The Dose Makes the Poison: Nutritional Overload Determines the Life Traits of Blood-Feeding Arthropods

Deployment and transcriptional evaluation of nitisinone, an FDA-approved drug, to control bed bugs

Bed bugs are blood-feeders that rapidly proliferate into large indoor infestations. Their bites can cause allergies, secondary infections and psychological stress, among other problems. Although several tactics for their management have been used, bed bugs continue to spread worldwide wherever humans reside. This is mainly due to human-mediated transport and their high resistance to several classes of insecticides. New treatment options with novel modes of action are required for their control. In this study, we evaluated the use of nitisinone (NTBC), an FDA-approved drug, for bed bug control in an insecticide-susceptible (HH) and an insecticide-resistant (CIN) population. Although NTBC was lethal to both populations when administered orally or applied topically in very low doses, we observed a slight but significant resistance in the CIN population. Transcriptomic analysis in both populations indicated that NTBC treatment elicited a broad suppression of genes associated with RNA post-transcriptional modifications, translation, endomembrane system, protein post-translational modifications and protein folding. The CIN population exhibited higher ATP production and xenobiotic detoxification. Feeding studies on a mouse model highlight that NTBC could be used as a control method of bed bugs by host treatment. The results demonstrate that NTBC can be used as a new active ingredient for bed bug control by topical or oral treatment and shed light on the molecular mechanisms of suppressed tyrosine metabolism following NTBC treatment.

The vector-symbiont affair: a relationship as (im)perfect as it can be

Vector-borne diseases are globally prevalent and represent a major socioeconomic problem worldwide. Blood-sucking arthropods transmit most pathogenic agents that cause these human infections. The pathogens transmission to their vertebrate hosts depends on how efficiently they infect their vector, which is particularly impacted by the microbiota residing in the intestinal lumen, as well as its cells or internal organs such as ovaries. The balance between costs and benefits provided by these interactions ultimately determines the outcome of the relationship. Here, we will explore aspects concerning the nature of microbe-vector interactions, including the adaptive traits required for their establishment, the varied outcomes of symbiotic interactions, as well as the factors influencing the transition of these relationships across a continuum from parasitism to mutualism.

Stochasticity, determinism, and contingency shape genome evolution of endosymbiotic bacteria

Evolution results from the interaction of stochastic and deterministic processes that create a web of historical contingency, shaping gene content and organismal function. To understand the scope of this interaction, we examine the relative contributions of stochasticity, determinism, and contingency in shaping gene inactivation in 34 lineages of endosymbiotic bacteria, Sodalis, found in parasitic lice, Columbicola, that are independently undergoing genome degeneration. Here we show that the process of genome degeneration in this system is largely deterministic: genes involved in amino acid biosynthesis are lost while those involved in providing B-vitamins to the host are retained. In contrast, many genes encoding redundant functions, including components of the respiratory chain and DNA repair pathways, are subject to stochastic loss, yielding historical contingencies that constrain subsequent losses. Thus, while selection results in functional convergence between symbiont lineages, stochastic mutations initiate distinct evolutionary trajectories, generating diverse gene inventories that lack the functional redundancy typically found in free-living relatives.

Genome analysis of "Candidatus Aschnera chinzeii," the bacterial endosymbiont of the blood-sucking bat fly Penicillidia jenynsii (Insecta: Diptera: Nycteribiidae)

Insect-microbe endosymbiotic associations are omnipresent in nature, wherein the symbiotic microbes often play pivotal biological roles for their host insects. In particular, insects utilizing nutritionally imbalanced food sources are dependent on specific microbial symbionts to compensate for the nutritional deficiency via provisioning of B vitamins in blood-feeding insects, such as tsetse flies, lice, and bedbugs. Bat flies of the family Nycteribiidae (Diptera) are blood-sucking ectoparasites of bats and shown to be associated with co-speciating bacterial endosymbiont "Candidatus Aschnera chinzeii," although functional aspects of the microbial symbiosis have been totally unknown. In this study, we report the first complete genome sequence of Aschnera from the bristled bat fly Penicillidia jenynsii. The Aschnera genome consisted of a 748,020 bp circular chromosome and a 18,747 bp circular plasmid. The chromosome encoded 603 protein coding genes (including 3 pseudogenes), 33 transfer RNAs, and 1 copy of 16S/23S/5S ribosomal RNA operon. The plasmid contained 10 protein coding genes, whose biological function was elusive. The genome size, 0.77 Mbp, was drastically reduced in comparison with 4-6 Mbp genomes of free-living γ-proteobacteria. Accordingly, the Aschnera genome was devoid of many important functional genes, such as synthetic pathway genes for purines, pyrimidines, and essential amino acids. On the other hand, the Aschnera genome retained complete or near-complete synthetic pathway genes for biotin (vitamin B7), tetrahydrofolate (vitamin B9), riboflavin (vitamin B2), and pyridoxal 5'-phosphate (vitamin B6), suggesting that Aschnera provides these vitamins and cofactors that are deficient in the blood meal of the host bat fly. Similar retention patterns of the synthetic pathway genes for vitamins and cofactors were also observed in the endosymbiont genomes of other blood-sucking insects, such as Riesia of human lice, Arsenophonus of louse flies, and Wigglesworthia of tsetse flies, which may be either due to convergent evolution in the blood-sucking host insects or reflecting the genomic architecture of Arsenophonus-allied bacteria.

