Flavin-dependent monooxygenases as a detoxification mechanism in insects: new insights from the arctiids (lepidoptera)

What one genus of showy moths can say about migration, adaptation, and wing pattern

The Ornate Moth, Utetheisa ornatrix, has served as a model species in chemical ecology studies for decades. Like in the widely publicized stories of the Monarch and other milkweed butterflies, the Ornate Moth and its relatives are tropical insects colonizing whole continents assisted by their chemical defenses. With the recent advances in genomic techniques and evo-devo research, it is becoming a model for studies in other areas, from wing pattern development to phylogeography, from toxicology to epigenetics. We used a genomic approach to learn about Utetheisa's evolution, detoxification, dispersal abilities, and wing pattern diversity. We present an evolutionary genomic analysis of the worldwide genus Utetheisa, then focusing on U. ornatrix. Our reference genome of U. ornatrix reveals gene duplications in the regions possibly associated with detoxification abilities, which allows them to feed on toxic food plants. Finally, comparative genomic analysis of over 100 U. ornatrix specimens from the museum with apparent differences in wing patterns suggest the potential roles of cortex and lim3 genes in wing pattern formation of Lepidoptera and the utility of museum-preserved collection specimens for wing pattern research.

Spontaneously hypertensive rats exhibit increased liver flavin monooxygenase expression and elevated plasma TMAO levels compared to normotensive and Ang II-dependent hypertensive rats

Background: Flavin monooxygenases (FMOs) are enzymes responsible for the oxidation of a broad spectrum of exogenous and endogenous amines. There is increasing evidence that trimethylamine (TMA), a compound produced by gut bacteria and also recognized as an industrial pollutant, contributes to cardiovascular diseases. FMOs convert TMA into trimethylamine oxide (TMAO), which is an emerging marker of cardiovascular risk. This study hypothesized that blood pressure phenotypes in rats might be associated with variations in the expression of FMOs. Methods: The expression of FMO1, FMO3, and FMO5 was evaluated in the kidneys, liver, lungs, small intestine, and large intestine of normotensive male Wistar-Kyoto rats (WKY) and two distinct hypertensive rat models: spontaneously hypertensive rats (SHRs) and WKY rats with angiotensin II-induced hypertension (WKY-ANG). Plasma concentrations of TMA and TMAO were measured at baseline and after intravenous administration of TMA using liquid chromatography-mass spectrometry (LC-MS). Results: We found that the expression of FMOs in WKY, SHR, and WKY-ANG rats was in the descending order of FMO3 > FMO1 >> FMO5. The highest expression of FMOs was observed in the liver. Notably, SHRs exhibited a significantly elevated expression of FMO3 in the liver compared to WKY and WKY-ANG rats. Additionally, the plasma TMAO/TMA ratio was significantly higher in SHRs than in WKY rats. Conclusion: SHRs demonstrate enhanced expression of FMO3 and a higher plasma TMAO/TMA ratio. The variability in the expression of FMOs and the metabolism of amines might contribute to the hypertensive phenotype observed in SHRs.

Kinetic characterization of a flavin-dependent monooxygenase from the insect food crop pest, Zonocerus variegatus

Zonocerus variegatus, or the painted grasshopper, is a food crop pest endemic in Western and Central Africa. Agricultural industries in these regions rely heavily on natural defense mechanisms to control the grasshopper population such as plant-secreted alkaloid compounds. In recent years, the Z. variegatus population has continued to rise due to acquired resistance to alkaloids. Here we focus on the kinetic characterization of a flavin-dependent monooxygenase, ZvFMO, that catalyzes the nitrogen oxidation of many of these alkaloid compounds and confers resistance to the insect. Expression and purification of ZvFMO through a traditional E. coli expression system was successful and provided a unique opportunity to characterize the catalytic properties of an FMO from insects. ZvFMO was found to catalyze oxidation reactions of tertiary nitrogen atoms and the sulfur of cysteamine. Using stopped-flow spectroscopy, we have determined the kinetic mechanism of ZvFMO. We assessed F383 for its involvement in substrate binding, which was previously proposed, and determined that this residue does not play a major role in binding substrates. Through molecular docking, we identified N304 and demonstrated that this residue plays a role in substrate binding. The role of K215 was studied and was shown that it plays a critical role in NAD(P)H binding and cofactor selectivity.

Evolution of chemosensory and detoxification gene families across herbivorous Drosophilidae

Herbivorous insects are exceptionally diverse, accounting for a quarter of all known eukaryotic species, but the genomic basis of adaptations that enabled this dietary transition remains poorly understood. Many studies have suggested that expansions and contractions of chemosensory and detoxification gene families-genes directly mediating interactions with plant chemical defenses-underlie successful plant colonization. However, this hypothesis has been challenging to test because the origins of herbivory in many insect lineages are ancient (>150 million years ago (mya)), obscuring genomic evolutionary patterns. Here, we characterized chemosensory and detoxification gene family evolution across Scaptomyza, a genus nested within Drosophila that includes a recently derived (<15 mya) herbivore lineage of mustard (Brassicales) specialists and carnation (Caryophyllaceae) specialists, and several nonherbivorous species. Comparative genomic analyses revealed that herbivorous Scaptomyza has among the smallest chemosensory and detoxification gene repertoires across 12 drosophilid species surveyed. Rates of gene turnover averaged across the herbivore clade were significantly higher than background rates in over half of the surveyed gene families. However, gene turnover was more limited along the ancestral herbivore branch, with only gustatory receptors and odorant-binding proteins experiencing strong losses. The genes most significantly impacted by gene loss, duplication, or changes in selective constraint were those involved in detecting compounds associated with feeding on living plants (bitter or electrophilic phytotoxins) or their ancestral diet (fermenting plant volatiles). These results provide insight into the molecular and evolutionary mechanisms of plant-feeding adaptations and highlight gene candidates that have also been linked to other dietary transitions in Drosophila.

Disentangling hindgut metabolism in the American cockroach through single-cell genomics and metatranscriptomics

Omnivorous cockroaches host a complex hindgut microbiota comprised of insect-specific lineages related to those found in mammalian omnivores. Many of these organisms have few cultured representatives, thereby limiting our ability to infer the functional capabilities of these microbes. Here we present a unique reference set of 96 high-quality single cell-amplified genomes (SAGs) from bacterial and archaeal cockroach gut symbionts. We additionally generated cockroach hindgut metagenomic and metatranscriptomic sequence libraries and mapped them to our SAGs. By combining these datasets, we are able to perform an in-depth phylogenetic and functional analysis to evaluate the abundance and activities of the taxa in vivo. Recovered lineages include key genera within Bacteroidota, including polysaccharide-degrading taxa from the genera Bacteroides, Dysgonomonas, and Parabacteroides, as well as a group of unclassified insect-associated Bacteroidales. We also recovered a phylogenetically diverse set of Firmicutes exhibiting a wide range of metabolic capabilities, including-but not limited to-polysaccharide and polypeptide degradation. Other functional groups exhibiting high relative activity in the metatranscriptomic dataset include multiple putative sulfate reducers belonging to families in the Desulfobacterota phylum and two groups of methanogenic archaea. Together, this work provides a valuable reference set with new insights into the functional specializations of insect gut symbionts and frames future studies of cockroach hindgut metabolism.

