Epicuticular lipids induce aggregation in Chagas disease vectors

Lipid Metabolism in Insect Vectors of Diseases

According to the World Health Organization vector-borne diseases account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually. Vectors are organisms that are able to transmit infectious pathogens between humans, or from animals to humans. Many of these vectors are hematophagous insects, which ingest the pathogen from an infected host during a blood meal, and later transmit it into a new host. Malaria, dengue, African trypanosomiasis, yellow fever, leishmaniasis, Chagas disease, and many others are examples of diseases transmitted by insects.Both the diet and the infection with pathogens trigger changes in many metabolic pathways, including lipid metabolism, compared to other insects. Blood contains mostly proteins and is very poor in lipids and carbohydrates. Thus, hematophagous insects attempt to efficiently digest and absorb diet lipids and also rely on a large de novo lipid biosynthesis based on utilization of proteins and carbohydrates as carbon source. Blood meal triggers essential physiological processes as molting, excretion, and oogenesis; therefore, lipid metabolism and utilization of lipid storage should be finely synchronized and regulated regarding that, in order to provide the necessary energy source for these events. Also, pathogens have evolved mechanisms to hijack essential lipids from the insect host by interfering in the biosynthesis, catabolism, and transport of lipids, which pose challenges to reproduction, survival, fitness, and other insect traits.In this chapter, we have tried to collect and highlight the current knowledge and recent discoveries on the metabolism of lipids in insect vectors of diseases related to the hematophagous diet and pathogen infection.

Cuticular extracts induce aggregation in head lice

The human head lice Pediculus humanus capitis (De Geer) (Phthiraptera: Pediculidae) are strict, obligate human ectoparasites that spends their entire life cycle in the host and cause skin irritation and derived infections. Despite the health-related importance, few studies have evaluated the chemical communication among these insects. Here, we evaluate the response of lice of both sexes to cuticular extracts using two solvents of different polarity (hexane and methanol). Cuticular extracts that elicited an attraction response towards head lice were analysed by gas chromatography-mass spectrometry (GC-MS) to determine the cuticular lipid profile. Both lice sexes were attracted to the hexane extracts but not the methanol extracts, suggesting the non-polarity of the compounds present in the cuticle. Chemical analyses of hexane extracts from males and females showed high similarity in major compounds. This study provides the first evidence that lice respond to cuticle extracts, which may be important to understand aggregation behaviour.

Lessons to be popular: the chemical basis of aggregation in Trypanosoma cruzi-infected and non-infected Chagasic bugs

Aggregation is one of the most remarkable behaviours in the animal kingdom-a process that is usually governed by pheromones. Triatomines are blood-sucking bugs that act as vectors of Trypanosoma cruzi, the etiological agent of Chagas disease in mammals, including humans. Triatomines usually gather in roosting refuges by using aggregation pheromones of unknown chemical structure. In terms of vector control, one option to reduce triatomine-human contact is via capturing the insects into traps baited with lures based on such aggregation pheromones. As a first step towards this aim, we elucidated the aggregation pheromone in the triatomine Triatoma pallidipennis, using T. cruzi-infected and non-infected bugs. We used different extraction techniques and gas chromatography coupled to mass spectrometry for the identification. Also, two different bioassays were implemented for evaluating the attractant and arrestant activity of the pheromone. We found that T. pallidipennis produced short-chain aldehydes as attractants, and nitrogen-derived compounds as arrestants. We detected differences in the production and perception of these compounds according to whether animals were infected or not. These findings show that T. cruzi may influence triatomine chemical ecology and are promising tools for triatomine control.

Highlights, challenges, and perspectives in basic and applied chemical ecology of triatomines

Triatomines are vectors of Chagas disease. Due to failures in their control, there is an urgent need for more efficient and environmentally friendly monitoring and control tools. These hematophagous insects rely heavily on chemical information from the environment to detect hosts and cues/signals from conspecifics. Chemical ecology includes the elucidation of the functional role of chemicals mediating interactions between organisms. Studies on the chemical ecology of triatomines are leading to novel methods for their monitor and control. Thus, laboratory tests to develop chemical attractants and repellents are promissory and have led to the design of, for example, efficient baited traps. However, the monitoring and control tools proposed until now have not been as effective in the field.

