MicroRNA miR-7-5p targets MARK2 to control metamorphosis in Galeruca daurica

The MARK2 gene, coding microtubule affinity-regulating kinase or serine/threonine protein kinase, is an important modulator in organism microtubule generation and cell polarity. However, its role in the metamorphosis of insects remains unknown. In this study, we found a conserved miRNA, miR-7-5p, which targets MARK2 to participate in the regulation of the larval-pupal metamorphosis in Galeruca daurica. The dual luciferase reporter assay showed that miR-7-5p interacted with the 3' UTR of MARK2 and repressed its expression. The expression profiling of miR-7-5p and MARK2 displayed an opposite trend during the larval-adult development process. In in-vivo experiments, overexpression of miR-7-5p by injecting miR-7-5p agomir in the final instar larvae down-regulated MARK2 and up-regulated main ecdysone signaling pathway genes including E74, E75, ECR, FTZ-F1 and HR3, which was similar to the results from knockdown of MARK2 by RNAi. In contrast, repression of miR-7-5p by injecting miR-7-5p antagomir obtained opposite effects. Notably, both overexpression and repression of miR-7-5p in the final instar larvae caused abnormal molting and high mortality during the larval-pupal transition, and high mortality during the pupal-adult transition. The 20-hydroxyecdysone (20E) injection experiment showed that 20E up-regulated miR-7-5p whereas down-regulated MARK2. This study reveals that the accurate regulation of miRNAs and their target genes is indispensable for insect metamorphosis.

MicroRNA miR-285 modulates the metamorphosis in Galeruca daurica by targeting Br-C

BACKGROUND

Galeruca daurica has become a new pest on the Inner Mongolia grasslands since an abrupt outbreak in 2009 caused serious damage. As a pupa indicator during insect metamorphosis, the early response gene of the ecdysone signaling pathway, Broad-Complex (Br-C), plays a vital role in the growth and development of insects. MicroRNAs (miRNAs) are small non-coding RNAs which mediate various biological activities, but it is unknown whether and how Br-C is regulated by miRNAs.

RESULTS

Temporal expression profiles revealed that miR-285 and Br-C basically displayed an opposite trend during larval-adult development, and Br-C was sharply up-regulated on the last day of final-instar larvae while miR-285 was significantly down-regulated. Both dual-luciferase reporter assay and miRNA-mRNA interaction assay indicated that miR-285 interacts with the coding sequence of Br-C and represses its expression. Not only overexpression but also downexpression of miR-285 led to the failure of larval to pupal to adult metamorphosis. In addition, both overexpression of miR-285 and silence of Br-C inhibited the expression of Br-C and other ecdysone signaling pathway genes, including E74, E75, ECR, FTZ-F1, and HR3. On the contrary, suppressing miR-285 obtained opposite results. Further experiments showed that 20-hydroxyecdysone down-regulated miR-285 and up-regulated Br-C and above-mentioned genes, whereas juvenile hormone alalogue (JHA) resulted in opposite effects.

CONCLUSION

Our results reveal that miR-285 is involved in mediating the metamorphosis in G. daurica by targeting Br-C in the ecdysone signaling pathway. miR-285 and its target Br-C could be as a potential target for G. daurica management. © 2024 Society of Chemical Industry.

MicroRNA miR-2765-3p regulates reproductive diapause by targeting FoxO in Galeruca daurica

The forkhead box O (FoxO), as a conserved transcription factor, plays an indispensable role in regulating insect diapause. However, how FoxO is regulated to control diapause in insects remains unknown. In this study, we discovered functional binding sites for miR-2765-3p in the 3' untranslated region of FoxO in Galeruca daurica. The luciferase reporter assay showed that miR-2765-3p targeted FoxO and suppressed its expression. The expression profiles of miR-2765-3p and FoxO displayed opposite patterns during the female developmental process. Overexpression of miR-2765-3p by the injection of the miR-2765-3p agomir into adult females reduced FoxO expression, leading to the suppression of lipid accumulation, promotion of ovarian development, and inhibition of reproductive diapause. This is similar to the phenotype that results from the depletion of FoxO by injecting dsFoxO into adult females. In addition, the repression of miR-2765-3p by injecting the miR-2765-3p antagomir increased the FoxO transcript level, leading to the stimulation of lipid accumulation, depression of ovarian development, and induction of reproductive diapause. A hormone injection assay showed that the juvenile hormone (JH) agonist (methoprene) upregulated miR-2765-3p and downregulated FoxO. Notably, injecting methoprene rescued ovarian development defects associated with miR-2765-3p inhibition. These findings indicate that the JH/miR-2765-3p/FoxO axis plays a vital role in the regulation of reproductive diapause in G. daurica.