Insight into the genomes of dominant yeast symbionts of European spruce bark beetle, Ips typographus

Spruce bark beetle Ips typographus can trigger outbreaks on spruce that results in significant losses in the forest industry. It has been suggested that symbiotic microorganisms inhabiting the gut of bark beetles facilitate the colonization of plant tissues as they play a role in the detoxification of plant secondary metabolites, degrade plant cell wall and ameliorate beetle's nutrition. In this study, we sequenced and functionally annotated the genomes of five yeasts Kuraishia molischiana, Cryptococcus sp., Nakazawaea ambrosiae, Ogataea ramenticola, and Wickerhamomyces bisporus isolated from the gut of Ips typographus. Genome analysis identified 5314, 7050, 5722, 5502, and 5784 protein coding genes from K. molischiana, Cryptococcus sp., N. ambrosiae, O. ramenticola, and W. bisporus, respectively. Protein-coding sequences were classified into biological processes, cellular and molecular function based on gene ontology terms enrichment. Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation was used to predict gene functions. All analyzed yeast genomes contain full pathways for the synthesis of essential amino acids and vitamin B6, which have nutritional importance to beetle. Furthermore, their genomes contain diverse gene families related to the detoxification processes. The prevalent superfamilies are aldo-keto reductase, ATP-binding cassette and the major facilitator transporters. The phylogenetic relationships of detoxification-related enzymes aldo-keto reductase, and cytochrome P450 monooxygenase, and ATP-binding cassette are presented. Genome annotations also revealed presence of genes active in lignocellulose degradation. In vitro analyses did not confirm enzymatic endolytic degradation of lignocellulose; however, all species can utilize and pectin and produce a large spectrum of exolytic enzymes attacking cellulose, chitin, and lipids.

Evolution of enzyme functionality in the flavin-containing monooxygenases

Among the molecular mechanisms of adaptation in biology, enzyme functional diversification is indispensable. By allowing organisms to expand their catalytic repertoires and adopt fundamentally different chemistries, animals can harness or eliminate new-found substances and xenobiotics that they are exposed to in new environments. Here, we explore the flavin-containing monooxygenases (FMOs) that are essential for xenobiotic detoxification. Employing a paleobiochemistry approach in combination with enzymology techniques we disclose the set of historical substitutions responsible for the family's functional diversification in tetrapods. Remarkably, a few amino acid replacements differentiate an ancestral multi-tasking FMO into a more specialized monooxygenase by modulating the oxygenating flavin intermediate. Our findings substantiate an ongoing premise that enzymatic function hinges on a subset of residues that is not limited to the active site core.

Transcriptional responses are oriented towards different components of the rearing environment in two Drosophila sibling species

Background

The chance to compare patterns of differential gene expression in related ecologically distinct species can be particularly fruitful to investigate the genetics of adaptation and phenotypic plasticity. In this regard, a powerful technique such as RNA-Seq applied to ecologically amenable taxa allows to address issues that are not possible in classic model species. Here, we study gene expression profiles and larval performance of the cactophilic siblings Drosophila buzzatii and D. koepferae reared in media that approximate natural conditions and evaluate both chemical and nutritional components of the diet. These closely related species are complementary in terms of host-plant use since the primary host of one is the secondary of the other. D. koepferae is mainly a columnar cactus dweller while D. buzzatii prefers Opuntia hosts.

Results

Our comparative study shows that D. buzzatii and D. koepferae have different transcriptional strategies to face the challenges posed by their natural resources. The former has greater transcriptional plasticity, and its response is mainly modulated by alkaloids of its secondary host, while the latter has a more canalized genetic response, and its transcriptional plasticity is associated with the cactus species.

Conclusions

Our study unveils a complex pleiotropic genetic landscape in both species, with functional links that relate detox responses and redox mechanisms with developmental and neurobiological processes. These results contribute to deepen our understanding of the role of host plant shifts and natural stress driving ecological specialization.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08745-9.

Sequestration of Plant Defense Compounds by Insects: From Mechanisms to Insect-Plant Coevolution

Plant defense compounds play a key role in the evolution of insect-plant associations by selecting for behavioral, morphological, and physiological insect adaptations. Sequestration, the ability of herbivorous insects to accumulate plant defense compounds to gain a fitness advantage, represents a complex syndrome of adaptations that has evolved in all major lineages of herbivorous insects and involves various classes of plant defense compounds. In this article, we review progress in understanding how insects selectively accumulate plant defense metabolites and how the evolution of specific resistance mechanisms to these defense compounds enables sequestration. These mechanistic considerations are further integrated into the concept of insect-plant coevolution. Comparative genome and transcriptome analyses, combined with approaches based on analytical chemistry that are centered in phylogenetic frameworks, will help to reveal adaptations underlying the sequestration syndrome, which is essential to understanding the influence of sequestration on insect-plant coevolution.

Influence of formic acid treatment on the proteome of the ectoparasite Varroa destructor

The ectoparasite Varroa destructor Anderson and Trueman is the most important parasites of the western honey bee, Apis mellifera L. The most widely currently used treatment uses formic acid (FA), but the understanding of its effects on V. destructor is limited. In order to understand the mechanism of action of FA, its effect on Varroa mites was investigated using proteomic analysis by liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). V. destructor was collected from honey bee colonies with natural mite infestation before and 24 h after the initiation of FA treatment and subjected to proteome analysis. A total of 2637 proteins were identified. Quantitative analysis of differentially expressed candidate proteins (fold change ≥ 1.5; p ≤ 0.05) revealed 205 differentially expressed proteins: 91 were induced and 114 repressed in the FA-treated group compared to the untreated control group. Impaired protein synthesis accompanied by increased protein and amino acid degradation suggest an imbalance in proteostasis. Signs of oxidative stress included significant dysregulation of candidate proteins of mitochondrial cellular respiration, increased endocytosis, and induction of heat shock proteins. Furthermore, an increased concentration of several candidate proteins associated with detoxification was observed. These results suggest dysregulated cellular respiration triggered by FA treatment as well as an increase in cellular defense mechanisms, including induced heat shock proteins and detoxification enzymes.