Exploring dose-response relationships in Aedes aegypti survival upon bacteria and arbovirus infection

A detailed understanding of how host fitness changes in response to variations in microbe density (an ecological measure of disease tolerance) is an important aim of infection biology. Here, we applied dose-response curves to study Aedes aegypti survival upon exposure to different microbes. We challenged female mosquitoes with Listeria monocytogenes, a model bacterial pathogen, Dengue 4 virus and Zika virus, two medically relevant arboviruses, to understand the distribution of mosquito survival following microbe exposure. By correlating microbe loads and host health, we found that a blood meal promotes disease tolerance in our systemic bacterial infection model and that mosquitoes orally infected with bacteria had an enhanced defensive capacity than insects infected through injection. We also showed that Aedes aegypti displays a higher survival profile following arbovirus infection when compared to bacterial infections. Here, we applied a framework for investigating microbe-induced mosquito mortality and details how the lifespan of Aedes aegypti varies with different inoculum sizes of bacteria and arboviruses.

Tyrosine transfer RNA levels and modifications during blood-feeding and vitellogenesis in the mosquito, Aedes aegypti

Mosquitoes such as Aedes aegypti must consume a blood meal for the nutrients necessary for egg production. Several transcriptome and proteome changes occur post blood meal that likely corresponds with codon usage alterations. Transfer RNA (tRNA) is the adapter molecule that reads messenger RNA (mRNA) codons to add the appropriate amino acid during protein synthesis. Chemical modifications to tRNA enhance codons' decoding, improving the accuracy and efficiency of protein synthesis. Here, we examined tRNA modifications and transcripts associated with the blood meal and subsequent periods of vitellogenesis in A. aegypti. More specifically, we assessed tRNA transcript abundance and modification levels in the fat body at critical times post blood-feeding. Based on a combination of alternative codon usage and identification of particular modifications, we identified that increased transcription of tyrosine tRNAs is likely critical during the synthesis of egg yolk proteins in the fat body following a blood meal. Altogether, changes in both the abundance and modification of tRNA are essential factors in the process of vitellogenin production after blood-feeding in mosquitoes.

eIF3 subunit M regulates blood meal digestion in Rhodnius prolixus affecting ecdysis, reproduction, and survival

In triatomines, blood-feeding triggers many physiological processes including post embryonic development and reproduction. Different feeding habits, such as hematophagy, can shape gene functions to meet the challenges of each type of diet. The gut of blood-sucking insects faces particular challenges after feeding due to the quantity and the quality of the food ingested. A comparison of transcriptomic and proteomic data indicates that post transcriptional regulation of gene expression is crucial in the triatomine gut. It was proposed that eukaryotic translation initiation factor 3 subunit m (eIF3m) and eIF3e define 2 different eIF3 complexes with a distinct affinity for the different mRNAs, thus selecting the set of mRNAs to be translated and constituting a post transcriptional mode of regulation of gene expression. Because the eIF3m is mainly expressed in the gut, we evaluated its relevance in Rhodnius prolixus physiology through RNA interference-mediated gene silencing. The knockdown of eIF3m reduced the digestion rate, affecting the processes triggered by a blood meal. Its silencing inhibited molting and caused premature death in nymphs while impaired ovary development, oviposition and increased resistance to starvation in adult females. The survival of males after feeding (resistance to starvation) was not affected by eIF3m knockdown. The information regarding the eIF3m function in insects is scarce and the phenotypes observed in R. prolixus upon eIF3m silencing are different and more severe than those previously described in Drosophila melanogaster, indicating a pleiotropic role of this gene in triatomines.

