FlyNap (triethylamine) increases the heart rate of mosquitoes and eliminates the cardioacceleratory effect of the neuropeptide CCAP

Repellency, toxicity, and physiological actions of low molecular weight basic amines in mosquitoes

BACKGROUND

This study investigated the behavioral responses and toxicity of three basic amines: 1-methylpiperazine, 1-methylpyrrolidine, and triethylamine (TEA), compounds suggested previously to be anosmic in vapor exposures to caged mosquitoes.

RESULTS

These compounds showed repellency of Aedes aegypti mosquitoes, followed by flightlessness, knockdown, and paralysis, all increasing with exposure time and dosage. Electrophysiological experiments showed a blocking effect on nerve discharge of the Drosophila melanogaster larval central nervous system (CNS) with little evidence of hyperexcitation. Blockage of voltage-gated (Kv2) potassium channel currents under patch clamp occurred at similar concentrations. Involvement of K+ channels in the action of basic amines was supported by behavior and CNS recordings of a Shaker Kv1 mutant exposed to TEA, where instead of blockage, a hyperexcitation of nerve firing was observed. Experiments on cockroach leg mechanoreceptors demonstrated neuronal excitation and on mosquito antennae strong electroantennogram (EAG) signals with an augmentation of blank air responses after a single puff of basic amine.

CONCLUSIONS

The neurophysiological effects of basic amines are consistent with K+ channel block, whereas the antennal EAG response was not obviously associated with anosmia. The low-dose effects of basic amines appear to be repellency and bradykinesia. Overall, the findings provide key insights into the mechanisms underlying the biological activity of basic amines. © 2024 Society of Chemical Industry.

Testing new compounds for efficacy against Varroa destructor and safety to honey bees (Apis mellifera)

BACKGROUND

Varroa destructor is among the greatest threats to honey bee health worldwide. Acaricides used to control Varroa are becoming increasingly ineffective due to resistance issues, prompting the need for new compounds that can be used for control purposes. Ideally, such compounds would exhibit high toxicity to Varroa while maintaining relatively low toxicity to bees and beekeepers. We characterized the lethal concentrations (LC₅₀ ) of amitraz, matrine, FlyNap®, the experimental carbamates 2-((2-ethylbutyl)thio)phenyl methylcarbamate (1) and 2-(2-ethylbutoxy)phenyl methylcarbamate (2), and dimethoate (positive control) for Varroa using a glass vial assay. The test compounds also were applied to honey bees using an acute contact toxicity assay to determine the adult bee LD₅₀ for each compound.

RESULTS

Amitraz was the most toxic compound to Varroa, but carbamate 2 was nearly as active (within 2-fold) and the most selective due to its lower bee toxicity, demonstrating its promise as a Varroa control. While carbamate 1 was less toxic to honey bees than was amitraz, it was also 4.7-fold less toxic to the mites. Both matrine and FlyNap® were relatively ineffective at killing Varroa and were moderately toxic to honey bees.

CONCLUSION

Additional testing is required to determine if carbamate 2 can be used as an effective Varroa control. As new chemical treatments are identified, it will be necessary to determine how they can be utilized best alongside other control techniques as part of an integrated pest management program. © 2021 Society of Chemical Industry.

Integrated Pest Management Control of Varroa destructor (Acari: Varroidae), the Most Damaging Pest of (Apis mellifera L. (Hymenoptera: Apidae)) Colonies

Varroa destructor is among the greatest biological threats to western honey bee (Apis mellifera L.) health worldwide. Beekeepers routinely use chemical treatments to control this parasite, though overuse and mismanagement of these treatments have led to widespread resistance in Varroa populations. Integrated Pest Management (IPM) is an ecologically based, sustainable approach to pest management that relies on a combination of control tactics that minimize environmental impacts. Herein, we provide an in-depth review of the components of IPM in a Varroa control context. These include determining economic thresholds for the mite, identification of and monitoring for Varroa, prevention strategies, and risk conscious treatments. Furthermore, we provide a detailed review of cultural, mechanical, biological, and chemical control strategies, both longstanding and emerging, used against Varroa globally. For each control type, we describe all available treatments, their efficacies against Varroa as described in the primary scientific literature, and the obstacles to their adoption. Unfortunately, reliable IPM protocols do not exist for Varroa due to the complex biology of the mite and strong reliance on chemical control by beekeepers. To encourage beekeeper adoption, a successful IPM approach to Varroa control in managed colonies must be an improvement over conventional control methods and include cost-effective treatments that can be employed readily by beekeepers. It is our intention to provide the most thorough review of Varroa control options available, ultimately framing our discussion within the context of IPM. We hope this article is a call-to-arms against the most damaging pest managed honey bee colonies face worldwide.

