Ecological Significance of Induction of Broad-Substrate Cytochrome P450s by Natural and Synthetic Inducers in Helicoverpa zea

Homology-based characterization of the cis-regulatory elements modulate flavone induction of CYP321A1 in Helicoverpa armigera

Xenobiotic response element (XRE) to flavone was the cis- regulatory elements that mediates the induction of the allelochemical-metabolizing CYP321A1 gene from Helicoverpa zea. However, it was unknown whether the XRE-Fla element existed in other species. Recently we have identified and cloned the CYP321A1 gene with promoter region in a related species, Helicoverpa armigera. Sequence similarity of two orthologous CYP321A1 genes was 97.27%, but the promoter sequence similarity was only 56.32%. Sequence alignment showed the XRE-Fla like element owns three mutations in H. armigera compared with H. zea. Progressive 5' deletions and internal mutation indicated that H. armigera XRE-Fla was the essential element of CYP321A1 gene in response to flavone. XRE-Fla mutations and EMSA analysis confirmed that the H. armigera XRE-Fla element binding factor was stronger than H. zea. The findings indicate the XRE element mutations mainly contribute to the differences between the flavone-induced expressions of two CYP321A1 genes, which improve the flexibility and adaptability for allelochemical response of H. armigera.

A transposon-introduced G-quadruplex motif is selectively retained and constrained to downregulate CYP321A1

Insects utilize xenobiotic compounds to up- and downregulate cytochrome P450 monooxygenases (P450s) involved in detoxification of toxic xenobiotics including phytochemicals and pesticides. G-quadruplexes (G4)-forming DNA motifs are enriched in the promoter regions of transcription factors and function as cis-acting elements to regulate these genes. Whether and how P450s gain and keep G4 DNA motifs to regulate their expression still remain unexplored. Here, we show that CYP321A1, a xenobiotic-metabolizing P450 from Helicoverpa zea, a polyphagous insect of economic importance, has acquired and preserved a G4 DNA motif by selectively retaining a transposon known as HzIS1-3 that carries this G4 DNA motif in its promoter region. The HzIS1-3 G4 DNA motif acts as a silencer to suppress the constitutive and induced expression of CYP321A1 by plant allelochemicals flavone and xanthotoxin through folding into an intramolecular parallel or hybrid-1 conformation in the absence or presence of K⁺ . The G4 ligand N-methylmesoporphyrin IX (NMM) strengthens the silencing effect of HzIS1-3 G4 DNA motif by switching its structure from hybrid-1 to hybrid-2. The enrichment of transposons in P450s and other environment-adaptation genes implies that selective retention of G4 DNA motif-carrying transposons may be the main evolutionary route for these genes to obtain G4 DNA motifs.

Overexpression of Multiple Cytochrome P450 Genes Conferring Clothianidin Resistance in Bradysia odoriphaga

Cytochrome P450 monooxygenases (P450s) play important roles in the detoxification metabolism of xenobiotics and are involved in the resistance of insects to many insecticides. In this study, piperonyl butoxide (PBO), an inhibitor of P450 enzyme activity, significantly increased the toxicity of clothianidin in the clothianidin-resistant (CL-R) population of Bradysia odoriphaga. The enzyme activity of P450 in the CL-R population was significantly higher than that in the SS population. Furthermore, four P450 genes were found to be significantly overexpressed in the CL-R population. Tissue-specific expression analysis indicates that CYP9J57, CYP3828A1, CYP6SX1, and CYP6QE1 were most highly expressed in the midgut and/or Malpighian tubules. After exposure to LC₃₀ of clothianidin, the expression levels of the four P450 genes were significantly upregulated. The RNAi-mediated knockdown of CYP9J57, CYP3828A1, and CYP6QE1 significantly increased the susceptibility of B. odoriphaga to clothianidin. These results suggest that P450 genes are involved in clothianidin resistance in B. odoriphaga. This provides a better understanding of P450-mediated clothianidin resistance in B. odoriphaga and will contribute to the management of insect resistance to insecticides.

Physiological and Population Responses of Nilaparvata lugens after Feeding on Drought-Stressed Rice

Simple Summary

Drought is considered a critical threat to crop growth and sustainable agriculture worldwide, and it also greatly impacts insect development and population growth. Brown planthopper (BPH), Nilaparvata lugens (Stål), is the predominant rice crop pest in China, and the damaging effects of BPH are enhanced by its strong migratory and reproductive capacities. Our results provide useful information about the effect of drought stress on the poor population growth and negative physiological changes in BPH. Negative changes to water balance and osmotic pressure can cause a decline in the quality of BPH; the GST content of BPH feeding on drought-stressed rice was significantly higher than BPH feeding on non-stressed control plants, and the length of flight muscle sarcomeres and mitochondrial content were decreased in BPH feeding on drought-stressed rice. These findings suggest that water management greatly impacts the physiology and population growth of BPH, and provide a basis for understanding physiological and population-wide responses in BPH during drought stress, which may be helpful in understanding the relationship between drought stress and BPH infestation.

Abstract

Drought stress greatly impacts insect development and population growth. Some studies have demonstrated increased reproductive capacity in drought-stressed insects; however, physiological changes in the brown planthopper (BPH), Nilaparvata lugens (Stål), during periods of drought are unclear. In this study, BPH fed on drought- stressed rice had lower population numbers than BPH feeding on non-stressed rice. Water content, osmotic pressure of hemolymph and total amino acid content of BPH were significantly lower when BPH fed on drought-stressed rice compared to the non-stressed control; however, glucose content and glutathione S-transferase (GST) activity were significantly higher in BPH fed on drought-stressed rice. The expression of Vitellogenin and Exuperantia in BPH fed on drought-stressed rice was higher than that in BPH feeding on non-stressed control plants. The size of myofibrils and the abundance of mitochondria in BPH flight muscles were significantly lower in BPH fed on drought-stressed rice compared to non-stressed plants. These results indicate that water management impacts the physiology of BPH, which may be useful in understanding the relationship between drought stress and this damaging herbivore.

