Identification of putative muscarinic acetylcholine receptor genes in Bactrocera dorsalis and functional analysis of Bdor-mAChR-B

Transgenic expression of mAChR-C dsRNA in maize confers efficient locust control

Plant-mediated RNA interference (RNAi), in which double-stranded RNAs (dsRNAs) targeting insect genes are expressed in plants for insect ingestion, has shown great potential for the control of herbivorous insect pests. Locusts, which are among the most destructive agricultural insect pests, appear to be resistant to orally delivered naked dsRNA. Moreover, the feasibility of using plant-mediated RNAi to suppress target gene expression in locusts remains unclear. Using the migratory locust Locusta migratoria, we report that the C-type muscarinic acetylcholine receptor (mAChR-C), a G protein-coupled receptor (GPCR) belonging to the bioamine receptor subfamily, plays a pivotal role in chitin metabolism by regulating genes responsible for chitin synthesis and degradation. Knockdown of locust mAChR-C via injection of dsRNA caused defective nymph molting and metamorphosis, accompanied by malformation, arrested development, and impaired motility. Notably, locusts fed transgenic maize expressing locust mAChR-C dsRNAs exhibited defective phenotypes similar to those subjected to mAChR-C dsRNA injection. In contrast, ingestion of transgenic maize expressing locust mAChR-C dsRNA had no significant effects on non-target insects, including the fall armyworm Spodoptera frugiperda, the cotton bollworm Helicoverpa armigera, the Asian corn borer Ostrinia furnacalis, and the oriental armyworm Mythimna separata. Our results suggest that transgenic expression of locust mAChR-C dsRNA is an effective RNAi approach for locust control and offers a promising eco-friendly strategy for locust management.

The Fatal Mushroom Neurotoxin Muscarine is Released from a Harmless Phosphorylated Precursor upon Cellular Injury

l-(+)-Muscarine (1)-producing mushrooms pose a severe threat to human health as ingestion can result in circulatory collapse or even death. However, their metabolic profile is surprisingly poorly understood, including knowledge of poison release and potentially toxic congeners. In the mycelium of the 1-producing fool's funnel mushroom Clitocybe rivulosa, we identified 4'-phosphomuscarine (2) as the major natural product. Its structure was elucidated by high-resolution mass spectrometry, nuclear magnetic resonance spectroscopy and by comparison with a synthesized reference. We also detected this previously overlooked phosphorylated compound in the fiber cap mushrooms Pseudosperma spectrale and Inocybe nitidiuscula. Studies on the activation of the muscarinic acetylcholine receptor M3 indicate only weak affinity of 2 to this target. Furthermore, we present biological evidence that muscaridine (3), a quaternary amine congener related to and co-occurring with 1, does not activate the muscarinic acetylcholine receptor M3 on human embryonic kidney cells. Our work provides important insight into the metabolic profile and the pharmacology of some of the most poisonous mushrooms. As the harmless 2 can liberate the potentially fatal 1 by unspecific enzymatic ester cleavage, these results are highly relevant for emergency medicine to estimate the true toxicity of these mushrooms.

NompC regulates locomotion and touch sensation in Bactrocera dorsalis

No mechanoreceptor potential C (NompC) is a major mechanotransduction channel with an important role in sensing of external mechanical stimuli by insects, which help these organisms to avoid injury and adapt to environmental changes. To explore the biological functions of NompC in Bactrocera dorsalis, a notorious agricultural pest, we successfully generated NompC knockout strains using clustered regularly interspaced small palindromic repeats (CRISPR) / CRISPR-associated nuclease 9 (Cas9) technology. BdorNompC knockout led to an adult lethal phenotype, with approximately 100% mortality at 3 d after eclosion. Morphological observation revealed that the legs and wings of BdorNompC knockout insects were deformed, while behavioral assays showed that the locomotion was impaired in both adults and larvae, relative to that of the wild-type strain. Moreover, BdorNompC knockout reduced gentle-touch response in larvae. These results suggest that BdorNompC is critical for B. dorsalis survival, and that this mechanosensation channel represents a potential new target for pest control agents. Our findings also represent novel evidence indicating that insect NompC is involved in modulating adult wing and leg morphology.