Behavioral response mediated by feces in Triatoma infestans (Hemiptera: Reduviidae: triatominae) susceptible and resistant to deltamethrin

Chemical cues from feces promote aggregation behavior in Triatoma infestans nymphs and adults. Given the importance of T. infestans resistant to pyrethroids in several areas of Argentina and Bolivia, it would be important to know if there is an association with specific attraction and aggregation behaviors. These behaviors, to and surrounding refuges, play an important role in triatomine population dynamics, an important factor to consider and model for vector control strategies. The aim of the present study was to analyze the behavior of orientation to chemical signals emitted by feces from deltamethrin resistant (R) and susceptible (S) T. infestans. The behavioral assays were performed in a circular glass arena divided in two equal sectors. Fecal signals emitted by both S and R feces are attractants to fifth-instar nymphs of both S and R populations. Both toxicological phenotypes remained significantly longer on R feces, as compared to S feces. This is the first evidence in a triatomine, for the association of an aggregation behavior and insecticide resistance and may be the result of pleiotropic effects surrounding resistance genes.

Synthesis and secretion of volatile short-chain fatty acids in Triatoma infestans infected with Beauveria bassiana

Physically disturbed Triatoma infestans (Hemiptera: Reduviidae) adults, as well as adults of other Chagas' disease vectors, secrete a mix of volatile organic compounds (VOCs) with alarm and possible sexual and defence functions. The aim of the present research was to test whether infection with the entomopathogenic fungus Beauveria bassiana (Ascomycota: Hypocreales: Clavicipitaceae) has an effect on VOC secretion in disturbed T. infestans and on the expression of two genes (Ti-brnq and Ti-bckdc) potentially involved in VOC biosynthesis. The volatiles released by insects at different time periods after fungal treatment were identified and their relative amounts measured. Isobutyric acid was the most abundant volatile found in both healthy and fungus-infected insects and underwent no significant relative changes through the infection process. The secretion of propionic acid, however, was significantly higher at 1-4 days post-infection (d.p. i.) compared with that in controls. A slight induction of both Ti-brnq and Ti-bckdc genes was found by real-time polymerase chain reaction at 4 d.p. i., with expression values reaching up to three-fold those in controls. The early stages of fungal infection seem to affect the composition of the alarm pheromone by changing the expression pattern of both genes analysed. These results help to elucidate the impact of fungal infections on the chemical ecology of triatomine bugs.

Transcriptome Analysis of the Triatoma infestans (Hemiptera: Reduviidae) Integument

The insect integument, formed by the cuticle and the underlying epidermis, is essential for insect fitness, regulation of lipid biosynthesis and storage, insect growth and feeding, together with development progress. Its participation in insecticide resistance has also been outlined. Triatoma infestans Klug (Hemiptera: Reduviidae) is one of the major vectors of Chagas disease in South America; however, genomic data are scarce. In this study, we performed a transcriptome analysis of the nymph integument in order to identify which genes are expressed and their putative role. Using the 454 GS-FLX sequencing platform, we obtained approximately 144,620 reads from the integument tissue. These reads were assembled into 6,495 isotigs and 8,504 singletons. Based on BLAST similarity searches, about 8,000 transcripts were annotated with known genes, conserved domains, and/or Gene Ontology terms.The most abundant transcripts corresponded to transcription factors and nucleic acid metabolism, membrane receptors, cell signaling, and proteins related to cytoskeleton, transport, and cell energy processes, among others. More than 10% of the transcripts-encoded proteins putatively involved in the metabolism of fatty acids and related components (fatty acid synthases, elongases, desaturases, fatty alcohol reductases), structural integument proteins, and the insecticide detoxification system (among them, cytochrome P450s, esterases, and glutathione transferases). Real-time qPCR assays were used to investigate their putative participation in the resistance mechanism. This preliminary study is the first transcriptome analysis of a triatomine integument, and together with prior biochemical information, will help further understandthe role of the integument in a wide array of mechanisms.