Functionally characterized arthropod pest and pollinator cytochrome P450s associated with xenobiotic metabolism

The cytochrome P450 family (P450s) of arthropods includes diverse enzymes involved in endogenous essential physiological functions and in the oxidative metabolism of xenobiotics, insecticides and plant allelochemicals. P450s can also establish insecticide selectivity in bees and pollinators. Several arthropod P450s, distributed in different phylogenetic groups, have been associated with xenobiotic metabolism, and some of them have been functionally characterized, using different in vitro and in vivo systems. The purpose of this review is to summarize scientific publications on arthropod P450s from major insect and mite agricultural pests, pollinators and Papilio sp, which have been functionally characterized and shown to metabolize xenobiotics and/or their role (direct or indirect) in pesticide toxicity or resistance has been functionally validated. The phylogenetic relationships among these P450s, the functional systems employed for their characterization and their xenobiotic catalytic properties are presented, in a systematic approach, including critical aspects and limitations. The potential of the primary P450-based metabolic pathway of target and non-target organisms for the development of highly selective insecticides and resistance-breaking formulations may help to improve the efficiency and sustainability of pest control.

Transcriptomic analysis of salivary gland and proteomic analysis of oral secretion in Helicoverpa armigera under cotton plant leaves, gossypol, and tannin stresses

Gossypol and tannin are involved in important chemical defense processes in cotton plants. In this study, we used transcriptomics and proteomics to explore the changes in salivary gland functional genes and oral secretion (OS) proteins after feeding with artificial diet (containing gossypols and tannins) and cotton plant leaves. We found that dietary cotton plant leaves, gossypols and tannins exerted adverse impacts on the genes that regulated the functions of peptidase, GTPase, glycosyl hydrolases in the salivary glands of the Helicoverpa armigera (H. armigera). However, GST, UGT, hydrolases, and lipase genes were up-regulated to participate in the detoxification and digestive of H. armigera. The oral secretory proteins of H. armigera were significantly inhibited under the stress of gossypol and tannin, such as enzyme activity, but some proteins (such as PZC71358.1) were up-regulated and involved in immune and digestive functions. The combined analysis of transcriptomics and metabolomics showed a weak correlation, and the genes and proteins involved were mainly in digestive enzyme activities. Our work clarifies the deleterious physiological impacts of gossypols and tannins on H. armigera and reveals the mechanism by which H. armigera effectively mitigate the phytotoxic effects through detoxification and immune systems.

The Effect of Santolina chamaecyparissus and Tagetes patula Essential Oils on Biochemical Markers of Oxidative Stress in Aphids

Simple Summary

Due to numerous side effects associated with extensive use of chemical insecticides, there is a need to develop eco-friendly alternative methods for insect pest control. One of these alternatives may be the use of essential oils (EOs). An important aspect of the efficient and safe application of EOs in plant protection is the elucidation of their toxicity mechanisms towards target pests. The present study aimed to determine the effects of Santolina chamaecyparissus (L.) and Tagetes patula (L.) EOs on development and physiology of two aphid species with different feeding specializations. Myzus persicae (Sulzer) is a broad generalist and Rhopalosiphum padi (L.) is a grass specialist. Exposure to the tested EOs limited the aphids’ development and led to induction of oxidative stress within their tissues. Analysis of the physiological parameters also showed that the oligophagous R. padi was more sensitive to EO treatment than the highly polyphagous M. persicae. The results suggest that the tested EOs can affect important biochemical processes within aphid tissues and have potential as eco-friendly aphicides.

Abstract

This study investigated the toxicity of essential oils (EOs) from Santolina chamaecyparissus (L.) and Tagetes patula (L.) towards the green peach aphid Myzus persicae (Sulzer) and the bird cherry-oat aphid Rhopalosiphum padi (L.). The effects of the EOs on aphid population parameters and levels of biochemical markers of oxidative stress within insect tissues were analyzed. In laboratory bioassays, application of the studied EOs at sublethal concentrations reduced daily fecundity and led to a decrease in the intrinsic rate of natural increase in both aphid species. Treatment with EOs also induced generation of reactive oxygen species (ROS) within aphid tissues. The highest levels of superoxide anion and hydrogen peroxide were noted after 24 and 48 h of exposure. Moreover, a significant increase in lipid peroxidation was shown in treated aphids, especially between 48 and 72 h after exposure. The increase was more pronounced after treatment with the essential oil of S. chamaecyparissus, which also exhibited higher aphicidal activity in toxicity tests. The activities of antioxidant enzymes—superoxide dismutase (SOD) and catalase (CAT)—were significantly elevated in both aphid species in response to the tested EOs. The obtained results suggest that oxidative stress evoked by treatment with the studied EOs may be an important factor determining their toxicity towards aphids.

The role of cytochrome P450-mediated detoxification in insect adaptation to xenobiotics

Insect cytochrome P450 monooxygenases (P450s) are well known to be involved in metabolic detoxification of xenobiotics, such as phytochemicals, insecticides and environmental pollutants. Enhanced metabolic detoxification is closely associated with the constitutive overexpression and induction of P450s. In general, multiple insect P450s are co-responsible for xenobiotic detoxification. Considering the capacity of P450s to respond to a wide range of xenobiotics, synergistic interactions between natural and synthetic xenobiotics and P450-mediated cross-tolerance/resistance are ubiquitous. Recent studies have indicated that both transcription factors and signaling pathways are involved in the regulation of P450 genes in xenobiotic responses. This article reviews our current understanding of P450-mediated detoxification in insect adaptation to xenobiotics and highlights recent progress in the molecular basis of P450 regulation.