Characterization of an agmatine N-acetyltransferase from Bactrocera dorsalis that modulates ovary development

Agmatine N-acetyltransferase (AgmNAT), which catalyzes the formation of N-acetylagmatine from acetyl-CoA and agmatine, is a member of the GCN5-related N-acetyltransferase family. So far, knowledge of the physiological roles of AgmNAT in insects is limited. Here, we identified one gene encoding protein homologous to that of Drosophila AgmNAT using sequence information from an activity-verified Drosophila AgmNAT in a BLAST search of the Bactrocera dorsalis genome. We expressed and purified B. dorsalis AgmNAT in Escherichia coli and used the purified enzyme to define the substrate specificity for acyl-CoA and amine substrates. Our application of the screening strategy to BdorAgmNAT led to the identification of agmatine as the best amine substrate for this enzyme, with the highest kcat/Km value. We successfully obtained a BdorAgmNAT knockout strain based on a wild-type strain (WT) using the CRISPR/Cas9 technique. The ovary development of the BdorAgmNAT knockout mutants was delayed for 10 days compared with the WT specimens. Moreover, mutants had a much smaller mature ovary size and laid far fewer eggs than WT. Loss of function of BdorAgmNAT caused by RNAi with mature WT females did not affect their fecundity. These findings indicate that BdorAgmNAT is critical for oogenesis. Our data provide the first evidence for AgmNAT in regulating ovary development.

Distribution analysis of TRH in Bactrocera dorsalis using a CRISPR/Cas9-mediated reporter knock-in strain

Although the study of many genes and their protein products is limited by the availability of high-quality antibodies, this problem could be solved by fusing a tag/reporter to an endogenous gene using a gene-editing approach. The type II bacterial CRISPR/Cas system has been demonstrated to be an efficient gene-targeting technology for many insects, including the oriental fruit fly Bactrocera dorsalis. However, knocking in, an important editing method of the CRISPR/Cas9 system, has lagged in its application in insects. Here, we describe a highly efficient homology-directed genome editing system for B. dorsalis that incorporates coinjection of embryos with Cas9 protein, guide RNA and a short single-stranded oligodeoxynucleotide donor. This one-step procedure generates flies carrying V5 tag (42 bp) in the BdorTRH gene. In insects, as in other invertebrates and in vertebrates, the neuronal tryptophan hydroxylase (TRH) gene encodes the rate-limiting enzyme for serotonin biosynthesis in the central nervous system. Using V5 monoclonal antibody, the distribution of TRH in B. dorsalis at different developmental stages was uncovered. Our results will facilitate the generation of insects carrying precise DNA inserts in endogenous genes and will lay foundation for the investigation of the neural mechanisms underlying the serotonin-mediated behaviour of B. dorsalis.

Toxicity of Beauveria bassiana to Bactrocera dorsalis and effects on its natural predators

Entomopathogenic fungi (EPF) are economical and environmentally friendly, forming an essential part of integrated pest management strategies. We screened six strains of Beauveria bassiana (B1-B6) (Hypocreales: Cordycipitaceae), of which B4 was the most virulent to Bactrocera dorsalis (Hendel) (Diptera: Tephritidae). We further assessed the biological characteristics of strain B4 and the environmental factors influencing its ability to infect B. dorsalis. We also evaluated the effects of B4 on two of the natural predators of B. dorsalis. We found that strain B4 was the most virulent to 3rd instar larvae, pupae, and adult B. dorsalis, causing mortality rates of 52.67, 61.33, and 90.67%, respectively. B4 was not toxic to B. dorsalis eggs. The optimum B4 effects on B. dorsalis were achieved at a relative humidity of 91-100% and a temperature of 25°C. Among the six insecticides commonly used for B. dorsalis control, 1.8% abamectin emulsifiable concentrate had the strongest inhibitory effect on B4 strain germination. B4 spraying affected both natural enemies (Amblyseius cucumeris and Anastatus japonicus), reducing the number of A. cucumeris and killing A. japonicus adults. We found a valuable strain of EPF (B4) that is virulent against many life stages of B. dorsalis and has great potential for the biological control of B. dorsalis. We also provide an important theoretical and practical base for developing a potential fungicide to control B. dorsalis.

