Identification and characterization of CYP307A1 as a molecular target for controlling the small hive beetle, Aethina tumida

RNAi-mediated knockdown of juvenile hormone acid methyltransferase depresses reproductive performance in female Aethina tumida

Small hive beetles, Aethina tumide, are free-flying parasites of social bee colonies where they feed and reproduce. In case of mass infestation, A. tumida can cause significant economic losses. There is an urgent need to explore novel green molecular approaches for sustainable control of A. tumida. It has been confirmed that juvenile hormone acid methyl transferase (JHAMT) plays a crucial role in regulating the synthesis of juvenile hormone (JH). However, its impact on female reproduction of A. tumida remains unclear. In the present study, a novel JHAMT gene was identified from A. tumida with an open reading frame of 978 bp, encoding a polypeptide of 325 amino acids containing a Methyltransferase domain. The deduced amino acid sequence of AtJHAMT shared 60 % and 33 % identity with homologs from Brassicogethes aeneus and Apis mellifera, respectively. The expression profile indicates that the transcription level of AtJHAMT increases in the adult stages, reaching its peak in 5-day-old female adults. AtJHAMT exhibits the highest expression levels in the ovaries, and fluorescence in situ hybridization (FISH) demonstrates that this gene shows a significant number of positive signals in the ovarian ducts and the head region. Furthermore, we investigated the function of AtJHAMT through RNA interference and methoprene rescue experiments. We also investigeted the off-target effects of the dsJHAMT. The results showed that silencing AtJHAMT through oral dsRNA delivery (feeding dsRNA-SPc mix) affected ovarian development and significantly reduced JH titers, female fecundity, female fertility, and egg hatchability. The application of methoprene partially rescued the negative effect of silencing AtJHAMT on reproduction. Several genes associated with ovarian development were significantly downregulated following interference with AtJHAMT, but their expression levels were restored after complementation experiments. Additionally, the off-target effects experiment showed that dsJHAMT from A. tumida had no adverse effects on ovaries development in honey bee queens. Overall, this study illustrates the functions of the JHAMT in A. tumida, which can serve as a potential target for controlling the reproduction of the most deleterious bee parasites, A. tumida.

Methuselah-like 2 mediated 20-hydroxyecdysone (20E) signaling regulates molting and fecundity in Nilaparvata lugens (Stål) (Hemiptera: Delphacidae)

BACKGROUND

G protein-coupled receptors (GPCRs) are very promising as the targets of endogenous neuropeptides/neuromodulators that, upon binding to receptors, induce profound changes in insect physiology. The Methuselan/Methuselan-like subfamily of GPCRs is reported to be associated with longevity and stress resistance. A previous study showed the fungicide jingangmycin-induced expression of Mthl2 and enhanced stress resistance in Nilaparvata lugens. However, the other physiological functions of Mthl2 remain unelucidated.

RESULTS

The Mthl2 was highly expressed before molting and decreased after that until the next ecdysis, showing a cyclical pattern related to molting behavior and predominantly distributed in cuticle-producing and reproductive tissues in N. lugens. Silencing Mthl2 by RNAi in nymphs disrupted the synthesis of 20E, caused downregulation of the 20E signaling-related genes, and further affected the transcription of cuticular proteins. Moreover, it led to the malformation of the integument structure and a declined emergence rate, whereas exogenous 20E could rescue the declined emergence rate caused by knockdown of Mthl2. Furthermore, depletion of Mthl2 through RNAi in the N. lugens nymphal stage influenced the development of the ovaries and fecundity in female adults. The soluble protein content in reproductive tissues, the protein and transcript levels of Vitellogenin (Vg) were significantly decreased after silencing of Mthl2, ultimately leading to a decline in the number of offspring with an obviously transgenerational consequence.

CONCLUSION

The current study revealed the physiological functions of Mthl2 in molting and fecundity of N. lugens, which can be used as an RNAi-based insecticide discovery to control this pest. © 2025 Society of Chemical Industry.

CYP303A1 regulates molting and metamorphosis through 20E signaling in Nilaparvata lugens Stål (Hemiptera: Delphacidae)

Cytochrome P450s play a crucial role in the breakdown of external substances and perform important activities in the hormone system of insects. It has been understood that P450s were essential in the metabolism of ecdysteroids. CYP303A1 is a highly conserved CYP in most insects, but its specific physiological functions remain poorly understood in Nilaparvata lugens Stål. In this study, NlCYP303A1 was identified and highly expressed in the pre-molt stages, predominantly in the cuticle-producing tissues. Silencing of NlCYP303A1 caused a lethal phenotype with a molting defect. Moreover, the 20E titers, the expression levels of Halloween genes, and critical genes associated with the 20E signaling pathway in N. lugens nymphs were significantly decreased with the silencing NlCYP303A1. We further performed additional backfilling of 20E to rescue the RNAi effects on NlCYP303A1. The gene expression levels that were previously reduced caused by silencing NlCYP303A1 were significantly elevated. However, the molting defects of nymphs were not effectively improved. The results demonstrated NlCYP303A1 plays a crucial role in the molting and metamorphosis of N. lugens by regulating the 20E signaling pathway and cuticular formation, enhances the understanding of the functional role of CYP 2 clans, and identifies candidate gene for RNAi-based control of N. lugens.

