Coupling clearing and hybridization chain reaction approaches to investigate gene expression in organs inside intact insect heads

Detecting RNA molecules within their natural environment inside intact arthropods has long been challenging, particularly in small organisms covered by a tanned and pigmented cuticle. Here, we have developed a methodology that enables high-resolution analysis of the spatial distribution of transcripts of interest without having to dissect tiny organs or tissues, thereby preserving their integrity. We have combined an in situ amplification approach based on hybridization chain reaction, which enhances the signal-to-noise ratio, and a clearing approach that allows the visualization of inner organs beneath the cuticle. We have implemented this methodology for the first time in Hemiptera, mapping two salivary aphid (Acyrthosiphon pisum) transcripts, the effector c002 and the salivary sheath protein SHP. With a multiplex approach, we could simultaneously detect different mRNAs in mounted pea aphid head-thorax samples and show that they were distributed in distinct secretory cells of salivary glands. RESEARCH HIGHLIGHTS: Combining hybridisation chain reaction and clearing allows the detection of transcripts in intact aphids heads. The transcripts of the two salivary proteins c002 and SHP are compartmentalized in distinct secretory cells of the principal glands.

Histamine excretion in common indoor and hematophagous arthropods

Histamine is a biogenic amine that regulates multiple physiological functions in diverse organisms, specifically playing a central role in the mammalian immune response. The common bed bug, Cimex lectularius L. (Hemiptera: Cimicidae), excretes histamine in large amounts in its feces as a component of its aggregation pheromone, which contaminates homes. The potential health risks associated with the presence of indoor histamine are unclear, but to predict future exposure risks, it is critical that we understand if other arthropods excrete histamine in any discernible phylogenetic pattern. In the present study, we evaluated histamine excretion by various arthropods; specifically those commonly found in large numbers indoors, other hematophagous species, and other species in the order Hemiptera. To evaluate arthropods for histamine excretion, rearing containers for each arthropod were swabbed and/or the harborage substrates were collected. Samples were then analyzed for the presence of histamine using gas chromatography-mass spectrometry. For those arthropods where histamine was present above the method detection limit, total histamine excretion was quantified over a period of 2 wk. Our results indicate that histamine excretion is limited to hematophagous hemipterans (bed bugs, bat bugs, tropical bed bugs, and kissing bugs), suggesting that indoor histamine contamination in the United States can be primarily linked to bed bugs.

Transmission of Xanthomonas albilineans by the spittlebug, Mahanarva fimbriolata (Hemiptera: Cercopidae), in Brazil: first report of an insect vector for the causal agent of sugarcane leaf scald

Leaf scald is a destructive sugarcane disease caused by the bacterium Xanthomonas albilineans (Ashby) Dowson. This pathogen presents the gene cluster SPI-1 T3SS, a conserved feature in pathogens vectored by animals. In this study, the competence of Mahanarva fimbriolata (Stål), a spittlebug commonly found in sugarcane fields in Brazil, was evaluated for the transmission of X. albilineans. Artificial probing assays were conducted to investigate the ability of M. fimbriolata adults to acquire X. albilineans from artificial diets containing the pathogen with subsequent inoculation of X. albilineans into pathogen-free diets. Plant probing assays with M. fimbriolata adults were conducted to evaluate the acquisition of X. albilineans from diseased source plants and subsequent inoculation of healthy recipient sugarcane plants. The presence of X. albilineans DNA in saliva/diet mixtures of the artificial probing assays and both insects and plants of the plant probing assays were checked using TaqMan assays. The artificial probing assays showed that M. fimbriolata adults were able to acquire and inoculate X. albilineans in diets. Plant probing assays confirmed the competence of M. fimbriolata to transmit X. albilineans to sugarcane. Over the entire experiment, 42% of the insects had acquired the pathogen and successful inoculation of the pathogen occurred in 18% of the recipient-susceptible sugarcane plants at 72 or 96 h of inoculation access period. Assays evidenced the vector competence of M. fimbriolata for transmission of X. albilineans, opening new pathways for investigating the biology and the economic impacts of the interaction between X. albilineans and M. fimbriolata.