Expression Profiling and Functional Analysis of Candidate Odorant Receptors in Galeruca daurica

Simple Summary

Odorant receptors (ORs) play an important role in the olfactory system in insects. However, there is no functional research on the odorant receptors of Galeruca daurica. In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults. Most GdauORs were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased.

Abstract

Galeruca daurica (Joannis) is an oligophagous pest in the grasslands of Inner Mongolia, China, which feed mainly on Allium spp. Odorant receptors (ORs) play an important role in the olfactory system in insects, and function together with olfactory co-receptor (ORco). In this study, 21 OR genes were identified from the transcriptome database of G. daurica adults, and named GdauOR1-20 and GdauORco. The expression profiles were examined by RT-qPCR and RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted to further identify the olfactory functions of GdauOR4, GdauOR11, GdauOR15, and GdauORco. It was found that 15 GdauORs (OR1, OR3-6, OR8, OR11-13, OR15, OR17-20, and ORco) were mainly expressed in antennae, and the expression levels of GdauORs in adults were affected by age. When GdauOR4, GdauOR15, and GdauORco were silenced by RNAi, the electrophysiological responses to host plant volatiles were significantly decreased in G. daurica. This study lays a necessary foundation for clarifying the mechanism on finding host plants in G. daurica.

Transcriptome-wide identification of microRNAs in response to 20-hydroxyecdysone in Galeruca daurica

Both 20-hydroxyecdysone (20E) and miRNAs have multiple functions in the regulation of various physiological processes in insects. However, little is known about the interaction between 20E and miRNAs. In this study, six small RNA libraries were constructed from the adult Galeruca daurica treated with 20E and dimethyl sulfoxide (DMSO), respectively. Using small RNA sequencing, a total of 183 miRNAs, including 140 known and 43 novel miRNAs, were identified. Compared with the control (DMSO), 52 miRNAs (21 up-regulated and 31 down-regulated) were significantly differentially expressed after 20E treatment. The KEGG and GO analysis of the predicted genes targeted by 20E-responsive miRNAs indicate that 20E may influence the metabolic change during reproductive diapause in G. daurica via regulating miRNAs.

MicroRNA let-7-5p targets the juvenile hormone primary response gene Krüppel homolog 1 and regulates reproductive diapause in Galeruca daurica

MicroRNAs (miRNAs) regulate various biological processes in insects. However, their roles in the regulation of insect diapause remain unknown. In this study, we address the biological function of a conserved miRNA, let-7-5p in the regulation of a juvenile hormone primary response gene, Krüppel homolog 1 (Kr-h1), which modulates reproductive diapause in Galeruca daurica. The dual luciferase reporter assay showed that let-7-5p depressed the expression of Kr-h1. The expression profiles of let-7-5p and Kr-h1 displayed opposite patterns in the adult developmental stage. Injection of let-7-5p agomir in pre-diapause adult females inhibited the expression of Kr-h1, which consequently led to delay ovarian development, increase lipid accumulation, expand fat body, and induce reproductive diapause just as depleting Kr-h1 did. Conversely, injection of let-7-5p antagomir resulted in opposite effects by reducing fat storage and stimulating reproduction. Moreover, JH receptor agonist methoprene reduced the expression of let-7-5p, and rescued the ovarian development defects associated with let-7-5p overexpression. These results indicate that let-7-5p plays an important role in the regulation of reproductive diapause and development of G. daurica adults through its target gene Kr-h1.

Functional Characterization of Olfactory Proteins Involved in Chemoreception of Galeruca daurica