Identification of Wild-Type CYP321A2 and Comparison of Allelochemical-Induced Expression Profiles of CYP321A2 with Its Paralog CYP321A1 in Helicoverpa zea

One possible way to overcome the diversity of toxic plant allelochemicals idiosyncratically distributed among potential host plants is to have more counterdefense genes via gene duplication or fewer gene losses. Cytochrome P450 is the most important gene family responsible for detoxification of the diversity of plant allelochemicals. We have recently reported the identification and cloning of the transposon (HzSINE1)-disrupted non-functional CYP321A2, a duplicated paralog of the xenobiotic-metabolizing P450 CYP321A1 from a laboratory colony of Helicoverpa zea. Here we report the identification of the wild-type intact allele of CYP321A2 from another H. zea colony. This CYP321A2 allele encodes a deduced protein of 498 amino acids and has the P450 signature motifs. Quantitative RT-PCR experiments showed that this CYP321A2 allele was highly expressed in midgut and fat body and achieved the highest expression level in the developmental stage of 5th and 3rd instar larvae. CYP321A2 and CYP321A1 were constitutively expressed in low levels but can be differentially and significantly induced by a range of the plant allelochemicals and plant signal molecules, among which xanthotoxin, flavone, and coumarin were the most prominent inducers of CYP321A2 both in midgut and fat body, whereas flavone, coumarin, and indole-3-carbinol were the prominent inducers of CYP321A1 in midgut and fat body. Moreover, xanthotoxin- and flavone-responsive regulatory elements of CYP321A1 were also detected in the promoter region of CYP321A2. Our results enrich the P450 inventory by identifying an allelochemical broadly induced CYP321A2, a paralog of CYP321A1 in H. zea. Our data also suggest that the CYP321A2/CYP321A1 paralogs are a pair of duplicated genes of multigene families and CYP321A2 could potentially be involved in the detoxification of plant allelochemicals and adaptation of H. zea to its chemical environment.

Insects for Food and Feed-Safety Aspects Related to Mycotoxins and Metals

Edible insects as an alternative source of protein are discussed as an important contribution to future feed and food security. However, it has to be ensured that the consumption is non-hazardous. This systematic review summarizes findings concerning contaminations of insects with mycotoxins and heavy metal ions (SciFinder, Pubmed, until 26 June 2019). Both kinds of contaminants were reported to reduce growth performance and increase mortality in insects. There was no evidence for accumulation of various mycotoxins analyzed in distinct insect species. However, further research is necessary due to limitation of data. Since the gut content contributes relevantly to the total body burden of contaminants in insects, a starving period before harvesting is recommended. Contrary, accumulation of heavy metal ions occurred to a varying extent dependent on metal type, insect species, and developmental stage. Examples are the accumulation of cadmium (black soldier fly) and arsenic (yellow mealworm). The reported species-specific accumulation and metabolism patterns of contaminants emphasize the importance to assess potential safety hazards in a case-by-case approach. Subject to regular monitoring of contaminants, the general ban in the European Union to use waste in animal feed should also be questioned regarding insect farming.

Bioactivation of aflatoxin B1 by a cytochrome P450, CYP6AE19 induced by plant signaling methyl jasmonate in Helicoverpa armigra (Hübner)

Herbivore attack leads to enhanced production of defensive compounds to mount anti-herbivore defense in plants via activation of the jasmonate signaling pathway. On the other hand, some herbivores can eavesdrop on plants defense signaling and up-regulate their cytochrome P450 genes to increase detoxification of defensive compounds. However, the ecological risk of eavesdropping on plant defense signaling is largely unknown. In this study, we examined the induction of cytochrome P450s by methyl jasmonate (MeJA) and its consequence on the toxicity of aflatoxin B1 (AFB1) to Helicoverpa armigra larvae. The results show that MeJA applications either in a diet or volatile exposure enhanced the toxicity of AFB1 to the larvae. RNA sequences analysis revealed that cytochrome P450 CYP6AE19 was highly induced when MeJA was applied with AFB1. In addition, HaGST encoding glutathione-S-transferase that mainly transforms aflatoxin B1 exo-8,9-epoxide to aflatoxin B1 exo-8,9-glutathione was also induced. RNA interference of CYP6AE19 via injecting a double-stranded RNA decreased mortality of larvae exposed to AFB1; while injecting a double-stranded RNA of HaGST increased larval mortality. Furthermore, a protein model was generated and a subsequent docking simulation for AFB1 suggests the bioactivation as a major mechanism of AFB1. This study provides evidence that MeJA increased larval mortality of H. armigera via induction of CYP6AE19 that can bioactivate AFB1.

Plant extracts for developing mosquito larvicides: From laboratory to the field, with insights on the modes of action

In the last decades, major research efforts have been done to investigate the insecticidal activity of plant-based products against mosquitoes. This is a modern and timely challenge in parasitology, aimed to reduce the frequent overuse of synthetic pesticides boosting resistance development in mosquitoes and causing serious threats to human health and environment. This review covers the huge amount of literature available on plant extracts tested as mosquito larvicides, particularly aqueous and alcoholic ones, due to their easy formulation in water without using surfactants. We analysed results obtained on more than 400 plant species, outlining that 29 of them have outstanding larvicidal activity (i.e., LC₅₀ values below 10 ppm) against major vectors belonging to the genera Anopheles, Aedes and Culex, among others. Furthermore, synergistic and antagonistic effects between plant extracts and conventional pesticides, as well as among selected plant extracts are discussed. The efficacy of pure compounds isolated from the most effective plant extracts and - when available - their mechanism of action, as well as the impact on non-target species, is also covered. These belong to the following class of secondary metabolites: alkaloids, alkamides, sesquiterpenes, triterpenes, sterols, flavonoids, coumarins, anthraquinones, xanthones, acetogenonins and aliphatics. Their mode of action on mosquito larvae ranges from neurotoxic effects to inhibition of detoxificant enzymes and larval development and/or midugut damages. In the final section, current drawbacks as well as key challenges for future research, including technologies to synergize efficacy and improve stability - thus field performances - of the selected plant extracts, are outlined. Unfortunately, despite the huge amount of laboratory evidences about their efficacy, only a limited number of studies was aimed to validate their efficacy in the field, nor the epidemiological impact potentially arising from these vector control operations has been assessed. This strongly limits the development of commercial mosquito larvicides of botanical origin, at variance with plant-borne products developed in the latest decades to kill or repel other key arthropod species of medical and veterinary importance (e.g., ticks and lice), as well as mosquito adults. Further research on these issues is urgently needed.