Lethal and sublethal effects of broflanilide on four tephritid pests (Diptera: Tephritidae)

BACKGROUND

Fruit flies are internationally important quarantine or invasive pests of many fruits and vegetables and can cause serious economic losses. Long-term reliance on insecticides for controlling these pests has led to increasing resistance to multiple insecticides; hence, a new agent is needed. In this study, the acute toxicity and sublethal effects of the novel insecticide broflanilide on four adult fruit fly species, Bactrocera dorsalis, Bactrocera cucurbitae, Bactrocera tau, and Bactrocera correcta, were evaluated.

RESULTS

Broflanilide was effective against B. dorsalis and B. correcta, with lethal concentration values (amount required to kill 50% of animals; LC₅₀ ) of 0.390 and 1.716 mg/L. However, for B. cucurbitae (19.673 mg/L) and B. tau (24.373 mg/L), the LC₅₀ was 50-60 times higher than that of B. dorsalis. The survival rates of B. correcta and B. cucurbitae were significantly lower under LC₅₀ treatment than those of the control (corrected for mortality rate). Sublethal concentrations of broflanilide stimulated fecundity in all species except B. tau. The hatching rate at LC₅₀ was significantly lower for B. correcta and B. tau compared with the control and even more so for B. correcta, which was zero.

CONCLUSION

Broflanilide is potentially an effective insecticide for controlling B. dorsalis and B. correcta. However, the variation in toxicity of broflanilide to the four fruit flies suggests that species variation needs to be carefully considered. Our results highlight the importance of clarifying the sublethal effects of insecticides on target insects to ensure the comprehensive evaluation and rational use of insecticides. © 2023 Society of Chemical Industry.

Comparative transcriptome analysis reveals the non-neuronal cholinergic system in the ovary of the oriental armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae)

BACKGROUND

Acetylcholine (ACh), as a classical neurotransmitter, plays great roles in the nervous system. There is increasing evidence of its non-neuronal roles in regulating basic cell functions in vertebrates. However, knowledge about the non-neuronal cholinergic system in insects is scarce.

RESULTS

A comparative transcriptome analysis was performed to investigate differences in the key molecular components of the cholinergic system between the head and ovary. The results showed that expression levels of most cholinergic system-related genes were higher in the head than in the ovary, and some cholinergic components were absent in the ovary. ACh contents ranged from 0.1 to 1.3 μg mg^-1 of wet weight during the development of the ovary, and weak acetylcholinesterase activity was also detected. Moreover, the ovary has a capacity for ACh synthesis. Bromoacetylcarnitine (BrACar), a specific carnitine acetyltransferase (CarAT) inhibitor, greatly inhibits ACh synthesis by 83.83% in ovary homogenates, but bromoacetylcholine (BrACh), a specific choline acetyltransferase (ChAT) inhibitor, has no effect on ACh synthesis in the ovary. These findings indicate that non-neuronal ACh in the ovary is only catalyzed by CarAT.

CONCLUSION

This study reveals the existence of the non-neuronal cholinergic system in the ovary of M. separata, whose synthesis and release mechanisms are different from those of the head. These results provide novel insights into the non-neuronal cholinergic system in insects, and will be valuable in the discovery of new target genes and the future development of green pest control. © 2022 Society of Chemical Industry.