Microbiota in disease-transmitting vectors

Haematophagous arthropods, including mosquitoes, ticks, flies, triatomine bugs and lice (here referred to as vectors), are involved in the transmission of various pathogens to mammals on whom they blood feed. The diseases caused by these pathogens, collectively known as vector-borne diseases (VBDs), threaten the health of humans and animals. Although the vector arthropods differ in life histories, feeding behaviour as well as reproductive strategies, they all harbour symbiotic microorganisms, known as microbiota, on which they depend for completing essential aspects of their biology, such as development and reproduction. In this Review, we summarize the shared and unique key features of the symbiotic associations that have been characterized in the major vector taxa. We discuss the crosstalks between microbiota and their arthropod hosts that influence vector metabolism and immune responses relevant for pathogen transmission success, known as vector competence. Finally, we highlight how current knowledge on symbiotic associations is being explored to develop non-chemical-based alternative control methods that aim to reduce vector populations, or reduce vector competence. We conclude by highlighting the remaining knowledge gaps that stand to advance basic and translational aspects of vector-microbiota interactions.

Defining the toxicological profile of 4-hydroxyphenylpyruvate dioxygenase-directed herbicides to Aedes aegypti and Amblyomma americanum

Inhibitors targeting the 4-hydroxyphenyl pyruvate dioxygenase (HPPD) enzyme are well established herbicides and HPPD is also a primary enzyme within the tyrosine metabolism pathway in hematophagous arthropods, which is an essential metaboilic pathway post-blood feeding to prevent tyrosine-mediated toxicity. The objective of this study was to characterize the toxicity of triketone, pyrazole, pyrazolone, isoxazole, and triazole herbicides that inhibit HPPD to blood-fed mosquitoes and ticks. Topical exposure of nitisinone to blood-fed Aedes aegypti yielded high toxicity with an LD50 of 3.81 ng/insect (95% CI: 3.09 to 4.67 ng; Hillslope: 0.97, r2: 0.99), yet was non-toxic to non-blood fed (NBF) mosquitoes. The rank order of toxicity was nitisinone > tembotrione > pyrazoxyfen > tebuconazole > mesotrione against blood-fed Ae. Aegypti, but nitisinone was approximately 30-fold more toxic than other chemicals tested. We also assessed the toxicity of HPPD-inhibiting herbicides to the lone star tick, Amblyomma americanum and similarly, nitisinone was toxic to Am. americanum with a lethal time to kill 50% of subjects (LT50) of 23 h at 10 μM. Knockdown of the gene encoding the HPPD enzyme was performed through RNA-interference led to significant mortality after blood feeding in both, Ae. aegypti and Am. americanum. Lastly, a fluorescence assay was developed to determine relative quantities of L-tyrosine in Ae. aegypti and Am. americanum treated with HPPD inhibitors. L-tyrosine levels correlated with toxicity with nitisinone exposure leading to increased tyrosine concentrations post-blood feeding. Taken together, these data support previous work suggesting HPPD-inhibitors represent a novel mode of toxicity to mosquitoes and ticks and may represent base scaffolds for development of novel insecticides specific for hematophagous arthropods.

Three-dimensional images reveal the impact of the endosymbiont Midichloria mitochondrii on the host mitochondria

The hard tick, Ixodes ricinus, a main Lyme disease vector, harbors an intracellular bacterial endosymbiont. Midichloria mitochondrii is maternally inherited and resides in the mitochondria of I. ricinus oocytes, but the consequences of this endosymbiosis are not well understood. Here, we provide 3D images of wild-type and aposymbiotic I. ricinus oocytes generated with focused ion beam-scanning electron microscopy. Quantitative image analyses of endosymbionts and oocyte mitochondria at different maturation stages show that the populations of both mitochondrion-associated bacteria and bacterium-hosting mitochondria increase upon vitellogenisation, and that mitochondria can host multiple bacteria in later stages. Three-dimensional reconstructions show symbiosis-dependent morphologies of mitochondria and demonstrate complete M. mitochondrii inclusion inside a mitochondrion. Cytoplasmic endosymbiont located close to mitochondria are not oriented towards the mitochondria, suggesting that bacterial recolonization is unlikely. We further demonstrate individual globular-shaped mitochondria in the wild type oocytes, while aposymbiotic oocytes only contain a mitochondrial network. In summary, our study suggests that M. mitochondrii modulates mitochondrial fragmentation in oogenesis possibly affecting organelle function and ensuring its presence over generations.