CYP4CJ1-mediated gossypol and tannic acid tolerance in Aphis gossypii Glover

Cytochrome P450 monooxygenases play a key role in herbivorous pest adaptation to host plants by the detoxification against plant allelochemicals. A new P450 gene (CYP4CJ1) was identified from Aphis gossypii, which displayed a positive response to plant allelochemicals. The transcript levels of CYP4CJ1 could be significantly induced by both gossypol and tannic acid. Knockdown of CYP4CJ1 increased the sensitivity of A. gossypii to these two plant allelochemicals. These results suggest that CYP4CJ1 could be involved in the tolerance of A. gossypii to some plant allelochemicals. Subsequently, we examined the regulatory mechanism of CYP4CJ1 based on the transcriptional and post-transcriptional level. A promoter region from -1422 to -1166 of CYP4CJ1 was identified, which was an essential plant allelochemical responsive region. In addition, miR-4133-3p was found to participate in the regulation of CYP4CJ1 post-transcriptionally. Our results suggest that the transcript abundance of CYP4CJ1, following the exposure of A. gossypii to gossypol and tannic acid can be attributed to both the transcriptional and post-transcriptional regulation mechanisms. These results are important for understanding the roles of P450s in the plant allelochemical tolerance of A. gossypii.

An overexpressed cytochrome P450 CYP439A1v3 confers deltamethrin resistance in Laodelphax striatellus Fallén (Hemiptera: Delphacidae)

Deltamethrin resistance in Laodelphax striatellus had been associated with its oxidative detoxification by overexpression of four cytochrome P450 monooxygenases like CYP353D1v2, CYP6FU1, CYP6AY3v2, and CYP439A1v3. The first three P450s have been validated for insecticide-metabolizing capability and only CYP6FU1 was found to degrade deltamethrin. In this study, an investigation was conducted to confirm the capability of CYP439A1v3 to degrade deltamethrin. The CYP439A1v3 was first expressed in Sf9 cell line and its recombinant enzyme was tested for metabolic activity against different insecticides using substrate depletion assay combined with metabolite identification. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and carbon monoxide (CO)-difference spectra analysis showed that the intact cytochrome P450 protein was successfully expressed. Tests with probe substrates proved its enzyme activity, as p-nitroanisole, ethoxycoumarin, and ethoxyresorufin were preferentially metabolized (specific activity 7.767 ± 1.22, 1.325 ± 0.37, and 0.355 ± 0.37 nmol/min per mg of protein, respectively) while only luciferin-HEGE was not. In vitro incubation of the recombinant CYP439A1v3 protein with deltamethrin revealed hydroxylation by producing hydroxydeltamethrin. On the contrary, no metabolite/metabolism was seen with nonpyrethroid insecticide, including imidacloprid, buprofezin, chlorpyrifos, and fipronil. To the best of our knowledge, this is the first study to link a CYP450 from family 439 to confer pyrethroid resistance to L. striatellus. This finding should help in the design of appropriate insecticide resistance management for control of this strain of L. striatellus.

Identification of Two Cytochrome Monooxygenase P450 Genes, CYP321A7 and CYP321A9, from the Tobacco Cutworm Moth (Spodoptera Litura) and Their Expression in Response to Plant Allelochemicals

Larvae of the polyphagous tobacco cutworm moth, Spodoptera litura (S. litura), encounter potentially toxic allelochemicals in food. It is therefore important for S. litura to produce detoxification enzymes such as cytochrome P450 monooxygenases (P450s). In this study, we have identified two novel cytochrome P450 genes of S. litura, named CYP321A7 and CYP321A9. Phylogenetic analysis indicated that they belong to the CYP321A subfamily. Expression levels of these genes at different development stages were determined by real-time quantitative polymerase chain reaction (PCR). The highest expression was found in the midgut and the fat body. Larvae fed with a diet supplemented with xanthotoxin or coumarin showed a strongly increased expression of CYP321A7 and CYP321A9 in the midgut and fat body as compared to larvae that consumed a control diet. In contrast, larvae consuming a diet containing aflatoxin B1 or quercetin did not induce the expression of these genes. CYP321A7 and CYP321A9 showed different expression profiles with respect to certain allelochemicals. For example, a diet containing cinnamic acid stimulated the expression of CYP321A9, whereas no changes were observed for CYP321A7. We suggest that the fine tuning of P450 gene expression is an important adaptation mechanism that allows polyphagous S. litura larvae to survive in a changing chemical environment.

Resistance irrelevant CYP417A2v2 was found degrading insecticide in Laodelphax striatellus

Cytochrome P450 monooxygenases (CYPs) usually overexpressed in resistant strain were found involved in oxidative detoxification of insecticides. In this study, an investigation was conducted to confirm if resistance irrelevant CYPs which were not overexpressed in resistant strain before, were capable of degrading insecticides. Three resistance irrelevant CYPs viz. CYP417A2v2, CYP425A1v2, and CYP4DJ1 from CYP4 family of Laodelphax striatellus were randomly selected for experiments. CYP417A2v2 and CYP425A1v2 were found expressed successfully in Sf9 cell line while CYP4DJ1 was not expressed successfully and out of two expressed CYPs, only CYP417A2v2 showed its efficient catalytic activity. For catalytic activity, three traditional model probe substrates and five insecticides were assayed. For the probe substrates screened, p-nitroanisole and ethoxycoumarin were preferentially metabolized by CYP417A2v2 (specific activity 3.76 ± 1.22 and 1.63 ± 0.37 nmol min-1 mg protein-1, respectively) and they may be potential diagnostic probes for this enzyme. Among insecticides, only imidacloprid was efficiently degraded by CYP417A2v2. Incubation of imidacloprid with CYP417A2v2 of L. striatellus and subsequent HPLC, LC-MS, and MS/MS analysis revealed the formation of imidacloprid metabolites, that is, 4' or 5'hydroxy-imidacloprid by hydroxylation. This result implies the exemption of CYPs character that it is not always, all the CYPs degrading insecticides being selected and overexpressed in resistant strains and the degrading CYPs without mutations to upregulate could be candidates during insecticide resistance evolution. This characterization of individual insect CYPs in insecticide degradation can provide insight for better understand of insecticide resistance development.