Neuroethology of Olfactory-Guided Behavior and Its Potential Application in the Control of Harmful Insects

Harmful insects include pests of crops and storage goods, and vectors of human and animal diseases. Throughout their history, humans have been fighting them using diverse methods. The fairly recent development of synthetic chemical insecticides promised efficient crop and health protection at a relatively low cost. However, the negative effects of those insecticides on human health and the environment, as well as the development of insect resistance, have been fueling the search for alternative control tools. New and promising alternative methods to fight harmful insects include the manipulation of their behavior using synthetic versions of "semiochemicals", which are natural volatile and non-volatile substances involved in the intra- and/or inter-specific communication between organisms. Synthetic semiochemicals can be used as trap baits to monitor the presence of insects, so that insecticide spraying can be planned rationally (i.e., only when and where insects are actually present). Other methods that use semiochemicals include insect annihilation by mass trapping, attract-and- kill techniques, behavioral disruption, and the use of repellents. In the last decades many investigations focused on the neural bases of insect's responses to semiochemicals. Those studies help understand how the olfactory system detects and processes information about odors, which could lead to the design of efficient control tools, including odor baits, repellents or ways to confound insects. Here we review our current knowledge about the neural mechanisms controlling olfactory responses to semiochemicals in harmful insects. We also discuss how this neuroethology approach can be used to design or improve pest/vector management strategies.

Chemical Prey Luring in Jackson's Chameleons

Lizards in the family Chamaeleonidae have been described as wiping a viscous substance from a pouch (the temporal pouch) at the angle of the jaw on branches and then capturing flies that land near the area where the wiping occurs. We confirmed the presence of this pouch in Jackson's chameleons. Histological work suggested that the material contained within is a result of decomposition of food and sloughed skin that has been trapped in the pouch rather than a glandular secretion. Gas chromatography-mass spectrometry indicated the presence of compounds that are both volatile and odiferous and similar to insect pheromones. Choice tests with houseflies revealed attraction to the temporal pouch material. Some authors have speculated that the temporal pouch material serves a function in territory marking and/or predator deterrence. While it may play these roles, our results suggest that it also plays a role in chemical luring of prey.

Age and aggregation trigger mating behaviour in the small hive beetle, Aethina tumida (Nitidulidae)

This study aimed to investigate the poorly documented reproductive behaviour of the small hive beetle, Aethina tumida (Nitidulidae), a honey bee (Apis mellifera) parasite. We described the mating behaviour in detail and tested the hypothesis that beetle aggregation plays a vital role in mating in this species. Gender preference was examined in the context of age-dependency and possible chemical communication. Beetles started mating at a high frequency 18 days after emergence from the soil but only if they were aggregated (p < 0.001); mating was infrequent when beetles were paired. Males in aggregation also tried to copulate with males and only copulated more frequently with females at 18 days after emergence from soil (p < 0.001) in contrast to newly emerged, 7-day-old and 60-day-old beetles. Males and females spent more time in social contact with the opposite sex (p < 0.01) when they were 18 days old in contrast to 7-day-old beetles. Filter papers which had been in contact with 21-day-old beetles were highly attractive to similar-aged beetles of the opposite sex (p < 0.01). This suggests that chemical substances produced by the beetles themselves play a role in mating. Mating behaviour was characterised by a short pre-copulation courtship and female aggression towards other females and copulating couples. Both behaviours may be indicative of cryptic female choice. Delayed onset of reproductive behaviour is typical of many polygamous species, whilst the indispensability of aggregation for onset of sexual behaviour seems to be a feature unique to A. tumida. Both strategies support mass reproduction in this parasitic species, enabling A. tumida to overcome its honey bee host colony, and are probably triggered by chemotactic cues..

Biological Control of the Chagas Disease Vector Triatoma infestans with the Entomopathogenic Fungus Beauveria bassiana Combined with an Aggregation Cue: Field, Laboratory and Mathematical Modeling Assessment

BACKGROUND

Current Chagas disease vector control strategies, based on chemical insecticide spraying, are growingly threatened by the emergence of pyrethroid-resistant Triatoma infestans populations in the Gran Chaco region of South America.

METHODOLOGY AND FINDINGS

We have already shown that the entomopathogenic fungus Beauveria bassiana has the ability to breach the insect cuticle and is effective both against pyrethroid-susceptible and pyrethroid-resistant T. infestans, in laboratory as well as field assays. It is also known that T. infestans cuticle lipids play a major role as contact aggregation pheromones. We estimated the effectiveness of pheromone-based infection boxes containing B. bassiana spores to kill indoor bugs, and its effect on the vector population dynamics. Laboratory assays were performed to estimate the effect of fungal infection on female reproductive parameters. The effect of insect exuviae as an aggregation signal in the performance of the infection boxes was estimated both in the laboratory and in the field. We developed a stage-specific matrix model of T. infestans to describe the fungal infection effects on insect population dynamics, and to analyze the performance of the biopesticide device in vector biological control.