Arylalkylamine N-acetyltransferase 1 gene (AANAT1) regulates cuticle pigmentation and ovary development of the adult oriental fruit fly, Bactrocera dorsalis

The arylalkylamine N-acetyltransferase (AANAT) enzymes catalyze the acetyl-CoA-dependent acetylation of an amine or arylalkylamine, which is involved in important biological processes of insects. Here, we carried out the molecular and biochemical identification of an arylalkylamine N-acetyltransferase (AANAT) from the oriental fruit fly, Bactrocera dorsalis. Using a bacterial expression system, we expressed and purified the encoded recombinant BdorAANAT1-V3 protein. The purified recombinant protein acts on a wide range of substrates, including dopamine, tyramine, octopamine, serotonin, methoxytryptamine, and tryptamine, and shows similar substrate affinity (i.e., K_m values: 0.16-0.26 mM) except for serotonin (K_m = 0.74 mM) and dopamine (K_m = 0.84 mM). Transcriptional profile analysis of BdorAANAT1 revealed that this gene is most prevalent in adults and abundant in the adult brain, gut, and ovary. Using the CRISPR/Cas9 technique, we successfully obtained a BdorAANAT1 knockout strain based on a wild-type strain (WT). Compared with the WT, the cuticle color of larvae and pupae is normal; however, in adult mutants, the yellow region of their thorax is darkly pigmented, and two black spots were evident at the abdomen's end. Moreover, the female BdorAANAT1 knockout mutant had a smaller ovary than the WT, and laid far fewer eggs. Loss of function of BdorAANAT1 caused by RNAi with mature adult females in which the reproductive system is fully developed had no effect on their fecundity. Altogether, these results indicate that BdorAANAT1 regulates ovary development. Our findings provide evidence for the insect AANAT1 modulating adult cuticle pigmentation and female fecundity.

Effects of four chemosterilants on Bactrocera tau

Bactrocera tau (Walker) is a fly pest species mainly distributed in Southeast Asia and the South Pacific; it causes substantial ecological and economic issues because of its destructiveness and rapid reproduction. Chemical sterilization technology can reduce the use of insecticides and is widely applied for insect pest control. In this study, the sterilization efficacy of varying concentrations of four chemosterilants, namely, hexamethylphosphoramide (HMPA), CSII Aqua, 5-fluorouracil (5-FU), and colchicine, on adult pumpkin flies was investigated. The results indicated that a solution of 0.03% HMPA had the highest sterilization efficacy. When the number of sterile males was equal to or exceeded 20 times that of untreated males, the hatching rate of offspring eggs was less than 10%. Chemosterilant treatment significantly altered the levels of acid phosphatase (ACP), alkaline phosphatase (AKP), and B. tau vitellogenin (BtVg); these substances have an important impact on reproductive development. The treatment also decreased the size of the reproductive organs (i.e., testes and ovaries). Our results suggest that 0.03% HMPA has unique sterilization properties and may represent a new chemical agent for the control of B. tau populations in agricultural settings.

Pirenzepine Binding Sites in the Brain of the Honeybee Apis mellifera: Localization and Involvement in Non-Associative Learning

Muscarinic acetylcholine receptors (mAChRs) play a central role in learning and memory in mammals as in honeybees. The results obtained in the honeybee Apis mellifera are based on the detrimental effects of the mAChR antagonists, atropine and scopolamine, on olfactory associative memory. Binding sites for the mAChR antagonist BODIPY^® FL pirenzepine were localized in the brain of the honeybee forager. Pirenzepine binding sites were detected indifferently in several somata and neuropilar areas. The highest binding site densities were present in the central complex and in somata of the dorsomedial border of the antennal lobes. An additional binding pattern was found in somata of the subesophageal ganglion. By contrast, Kenyon cell (KC) somata were not stained. Pirenzepine (PZ) effects on non-associative learning were evaluated. Treated animals required more trials for the habituation of the proboscis extension reflex (PER) than controls, and the duration of the PER increased after PZ brain injection. These results suggest that the network mediating habituation of the PER involves PZ binding sites that are not necessarily present on the circuitry mediating olfactory conditioning of the PER.