Adaptation to toxic hosts as a factor in the evolution of insecticide resistance

Insecticide resistance is a serious economic problem that jeopardizes sustainability of chemical control of herbivorous insects and related arthropods. It can be viewed as a specific case of adaptation to toxic chemicals, which has been driven in large part, but not exclusively, by the necessity for insect pests to tolerate defensive compounds produced by their host plants. Synthetic insecticides may simply change expression of specific sets of detoxification genes that have evolved due to ancestral associations with host plants. Feeding on host plants with more abundant or novel secondary metabolites has even been shown to prime insect herbivores to tolerate pesticides. Clear understanding of basic evolutionary processes is important for achieving lasting success in managing herbivorous arthropods.

The retardant effect of 2-Tridecanone, mediated by Cytochrome P450, on the Development of Cotton bollworm, Helicoverpa armigera

BACKGROUND

Plant allelochemicals act as toxins, inhibitors of digestion, and deterrents that affect the fecundity of insects. These compounds have attracted significant research attention in recent decades, and much is known about the effects of these xenobiotic plant secondary metabolites on insect development. To date, although ecological interactions between xenobiotic plant secondary chemicals that retard insect growth have been observed in many species, it remains unclear how particular allelochemicals influence insect development in a life stage-dependent manner.

RESULTS

We found that 2-tridecanone can affect insect development; this effect appears similar to the symptoms induced by the physiological imbalance between juvenile and molting hormones in cotton bollworm. We later detected that a decrease in the concentration of 20-hydroxyecdysone occurred alongside the observed symptoms. We next identified the transcriptome of Helicoverpa armigera and eightdigital gene expression libraries for shading light on how 2-tridecanone retarded the development of cotton bollworm. The expression of CYP314A1, CYP315A1, CYP18A1, CYP307A1, and CYP306A1 (unigenes 16487, 15409, 40026, 41217, 35643, 16953, 8199, 13311, and 13036) were found to be induced by 2-tridecanone; these are known to be related to the biosynthesis or metabolism of 20-hydroxyecdysone. Expression analysis and RNA interference studies established that the retardant effect of 2-tridecanone on the development of cotton bollworm is mediated by P450 genes.

CONCLUSIONS

The candidate P450 gene approach described and exploited here is useful for identifying potential causal genes for the influence of plant allelochemicals on insect development.

Cytochrome P450 CYP6DA2 regulated by cap 'n'collar isoform C (CncC) is associated with gossypol tolerance in Aphis gossypii Glover

Cotton plants accumulate phytotoxins, such as gossypol and related sesquiterpene aldehydes, to resist insect herbivores. The survival of insects exposed to toxic secondary metabolites depends on the detoxification metabolism mediated by limited groups of cytochrome P450. Gossypol has an antibiotic effect on Aphis gossypii, and as the concentrations of gossypol were increased in the present study, the mortality of cotton aphids increased from 4 to 28%. The fecundity of the cotton aphids exposed to gossypol was also significantly reduced compared with the control. The transcriptional levels of CYP6DA2 in cotton aphids were significantly induced when exposed to gossypol, and knockdown of the CYP6DA2 transcripts by RNA interference (RNAi) significantly increased the toxicity of gossypol to cotton aphids. To further understand the gossypol regulatory cascade, the 5'-flanking promoter sequences of CYP6DA2 were isolated with a genome walker, and the promoter was very active and was inducible by gossypol. Co-transfection of the cap 'n' collar isoform C (CncC) and CYP6DA2 promoters dramatically increased the expression of CYP6DA2, and suppression of the CncC transcripts by RNAi significantly decreased the expression levels of CYP6DA2, and significantly increased the toxicity of gossypol to cotton aphids. Thus, the transcriptional regulation of CYP6DA2 involved the transcriptional factor CncC.

Only a minority of broad-range detoxification genes respond to a variety of phytotoxins in generalist Bemisia tabaci species

Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode. Here, we used the Bemisia tabaci sibling species MEAM1 and MED, as a model system for studying constitutive and induced detoxification resistance and their associated tradeoffs. B. tabaci adults were allowed to feed through membranes for 24 h on diet containing only sucrose or sucrose with various phytotoxins. Quantitative real-time PCR analyses of 18 detoxification genes, indicated that relatively few transcripts were changed in both the MEAM1 and MED species, in response to the addition of phytotoxins to the diet. Induced transcription of detoxification genes only in the MED species, in response to the presence of indole-3-carbinol in the insect’s diet, was correlated with maintenance of reproductive performance in comparison to significant reduction in performance of the MEAM1 species. Three genes, COE2, CYP6-like 5 and BtGST2, responded to more than one compound and were highly transcribed in the insect gut. Furthermore, functional assays showed that the BtGST2 gene encodes a protein capable of interacting with both flavonoids and glucosinolates. In conclusion, several detoxification genes were identified that could potentially be involved in the adaptation of B. tabaci to its host plants.