Collection and Rearing of Container Mosquitoes and a 24-h Addition to the CDC Bottle Bioassay

Container mosquitoes (Diptera: Culicidae) oviposit their eggs in both natural and artificial containers. Many container mosquito species also serve as important vectors of disease-causing pathogens including Aedes aegypti, Ae. albopictus, and Ae. triseriatus. Control of these species can be done through the use of adulticide sprays. The efficacy of these treatments is highly dependent on the insecticide susceptibility status of the local mosquito populations. This paper provides protocols on collecting and rearing container mosquitoes for use in the Centers for Disease Control and Prevention (CDC) bottle bioassay. A brief description of the CDC bottle bioassay is provided as well as a standardized protocol for the incorporation of a 24-h mortality to the CDC bottle bioassay. Results from this 24-h holding addition to the CDC bottle bioassay reveal that some forms of resistance may be missed without the incorporation of the additional mortality reading. These protocols provide a foundation for new laboratories to establish rearing protocols and begin conducting resistance monitoring.

Nitric oxide produced by periostial hemocytes modulates the bacterial infection-induced reduction of the mosquito heart rate

The circulatory and immune systems of mosquitoes are functionally integrated. An infection induces the migration of hemocytes to the dorsal vessel, and specifically, to the regions surrounding the ostia of the heart. These periostial hemocytes phagocytose pathogens in the areas of the hemocoel that experience the highest hemolymph flow. Here, we investigated whether a bacterial infection affects cardiac rhythmicity in the African malaria mosquito, Anopheles gambiae We discovered that infection with Escherichia coli, Staphylococcus aureus and Staphylococcus epidermidis, but not Micrococcus luteus, reduces the mosquito heart rate and alters the proportional directionality of heart contractions. Infection does not alter the expression of genes encoding crustacean cardioactive peptide (CCAP), FMRFamide, corazonin, neuropeptide F or short neuropeptide F, indicating that they do not drive the cardiac phenotype. Infection upregulates the transcription of two superoxide dismutase (SOD) genes, catalase and a glutathione peroxidase, but dramatically induces upregulation of nitric oxide synthase (NOS) in both the heart and hemocytes. Within the heart, nitric oxide synthase is produced by periostial hemocytes, and chemically inhibiting the production of nitric oxide using l-NAME reverses the infection-induced cardiac phenotype. Finally, infection induces the upregulation of two lysozyme genes in the heart and other tissues, and treating mosquitoes with lysozyme reduces the heart rate in a manner reminiscent of the infection phenotype. These data demonstrate an exciting new facet of the integration between the immune and circulatory systems of insects, whereby a hemocyte-produced factor with immune activity, namely nitric oxide, modulates heart physiology.

Crustacean cardioactive peptide (CCAP) of the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae): Molecular characterization, distribution and its potential roles in larva-pupa ecdysis

Insects must undergo ecdysis for successful development and growth, and the crustacean cardioactive peptide (CCAP) is one of the most important hormone in this process. Here we reported a cDNA encoding for the CCAP precursor cloned from the oriental fruit fly, Bactrocera dorsalis, a most destructive insect pest of agriculture. The CCAP mature peptide (PFCNAFTGC-NH2) of B. dorsalis was generated by post-translational processing and found to be highly comparable with other insects. RT-qPCR showed that mRNA of CCAP in B. dorsalis (BdCCAP) was predominantly expressed in the central nervous system (CNS) and midgut of 3rd-instar larvae. By using immunohistochemical analysis, we also localized the endocrine cells that produce CCAP in the CNS, ring gland and midgut of 3rd-instar larvae of B. dorsalis. The synthetic CCAP mature peptide could induce the expression of mRNA of adipokinetic hormone (AKH), the metabolic neuropeptides in insects. The expression of BdCCAP mRNA in the CNS, but not in the midgut, could be upregulated in the response to the challenge of insect molting hormone, 20-hydroxyecdysone.