Engineering cellular metabolite transport for biosynthesis of computationally predicted tropane alkaloid derivatives in yeast

Microbial biosynthesis of plant natural products (PNPs) can facilitate access to valuable medicinal compounds and derivatives. Such efforts are challenged by metabolite transport limitations, which arise when complex plant pathways distributed across organelles and tissues are reconstructed in unicellular hosts without concomitant transport machinery. We recently reported an engineered yeast platform for production of the tropane alkaloid (TA) drugs hyoscyamine and scopolamine, in which product accumulation is limited by vacuolar transport. Here, we demonstrate that alleviation of transport limitations at multiple steps in an engineered pathway enables increased production of TAs and screening of useful derivatives. We first show that supervised classifier models trained on a tissue-delineated transcriptome from the TA-producing plant Atropa belladonna can predict TA transporters with greater efficacy than conventional regression- and clustering-based approaches. We demonstrate that two of the identified transporters, AbPUP1 and AbLP1, increase TA production in engineered yeast by facilitating vacuolar export and cellular reuptake of littorine and hyoscyamine. We incorporate four different plant transporters, cofactor regeneration mechanisms, and optimized growth conditions into our yeast platform to achieve improvements in de novo hyoscyamine and scopolamine production of over 100-fold (480 μg/L) and 7-fold (172 μg/L). Finally, we leverage computational tools for biosynthetic pathway prediction to produce two different classes of TA derivatives, nortropane alkaloids and tropane N-oxides, from simple precursors. Our work highlights the importance of cellular transport optimization in recapitulating complex PNP biosyntheses in microbial hosts and illustrates the utility of computational methods for gene discovery and expansion of heterologous biosynthetic diversity.

From Global to Local-New Insights into Features of Pyrethroid Detoxification in Vector Mosquitoes

The threat of mosquito-borne diseases continues to be a problem for public health in subtropical and tropical regions of the world; in response, there has been increased use of adulticidal insecticides, such as pyrethroids, in human habitation areas over the last thirty years. As a result, the prevalence of pyrethroid-resistant genetic markers in natural mosquito populations has increased at an alarming rate. This review details recent advances in the understanding of specific mechanisms associated with pyrethroid resistance, with emphasis on features of insecticide detoxification and the interdependence of multiple cellular pathways. Together, these advances add important context to the understanding of the processes that are selected in resistant mosquitoes. Specifically, before pyrethroids bind to their targets on motoneurons, they must first permeate the outer cuticle and diffuse to inner tissues. Resistant mosquitoes have evolved detoxification mechanisms that rely on cytochrome P450s (CYP), esterases, carboxyesterases, and other oxidation/reduction (redox) components to effectively detoxify pyrethroids to nontoxic breakdown products that are then excreted. Enhanced resistance mechanisms have evolved to include alteration of gene copy number, transcriptional and post-transcriptional regulation of gene expression, as well as changes to cellular signaling mechanisms. Here, we outline the variety of ways in which detoxification has been selected in various mosquito populations, as well as key gene categories involved. Pathways associated with potential new genes of interest are proposed. Consideration of multiple cellular pathways could provide opportunities for development of new insecticides.

Comparative transcriptomics indicates endogenous differences in detoxification capacity after formic acid treatment between honey bees and varroa mites

Formic acid (FA) has been used for decades to control Varroa destructor, one of the most important parasites of the western honey bee, Apis mellifera. The rather unselective molecular mode of action of FA and its possible effects on honeybees have long been a concern of beekeepers, as it has undesirable side effects that affect the health of bee colonies. This study focuses on short-term transcriptomic changes as analysed by RNAseq in both larval and adult honey bees and in mites after FA treatment under applied conditions. Our study aims to identify those genes in honey bees and varroa mites differentially expressed upon a typical FA hive exposure scenario. Five detoxification-related genes were identified with significantly enhanced and one gene with significantly decreased expression under FA exposure. Regulated genes in our test setting included members of various cytochrome P450 subfamilies, a flavin-dependent monooxygenase and a cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH), known to be involved in formate metabolism in mammals. We were able to detect differences in the regulation of detoxification-associated genes between mites and honey bees as well as between the two different developmental stages of the honey bee. Additionally, we detected repressed regulation of Varroa genes involved in cellular respiration, suggesting mitochondrial dysfunction and supporting the current view on the mode of action of FA-inhibition of oxidative phosphorylation. This study shows distinct cellular effects induced by FA on the global transcriptome of both host and parasite in comparison. Our expression data might help to identify possible differences in the affected metabolic pathways and thus make a first contribution to elucidate the mode of detoxification of FA.

Transcriptome profiling revealed potentially important roles of defensive gene expression in the divergence of insect biotypes: a case study with the cereal aphid Sitobion avenae

Background

Many insects can develop differential biotypes on variable host plants, but the underlying molecular factors and mechanisms are not well understood. To address this issue, transcriptome profiling analyses were conducted for two biotypes of the cereal aphid, Sitobion avenae (Fabricius), on both original and alternative plants.

Results

Comparisons between both biotypes generated 4174 differentially expressed unigenes (DEGs). In their response to host plant shift, 39 DEGs were shared by both biotypes, whereas 126 and 861 DEGs occurred only in biotypes 1 and 3, respectively. MMC (modulated modularity clustering) analyses showed that specific DEGs of biotypes 1 and 3 clustered into five and nine transcriptional modules, respectively. Among these DEGs, defense-related genes underwent intensive expression restructuring in both biotypes. However, biotype 3 was found to have relatively lower gene transcriptional plasticity than biotype 1. Gene enrichment analyses of the abovementioned modules showed functional divergence in defensive DEGs for the two biotypes in response to host transfer. The expression plasticity for some defense related genes was showed to be directly related to fecundity of S. avenae biotypes on both original and alternative plants, suggesting that expression plasticity of key defensive genes could have significant impacts on the adaptive potential and differentiation of S. avenae biotypes on different plants.

Conclusions

The divergence patterns of transcriptional plasticity in defense related genes may play important roles in the phenotypic evolution and differentiation of S. avenae biotypes. Our results can provide insights into the role of gene expression plasticity in the divergence of insect biotypes and adaptive evolution of insect populations.

Microbial Engineering for Production of N-Functionalized Amino Acids and Amines

N-functionalized amines play important roles in nature and occur, for example, in the antibiotic vancomycin, the immunosuppressant cyclosporine, the cytostatic actinomycin, the siderophore aerobactin, the cyanogenic glucoside linamarin, and the polyamine spermidine. In the pharmaceutical and fine-chemical industries N-functionalized amines are used as building blocks for the preparation of bioactive molecules. Processes based on fermentation and on enzyme catalysis have been developed to provide sustainable manufacturing routes to N-alkylated, N-hydroxylated, N-acylated, or other N-functionalized amines including polyamines. Metabolic engineering for provision of precursor metabolites is combined with heterologous N-functionalizing enzymes such as imine or ketimine reductases, opine or amino acid dehydrogenases, N-hydroxylases, N-acyltransferase, or polyamine synthetases. Recent progress and applications of fermentative processes using metabolically engineered bacteria and yeasts along with the employed enzymes are reviewed and the perspectives on developing new fermentative processes based on insight from enzyme catalysis are discussed.