Phylogenetic analysis reveals positive correlations between adaptations to diverse hosts in a group of pathogen-like herbivores

A jack of all trades can be master of none-this intuitive idea underlies most theoretical models of host-use evolution in plant-feeding insects, yet empirical support for trade-offs in performance on distinct host plants is weak. Trade-offs may influence the long-term evolution of host use while being difficult to detect in extant populations, but host-use evolution may also be driven by adaptations for generalism. Here we used host-use data from insect collection records to parameterize a phylogenetic model of host-use evolution in armored scale insects, a large family of plant-feeding insects with a simple, pathogen-like life history. We found that a model incorporating positive correlations between evolutionary changes in host performance best fit the observed patterns of diaspidid presence and absence on nearly all focal host taxa, suggesting that adaptations to particular hosts also enhance performance on other hosts. In contrast to the widely invoked trade-off model, we advocate a "toolbox" model of host-use evolution in which armored scale insects accumulate a set of independent genetic tools, each of which is under selection for a single function but may be useful on multiple hosts.

Expression analysis of two P450 monooxygenase genes of the tobacco cutworm moth (Spodoptera litura) at different developmental stages and in response to plant allelochemicals

Cytochrome P450 monooxygenases (P450s) of insects are known to be involved in the metabolism or detoxification of plant allelochemicals and insecticides. Spodoptera litura (Lepidoptera, Noctuidae) is a polyphagous moth responsible for severe yield losses in many crops. In this study, two full-length P450 genes, CYP6B48 and CYP6B58, were cloned from S. litura. The cDNA sequences encode proteins with 503 and 504 amino acids, respectively. Phylogenetic analysis revealed that CYP6B48 and CYP6B58 belong to the CYP6B subfamily of P450s. Quantitative real-time PCR analyses showed that CYP6B48 and CYP6B58 were expressed only at larval stage, but not at pupal and adult stages. The highest levels of transcripts were found in the midguts and fat bodies of the larvae. No expression was detected in the ovary or hemolymph. Feeding with diets containing cinnamic acid, quercetin, or coumarin did not affect expression of CYP6B48. In contrast, diet supplemented with xanthotoxin dramatically increased the levels of CYP6B48 transcript in the midgut and fat bodies. Larvae fed with flavone had high levels of transcript of CYP6B48 in the midgut, whereas only slightly elevated levels were found in the fat bodies. Effects of the tested allelochemicals on CYP6B58 expression were minor. Hence, our findings show that S. litura responds to specific allelochemicals such as xanthotoxin with the accumulation of CYP6B48 transcripts, suggesting that specific signals in the food control the insect's ability to convert toxic allelochemicals to less harmful forms at the transcriptional level.

Common and unique cis-acting elements mediate xanthotoxin and flavone induction of the generalist P450 CYP321A1

How polyphagous herbivores up-regulate their counterdefense genes in response to a broad range of structurally different allelochemicals remains largely unknown. To test whether this is accomplished by having more allelochemical-response elements or the similar number of functionally more diverse elements, we mapped out the cis-acting elements mediating the induction of the allelochemical-metabolizing CYP321A1 from the generalist Helicoverpa zea by xanthotoxin and flavone, two structurally distinct allelochemicals with very different encounter rate by this species. Seven xanthotoxin-responsive elements were localized by analyzing promoter activities of varying length of CYP321A1 promoter in H. zea fatbody cells. Compared with the 5 flavone-responsive elements mapped out previously, there are four common elements (1 essential element, 2 enhancers, and 1 negative element) mediating induction of CYP321A1 by both of the two allelochemicals. The remaining four elements (3 enhancers and 1 negative element), however, only regulate induction of CYP321A1 by either of the two allelochemicals. Co-administration of the two allelochemicals resulted in an induction fold that is significantly lower than the expected additive value of the two allelochemicals. These results indicate that xanthotoxin- and flavone-induced expressions of CYP321A1 are mediated mainly by the functionally more diverse common elements although the allelochemical-unique elements also play a role.

Transcription profiling of 12 asian gypsy moth (Lymantria dispar) cytochrome P450 genes in response to insecticides

As the main group of detoxification enzymes, cytochrome P450 monoxygenases (P450s) catalyse an extremely diverse range of reactions that play an important role in the detoxification of foreign compounds. Transcription profiling of 12 Lymantria dispar P450 genes from the CYP6 subfamily believed to be involved in insecticide metabolism was performed in this study. Life-stage transcription profiling of CYP6 genes revealed significant variations between eggs, larvae, pupae, and adult males and females. Exposure of larvae to sublethal doses of deltamethrin, omethoate, and carbaryl enhanced the transcription of most of the CYP6 P450 genes, with induction peaking between 24 and 72 h after exposure. Transcription profiles were dependent on the levels of insecticide exposure and the various developmental stages.

Aflatoxin B1: toxicity, bioactivation and detoxification in the polyphagous caterpillar Trichoplusia ni

Trichoplusia ni caterpillars are polyphagous foliage-feeders and rarely likely to encounter aflatoxin B1 (AFB1), a mycotoxin produced by Aspergillus flavus and A. parasiticus, in their host plants. To determine how T. ni copes with AFB1, we evaluated the toxicity of AFB1 to T. ni caterpillars at different developmental stages and found that AFB1 tolerance significantly increases with larval development. Diet incorporation of AFB1 at 1 μg/g completely inhibited larval growth and pupation of newly hatched larvae, but 3 μg/g AFB1 did not have apparent toxic effects on larval growth and pupation of caterpillars that first consume this compound 10 days after hatching. Piperonyl butoxide, a general inhibitor of cytochrome P450 monooxygenases (P450s), reduced the toxicity of AFB1, suggesting that AFB1 is bioactivated in T. ni and this bioactivation is mediated by P450s. Some plant allelochemicals, including flavonoids such as flavones, furanocoumarins such as xanthotoxin and imperatorin, and furanochromones such as visnagin, that induce P450s in other lepidopteran larvae ameliorated AFB1 toxicity, suggesting that P450s are also involved in AFB1 detoxification in T. ni.