Odorant-binding proteins (OBPs) and chemosensory proteins (CSPs) play a fundamental role in insect olfaction. Galeruca daurica (Joannis) is a new pest with outbreak status in the Inner Mongolia grasslands, northern China. In this study, six olfactory protein genes (GdauOBP1, GdauOBP6, GdauOBP10, GdauOBP15, GdauCSP4, and GdauCSP5) were cloned by RACE and expressed by constructing a prokaryotic expression system. Their binding affinities to 13 compounds from host volatiles (Allium mongolicum) were determined by fluorescence-binding assay. In order to further explore the olfactory functions of GdauOBP15 and GdauCSP5, RNA interference (RNAi) and electroantennogram (EAG) experiments were conducted. Ligand-binding assays showed that the binding properties of the six recombinant proteins to the tested volatiles were different. GdauOBP6, GdauOBP15, GdauCSP4, and GdauCSP5 could bind several tested ligands of host plants. It was suspected that GdauOBP6, GdauOBP15, GdauCSP4, and GdauCSP5 were related to the host location in G. daurica. We also found that there were different EAG responses between males and females when the GdauOBP15 and GdauCSP5 genes were silenced by RNAi. The EAG response of G. daurica females to 2-hexenal was significantly decreased in dsRNA-OBP15-injected treatment compared to the control, and the dsRNA-CSP5-treated females significantly reduced EAG response to eight tested host volatiles (1,3-dithiane, 2-hexenal, methyl benzoate, dimethyl trisulfide, myrcene, hexanal, 1,3,5-cycloheptatriene, and p-xylene). However, the EAG response had no significant difference in males. Both GdauOBP15 and GdauCSP5 may have different functions between males and females in G. daurica and may play more important roles in females searching for host plants.

Laboratory Selection, Cross-Resistance, Risk Assessment to Lambda-Cyhalothrin Resistance, and Monitoring of Insecticide Resistance for Plant Bug Lygus pratensis (Hemiptera: Miridae) in Farming-Pastoral Ecotones of Northern China

The plant bug Lygus pratensis Linnaeus (Hemiptera: Miridae) is an important insect pest of alfalfa in grassland farming in northern China. A field population of L. pratensis was selected in the laboratory for 14 consecutive generations with lambda-cyhalothrin to generate 42.555-fold resistance. Selection also induced low cross-resistance to imidacloprid and beta-cypermethrin, and medium cross-resistance to deltamethrin. Realized heritability (h2) of lambda-cyhalothrin resistance was 0.339. Susceptible baselines of L. pratensis were established for five insecticides using the glass-vial method, the values of which were 6.849, 3.423, 8.778, 3.559, and 117.553 ng/cm2 for phoxim, methomyl, imidacloprid, lambda-cyhalothrin, and avermectin, respectively, along with the calculated LC99 diagnostic doses. This resistance risk assessment study suggests that a high risk of lambda-cyhalothrin resistance exists in the field. In addition, a 5-year field investigation of resistance monitoring of L. pratensis was conducted in seven alfalfa regions in farming-pastoral ecotones in northern China. The resistance levels of most populations were very low for phoxim, methomyl, and avermectin, with an upward trend for lambda-cyhalothrin resistance in the DK (Dengkou County), TKT (Tuoketuo County), XL (Xilinhot), and LX (Linxi County) populations during 2015-2019, and medium resistance level to imidacloprid in the TKT population in five years we sampled. The study provided information on chemical control, lambda-cyhalothrin resistance development, baseline susceptibility, and the status of resistance to five commonly-used insecticides against L. pratensis. These results could be used to optimize pyrethroid insecticide use as part of a pest integrated resistance management strategy against this key insect pest of alfalfa.

Identification and functional analysis of microRNAs in the regulation of summer diapause in Galeruca daurica

MicroRNAs (miRNAs) regulate gene expression at the post-transcriptional level. Although the regulatory roles of miRNAs in various physiological processes throughout insect development have been investigated, it is almost unknown about the roles of miRNAs involved in regulation of diapause in insects. We constructed nine small RNA libraries from Galeruca daurica adults at different diapause stages: pre-diapause (PD), diapause (D), and post-diapause (TD). Using Illumina sequencing, a total of 95.06 million valid reads was obtained, and 222 miRNAs, including 135 conserved and 87 novel miRNAs, were identified from G. daurica. The expression profiles of these miRNAs were analyzed across different diapause stages. The 30 and 13 miRNAs were differentially expressed in the D/PD and TD/D comparisons, respectively. The KEGG and GO analysis of the predicted target genes suggested the essential roles of miRNAs in the regulation of summer diapause in G. daurica, especially via the juvenile hormone, ribosome, MAPK signaling, and Ca²⁺ signaling pathways. Our research results indicate that miRNAs may be involved in the regulation of summer diapause in G. daurica, and these results also provide an important new small RNA genomics resource for further studies on insect diapause.