A flavin-dependent monooxgenase confers resistance to chlorantraniliprole in the diamondback moth, Plutella xylostella

The diamondback moth, Plutella xylostella, is a damaging pest of cruciferous crops, and has evolved resistance to many of the insecticides used for control, including members of the diamide class. Previous work on the molecular basis of resistance to diamides has documented mutations in the target-site, the ryanodine receptor, in resistant populations of P. xylostella worldwide. In contrast the role of metabolic resistance to this insecticide class is significantly less clear. Here we show that overexpression of a flavin-dependent monooxgenase (FMO) confers resistance to the diamide chlorantraniliprole in P. xylostella. Transcriptome profiling of diamide resistant strains, with and without target-site resistance, revealed constitutive over-expression of several transcripts encoding detoxification enzymes compared to susceptible strains. Two of these, CYP6BG1, and PxFMO2 were particularly highly overexpressed (33,000 and 14,700-fold, respectively) in a resistant strain (HAW) lacking target-site resistance. After 17 generations without diamide selection the resistance of the HAW strain fell by 52-fold and the expression of PxFMO2 by > 1300-fold, however, the expression of CYP6BG1 declined by only 3-fold. Generation of transgenic Drosophila melanogaster expressing these genes demonstrated that PxFMO2, but not CYP6BG1, confers resistance in vivo. Overexpression of PxFMO2 in the HAW strain is associated with mutations, including a putative transposable element insertion, in the promoter of this gene. These enhance the expression of a reporter gene when expressed in a lepidopteran cell line suggesting they are, at least in part, responsible for the overexpression of PxFMO2 in the resistant strain. Our results provide new evidence that insect FMOs can be recruited to provide resistance to synthetic insecticides.

An Overview of Genes From Cyberlindnera americana, a Symbiont Yeast Isolated From the Gut of the Bark Beetle Dendroctonus rhizophagus (Curculionidae: Scolytinae), Involved in the Detoxification Process Using Genome and Transcriptome Data

Bark beetles from Dendroctonus genus promote ecological succession and nutrient cycling in coniferous forests. However, they can trigger outbreaks leading to important economic losses in the forest industry. Conifers have evolved resistance mechanisms that can be toxic to insects but at the same time, bark beetles are capable of overcoming tree barriers and colonize these habitats. In this sense, symbiont yeasts present in the gut of bark beetles have been suggested to play a role in the detoxification process of tree defensive chemicals. In the present study, genes related to this process were identified and their response to a terpene highly toxic to bark beetles and their symbionts was analyzed in the Cyberlindnera americana yeast. The genome and transcriptome of C. americana (ChDrAdgY46) isolated from the gut of Dendroctonus rhizophagus were presented. Genome analysis identified 5752 protein-coding genes and diverse gene families associated with the detoxification process. The most abundant belonged to the Aldo-Keto Reductase Superfamily, ATP-binding cassette Superfamily, and the Major Facilitator Superfamily transporters. The transcriptome analysis of non-α-pinene stimulated and α-pinene stimulated yeasts showed a significant expression of genes belonging to these families. The activities demonstrated by the genes identified as Aryl-alcohol dehydrogenase and ABC transporter under (+)-α-pinene suggest that they are responsible, that C. americana is a dominant symbiont that resists high amounts of monoterpenes inside the gut of bark beetles.

Modified neonicotinoid insecticide with bi-directional selective toxicity and drug resistance

A three-dimensional quantitative structure-activity relationship (3D-QSAR) model was established based on the molecular structures and the negative logarithm of experimental lethal concentration 50 values (pLC₅₀) of neonicotinoid insecticides. Then, the mechanisms of bi-directional selective toxic effects and drug resistance were determined using homology modeling and molecular docking analyses. The results of the model showed that the 1-, 2-, 4-, and 12- positions of neonicotinoid insecticides strongly affected their toxicity, and that the introduction of bulky or electropositive groups at these positions could increase the pLC₅₀ values. Using Compound 19 as a template, we designed 37 derivatives with greater toxicity (increased by 0.04-11.45%). Among them, 20 derivatives had bioconcentrations lower than that of Compound 19 (reduced by 0.38-147.88%). Further screening of Compound 19 and the 20 derivatives mentioned above by homology modeling and acetylcholine receptors (AChRs) molecular docking analyses showed that 10 derivatives had bi-directional selective toxic effects against pests and bees. Further docking analyses of Compound 19 and these 10 derivatives identified that Derivative-33 showed decreased docking with superoxide dismutase (SOD) and glutathione S transferase (GST) in pests and enhanced docking with these enzymes in bees, indicating bi-directional selective resistance for pests and bees. Accordingly, Derivative-33 was selected as a new insecticide with high toxicity to pests and low toxicity to bees (bi-directional selective toxicity), low resistance in pest populations, and high resistance in bee populations. This study provides valuable reference data and will be useful for the development of strategies to produce new environmentally friendly pesticides.

Molecular characterization, expression pattern and metabolic activity of flavin-dependent monooxygenases in Spodoptera exigua

Enhanced detoxification is one of the important mechanisms for insecticide resistance. Most research in this field to date has focused on the role of cytochrome P450s. Our previous work revealed that flavin-dependent monooxygenases (FMOs) were involved in metabolic resistance of Spodoptera exigua. In the present study we investigated the molecular characteristics, expression patterns and oxidative activities of SeFMO on insecticides. Three FMO genes, which encode proteins with the typical FMO motifs, were cloned from S. exigua. The oxidative activities of eukaryotically expressed SeFMO enzymes were verified with the model substrate of FMO. Importantly, the SeFMOs had significantly higher oxidative activities on metaflumizone and lambda-cyhalothrin than on model substrates and other insecticides tested. The three SeFMOs were mainly expressed in the midgut, fat body and Malpighian tubules. The tissues responsible for xenobiotic metabolism and their expression characteristics were similar to those of P450s acting as detoxification genes. The study also revealed that the expression of SeFMOs could be induced by insecticide exposure, and that SeFMOs were over-expressed in a metaflumizone-resistant strain of S. exigua. These results suggest that SeFMOs are important insecticide detoxifying enzymes, and that over-expression of FMO genes may be one of the mechanisms for metabolic resistance in S. exigua.