Impact of environment on mosquito response to pyrethroid insecticides: facts, evidences and prospects

By transmitting major human diseases such as malaria, dengue fever and filariasis, mosquito species represent a serious threat worldwide in terms of public health, and pose a significant economic burden for the African continent and developing tropical regions. Most vector control programmes aiming at controlling life-threatening mosquitoes rely on the use of chemical insecticides, mainly belonging to the pyrethroid class. However, resistance of mosquito populations to pyrethroids is increasing at a dramatic rate, threatening the efficacy of control programmes throughout insecticide-treated areas, where mosquito-borne diseases are still prevalent. In the absence of new insecticides and efficient alternative vector control methods, resistance management strategies are therefore critical, but these require a deep understanding of adaptive mechanisms underlying resistance. Although insecticide resistance mechanisms are intensively studied in mosquitoes, such adaptation is often considered as the unique result of the selection pressure caused by insecticides used for vector control. Indeed, additional environmental parameters, such as insecticides/pesticides usage in agriculture, the presence of anthropogenic or natural xenobiotics, and biotic interactions between vectors and other organisms, may affect both the overall mosquito responses to pyrethroids and the selection of resistance mechanisms. In this context, the present work aims at updating current knowledge on pyrethroid resistance mechanisms in mosquitoes and compiling available data, often from different research fields, on the impact of the environment on mosquito response to pyrethroids. Key environmental factors, such as the presence of urban or agricultural pollutants and biotic interactions between mosquitoes and their microbiome are discussed, and research perspectives to fill in knowledge gaps are suggested.

Insect P450s: mounted for battle in their war against toxins

Transcriptome profiling methods are rapidly changing the ways in which insect responses to the environment can be assessed. One article featured in this issue of Molecular Ecology utilizes global expression analysis to extend previous studies examining the basis of insecticide resistance in Helicoverpa species that are devastating crop pests worldwide. In this issue Tao et al. (2012) have used a wide spectrum of biochemical and molecular techniques to examine the extent to which naturally occurring plant compounds and one synthetic insecticide change transcriptome profiles of Helicoverpa armigera larvae (Fig. 1) to enhance their adaptation to plant toxins and insecticides in their diet and environment. With their data highlighting an array of P450 transcripts and a few esterase transcripts that are correlated with increased tolerance to the insecticide deltamethrin, these researchers provide the first comprehensive evaluation of cytochrome P450 monooxygenase (P450), glutathione S-transferase (GST) and esterase (Est) gene responses in a lepidopteran species repeatedly subjected to pyrethroid selection. Building on this information, these researchers venture into the realm of the biorational in showing that RNAi-mediated destruction of one prominent toxin-inducible P450 transcript can increase sensitivity to deltamethrin, thereby compromising larval growth in the presence of this insecticide. The fact that the resulting levels of deltamethrin tolerance in the RNAi-silenced larvae are not restored completely to the levels in larvae whose defences are not stimulated by consuming plant compounds provides further evidence that their adaptive strategies for survival in toxin-rich environments include many different P450, esterase and, possibly, other genes. Together, their data highlight the complexity of the defence systems evolving in lepidopterans and other insect pests.

Resistance of Australian Helicoverpa armigera to fenvalerate is due to the chimeric P450 enzyme CYP337B3

Worldwide, increasing numbers of insects have evolved resistance to a wide range of pesticides, which hampers their control in the field and, therefore, threatens agriculture. Members of the carboxylesterase and cytochrome P450 monooxygenase superfamilies are prominent candidates to confer metabolic resistance to pyrethroid insecticides. Both carboxylesterases and P450 enzymes have been shown to be involved in pyrethroid resistance in Australian Helicoverpa armigera, the noctuid species possessing by far the most reported resistance cases worldwide. However, specific enzymes responsible for pyrethroid resistance in field populations of this species have not yet been identified. Here, we show that the resistance toward fenvalerate in an Australian strain of H. armigera is due to a unique P450 enzyme, CYP337B3, which arose from unequal crossing-over between two parental P450 genes, resulting in a chimeric enzyme. CYP337B3 is capable of metabolizing fenvalerate into 4'-hydroxyfenvalerate, which exhibits no toxic effect on susceptible larvae; enzymes from the parental P450 genes showed no detectable fenvalerate metabolism. Furthermore, a polymorphic H. armigera strain could be bred into a susceptible line possessing the parental genes CYP337B1 and CYP337B2 and a resistant line possessing only CYP337B3. The exclusive presence of CYP337B3 in resistant insects of this strain confers a 42-fold resistance to fenvalerate. Thus, in addition to previously documented genetic mechanisms of resistance, recombination can also generate selectively advantageous variants, such as this chimeric P450 enzyme with an altered substrate specificity leading to a potent resistance mechanism.

Cyanogenic glucosides in the biological warfare between plants and insects: the Burnet moth-Birdsfoot trefoil model system

Cyanogenic glucosides are important components of plant defense against generalist herbivores due to their bitter taste and the release of toxic hydrogen cyanide upon tissue disruption. Some specialized herbivores, especially insects, preferentially feed on cyanogenic plants. Such herbivores have acquired the ability to metabolize cyanogenic glucosides or to sequester them for use in their own predator defense. Burnet moths (Zygaena) sequester the cyanogenic glucosides linamarin and lotaustralin from their food plants (Fabaceae) and, in parallel, are able to carry out de novo synthesis of the very same compounds. The ratio and content of cyanogenic glucosides is tightly regulated in the different stages of the Zygaena filipendulae lifecycle and the compounds play several important roles in addition to defense. The transfer of a nuptial gift of cyanogenic glucosides during mating of Zygaena has been demonstrated as well as the possible involvement of hydrogen cyanide in male assessment and nitrogen metabolism. As the capacity to de novo synthesize cyanogenic glucosides was developed independently in plants and insects, the great similarities of the pathways between the two kingdoms indicate that cyanogenic glucosides are produced according to a universal route providing recruitment of the enzymes required. Pyrosequencing of Z. filipendulae larvae de novo synthesizing cyanogenic glucosides served to provide a set of good candidate genes, and demonstrated that the genes encoding the pathway in plants and Z. filipendulae are not closely related phylogenetically. Identification of insect genes involved in the biosynthesis and turn-over of cyanogenic glucosides will provide new insights into biological warfare as a determinant of co-evolution between plants and insects.