CONCLUSIONS

The pheromone-containing infective box is a promising new tool against indoor populations of this Chagas disease vector, with the number of boxes per house being the main driver of the reduction of the total domestic bug population. This ecologically safe approach is the first proven alternative to chemical insecticides in the control of T. infestans. The advantageous reduction in vector population by delayed-action fungal biopesticides in a contained environment is here shown supported by mathematical modeling.

Measuring information flow in cellular networks by the systems biology method through microarray data

In general, it is very difficult to measure the information flow in a cellular network directly. In this study, based on an information flow model and microarray data, we measured the information flow in cellular networks indirectly by using a systems biology method. First, we used a recursive least square parameter estimation algorithm to identify the system parameters of coupling signal transduction pathways and the cellular gene regulatory network (GRN). Then, based on the identified parameters and systems theory, we estimated the signal transductivities of the coupling signal transduction pathways from the extracellular signals to each downstream protein and the information transductivities of the GRN between transcription factors in response to environmental events. According to the proposed method, the information flow, which is characterized by signal transductivity in coupling signaling pathways and information transductivity in the GRN, can be estimated by microarray temporal data or microarray sample data. It can also be estimated by other high-throughput data such as next-generation sequencing or proteomic data. Finally, the information flows of the signal transduction pathways and the GRN in leukemia cancer cells and non-leukemia normal cells were also measured to analyze the systematic dysfunction in this cancer from microarray sample data. The results show that the signal transductivities of signal transduction pathways change substantially from normal cells to leukemia cancer cells.

Behavioural biology of Chagas disease vectors

Many arthropod species have adopted vertebrate blood as their main food source. Blood is rich in nutrients and, except for the presence of parasites, sterile. However, this food source is not freely available, nor is obtaining it devoid of risk. It circulates inside vessels hidden underneath the skin of mobile hosts that are able to defend themselves and even predate the insects that try to feed on them. Thus, the haematophagous lifestyle is associated with major morphological, physiological and behavioural adaptations that have accumulated throughout the evolutionary history of the various lineages of blood-sucking arthropods. These adaptations have significant consequences for the evolution of parasites as well as for the epidemiology of vector-transmitted diseases. In this review article, we analyse various aspects of the behaviour of triatomine bugs to illustrate how each behavioural trait represents a particular adaptation to their close association with their hosts, which may easily turn into predators. Our aim is to offer to the reader an up-to-date integrative perspective on the behaviour of Chagas disease vectors and to propose new research avenues to encourage both young and experienced colleagues to explore this aspect of triatomine biology.

Kissing bugs in the United States: risk for vector-borne disease in humans

Eleven species of kissing bugs are found in the United States. Their home ranges may be expanding northward, perhaps as a consequence of climate change. At least eight of the species, perhaps all, are reported to harbor Trypanosoma cruzi, the parasite that causes Chagas disease. Because humans are encroaching on kissing bug habitat, there is concern for vector-transmitted Chagas disease in the United States. To date, documented autochthonous cases of Chagas in humans in the United States are rare. Kissing bugs are capable of adapting to new habitats such as human domiciles; however, they do not colonize homes in the United States as in Central and South America. We review the biology, behavior, and medical importance of kissing bugs and the risk they pose for transmission of Chagas disease in the United States. Where possible, descriptions of US species are compared to the epidemiologically important Latin American species.

Genetic basis of triatomine behavior: lessons from available insect genomes

Triatomines have been important model organisms for behavioural research. Diverse reports about triatomine host search, pheromone communication in the sexual, shelter and alarm contexts, daily cycles of activity, refuge choice and behavioural plasticity have been published in the last two decades. In recent times, a variety of molecular genetics techniques has allowed researchers to investigate elaborate and complex questions about the genetic bases of the physiology of insects. This, together with the current characterisation of the genome sequence of Rhodnius prolixus allows the resurgence of this excellent insect physiology model in the omics era. In the present revision, we suggest that studying the molecular basis of behaviour and sensory ecology in triatomines will promote a deeper understanding of fundamental aspects of insect and, particularly, vector biology. This will allow uncovering unknown features of essential insect physiology questions for a hemimetabolous model organism, promoting more robust comparative studies of insect sensory function and cognition.