F-actin dynamics in midgut cells enables virus persistence in vector insects

Hemipteran insects that transmit plant viruses in a persistent circulative manner acquire, retain and transmit viruses for their entire life. The mechanism enabling this persistence has remained unclear for many years. Here, we determined how wheat dwarf virus (WDV) persists in its leafhopper vector Psammotettix alienus. We found that WDV caused the up-regulation of actin-depolymerizing factor (ADF) at the mRNA and protein levels in the midgut cells of leafhoppers after experiencing a WDV acquisition access period (AAP) of 6, 12 or 24 h. Experimental inhibition of F-actin depolymerization by jasplakinolide and dsRNA injection led to lower virus accumulation levels and transmission efficiencies, suggesting that depolymerization of F-actin regulated by ADF is essential for WDV invasion of midgut cells. Exogenous viral capsid protein (CP) inhibited ADF depolymerization of actin filaments in vitro and in Spodoptera frugiperda 9 (Sf9) cells because the CP competed with actin to bind ADF and then blocked actin filament disassembly. Interestingly, virions colocalized with ADF after a 24-h AAP, just as actin polymerization occurred, indicating that the binding of CP with ADF affects the ability of ADF to depolymerize F-actin, inhibiting WDV entry. Similarly, the luteovirus barley yellow dwarf virus also induced F-actin depolymerization and then polymerization in the gut cells of its vector Schizaphis graminum. Thus, F-actin dynamics are altered by nonpropagative viruses in midgut cells to enable virus persistence in vector insects.

Insecticide-contaminated honeydew: risks for beneficial insects

Honeydew is the sugar-rich excretion of phloem-feeding hemipteran insects such as aphids, mealybugs, whiteflies, and psyllids, and can be a main carbohydrate source for beneficial insects in some ecosystems. Recent research has revealed that water-soluble, systemic insecticides contaminate honeydew excreted by hemipterans that feed on plants treated with these insecticides. This contaminated honeydew can be toxic to beneficial insects, such as pollinators, parasitic wasps and generalist predators that feed on it. This route of exposure has now been demonstrated in three plant species, for five systemic insecticides and four hemipteran species; therefore, we expect this route to be widely available in some ecosystems. In this perspective paper, we highlight the importance of this route of exposure by exploring: (i) potential pathways through which honeydew might be contaminated with insecticides; (ii) hemipteran families that are more likely to excrete contaminated honeydew; and (iii) systemic insecticides with different modes of action that might contaminate honeydew through the plant. Furthermore, we analyse several model scenarios in Europe and/or the USA where contaminated honeydew could be problematic for beneficial organisms that feed on this ubiquitous carbohydrate source. Finally, we explain why this route of exposure might be important when exotic, invasive, honeydew-producing species are treated with systemic insecticides. Overall, this review opens a new area of research in the field of ecotoxicology to understand how insecticides can reach non-target beneficial insects. In addition, we aim to shed light on potential undescribed causes of insect declines in ecosystems where honeydew is an important carbohydrate source for insects, and advocate for this route of exposure to be included in future environmental risk assessments.

Next Generation dsRNA-Based Insect Control: Success So Far and Challenges

RNA interference (RNAi) is a method of gene silencing where dsRNA is digested into small interfering RNA (siRNA) in the presence of enzymes. These siRNAs then target homologous mRNA sequences aided by the RNA-induced silencing complex (RISC). The mechanism of dsRNA uptake has been well studied and established across many living organisms including insects. In insects, RNAi is a novel and potential tool to develop future pest management means targeting various classes of insects including dipterans, coleopterans, hemipterans, lepidopterans, hymenopterans and isopterans. However, the extent of RNAi in individual class varies due to underlying mechanisms. The present review focuses on three major insect classes viz hemipterans, lepidopterans and coleopterans and the rationale behind this lies in the fact that studies pertaining to RNAi has been extensively performed in these groups. Additionally, these classes harbour major agriculturally important pest species which require due attention. Interestingly, all the three classes exhibit varying levels of RNAi efficiencies with the coleopterans exhibiting maximum response, while hemipterans are relatively inefficient. Lepidopterans on the other hand, show minimum response to RNAi. This has been attributed to many facts and few important being endosomal escape, high activity dsRNA-specific nucleases, and highly alkaline gut environment which renders the dsRNA unstable. Various methods have been established to ensure safe delivery of dsRNA into the biological system of the insect. The most common method for dsRNA administration is supplementing the diet of insects via spraying onto leaves and other commonly eaten parts of the plant. This method is environment-friendly and superior to the hazardous effects of pesticides. Another method involves submergence of root systems in dsRNA solutions and subsequent uptake by the phloem. Additionally, more recent techniques are nanoparticle- and Agrobacterium-mediated delivery systems. However, due to the novelty of these biotechnological methods and recalcitrant nature of certain crops, further optimization is required. This review emphasizes on RNAi developments in agriculturally important insect species and the major hurdles for efficient RNAi in these groups. The review also discusses in detail the development of new techniques to enhance RNAi efficiency using liposomes and nanoparticles, transplastomics, microbial-mediated delivery and chemical methods.