Climate and host-plant associations shaped the evolution of ceutorhynch weevils throughout the Cenozoic

Using molecular phylogenetic data and methods we inferred divergence times and diversification patterns for the weevil subfamily Ceutorhynchinae in the context of host‐plant associations and global climate over evolutionary time. We detected four major diversification shifts that correlate with both host shifts and major climate events. Ceutorhynchinae experienced an increase in diversification rate at ∼53 Ma, during the Early Eocene Climate Optimum, coincident with a host shift to Lamiaceae. A second major diversification phase occurred at the end of the Eocene (∼34 Ma). This contrasts with the overall deterioration in climate equability at the Eocene‐Oligocene boundary, but tracks the diversification of important host plant clades in temperate (higher) latitudes, leading to increased diversification rates in the weevil clades infesting temperate hosts. A third major phase of diversification is correlated with the rising temperatures of the Late Oligocene Warming Event (∼26.5 Ma); diversification rates then declined shortly after the Middle Miocene Climate Transition (∼14.9 Ma). Our results indicate that biotic and abiotic factors together explain the evolution of Ceutorhynchinae better than each of these drivers viewed in isolation.

Plant-Derived Substances Used Against Beetles-Pests of Stored Crops and Food-and Their Mode of Action: A Review

Plants are sources of numerous active substances that are used to protect crops. Currently, due to the limitations of using synthetic insecticides, plant products have attracted increasing attention as possible pesticides. In this review, we discuss some of the most interesting plant products (for example, Solanaceae, or Asteraceae extracts, Artemisia absinthium or Citrus spp. essential oils, and single compounds like α-chaconine, or α-solanine) that exhibit insecticidal activity against beetles that are pests of stored food products. Next, we describe and discuss the mode of action of these products, including lethal and sublethal effects, such as antifeedant or neurotoxic activity, ultrastructural malformation, and effects on prooxidant/antioxidant balance. Furthermore, the methods of application of plant-derived substances in food storage areas are presented.

Crystal structure of pyrrolizidine alkaloid N-oxygenase from the grasshopper Zonocerus variegatus

The high-resolution crystal structure of the flavin-dependent monooxygenase (FMO) from the African locust Zonocerus variegatus is presented and the kinetics of structure-based protein variants are discussed. Z. variegatus expresses three flavin-dependent monooxygenase (ZvFMO) isoforms which contribute to a counterstrategy against pyrrolizidine alkaloids (PAs). PAs are protoxic compounds produced by some angiosperm lineages as a chemical defence against herbivores. N-Oxygenation of PAs and the accumulation of PA N-oxides within their haemolymph result in two evolutionary advantages for these insects: (i) they circumvent the defence mechanism of their food plants and (ii) they can use PA N-oxides to protect themselves against predators, which cannot cope with the toxic PAs. Despite a high degree of sequence identity and a similar substrate spectrum, the three ZvFMO isoforms differ greatly in enzyme activity. Here, the crystal structure of the Z. variegatus PA N-oxygenase (ZvPNO), the most active ZvFMO isoform, is reported at 1.6 Å resolution together with kinetic studies of a second isoform, ZvFMOa. This is the first available crystal structure of an FMO from class B (of six different FMO subclasses, A-F) within the family of flavin-dependent monooxygenases that originates from a more highly developed organism than yeast. Despite the differences in sequence between family members, their overall structure is very similar. This indicates the need for high conservation of the three-dimensional structure for this type of reaction throughout all kingdoms of life. Nevertheless, this structure provides the closest relative to the human enzyme that is currently available for modelling studies. Of note, the crystal structure of ZvPNO reveals a unique dimeric arrangement as well as small conformational changes within the active site that have not been observed before. A newly observed kink within helix α8 close to the substrate-binding path might indicate a potential mechanism for product release. The data show that even single amino-acid exchanges in the substrate-entry path, rather than the binding site, have a significant impact on the specific enzyme activity of the isoforms.

Does host plant adaptation lead to pesticide resistance in generalist herbivores?

Most herbivorous arthropods feed on one or a few closely related plant species; however, certain insect and mite species have a greatly expanded host range. Several of these generalists also show a remarkable propensity to evolve resistance to chemical pesticides. In this review, we ask if the evolution of mechanisms to tolerate the diversity of plant secondary metabolites that generalist herbivores encounter, has pre-adapted them to resist synthetic pesticides. Critical examination of the evidence suggests that a generalist life-style per se is not a predictor of rapid resistance evolution to pesticides. Rather the prevalence of pesticide resistance in generalist herbivores probably reflects their economic importance as pests and thus the strong selection imposed by intensive pesticide use.

Trophic transfer of hexabromocyclododecane in the terrestrial and aquatic food webs from an e-waste dismantling region in East China

Trophic transfer of hexabromocyclododecane (HBCD) was investigated in both the terrestrial and aquatic food webs from an e-waste dismantling region in East China. The mean Σ₃HBCD concentrations in the terrestrial species varied from 0.91 (0.16-1.85) ng g^-1 lipid weight (lw) in dragonflies (Pantala flavescens) to 40.3 (22.1-51.1) ng g^-1 lw in rats (Rattus norvegicus). The isomeric profile indicated that α-HBCD presented a decreasing trend along the trophic level (TL) (from 97.2% to 16.3% of Σ₃HBCDs), while γ-HBCD showed a reverse trend (from 2.8% to 73.6% of Σ₃HBCDs). The trophic magnification factor (TMF) derived from the slope of the regression line between TLs and ln-transferred Σ₃HBCDs was 0.10, suggesting a trophic dilution of HBCD in the terrestrial food web. By contrast, in the aquatic species, Σ₃HBCD concentrations varied from 5.02 (3.5-6.55) ng g^-1 lw in apple snails (Ampullaria gigas spix) to 45.9 (14.9-67.8) ng g^-1 lw in grass carps (Ctenopharyngodon idellus). α-HBCD was the dominant isomer, followed by γ-HBCD in the majority of species. A positive linear relationship was observed in the plots of ln Σ₃HBCDs versus TLs (R² = 0.81, p = 0.06). The TMF for Σ₃HBCDs was 6.36, indicating a trophic magnification of HBCD in the aquatic food web. Although these results demonstrated the distinct trophic transfer of Σ₃HBCDs in different ecosystems, further research is needed to eliminate the uncertainty of the tendencies, due to the non-significant relationship and limited species.