Fatty-acid preference changes during development in Drosophila melanogaster

Fatty-acids (FAs) are required in the diet of many animals throughout their life. However, the mechanisms involved in the perception of and preferences for dietary saturated and unsaturated FAs (SFAs and UFAs, respectively) remain poorly explored, especially in insects. Using the model species Drosophila melanogaster, we measured the responses of wild-type larvae and adults to pure SFAs (14, 16, and 18 carbons) and UFAs (C18 with 1, 2, or 3 double-bonds). Individual and group behavioral tests revealed different preferences in larvae and adults. Larvae preferred UFAs whereas SFAs tended to induce both a strong aversion and a persistent aggregation behavior. Adults generally preferred SFAs, and laid more eggs and had a longer life span when ingesting these substances as compared to UFAs. Our data suggest that insects can discriminate long-chain dietary FAs. The developmental change in preference shown by this species might reflect functional variation in use of FAs or stage-specific nutritional requirements, and may be fundamental for insect use of these major dietary components.

Contribution of the horizontal transmission of the entomopathogenic fungus Beauveria bassiana to the overall performance of a fungal powder formulation against Triatoma infestans

Control of domiciliated Triatoma infestans, the major Chagas disease vector in southern South America, is currently achieved by indoor residual spraying of infested houses with chemical insecticides. However, in recent years this strategy has been threatened by the emergence of pyrethroid-resistant bug populations. As an alternative approach, we have previously demonstrated the efficacy of the entomopathogenic fungus Beauveria bassiana to control T. infestans bugs regardless of their pyrethroid susceptibility. In this work, we tested the virulence and residual activity of a powdered fungal formulation, and studied the significance of the horizontal transmission process (autodissemination) to fungal infection of bugs. The B. bassiana-based formulation was highly virulent against all T. infestans stages, and maintained its insecticidal capability for at least 5 months under natural ambient conditions. We showed that horizontal transmission of conidia is associated to bug density, and contributes significantly to the overall population infection event.

Gene-silencing reveals the functional significance of pheromone biosynthesis activating neuropeptide receptor (PBAN-R) in a male moth

The role of pheromone biosynthesis activating neuropeptide (PBAN) in the regulation of pheromone biosynthesis of several female moth species is well elucidated, but its role in the males has been a mystery for over two decades since its discovery from both male and female central nervous systems. In previous studies we have identified the presence of the gene transcript for the PBAN-G-protein coupled receptor (PBAN-R) in Helicoverpa armigera male hair-pencil-aedaegus complexes (male complexes), a tissue structurally homologous to the female pheromone gland. Moreover, we showed that this transcript is up-regulated during pupal-adult development, analogous to its regulation in the female pheromone-glands, thereby indicating a likely functional gene. Here we argue in favor of PBAN's role in regulating the free fatty-acid components (myristic, palmitic, stearic, and oleic acids) and alcohol components (hexadecanol, cis-11 hexadecanol, and octadecanol) in male complexes. We demonstrate the diel periodicity in levels of male components, with peak titers occurring during the 7th-9th h in the scotophase, coincident with female pheromone production. In addition, we show significant stimulation of component levels by synthetic HezPBAN. Furthermore, we confirm PBAN's function in this tissue through knockdown of the PBAN-R gene using RNAi-mediated gene-silencing. Injections of PBAN-R dsRNA into the male hemocoel significantly inhibited levels of the various male components by 58%-74%. In conclusion, through gain and loss of function we revealed the functionality of the PBAN-R and the key components that are up-regulated by PBAN.