Real-time telemetry monitoring of oxygen in the central complex of freely-walking Gromphadorhina portentosa

A new telemetric system for the electrochemical monitoring of dissolved oxygen is showed. The device, connected with two amperometric sensors, has been successfully applied to the wireless detection of the extracellular oxygen in the central complex of freely-walking Gromphadorhina portentosa. The unit was composed of a potentiostat, a two-channel sensor conditioning circuit, a microprocessor module, and a wireless serial transceiver. The amperometric signals were digitalized and sent to a notebook using a 2.4 GHz transceiver while a serial-to-USB converter was connected to a second transceiver for completing the communication bridge. The software, running on the laptop, allowed to save and graph the oxygen signals. The electronics showed excellent stability and the acquired data was linear in a range comprised between 0 and -165 nA, covering the entire range of oxygen concentrations. A series of experiments were performed to explore the dynamics of dissolved oxygen by exposing the animals to different gases (nitrogen, oxygen and carbon dioxide), to low temperature and anesthetic agents (chloroform and triethylamine). The resulting data are in agreement with previous O₂ changes recorded in the brain of awake rats and mice. The proposed system, based on simple and inexpensive components, can constitute a new experimental model for the exploration of central complex neurochemistry and it can also work with oxidizing sensors and amperometric biosensors.

Genomics of Early Cardiac Dysfunction and Mortality in Obese Drosophila melanogaster

In experimental evolution, we impose functional demands on laboratory populations of model organisms using selection. After enough generations of such selection, the resulting populations constitute excellent material for physiological research. An intense selection regime for increased starvation resistance was imposed on 10 large outbred Drosophila populations. We observed the selection responses of starvation and desiccation resistance, metabolic reserves, and heart robustness via electrical pacing. Furthermore, we sequenced the pooled genomes of these populations. As expected, significant increases in starvation resistance and lipid content were found in our 10 intensely selected SCO populations. The selection regime also improved desiccation resistance, water content, and glycogen content among these populations. Additionally, the average rate of cardiac arrests in our 10 obese SCO populations was double the rate of the 10 ancestral CO populations. Age-specific mortality rates were increased at early adult ages by selection. Genomic analysis revealed a large number of single nucleotide polymorphisms across the genome that changed in frequency as a result of selection. These genomic results were similar to those obtained in our laboratory from less direct selection procedures. The combination of extensive genomic and phenotypic differentiation between these 10 populations and their ancestors makes them a powerful system for the analysis of the physiological underpinnings of starvation resistance.

Effects of evolutionary history on genome wide and phenotypic convergence in Drosophila populations

BACKGROUND

Studies combining experimental evolution and next-generation sequencing have found that adaptation in sexually reproducing populations is primarily fueled by standing genetic variation. Consequently, the response to selection is rapid and highly repeatable across replicate populations. Some studies suggest that the response to selection is highly repeatable at both the phenotypic and genomic levels, and that evolutionary history has little impact. Other studies suggest that even when the response to selection is repeatable phenotypically, evolutionary history can have significant impacts at the genomic level. Here we test two hypotheses that may explain this discrepancy. Hypothesis 1: Past intense selection reduces evolutionary repeatability at the genomic and phenotypic levels when conditions change. Hypothesis 2: Previous intense selection does not reduce evolutionary repeatability, but other evolutionary mechanisms may. We test these hypotheses using D. melanogaster populations that were subjected to 260 generations of intense selection for desiccation resistance and have since been under relaxed selection for the past 230 generations.

RESULTS

We find that, with the exception of longevity and to a lesser extent fecundity, 230 generations of relaxed selection has erased the extreme phenotypic differentiation previously found. We also find no signs of genetic fixation, and only limited evidence of genetic differentiation between previously desiccation resistance selected populations and their controls.

CONCLUSION

Our findings suggest that evolution in our system is highly repeatable even when populations have been previously subjected to bouts of extreme selection. We therefore conclude that evolutionary repeatability can overcome past bouts of extreme selection in Drosophila experimental evolution, provided experiments are sufficiently long and populations are not inbred.

Insect heart rhythmicity is modulated by evolutionarily conserved neuropeptides and neurotransmitters

Insects utilize an open circulatory system to transport nutrients, waste, hormones and immune factors throughout the hemocoel. The primary organ that drives hemolymph circulation is the dorsal vessel, which is a muscular tube that traverses the length of the body and is divided into an aorta in the head and thorax, and a heart in the abdomen. The dorsal vessel is myogenic, but its rhythmicity is modulated by neuropeptides and neurotransmitters. This review summarizes how neuropeptides such as crustacean cardioactive peptide (CCAP), FMRFamide-like peptides, proctolin, allatotropin and allatostatin modulate the heart contraction rate and the directionality of heart contractions. Likewise, it discusses how neurotransmitters such as serotonin, octopamine, glutamate and nitric oxide influence the heart rate, and how transcriptomic and proteomic approaches are advancing our understanding of insect circulatory physiology. Finally, this review argues that the immune system may modulate heart rhythmicity, and discusses how the myotropic activity of cardioactive factors extends to the accessory pulsatile organs, such as the auxiliary hearts of the antennae.