Leaf-footed bugs possess multiple hidden contrasting color signals, but only one is associated with increased body size

Antipredatory displays that incorporate hidden contrasting coloration are found in a variety of different animals. These displays are seen in organisms that have drab coloration at rest, but when disturbed reveal conspicuous coloration. Examples include the bright abdomens of mountain katydids and the colorful underwings of hawk moths. Such hidden displays can function as secondary defenses, enabling evasion of a pursuant predator. To begin to understand why some species have these displays while others do not, we conducted phylogenetic comparative analyses to investigate factors associated with the evolution of hidden contrasting coloration in leaf-footed bugs. First, we investigated whether hidden contrasting coloration was associated with body size because these displays are considered to be more effective in larger organisms. We then investigated whether hidden contrasting coloration was associated with an alternative antipredatory defense, in this case rapid autotomy. We found that leaf-footed bugs with hidden contrasting coloration tended to autotomize more slowly, but this result was not statistically significant. We also found that the presence of a body size association was dependent upon the form of the hidden color display. Leaf-footed bugs that reveal red/orange coloration were the same size, on average, as species without a hidden color display. However, species that reveal white patches on a black background were significantly larger than species without a hidden color display. These results highlight the diversity of forms that hidden contrasting color signal can take, upon which selection may act differently.

Agamermis (Nematoda: Mermithidae) Infection in South Carolina Agricultural Pests

Native and invasive stink bugs (Hemiptera: Pentatomidae) and the closely related invasive Megacopta cribraria (Hemiptera: Plataspidae) are agricultural pests in the southeastern United States. Natural enemies, from various phyla, parasitize these pests and contribute to population regulation. We specifically investigated Nematoda infections in pentatomid and plataspid pests in one soybean field in South Carolina in 2015. Nematodes were identified through molecular and morphological methods and assigned to family Mermithidae, genus Agamermis. This study reports mermithid nematode infection in immature M. cribraria for the first time and provides the first mermithid host record for the stink bugs Chinavia hilaris, Euschistus servus, and another Euschistus species, and a grasshopper (Orthoptera: Acrididae) in South Carolina. The same Agamermis species infected all hosts. The broad host range and prevalence suggests that Agamermis may be an important contributor to natural mortality of pentatomid and plataspid pests. Previous mermithid host records for the Pentatomidae and Plataspidae worldwide are summarized. Further work is needed to assess the impact of infection on populations over a broader range of agricultural fields and geographic localities.

Identification, characterization and expression of a receptor for the unusual myosuppressin in the blood-feeding bug, Rhodnius prolixus

Myosuppressins are a family of the FMRFamide-like peptides. They have been characterized in many insects and shown to inhibit visceral muscle contraction. Rhodnius prolixus possesses an unusual myosuppressin in that the typical FLRFamide C-terminal motif is unique and ends with FMRFamide. In the present study, we isolated the cDNA sequence for the R. prolixus receptor for this unusual myosuppressin (RhoprMSR). Quantitative PCR indicates high relative transcript expression of RhoprMSR in the central nervous system and also supports the previously described physiological effects of RhoprMS on the digestive system, with expression of the RhoprMSR transcript in the midgut and hindgut. Expression of the RhoprMSR transcript was also found in the female and male reproductive system of 5th instar nymphs, with transcript expression greater in the female reproductive tissues. No expression was found in the salivary glands or Malpighian tubules. A functional receptor expression assay confirmed that the cloned RhoprMSR is indeed activated by RhoprMS (half maximum effective concentration = 42.7 nM). Structure-activity studies based upon both functional receptor assays and physiological assays showed the importance of the HVFMRFamide moiety, as further N-terminal truncation removed all activity.

Evidence for a conserved CCAP-signaling pathway controlling ecdysis in a hemimetabolous insect, Rhodnius prolixus

A vital feature in the success of Ecdysozoa is their ability to shed their exoskeleton (a process called ecdysis) such that they can grow or change their morphology. In holometabolous insects, these behaviors are orchestrated by the sequential actions of neuropeptides, one of which is crustacean cardioactive peptide (CCAP). Little is known about the control of ecdysis in hemimetabolous insects. Here, we report that CCAP is essential for successful ecdysis in the hemimetabolous insect, Rhodnius prolixus; the vector of Chagas disease. The first indication of CCAP's involvement in ecdysis was the observation of decreased staining intensity of CCAP-containing neurons immediately following ecdysis, indicative of the release of CCAP. The critical importance of the CCAP signaling pathway was further demonstrated by knockdown (as determined by qPCR and immunohistochemistry) of the CCAP and CCAPR transcripts utilizing dsRNA. This technique reduced the staining intensity of CCAP-containing neurons, and knocked down the transcript levels by up to 92%, with lethal consequences to the insect. Insects with these transcripts knocked down had very high mortality (up to 84%), typically at the expected time of the ecdysis sequence, or had ecdysis extremely delayed. This is the first report of the susceptibility of R. prolixus to dsRNA knockdown of neuropeptide and receptor transcripts, and the data clearly demonstrates the conserved nature of the CCAP signaling pathway in ecdysis between holometabolous and hemimetabolous insects.