HPLC-MS Analysis of Lichen-Derived Metabolites in the Life Stages of Crambidia cephalica (Grote & Robinson)

Tiger moths (Lepidoptera: Erebidae: Arctiinae: Arctiini) are notable for their specialized associations with hosts that produce toxic secondary compounds, and are thus an ideal study system for understanding insect-plant interactions and the evolution of antipredatory defense. Likewise, their sister lineage (Arctiinae: Lithosiini) has been documented feeding on algae and lichens, and is known to sequester lichen-derived secondary compounds from the larval to adult stages. Prevalence of lichenivory in this early radiation (ca. 3000 species) may provide clues to the phylogenetic basis for storied chemical sequestration within all tiger moths. Despite the evolutionary significance of this trait, we lack a basic understanding of the extent of lichenivory among lithosiines, and the distribution of sequestered chemicals among life stages. The dynamics of chemical sequestration throughout the lifecycle for the lichen moth Crambidia cephalica were investigated by testing the hypothesis that lichen-derived metabolites are unequally distributed among life stages, and that laboratory-reared C. cephalica have less metabolite diversity than wild-caught individuals. Crambidia cephalica was reared on Physcia, and analyzed using high-performance liquid chromatography coupled to mass spectrometry (HPLC-MS). Several putative lichen-derived metabolites were detected across three life stages, i.e., larval, pupal, and adult, and differences among life stages and lichen host were observed. These results provide evidence that multiple lichen-derived metabolites are sequestered by C. cephalica; some metabolites are retained through adulthood, and others are lost or modified in earlier life stages. The presence of differing lichen-derived metabolites across life stages may indicate functional properties of the metabolites for C. cephalica with regards to chemical protection from antagonists, and other physiological processes.

Genes Expressed Differentially in Hessian Fly Larvae Feeding in Resistant and Susceptible Plants

The Hessian fly, Mayetiola destructor, is a destructive pest of wheat worldwide and mainly controlled by deploying resistant cultivars. In this study, we investigated the genes that were expressed differentially between larvae in resistant plants and those in susceptible plants through RNA sequencing on the Illumina platform. Informative genes were 11,832, 14,861, 15,708, and 15,071 for the comparisons between larvae in resistant versus susceptible plants for 0.5, 1, 3, and 5 days, respectively, after larvae had reached the feeding site. The transcript abundance corresponding to 5401, 6902, 8457, and 5202 of the informative genes exhibited significant differences (p ≤ 0.05) in the respective paired comparisons. Overall, genes involved in nutrient metabolism, RNA and protein synthesis exhibited lower transcript abundance in larvae from resistant plants, indicating that resistant plants inhibited nutrient metabolism and protein production in larvae. Interestingly, the numbers of cytochrome P450 genes with higher transcript abundance in larvae from resistant plants were comparable to, or higher than those with lower transcript abundance, indicating that toxic chemicals from resistant plants may have played important roles in Hessian fly larval death. Our study also identified several families of genes encoding secreted salivary gland proteins (SSGPs) that were expressed at early stage of 1^st instar larvae and with more genes with higher transcript abundance in larvae from resistant plants. Those SSGPs are candidate effectors with important roles in plant manipulation.

C. elegans flavin-containing monooxygenase-4 is essential for osmoregulation in hypotonic stress

Studies in Caenorhabditis elegans have revealed osmoregulatory systems engaged when worms experience hypertonic conditions, but less is known about measures employed when faced with hypotonic stress. Inactivation of fmo-4, which encodes flavin-containing monooxygenase-4, results in dramatic hypoosmotic hypersensitivity; worms are unable to prevent overwhelming water influx and swell rapidly, finally rupturing due to high internal hydrostatic pressure. fmo-4 is expressed prominently in hypodermis, duct and pore cells but is excluded from the excretory cell. Thus, FMO-4 plays a crucial osmoregulatory role by promoting clearance of excess water that enters during hypotonicity, perhaps by synthesizing an osmolyte that acts to establish an osmotic gradient from excretory cell to duct and pore cells. C. elegans FMO-4 contains a C-terminal extension conserved in all nematode FMO-4s. The coincidently numbered human FMO4 also contains an extended C-terminus with features similar to those of FMO-4. Although these shared sequence characteristics suggest potential orthology, human FMO4 was unable to rescue the fmo-4 osmoregulatory defect. Intriguingly, however, mammalian FMO4 is expressed predominantly in the kidney - an appropriate site if it too is, or once was, involved in osmoregulation.

Xenobiotic detoxification pathways in honey bees

Relative to most other insect genomes, the western honey bee Apis mellifera has a deficit of detoxification genes spanning Phase I (functionalization), II (conjugation) and III (excretion) gene families. Although honeybees do not display across-the-board greater sensitivity to pesticides, this deficit may render them vulnerable to synergistic interactions among xenobiotics. Diet quality, in terms of protein and phytochemical content, has a pronounced influence on tolerance of toxic compounds. Detoxification gene inventory reduction may reflect an evolutionary history of consuming relatively chemically benign nectar and pollen, as other apoid pollinators display comparable levels of cytochrome P450 gene reduction. Enzymatic detoxification in the eusocial A. mellifera may be complemented by behaviors comprising a 'social detoxification system,' including forager discrimination, dilution by pollen mixing, and colony food processing via microbial fermentation, that reduces the number or quantity of ingested chemicals requiring detoxification.

Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities

BACKGROUND

Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

SCOPE

The objective of this review is to evaluate (1) which strategies plants have evolved to cope with herbivores and (2) which traits herbivores have evolved that enable them to counter these defences. The primary focus is on the induction and suppression of plant defences and the review outlines how the palette of traits that determine induction/suppression of, and resistance/susceptibility of herbivores to, plant defences can give rise to exploitative competition and facilitation within ecological communities "inhabiting" a plant.

CONCLUSIONS

Herbivores have evolved diverse strategies, which are not mutually exclusive, to decrease the negative effects of plant defences in order to maximize the conversion of plant material into offspring. Numerous adaptations have been found in herbivores, enabling them to dismantle or bypass defensive barriers, to avoid tissues with relatively high levels of defensive chemicals or to metabolize these chemicals once ingested. In addition, some herbivores interfere with the onset or completion of induced plant defences, resulting in the plant's resistance being partly or fully suppressed. The ability to suppress induced plant defences appears to occur across plant parasites from different kingdoms, including herbivorous arthropods, and there is remarkable diversity in suppression mechanisms. Suppression may strongly affect the structure of the food web, because the ability to suppress the activation of defences of a communal host may facilitate competitors, whereas the ability of a herbivore to cope with activated plant defences will not. Further characterization of the mechanisms and traits that give rise to suppression of plant defences will enable us to determine their role in shaping direct and indirect interactions in food webs and the extent to which these determine the coexistence and persistence of species.

Molecular mechanisms of insect adaptation to plant secondary compounds

During feeding, herbivorous insects are exposed to an array of plant defensive compounds. In this review, we examine molecular mechanisms of insect adaptation to these toxic metabolites. We discuss both the importance of evolutionary variation of existing detoxification gene families, as well as the evolution of novel mechanisms through gene recruitment, neofunctionalization and horizontal gene transfer. The ability of insects to cope with the chemical diversity of their host plants and the different mechanisms that insects use to resist these toxins open new avenues for understanding fundamental aspects of insect-plant coevolutionary adaptation.