Three Chemosensory Proteins Involved in Chemoreception of Oedaleus asiaticus (Orthopera: Acridoidea)

Chemosensory proteins (CSPs) are thought to play roles in the insect olfactory system by binding and carrying hydrophobic odorants across the aqueous sensillar lymph. The band-winged grasshopper, Oedaleus asiaticus Bei-Bienko, is one of the most important grasshopper pests in northern China, but there is little information about its olfactory system. In order to investigate the olfactory functions of CSPs in this pest, three CSP genes (OasiCSP4, OasiCSP11 and OasiCSP12) were expressed in Escherichia coli, and the binding affinities of the three recombinant CSP proteins were measured for 16 volatiles from the host plant (Stipa krylovii), fecal material and body of live adult O. asiaticus using fluorescence competitive binding assays. To further verify their olfactory functions, RNA interference (RNAi) and electrophysiological recording were conducted. The three recombinant proteins displayed different degrees of binding to various volatiles in ligand-binding assays, with OasiCSP12 having higher binding affinities for more volatiles than OasiCSP4 and OasiCSP11. OasiCSP12 exhibited strong binding affinities (Ki < 20 μΜ) for five host plant volatiles and one volatile from the live body of adult O. asiaticus. The transcript levels of the three OasiCSP genes were significantly lower after silencing the individual genes by RNAi, which in turn reduced the EAG responses in adults of both sexes to most tested compounds. Our study indicates that these three OasiCSPs are involved in the detection of volatile semiochemicals, and may play important roles in finding host plants and in aggregation in O. asiaticus.

Comparative Analysis of Transcriptome Responses to Cold Stress in Galeruca daurica (Coleoptera: Chrysomelidae)

Galeruca daurica (Joannis) has become a new insect pest in the Inner Mongolia grasslands since 2009, and its larvae and eggs have strong cold tolerance. To get a deeper insight into its molecular mechanisms of cold stress responses, we performed de novo transcriptome assembly for G. daurica by RNA-Seq and compared the transcriptomes of its larvae exposed to five different temperature treatments (-10, -5, 0, 5, and 25°C for 1 h and then recovered at 25°C for 1 h), respectively. Compared with the control (25°C), the numbers of differentially expressed genes (DEGs) decreased from 1,821 to 882, with the temperature declining from 5 to -10°C. Moreover, we obtained 323 coregulated DEGs under different low temperatures. Under four low temperatures (-10, -5, 0, and 5°C), a large number of genes were commonly upregulated during recovery from cold stresses, including those related to cuticle protein, followed by cytochrome P450, clock protein, fatty acid synthase, and fatty acyl-CoA reductase; meanwhile, lots of genes encoding cuticle protein, RNA replication protein, RNA-directed DNA polymerase, and glucose dehydrogenase were commonly downregulated. Our findings provide important clues for further investigations of key genes and molecular mechanisms involved in the adaptation of G. daurica to harsh environments.

Effects of Temperature on the Termination of Egg Diapause and Post-Diapause Embryonic Development of Galeruca daurica (Coleoptera: Chrysomelidae)

Galeruca daurica (Joannis) is a new pest on the grasslands of Inner Mongolia, China. It is univoltine and overwinters in the egg stage. Larvae and adults feed on the foliage of Allium plants. To assess the requirements to terminate egg diapause and subsequent effects on post-diapause development rate, eggs were held at different temperature regimes. Exposure to low temperatures was required to terminate egg diapause. Prolonged exposure (2 mo vs 1 mo) to 5°C and outside ambient conditions (mean temperature: 10.5°C; range: -7.1-21.6°C) enhanced the termination of egg diapause. Prolonged exposure also reduced the time to egg hatch; e.g., eggs held for 2 mo versus 1 mo at 5°C developed more quickly when subsequently placed at warmer temperatures. Egg hatch was observed at 17, 21, 25, and 29°C, but not at 15°C. Regression analysis identified 16.2°C as the minimum temperature for post-diapause development. The temperature requirement to complete embryonic development (from diapause termination to egg hatch) was calculated to be 103.1 to 140.9 degree-days.