Molecular characterization and expression analysis of a suite of cytochrome P450 enzymes implicated in insect hydrocarbon degradation in the entomopathogenic fungus Beauveria bassiana

The insect epicuticle or waxy layer comprises a heterogeneous mixture of lipids that include abundant levels of long-chain alkanes, alkenes, wax esters and fatty acids. This structure represents the first barrier against microbial attack and for broad-host-range insect pathogens, such as Beauveria bassiana, it is the initial interface mediating the host–pathogen interaction, since these organisms do not require any specialized mode of entry and infect target hosts via the cuticle. B. bassiana is able to grow on straight chain alkanes up to n-C33 as a sole source of carbon and energy. The cDNA and genomic sequences, including putative regulatory elements, for eight cytochrome P450 enzymes, postulated to be involved in alkane and insect epicuticle degradation, were isolated and characterized. Expression studies using a range of alkanes as well as an insect-derived epicuticular extract from the blood-sucking bug Triatomas infestans revealed a differential expression pattern for the P450 genes examined, and suggest that B. bassiana contains a series of hydrocarbon-assimilating enzymes with overlapping specificity in order to target the surface lipids of insect hosts. Phylogenetic analysis of the translated ORFs of the sequences revealed that the enzyme which displayed the highest levels of induction on both alkanes and the insect epicuticular extract represents the founding member of a new cytochrome P450 family, with three of the other sequences assigned as the first members of new P450 subfamilies. The remaining four proteins clustered with known P450 families whose members include alkane monooxygenases.

Control of pyrethroid-resistant Chagas disease vectors with entomopathogenic fungi

Background

Triatoma infestans-mediated transmission of Tripanosoma cruzi, the causative agent of Chagas disease, remains as a major health issue in southern South America. Key factors of T. infestans prevalence in specific areas of the geographic Gran Chaco region—which extends through northern Argentina, Bolivia, and Paraguay—are both recurrent reinfestations after insecticide spraying and emerging pyrethroid-resistance over the past ten years. Among alternative control tools, the pathogenicity of entomopathogenic fungi against triatomines is already known; furthermore, these fungi have the ability to fully degrade hydrocarbons from T. infestans cuticle and to utilize them as fuel and for incorporation into cellular components.

Methodology and Findings

Here we provide evidence of resistance-related cuticle differences; capillary gas chromatography coupled to mass spectrometry analyses revealed that pyrethroid-resistant bugs have significantly larger amounts of surface hydrocarbons, peaking 56.2±6.4% higher than susceptible specimens. Also, a thicker cuticle was detected by scanning electron microscopy (32.1±5.9 µm and 17.8±5.4 µm for pyrethroid-resistant and pyrethroid-susceptible, respectively). In laboratory bioassays, we showed that the virulence of the entomopathogenic fungi Beauveria bassiana against T. infestans was significantly enhanced after fungal adaptation to grow on a medium containing insect-like hydrocarbons as the carbon source, regardless of bug susceptibility to pyrethroids. We designed an attraction-infection trap based on manipulating T. infestans behavior in order to facilitate close contact with B. bassiana. Field assays performed in rural village houses infested with pyrethroid-resistant insects showed 52.4% bug mortality. Using available mathematical models, we predicted that further fungal applications could eventually halt infection transmission.

Conclusions

This low cost, low tech, ecologically friendly methodology could help in controlling the spread of pyrethroid-resistant bugs.

Chagas disease, also known as American Trypanosomiasis, is the most relevant parasitic disease in Latin America, being a major burden that affects mostly poor human populations living in rural areas. The kissing-bugs of the Triatominae family transmit the parasite Trypanosoma cruzi by infectious blood-sucking; Triatoma infestans is the vector of major relevance in the southern Cone of South America. Current control strategies, heavily based on residual insecticide spraying, are threatened by the emergence of pyrethroid-resistant bug populations. Furthermore, ensuring the long-term and sustainable control of this overwhelming disease remains a major challenge. Here we show the utility of a simple, low-cost, biological control methodology against T. infestans bugs, regardless of their susceptibility to pyrethroid insecticides. It is based on the understanding of the initial contact interactions between a mycoinsecticide agent—the fungus Beauveria bassiana—and the host defense barrier, the bug cuticle. The proposed methodology is also supported by present data showing a relationship between the triatomine cuticle width and its hydrocarbon surface components, with insecticide resistance. These results will help to provide a safe and efficient alternative to overcome pyrethroid-resilience of these noxious bugs. A high transfer potential to immediate application in rural communities located in remote areas inaccessible to sanitary control teams, and to the control of other Chagas disease vectors as well, is also envisaged.