New aspect of plant-rhizobia interaction: alkaloid biosynthesis in Crotalaria depends on nodulation

Infection of legume hosts by rhizobial bacteria results in the formation of a specialized organ, the nodule, in which atmospheric nitrogen is reduced to ammonia. Nodulation requires the reprogramming of the plant cell, allowing the microsymbiont to enter the plant tissue in a highly controlled manner. We have found that, in Crotalaria (Fabaceae), this reprogramming is associated with the biosynthesis of pyrrolizidine alkaloids (PAs). These compounds are part of the plant's chemical defense against herbivores and cannot be regarded as being functionally involved in the symbiosis. PAs in Crotalaria are detectable only when the plants form nodules after infection with their rhizobial partner. The identification of a plant-derived sequence encoding homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, suggests that the plant and not the microbiont is the producer of PAs. Transcripts of HSS are detectable exclusively in the nodules, the tissue with the highest concentration of PAs, indicating that PA biosynthesis is restricted to the nodules and that the nodules are the source from which the alkaloids are transported to the above ground parts of the plant. The link between nodulation and the biosynthesis of nitrogen-containing alkaloids in Crotalaria highlights a further facet of the effect of symbiosis with rhizobia on the ecologically important trait of the plant's chemical defense.

Biosynthetic pathway of the phytohormone auxin in insects and screening of its inhibitors

Insect galls are abnormal plant tissues induced by galling insects. The galls are used for food and habitation, and the phytohormone auxin, produced by the insects, may be involved in their formation. We found that the silkworm, a non-galling insect, also produces an active form of auxin, indole-3-acetic acid (IAA), by de novo synthesis from tryptophan (Trp). A detailed metabolic analysis of IAA using IAA synthetic enzymes from silkworms indicated an IAA biosynthetic pathway composed of a three-step conversion: Trp → indole-3-acetaldoxime → indole-3-acetaldehyde (IAAld) → IAA, of which the first step is limiting IAA production. This pathway was shown to also operate in gall-inducing sawfly. Screening of a chemical library identified two compounds that showed strong inhibitory activities on the conversion step IAAld → IAA. The inhibitors can be efficiently used to demonstrate the importance of insect-synthesized auxin in gall formation in the future.

Biochemical mechanisms for metaflumizone resistance in beet armyworm, Spodoptera exigua

The metaflumizone, which belongs to the class of voltage-dependent sodium channel blockers, was registered to control Spodoptera exigua on vegetables in China in 2009. The present study revealed S. exigua has developed high resistance to this novel chemistry insecticide shortly after 2-3 years application in Guangdong Province of China. The metabolic mechanisms for metaflumizone resistance in this insect were analysed. The inhibitor of esterases greatly potentiates the toxicity of this chemical against the field resistant populations. The synergism ratio is 5.7 and 3.4-fold for S. exigua collected from Huizhou, Guangdong Province in 2011 and 2012, respectively. The activity of esterases in field populations (HZ12) is also significantly greater than that in the susceptible strain, and further significantly increased by challenge with metaflumizone for 3 generations. However, the inhibitor of P450s or GSTs only has slight synergism on metaflumizone toxicity against resistant populations, and there are no obvious differences in activities of P450s or GSTs between resistant populations and the susceptible strain. These results suggest that esterases might take pivotal role in conferring metabolic resistance to metaflumizone in the field populations of S. exigua, and P450s or GSTs are not involved in this resistance. Moreover, flavin-dependent monooxygenases (FMOs) are discovered to involve in metaflumizone resistance in the field populations of S. exigua. The FMO inhibitor, methimazole, potentiates metaflumizone toxicity in resistant larva of this species substantially. The synergism ratios for methimazole in resistant populations HZ11 and HZ12 were 3.1 and 1.9, respectively. Enzymatic assays also revealed higher FMO activities in resistant populations than in the susceptible strain, and successive selection with metaflumizone further increased the FMO activity in the field resistant population, but not significantly. The higher FMO activities in the older larval stages and in the larval midgut signify the importance of FMO in the detoxification of xenobiotic from food sources. The synergism assay and FMO activity analysis suggest that FMO contributes to metaflumizone detoxification in resistant populations of S. exigua and conferred metaflumizone resistance in S. exigua. A novel mechanism for insecticide resistance by insect was proposed.

Ecological immunology mediated by diet in herbivorous insects

A rapidly advancing area of ecological immunology concerns the effects of diet on animals' immunological responses to parasites and pathogens. Here, we focus on diet-mediated ecological immunology in herbivorous insects, in part because these organisms commonly experience nutritional limitations from their diets of plants. Nutritional immunology highlights nutrient-based trade-offs between immunological and other physiological processes as well as trade-offs among distinct immunological processes. This field reveals that nutrition influences the quality and quantity of immunological defense in herbivorous insects, and conversely that nutritional intake by herbivorous insects can be an adaptive response to the specific types of immune-challenge they face in the context of other physiological processes. Because the diets of herbivores challenge them physiologically with plants' secondary metabolites, another area of study analyzes constraints on immunological defense imposed by secondary metabolites of plants in the diets of herbivorous insects. Alternatively, some herbivores can use secondary metabolites as medicine against parasites or pathogens. Animal-medication theory makes an important contribution to ecological immunology by distinguishing prophylactic and therapeutic mechanisms of anti-parasite defense. Integrating ideas from animal-medication and nutritional immunology, we outline a conceptual framework in which the immunological role of the diet consists of mechanisms of prophylaxis, therapy, compensation, and combinations thereof. Then, we use this framework to organize findings from our own research on diet-mediated ecological immunology of woolly bear caterpillars. We show evidence that the woolly bear caterpillar, Grammia incorrupta (Hy. Edwards) (Lepidoptera, Erebidae, and Arctiinae), can employ both diet-mediated prophylaxis and therapy. First, increased consumption of carbohydrate-biased food prior to immune-challenge increased its melanization-response. Second, increased consumption of pyrrolizidine alkaloids (PAs) more than 24 h after parasitism by tachinid flies resulted in anti-parasite resistance. Caterpillars reduced feeding on protein-biased food within 24 h after immune-challenge, showing evidence of illness-induced anorexia. We synthesize our work to generate the hypothesis that a diet-mediated defense by the host against parasites acts as a temporally explicit, multi-stage process.