[Supercooling capacity and cold hardiness of the Pararcyplera microptera meridionalis (Orthoptera: Acrididae) eggs]

Using the thermocouple method, the supercooling point (SCP) and cold hardiness of Pararcyplera microptera meridionalis eggs were measured in the laboratory. The soil water content significantly affected the water content of pre-diapause eggs, but had no significant effect on the SCP, and the water content of pre-diapause eggs rose with the increasing soil water content. There were highly significant differences among the SCPs, water contents or fat contents in the eggs at different developmental stages. With the egg' s development, the water content decreased from 51.5% at oviposition to 46.8% in 120 days after oviposition, the fat content increased from 10.5% (fresh mass)/19.0% (dry mass) to 14.5% (fresh mass)/28.9% (dry mass), and the SCP declined from -23.5 degrees C to -30.0 degrees C. There was a significant correlation between the SCP and the water content or fat content. The SCPs of deep-diapause eggs were lower than those of pre- and early-diapause eggs. The different low temperatures and treatment durations significantly affected the survival rate of diapause eggs. The lethal low temperature (Ltemp50) for 12 h exposure was -27.3 degrees C and the lethal time (Ltime50) at -25 degrees C was 22.73 days. As the mean SCPs of diapause eggs was much similar to their Ltemp50, the SCP could be considered as a good indicator of cold hardiness for P. m. meridionalis eggs and this species is a freeze-intolerant insect.

Supercooling capacity and cold hardiness of band-winged grasshopper eggs (Orthoptera: Acrididae)

The band-winged grasshopper, Oedaleus asiaticus Bei-Bienko, is one of the most dominant and economically important grasshopper species in the steppe grasslands and farming-pastoral ecotone in northern China. It is a univoltine species and overwinters as eggs in soil. The cold hardiness of its eggs was examined in the laboratory. Water content in soil significantly affected the supercooling points (SCPs), water content and fat content of prediapause eggs. With the increase of water content in soil, the SCP, and water content of prediapause eggs rose whereas the fat content declined. There was a significant relationship between the SCP and water content or fat content of prediapause eggs. The SCPs of prediapause and diapause eggs varied from -7.6 to -28.4°C and the SCPs of eggs 30 d after oviposition could be divided into two groups. The means of high SCP group (-11.0 to -11.9°C) were much higher than those of low SCP group (-21.8 to -21.9°C), and the majority belonged to the latter (90.48-93.33%). The SCPs of prediapause eggs and early-diapause eggs 30 d after oviposition were significantly higher than those of deep-diapause eggs 60 d after oviposition. The survival rates of diapause eggs were significantly different among different temperature treatments. The survival rate was higher than 88% at greater than -20°C and declined significantly to 57% at -25°C, and suddenly dropped to zero at -30°C. The lower lethal temperature (Ltemp50) for 12 h exposure was -25.3°C and the lower lethal time (Ltime50) at -20°C was 32.8 d. As the mean SCPs of diapause eggs were similar to their Ltemp50, the SCP of eggs can be considered as a good indicator of cold hardiness for O. asiaticus and that this grasshopper is a freeze-intolerant insect.

[Effects of host plants on the life table parameters of experimental populations of Aphis gossypii]

A comparative study was conducted on the life table parameters of Aphis gossypii reared on five host plant species at (25 +/- 1) degrees C in laboratory. There existed significant differences in the durations of various developmental stages, adult longevity, mean offspring number per day, net reproductive rate, intrinsic rate of increase, finite rate of increase, mean generation time, and population doubling time for the A. gossypii populations reared on the host plants. For the aphids on Lagenaria siceraria var. turbinate, they needed the longest time (5.84 days) to complete one generation, but for those on the other four plants, no significant differences were observed, with the time needed ranged from 5.24 to 5.45 days. The adult longevity was the longest (20.04 days) on Cucumis sativus, but had no significant differences on the other four host plants, being from 14.76 to 16.03 days. The populations' survival curves on all test host plants were of Deevey I, i. e., the death mainly occurred during late period. The survival rate on C. sativus was higher than those on the other four host plants. Based on the intrinsic rates of increase of A. gossypii, its host suitability was in the order of Cucumis melo var. saccharinus > Lagenaria siceraria var. turbinate > Cucurbita moschata var. melonaeformis > Cucumis sativus > Cucurbita pepo var. medullosa.

[Relationships between host preference of Liriomyza huidobrensis (Blanchard) and nutrient and chlorophyll contents in host foliage]

The laboratory study with no free choice means showed that the host preference of Liriomyza huidobrensis (Blanchard) adult differed significantly with test plants. Phaseolus vulgaris, Chrysanthemum coronarium and Cucurbita pepo were the most preferred, while Lycopersicum esculenturn and Brassica oleracea were the least. Correlative analysis indicated that the host preference of L. huidobrensis adult had a significant positive correlation with the content of soluble sugar, but less correlation with soluble protein and chlorophyll contents in host foliage.