus sinensis, R. officinalis, Pinus eldarica and Lavandula angustifolia where the most effective in fumigant assays. Rosmarinus officinalis and Z. multiflora EOs were selected for further investigation and showed higher toxicity against a susceptible population, compared to two insecticide-resistant populations. Correlation analysis suggested cross-resistance between these EOs and pyrethroids in the resistant populations. The toxicity of both EOs on the resistant populations was synergized by three detoxification enzyme inhibitors. Further, in vitro inhibition studies showed that R. officinalis and Z. multiflora EOs more effectively inhibited the activities of the detoxification enzymes from flies of the susceptible population compared to those of the pyrethroid resistant populations. Synergistic and enzymatic assays further revealed that increased activities of P450s, GSTs, and CarEs are possibly involved in the cross-resistance between EOs and pyrethroids. Investigating the molecular mechanisms of P450s, GSTs, and CarEs in the resistance to EOs should be subject to further studies.

The Role of Cytochrome P450s in Insect Toxicology and Resistance

Insect cytochrome P450 monooxygenases (P450s) perform a variety of important physiological functions, but it is their role in the detoxification of xenobiotics, such as natural and synthetic insecticides, that is the topic of this review. Recent advances in insect genomics and postgenomic functional approaches have provided an unprecedented opportunity to understand the evolution of insect P450s and their role in insect toxicology. These approaches have also been harnessed to provide new insights into the genomic alterations that lead to insecticide resistance, the mechanisms by which P450s are regulated, and the functional determinants of P450-mediated insecticide resistance. In parallel, an emerging body of work on the role of P450s in defining the sensitivity of beneficial insects to insecticides has been developed. The knowledge gained from these studies has applications for the management of P450-mediated resistance in insect pests and can be leveraged to safeguard the health of important beneficial insects.

Involvement of CYP2 and mitochondrial clan P450s of Helicoverpa armigera in xenobiotic metabolism

Insect CYP2 and mitochondrial clan P450s are relatively conserved genes encoding enzymes generally thought to be involved in biosynthesis or metabolism of endobiotics. However, emerging evidence argues they have potential roles in chemical defense as well, but their actual detoxification functions remain largely unknown. Here, we focused on the full complement of 8 CYP2 and 10 mitochondrial P450s in the generalist herbivore, Helicoverpa armigera. Their varied spatiotemporal expression profiles were analyzed and reflected their specific functions. For functional study of the mitochondrial clan P450s, the redox partners, adrenodoxin reductase (AdR) and adrenodoxin (Adx), were identified from genomes of eight insects and an efficient in vitro electron transfer system of mitochondrial P450 was established by co-expression with Adx and AdR of H. armigera. All CYP2 clan P450s and 8 mitochondrial P450s were successfully expressed in Sf9 cells and compared functionally. In vitro metabolism assays showed that two CYP2 clan P450s (CYP305B1 and CYP18A1) and CYP333B3 (mito clan) could epoxidize aldrin to dieldrin, while CYP305B1 and CYP339A1 (mito clan) have limited but significant hydroxylation capacities to esfenvalerate. CYP303A1 of the CYP2 clan exhibits high metabolic efficiency to 2-tridecanone. Screening the xenobiotic metabolism competence of CYP2 and mitochondrial clan P450s not only provides new insights on insect chemical defense but also can give indications on their physiological functions in H. armigera and other insects.

Molecular characterization and functional analysis of cytochrome P450-mediated detoxification CYP302A1 gene involved in host plant adaptation in Spodoptera frugieprda

The fall armyworm (FAW) Spodoptera frugiperda is a destructive and polyphagous pest of many essential food crops including maize and rice. The FAW is hard to manage, control, or eradicate, due to its polyphagous nature and voracity of feeding. Here, we report the characterization and functional analysis of the detoxification gene CYP302A1 and how S. frugieprda larvae use a detoxification mechanism to adapt host plants. Results demonstrated that CYP302A1 expression levels were much higher in midgut tissue and the older S. frugiperda larvae. Our current studies revealed the enhanced P450 activity in the midguts of S. frugiperda larvae after exposure to rice plants as compared to corn plants and an artificial diet. Furthermore, higher mortality was observed in PBO treated larvae followed by the exposure of rice plants as compared to the corn plant. The dsRNA-fed larvae showed downregulation of CYP302A1 gene in the midgut. At the same time, higher mortality, reduced larval weight and shorter developmental time was observed in the dsRNA-fed larvae followed by the exposure of rice plant as compared to the corn plant and DEPC-water treated plants as a control. These results concluded that the inducible P450 enzyme system and related genes could provide herbivores with an ecological opportunity to adapt to diverse host plants by utilizing secondary compounds present in their host plants.