Mosquito aging modulates the heart rate and the proportional directionality of heart contractions

Mosquito aging impacts a myriad of physiological processes, including digestion, flight, mating, reproductive success, and immunity. In the present study, we conducted intravital video imaging in 1, 3, 5, 10, 15 and 20-day-old Anopheles gambiae female adults to assess whether aging impacts mosquito heart physiology. We found that the heart contraction rate increases over the first 15days of adulthood and then decreases. These changes occur for both contraction directions, although aging results in a relative change in the anterograde versus retrograde contraction rates. That is, whereas for the first 5days of life the anterograde and retrograde contraction rates are similar, from day 10 to day 20 the retrograde contraction rate is higher than the anterograde contraction rate. Aging also biases the proportional directionality of heart contractions, from approximately two thirds of the time being spent contracting in the anterograde direction and two thirds of the contractions propagating anterograde during the first 5days of life to an approximately even split between anterograde and retrograde when the mosquitoes have reached 10 to 20days of age. Transcriptional analyses of crustacean cardioactive peptide (CCAP), FMRFamide, calcium-calmodulin dependent kinase II (CaMKII), pygopus, manganese-iron superoxide dismutase (MnSOD1) and vinculin by quantitative RT-PCR revealed age-associated changes in gene expression, with MnSOD1 and vinculin expression showing a declining trend with age. RNAi-based knockdown of MnSOD1 or vinculin resulted in heart physiology that trended toward the aging phenotype for every parameter that was measured, suggesting that these two genes are involved in cardiac aging.

Starvation but not locomotion enhances heart robustness in Drosophila

Insects and vertebrates have multiple major physiological systems, each species having a circulatory system, a metabolic system, and a respiratory system that enable locomotion and survival in stressful environments, among other functions. Broadening our understanding of the physiology of Drosophila melanogaster requires the parsing of interrelationships among such major component physiological systems. By combining electrical pacing and flight exhaustion assays with manipulative conditioning, we have started to unpack the interrelationships between cardiac function, locomotor performance, and other functional characters such as starvation and desiccation resistance. Manipulative sequences incorporating these four physiological characters were applied to five D. melanogaster lab populations that share a common origin from the wild and a common history of experimental evolution. While exposure to starvation or desiccation significantly reduced flight duration, exhaustion due to flight only affected subsequent desiccation resistance. A strong association was found between flight duration and desiccation resistance, providing additional support for the hypothesis that these traits depend on glycogen and water content. However, there was negligible impact on rate of cardiac arrests from exhaustion by flight or exposure to desiccant. Brief periods of starvation significantly lowered the rate of cardiac arrest. These results provide suggestive support for the adverse impact of lipids on Drosophila heart robustness, a parallel result to those of many comparable studies in human cardiology. Overall, this study underscores clear distinctions among the connections between specific physiological responses to stress and specific types of physiological performance.

Mosquito Peptide Hormones: Diversity, Production, and Function

Mosquitoes, like other insects, produce a diversity of peptide hormones that are processed from different precursor proteins and have a range of activities. Early studies relied on purification of bioactive peptides for hormone identification, but more recently genomic data have provided the information needed to more comprehensively identify peptide hormone genes and associated receptors. The first part of this chapter summarizes the known or predicted peptide hormones that are produced by mosquitoes. The second part of this chapter discusses the sources of these molecules and their functions.

Comparative structural and functional analysis of the larval and adult dorsal vessel and its role in hemolymph circulation in the mosquito Anopheles gambiae

Hemolymph circulation in insects is driven primarily by the contractile action of a dorsal vessel, which is divided into an abdominal heart and a thoracic aorta. As holometabolous insects, mosquitoes undergo striking morphological and physiological changes during metamorphosis. This study presents a comprehensive structural and functional analysis of the larval and adult dorsal vessel in the malaria mosquito Anopheles gambiae. Using intravital video imaging we show that, unlike the adult heart, the larval heart contracts exclusively in the anterograde direction and does not undergo heartbeat directional reversals. The larval heart contracts 24% slower than the adult heart, and hemolymph travels across the larval dorsal vessel at a velocity that is 68% slower than what is seen in adults. By fluorescently labeling muscle tissue we show that although the general structure of the heart and its ostia are similar across life stages, the heart-associated alary muscles are significantly less robust in larvae. Furthermore, unlike the adult ostia, which are the entry points for hemolymph into the heart, the larval ostia are almost entirely lacking in incurrent function. Instead, hemolymph enters the larval heart through incurrent openings located at the posterior terminus of the heart. These posterior openings are structurally similar across life stages, but in adults have an opposite, excurrent function. Finally, the larval aorta and heart differ significantly in the arrangement of their cardiomyocytes. In summary, this study provides an in-depth developmental comparison of the circulatory system of larval and adult mosquitoes.