Natural history and intragenomic dynamics of the Transib transposon Hztransib in the cotton bollworm Helicoverpa zea

Hztransib, recently identified from Helicoverpa zea, represents the first intact and transcriptionally active Transib element. Its open reading frame was detected in Helicoverpa armigera, from which H. zea evolved, and in Helicoverpa assulta, the common ancestor of H. zea and H. armigera, but its remaining parts were found only in H. armigera. Thirty-nine Hztransib insertion sites, all of which are polymorphic, were detected from eight populations of H. zea. Out of the 39 insertion sites, 35 were not frequently occupied, with 1-33 occurrences in a total of 128 individuals from the eight populations (16 larvae per population). Its copy number ranged from 5.8 to 14.2 per individual, with putative intact copies always more abundant than internally deleted ones. Taking this evidence together, Hztransib probably transferred to H. zea from H. armigera and most likely still retains its capacity to maintain structural integrity, increase copy number and remobilize in H. zea.

General guidelines for authors for submission of manuscripts that contain molecular biological content

The Journal of Chemical Ecology (the Journal) has been publishing an increasing number of contributions that report experiments that use the methods of molecular biology. In 2004, a special issue of the Journal [JCE 30(12)] focused on molecular chemical ecology and documented the increasing trend of published submissions with molecular content (Seybold 2004). The guidelines presented here were developed to provide assistance to authors, with the goal of ensuring standardized and complete reporting of molecular information in the Journal. The fields of molecular biology and bioinformatics change rapidly as new technologies and protocols are continually developed and introduced. Thus, the Journal intends to revise and re-issue these guidelines periodically as significant changes in this area of science occur. Molecular biological content includes reports on specific nucleic acids and proteins involved in chemical ecological phenomena. The guidelines are separated into 3 parts: I. Types of Papers Published; II. Appropriate Nomenclature and Abbreviations; and III. Presentation of Molecular Biological Content.

Transcriptional overexpression of CYP6B8/CYP6B28 and CYP6B9 is a mechanism associated with cypermethrin survivorship in field-collected Helicoverpa zea (Lepidoptera: Noctuidae) moths

BACKGROUND

There are multiple reports of resistance to pyrethroid insecticides in Helicoverpa zea (Boddie); however, metabolic mechanisms responsible for insect survivorship have yet to be elucidated at the molecular level. Here, the transcriptional expression of two cytochrome P450 genes involved in pyrethroid metabolism, CYP6B8/CYP6B28 and CYP6B9, in H. zea male moths surviving discriminating dosages of cypermethrin in the adult vial test were investigated using quantitative PCR.

RESULTS

Insects classified as cypermethrin resistant in the adult vial assay contained significantly higher levels of both transcripts compared with the susceptible (ranging from a factor of 3.7 to 34.9 for CYP6B8/CYP6B28 and from 5.6 to 39.6 for CYP6B9), and in individual insects both transcripts were present at similar levels. Analysis of individual males collected across ecological regions and years in Texas showed overexpression of these genes.

CONCLUSIONS

This paper is the first to identify transcriptional overexpression of CYP genes associated with pyrethroid survivorship in field-collected H. zea. This discovery will allow for greater focus on increased metabolism studies at the population level. Understanding mechanisms responsible for resistance will greatly improve the ability to monitor resistance and make better control recommendations, as metabolic resistance may threaten the success not only of pyrethroids but also of other synthetic pesticides.

Functional characterization of cis-acting elements mediating flavone-inducible expression of CYP321A1

How plant allelochemicals elicit herbivore counterdefense genes remains largely unknown. To define the cis-acting elements for flavone inducibility of the allelochemical-metabolizing CYP321A1 from Helicoverpa zea, functions of varying length of CYP321A1 promoter are examined in H. zea fatbody cells. Progressive 3' deletions reveal presence of positive elements in the 5' untranslated region (UTR). Progressive 5' deletions map out regions of one essential element, four enhancers, and two silencers. Further progressive 5'deletions localize the essential element to a 36-bp region from -109 to -74. This essential element, designated as xenobiotic response element to flavone (XRE-Fla), contains a 5' AT-only TAAT inverted repeat, a GCT mirror repeat and a 3' antioxidant response element-like element. Internal deletions and substitution mutations show that the TAAT repeat is only necessary for the maximal flavone inducibility, whereas the other two components are necessary for the basal and flavone-induced expression of CYP321A1. Electrophoresis mobility shift assays demonstrate that XRE-Fla specifically binds to H. zea fatbody cell nuclear extracts and flavone treatment increases the nuclear concentrations of the yet-to-be characterized transcription factors binding to XRE-Fla. Taken together, CYP321A1 expression is regulated primarily by XRE-Fla and secondarily by other cis elements scattered in its promoter and 5' UTR.

Transcription profiling of eleven cytochrome P450s potentially involved in xenobiotic metabolism in the mosquito Aedes aegypti

Transcription profiles of 11 Aedes aegypti P450 genes from CYP6 and CYP9 subfamilies potentially involved in xenobiotic metabolism were investigated. Many genes were preferentially transcribed in tissues classically involved in xenobiotic metabolism including midgut and Malpighian tubules. Life-stage transcription profiling revealed important variations amongst larvae, pupae, and adult males and females. Exposure of mosquito larvae to sub-lethal doses of three xenobiotics induced the transcription of several genes with an induction peak after 48 to 72 h exposure. Several CYP genes were also induced by oxidative stress and one gene strongly responded to 20-hydroxyecdysone. Overall, this study revealed that these P450s show different transcription profiles according to xenobiotic exposures, life stages or sex. Their putative chemoprotective functions are discussed.