Activation of the ROS/CncC and 20-Hydroxyecdysone Signaling Pathways Is Associated with Xanthotoxin-Induced Tolerance to λ-Cyhalothrin in Spodoptera litura

Adaptation to phytochemicals in herbivorous insects can influence tolerance to insecticides. However, it is unclear how insects use phytochemicals as cues to activate their metabolic detoxification systems. In this study, we found that dietary exposure to xanthotoxin enhanced tolerance of Spodoptera litura larvae to λ-cyhalothrin. Xanthotoxin ingestion significantly elevated the mRNA levels of 35 detoxification genes as well as the transcription factors Cap 'n' collar isoform-C (CncC) and its binding factor small muscle aponeurosis fibromatosis isoform-K (MafK). Additionally, xanthotoxin exposure increased the levels of reactive oxygen species (ROS), while ROS inhibitor N-acetylcysteine (NAC) treatment blocked xanthotoxin-induced expression of CncC, MafK, and detoxification genes and also prevented xanthotoxin-enhanced larval tolerance to λ-cyhalothrin. The 20-hydroxyecdysone (20E) signaling pathway was effectively activated by xanthotoxin, while blocking of 20E signaling transduction prevented xanthotoxin-enhanced larval tolerance to λ-cyhalothrin. Application of 20E induced the expression of multiple xanthotoxin-induced detoxification genes and enhanced λ-cyhalothrin tolerance in S. litura. NAC treatment blocked xanthotoxin-induced 20E synthesis, while the CncC agonist curcumin activated the 20E signaling pathway. These results indicate that the ROS/CncC pathway controls the induction of metabolic detoxification upon exposure to xanthotoxin, at least in part, through its regulation of the 20E signaling pathway.

Comparative analysis of the detoxification gene inventory of four major Spodoptera pest species in response to xenobiotics

The genus Spodoptera (Lepidoptera: Noctuidae) comprises some of the most polyphagous and destructive agricultural pests worldwide. The success of many species of this genus is due to their striking abilities to adapt to a broad range of host plants. Superfamilies of detoxification genes play a crucial role in the adaption to overcome plant defense mechanisms mediated by numerous secondary metabolites and toxins. Over the past decade, a substantial amount of expression data in Spodoptera larvae was produced for those genes in response to xenobiotics such as plant secondary metabolites, but also insecticide exposure. However, this information is scattered throughout the literature and in most cases does not allow to clearly identify candidate genes involved in host-plant adaptation and insecticide resistance. In the present review, we analyzed and compiled information on close to 600 pairs of inducers (xenobiotics) and induced genes from four main Spodoptera species: S. exigua, S. frugiperda, S. littoralis and S. litura. The cytochrome P450 monooxygenases (P450s; encoded by CYP genes) were the most upregulated detoxification genes across the literature for all four species. Most of the data was provided from studies on S. litura, followed by S. exigua, S. frugiperda and S. littoralis. We examined whether these detoxification genes were reported for larval survival under xenobiotic challenge in forward and reverse genetic studies. We further analyzed whether biochemical assays were carried out showing the ability of corresponding enzymes and transporters to breakdown and excrete xenobiotics, respectively. This revealed a clear disparity between species and the lack of genetic and biochemical information in S. frugiperda. Finally, we discussed the biological importance of detoxification genes for this genus and propose a workflow to study the involvement of these enzymes in an ecological and agricultural context.

Insecticide resistance in the Cydia pomonella (L): Global status, mechanisms, and research directions

The codling moth, Cydia pomonella (Lepidoptera: Tortricidae) is a major pest of pome fruit and walnuts worldwide. Although environmentally compatible integrated control strategies, such as mating disruption, attract-kill strategy, and sterile insect technique have been conducted for management of this notorious pest, effects to control of codling moth have mainly relied on insecticides. In consequence, different levels of insecticide resistance towards organophosphates, neonicotinoids, hydrazines, benzoylureas, pyrethroids, diamides, spinosyns, avermectins, JH mimics, carbamates, oxadiazines and C. pomonella granulovirus (CpGVs) have developed in codling moth in different countries and areas. Both metabolic and target-site mechanisms conferring resistance have been revealed in the codling moth. In this review, we summarize the current global status of insecticide resistance, the biochemical and molecular mechanisms involved, and the implications for resistance management.

Pyrethroid metabolism by eleven Helicoverpa armigera P450s from the CYP6B and CYP9A subfamilies

Lepidopteran P450s of the CYP6B and CYP9A subfamilies are thought to play important roles in host plant adaptation and insecticide resistance. An increasing number of paralogs and orthologs with high levels of sequence identity have been found in these subfamilies by mining recent genome projects. However, the biochemical function of most of them remains unknown. A better understanding of the evolution of P450 genes and of the catalytic competence of the enzymes they encode is needed to facilitate studies of host plant use and insecticide resistance. Here, we focused on the full complement of CYP6B (4 genes) and CYP9A (7 genes) in the generalist herbivore, Helicoverpa armigera. These P450s were heterologously expressed in Sf9 cells and compared functionally. In vitro assays showed that all CYP6B and CYP9A P450s can metabolize esfenvalerate efficiently, except for the evolutionarily divergent CYP6B43. A new 2'-hydroxy-metabolite of esfenvalerate was identified and found to be the main metabolite produced by CYP9A12. All tested P450s showed only low induction responses to esfenvalerate. To put these results from H. armigera P450s in perspective, 158 complete CYP6B and 100 complete CYP9A genes from 34 ditrysian species were manually curated. The CYP9A subfamily was more widespread than the CYP6B subfamily and the latter showed dramatic gains and losses, with ten species lacking CYP6B genes. Two adjacent CYP6B loci were found on chromosome 21, with different fates during the evolution of Lepidoptera. The diversity and functional redundancy of CYP6B and CYP9A genes challenge resistance management and pest control strategies as many P450s are available to insects to cope with chemical stresses they encounter.