Daphnia longicephala neuropeptides: morphological description of crustacean cardioactive peptide (CCAP) and periviscerokinins in the Ctenodaphnia central nervous system

The publication of the Daphnia genome has driven research in this ecologically relevant model organism in many directions. However, information on this organism's physiology and the relevant controlling factors is limited. In this regard, especially neuropeptides are important biochemical regulators that control a variety of cellular processes, which in combination influence physiological conditions and allow the adaptation of the internal physiological state to external conditions. Thus, neuropeptides are prime in understanding an organism's physiology. We here aimed to detect and describe the distribution of evolutionary conserved neuropeptides including the crustacean cardioactive peptide (CCAP) and peptides of the family periviscerokinins (PVKs) in the central nervous system and the periphery of the Daphnia longicephala head region. We were able to identify a large pair of CCAP immunoreactive cells within central nervous system. In addition, in the periphery we found CCAP immunoreactive cells in the epidermis of the head with processes indicating cuticular secretion. Furthermore, we were able to identify and describe a complex neuronal circuit of PVK neuropeptides in the central nervous system. The data obtained in this study will provide important background information for future investigations aiming to unravel the cellular, neuronal and physiological pathways in a highly adaptive organism such as Daphnia.

Hemolymph circulation in insect sensory appendages: functional mechanics of antennal accessory pulsatile organs (auxiliary hearts) in the mosquito Anopheles gambiae

Mosquito antennae provide sensory input that modulates host-seeking, mating and oviposition behaviors. Thus, mosquitoes must ensure the efficient transport of molecules into and out of these appendages. To accomplish this, mosquitoes and other insects have evolved antennal accessory pulsatile organs (APOs) that drive hemolymph into the antennal space. This study characterizes the structural mechanics of hemolymph propulsion throughout the antennae of Anopheles gambiae. Using intravital video imaging, we show that mosquitoes possess paired antennal APOs that are located on each side of the head's dorsal midline. They are situated between the frons and the vertex in an area that is dorsal to the antenna but ventral to the medial-most region of the compound eyes. Antennal APOs contract in synchrony at 1 Hz, which is 45% slower than the heart. By means of histology and intravital imaging, we show that each antennal APO propels hemolymph into the antenna through an antennal vessel that traverses the length of the appendage and has an effective diameter of 1-2 μm. When hemolymph reaches the end of the appendage, it is discharged into the antennal hemocoel and returns to the head. Because a narrow vessel empties into a larger cavity, hemolymph travels up the antenna at 0.2 mm s(-1) but reduces its velocity by 75% as it returns to the head. Finally, treatment of mosquitoes with the anesthetic agent FlyNap (triethylamine) increases both antennal APO and heart contraction rates. In summary, this study presents a comprehensive functional characterization of circulatory physiology in the mosquito antennae.

Myotropic effects of FMRFamide containing peptides on the heart of the mosquito Anopheles gambiae

FMRFamide-like peptides (FLPs) are produced by invertebrate and vertebrate animals, and regulate diverse physiological processes. In insects, several FLPs modulate heart physiology, with some increasing and others decreasing dorsal vessel contraction dynamics. Here, we describe the FMRFamide gene structure in the mosquito, Anopheles gambiae, quantify the developmental and spatial expression of FMRFamide and its putative receptor (FMRFamideR), and show that the peptides FMRFamide and SALDKNFMRFamide have complex myotropic properties. RACE sequencing showed that the FMRFamide gene encodes eight putative FLPs and is alternatively spliced. Of the eight FLPs, only one is shared by A. gambiae, Aedes aegypti and Culex quinquefasciatus: SALDKNFMRFamide. Quantitative PCR showed that peak expression of FMRFamide and FMRFamideR occurs in second instar larvae and around eclosion. In adults, FMRFamide is primarily transcribed in the head and thorax, and FMRFamideR is primarily transcribed in the thorax. Intravital video imaging of mosquitoes injected FMRFamide and SALDKNFMRFamide revealed that at low doses these peptides increase heart contraction rates. At high doses, however, these peptides decrease heart contraction rates and alter the proportional directionality of heart contractions. Taken altogether, these data describe the FMRFamide gene in A. gambiae, and show that FLPs are complex modulators of mosquito circulatory physiology.