Detoxification and stress response genes expressed in a western North American bumble bee, Bombus huntii (Hymenoptera: Apidae)

Background

The Hunt bumble bee (Bombus huntii Greene, Hymenoptera: Apidae) is a holometabolous, social insect important as a pollinator in natural and agricultural ecosystems in western North America. Bumble bees spend a significant amount of time foraging on a wide variety of flowering plants, and this activity exposes them to both plant toxins and pesticides, posing a threat to individual and colony survival. Little is known about what detoxification pathways are active in bumble bees, how the expression of detoxification genes changes across life stages, or how the number of detoxification genes expressed in B. huntii compares to other insects.

Results

We found B. huntii expressed at least 584 genes associated with detoxification and stress responses. The expression levels of some of these genes, such as those encoding the cytochrome P450s, glutathione S-transferases (GSTs) and glycosidases, vary among different life stages to a greater extent than do other genes. We also found that the number of P450s, GSTs and esterase genes expressed by B. huntii is similar to the number of these genes found in the genomes of other bees, namely Bombus terrestris, Bombus impatiens, Apis mellifera and Megachile rotundata, but many fewer than are found in the fly Drosophila melanogaster.

Conclusions

Bombus huntii has transcripts for a large number of detoxification and stress related proteins, including oxidation and reduction enzymes, conjugation enzymes, hydrolytic enzymes, ABC transporters, cadherins, and heat shock proteins. The diversity of genes expressed within some detoxification pathways varies among the life stages and castes, and we typically identified more genes in the adult females than in larvae, pupae, or adult males, for most pathways. Meanwhile, we found the numbers of detoxification and stress genes expressed by B. huntii to be more similar to other bees than to the fruit fly. The low number of detoxification genes, first noted in the honey bee, appears to be a common phenomenon among bees, and perhaps results from their symbiotic relationship with plants. Many flowering plants benefit from pollinators, and thus offer these insects rewards (such as nectar) rather than defensive plant toxins.

Genomes of Ashbya fungi isolated from insects reveal four mating-type loci, numerous translocations, lack of transposons, and distinct gene duplications

The filamentous fungus Ashbya gossypii is a cotton pathogen transmitted by insects. It is readily grown and manipulated in the laboratory and is commercially exploited as a natural overproducer of vitamin B2. Our previous genome analysis of A. gossypii isolate ATCC10895, collected in Trinidad nearly 100 years ago, revealed extensive synteny with the Saccharomyces cerevisiae genome, leading us to use it as a model organism to understand the evolution of filamentous growth. To further develop Ashbya as a model system, we have investigated the ecological niche of A. gossypii and isolated additional strains and a sibling species, both useful in comparative analysis. We isolated fungi morphologically similar to A. gossypii from different plant-feeding insects of the suborder Heteroptera, generated a phylogenetic tree based on rDNA-ITS sequences, and performed high coverage short read sequencing with one A. gossypii isolate from Florida, a new species, Ashbya aceri, isolated in North Carolina, and a genetically marked derivative of ATCC10895 intensively used for functional studies. In contrast to S. cerevisiae, all strains carry four not three mating type loci, adding a new puzzle in the evolution of Ashbya species. Another surprise was the genome identity of 99.9% between the Florida strain and ATCC10895, isolated in Trinidad. The A. aceri and A. gossypii genomes show conserved gene orders rearranged by eight translocations, 90% overall sequence identity, and fewer tandem duplications in the A. aceri genome. Both species lack transposable elements. Finally, our work identifies plant-feeding insects of the suborder Heteroptera as the most likely natural reservoir of Ashbya, and that infection of cotton and other plants may be incidental to the growth of the fungus in its insect host.

Independent recruitment of a flavin-dependent monooxygenase for safe accumulation of sequestered pyrrolizidine alkaloids in grasshoppers and moths

Several insect lineages have developed diverse strategies to sequester toxic pyrrolizidine alkaloids from food-plants for their own defense. Here, we show that in two highly divergent insect taxa, the hemimetabolous grasshoppers and the holometabolous butterflies, an almost identical strategy evolved independently for safe accumulation of pyrrolizidine alkaloids. This strategy involves a pyrrolizidine alkaloid N-oxygenase that transfers the pyrrolizidine alkaloids to their respective N-oxide, enabling the insects to avoid high concentrations of toxic pyrrolizidine alkaloids in the hemolymph. We have identified a pyrrolizidine alkaloid N-oxygenase, which is a flavin-dependent monooxygenase, of the grasshopper Zonocerus variegatus. After heterologous expression in E. coli, this enzyme shows high specificity for pyrrolizidine alkaloids of various structural types and for the tropane alkaloid atropine as substrates, a property that has been described previously for a pyrrolizidine alkaloid N-oxygenase of the arctiid moth Grammia geneura. Phylogenetic analyses of insect flavin-dependent monooxygenase sequences suggest that independent gene duplication events preceded the establishment of this specific enzyme in the lineages of the grasshoppers and of arctiid moths. Two further flavin-dependent monooxygenase sequences have been identified from Z. variegatus sharing amino acid identities of approximately 78% to the pyrrolizidine alkaloid N-oxygenase. After heterologous expression, both enzymes are also able to catalyze the N-oxygenation of pyrrolizidine alkaloids, albeit with a 400-fold lower specific activity. With respect to the high sequence identity between the three Z. variegatus sequences this ability to N-oxygenize pyrrolizidine alkaloids is interpreted as a relict of a former bifunctional ancestor gene of which one of the gene copies optimized this activity for the specific adaptation to pyrrolizidine alkaloid containing food plants.

Evolutionary recruitment of a flavin-dependent monooxygenase for stabilization of sequestered pyrrolizidine alkaloids in arctiids

Pyrrolizidine alkaloids are secondary metabolites that are produced by certain plants as a chemical defense against herbivores. They represent a promising system to study the evolution of pathways in plant secondary metabolism. Recently, a specific gene of this pathway has been shown to have originated by duplication of a gene involved in primary metabolism followed by diversification and optimization for its specific function in the defense machinery of these plants. Furthermore, pyrrolizidine alkaloids are one of the best-studied examples of a plant defense system that has been recruited by several insect lineages for their own chemical defense. In each case, this recruitment requires sophisticated mechanisms of adaptations, e.g., efficient excretion, transport, suppression of toxification, or detoxification. In this review, we briefly summarize detoxification mechanism known for pyrrolizidine alkaloids and focus on pyrrolizidine alkaloid N-oxidation as one of the mechanisms allowing insects to accumulate the sequestered toxins in an inactivated protoxic form. Recent research into the evolution of pyrrolizidine alkaloid N-oxygenases of adapted arctiid moths (Lepidoptera) has shown that this enzyme originated by the duplication of a gene encoding a flavin-dependent monooxygenase of unknown function early in the arctiid lineage. The available data suggest several similarities in the molecular evolution of this adaptation strategy of insects to the mechanisms described previously for the evolution of the respective pathway in plants.