Three odorant-binding proteins of small hive beetles, Aethina tumida, participate in the response of bee colony volatiles

Aethina tumida (small hive beetle, SHB) is a rapidly spreading invasive parasite of bee colonies. The olfactory system plays a key role in insect behavior, and odorant-binding proteins (OBPs) are involved in the first step of the olfactory signal transduction pathway and the detection of host volatiles. However, the olfactory mechanism of OBPs in SHB-localized bee colonies is unclear. In this study, electroantennogram (EAG) and behavioral bioassay showed that only three compounds (2-heptanone, ocimene, and ethyl palmitate) from bee colonies triggered high electrophysiological and behavioral responses. Three antenna-specific OBP genes (OBP6, OBP11, and OBP19) were identified, and they were significantly expressed on adult days 6-7. Furthermore, by combining RNA interference (RNAi) with EAG, olfactometer bioassay, competitive fluorescence binding assays, and molecular docking, we found that these three OBP genes were involved in the recognition of 2-heptanone and ethyl palmitate, and AtumOBP6 is also involved in the recognition of ocimene. These data indicate that AtumOBP6, AtumOBP11, and AtumOBP19 play an important role in the olfactory response to bee colony volatiles. Our results provide new insights into the functions of the OBP families in A. tumida and help to explore more potential target genes for environmentally friendly pest control strategies.

Matrix metalloproteinase 2 contributes to adult eclosion and immune response in the small hive beetle, Aethina tumida

During the pupal-adult eclosion process of holometabolous insects, the old cuticle is shed and replaced by a completely different new cuticle that requires tanning and expansion, along with extensive extracellular matrix (ECM) remodeling. In vertebrates, matrix metalloproteinases (MMPs), a class of zinc-dependent endopeptidases, play key roles in regulating the ECM that surrounds cells. However, little is known about these extracellular proteinases available in insects. The small hive beetle (SHB), Aethina tumida, is a widespread invasive parasite of honey bees. In this study, 6 MMP homologs were identified in the SHB genome. RNA interference experiments showed that all 6 AtMmps are not required for the larval-pupal transition, only AtMmp2 was essential for pupal-adult eclosion in SHB. Knockdown of AtMmp2 resulted in eclosion defects and wing expansion failure, as well as mortality within 3 d of adult eclosion. Transcriptomic analysis revealed that knockdown of AtMmp2 significantly increased expression of the Toll and Imd pathways, chitin metabolism, and cross-linking (such as the pro-phenoloxidase activating cascade pathway and the tyrosine-mediated cuticle sclerotization and pigmentation pathway). These data revealed evolutionarily conserved functions of Mmp2 in controlling adult eclosion and wing expansion, also provided a preliminary exploration of the novel function of regulating Toll and Imd pathways, as well as new insights into how MMPs regulate insect development and defense barriers.

Knockdown of the ecdysone receptor disrupts development and causes mortality in the melon fly, Zeugodacus cucurbitae

Cucurbits are important economic plants that are attacked by numerous pests, among which the melon fly Zeugodacus cucurbitae is extremely problematic. New sustainable pest control strategies are necessary to replace chemical insecticides that are harmful to the environment, human health and nontarget species. The RNA interference (RNAi) technology is one of the most promising tools due to high efficiency and species specificity. We developed an RNAi strategy targeting the ecdysone receptor (ECR) of Z. cucurbitae, which plays an important role in moulting and reproduction. We identified, described and isolated the ECR gene of Z. cucurbitae and measured its expression pattern across developmental stages and tissues. ZcECR knockdown via dsZcECR ingestion caused a significant larval mortality and abnormal phenotypes in pupae and adults. About 68% of larvae fed with a dsZcECR-treated diet failed to enter the pupal stage and died. In addition, ZcECR knockdown dramatically reduced pupal weight (by 3.24 mg on average) and fecundity (by about 23%). RNAi targeting the ECR gene is therefore a promising method to control Z. cucurbitae, paving the way for the development of novel sustainable and highly specific control strategies.

Proteasome β3 subunit (PSMB3) controls female reproduction by promoting ecdysteroidogenesis during sexual maturation in Bactrocera dorsalis

Steroid hormone 20-hydroxyecdysone (20E) plays critical roles in reproductive development in dipterans and several other insect species. Ecdysteroidogenesis in the glands of larval or nymphal insects and other arthropods has been extensively studied, but that in the adult gonads remains largely unknown. Here we identified a proteasome β3 subunit (PSMB3) from a highly invasive pest Bactrocera dorsalis, and found that this gene was crucial for ecdysone production during female reproduction. PSMB3 was enriched in the ovary, and it was upregulated during sexual maturation. RNAi-mediated depletion of PSMB3 resulted in retarded ovarian development and decreased fecundity. Additionally, knockdown of PSMB3 reduced 20E titer in hemolymph of B. dorsalis. Molecularly, RNA sequencing and qPCR validation revealed that PSMB3 depletion suppressed the expression of 20E biosynthetic genes in the ovary and 20E responsive genes in the ovary and fat body. Furthermore, exogenous 20E rescued the inhibition of the ovarian development caused by PSMB3 depletion. Taken together, this study provides new insights into the adult reproductive development-related biological processes controlled by PSMB3, and proposed a potential eco-friendly control strategy against this notorious agricultural pest.