The Aedes aegypti peritrophic matrix controls arbovirus vector competence through HPx1, a heme-induced peroxidase

Aedes aegypti mosquitoes are the main vectors of arboviruses. The peritrophic matrix (PM) is an extracellular layer that surrounds the blood bolus. It acts as an immune barrier that prevents direct contact of bacteria with midgut epithelial cells during blood digestion. Here, we describe a heme-dependent peroxidase, hereafter referred to as heme peroxidase 1 (HPx1). HPx1 promotes PM assembly and antioxidant ability, modulating vector competence. Mechanistically, the heme presence in a blood meal induces HPx1 transcriptional activation mediated by the E75 transcription factor. HPx1 knockdown increases midgut reactive oxygen species (ROS) production by the DUOX NADPH oxidase. Elevated ROS levels reduce microbiota growth while enhancing epithelial mitosis, a response to tissue damage. However, simultaneous HPx1 and DUOX silencing was not able to rescue bacterial population growth, as explained by increased expression of antimicrobial peptides (AMPs), which occurred only after double knockdown. This result revealed hierarchical activation of ROS and AMPs to control microbiota. HPx1 knockdown produced a 100-fold decrease in Zika and dengue 2 midgut infection, demonstrating the essential role of the mosquito PM in the modulation of arbovirus vector competence. Our data show that the PM connects blood digestion to midgut immunological sensing of the microbiota and viral infections.

Iron Homeostasis in Insects

Iron is an essential micronutrient for all types of organisms; however, iron has chemical properties that can be harmful to cells. Because iron is both necessary and potentially damaging, insects have homeostatic processes that control the redox state, quantity, and location of iron in the body. These processes include uptake of iron from the diet, intracellular and extracellular iron transport, and iron storage. Early studies of iron-binding proteins in insects suggested that insects and mammals have surprisingly different mechanisms of iron homeostasis, including different primary mechanisms for exporting iron from cells and for transporting iron from one cell to another, and subsequent studies have continued to support this view. This review summarizes current knowledge about iron homeostasis in insects, compares insect and mammalian iron homeostasis mechanisms, and calls attention to key remaining knowledge gaps.

Transcriptomic analysis of the adaptation to prolonged starvation of the insect-dwelling Trypanosoma cruzi epimastigotes

Trypanosoma cruzi is a digenetic unicellular parasite that alternates between a blood-sucking insect and a mammalian, host causing Chagas disease or American trypanosomiasis. In the insect gut, the parasite differentiates from the non-replicative trypomastigote forms that arrive upon blood ingestion to the non-infective replicative epimastigote forms. Epimastigotes develop into infective non-replicative metacyclic trypomastigotes in the rectum and are delivered via the feces. In addition to these parasite stages, transitional forms have been reported. The insect-feeding behavior, characterized by few meals of large blood amounts followed by long periods of starvation, impacts the parasite population density and differentiation, increasing the transitional forms while diminishing both epimastigotes and metacyclic trypomastigotes. To understand the molecular changes caused by nutritional restrictions in the insect host, mid-exponentially growing axenic epimastigotes were cultured for more than 30 days without nutrient supplementation (prolonged starvation). We found that the parasite population in the stationary phase maintains a long period characterized by a total RNA content three times smaller than that of exponentially growing epimastigotes and a distinctive transcriptomic profile. Among the transcriptomic changes induced by nutrient restriction, we found differentially expressed genes related to managing protein quality or content, the reported switch from glucose to amino acid consumption, redox challenge, and surface proteins. The contractile vacuole and reservosomes appeared as cellular components enriched when ontology term overrepresentation analysis was carried out, highlighting the roles of these organelles in starving conditions possibly related to their functions in regulating cell volume and osmoregulation as well as metabolic homeostasis. Consistent with the quiescent status derived from nutrient restriction, genes related to DNA metabolism are regulated during the stationary phase. In addition, we observed differentially expressed genes related to the unique parasite mitochondria. Finally, our study identifies gene expression changes that characterize transitional parasite forms enriched by nutrient restriction. The analysis of the here-disclosed regulated genes and metabolic pathways aims to contribute to the understanding of the molecular changes that this unicellular parasite undergoes in the insect vector.

Comparative analysis of detoxification-related gene superfamilies across five hemipteran species

Background

Hemiptera is one of the most speciose orders of insects, and the most speciose considering Hemimetabola. Through their evolutive history, hemipterans with different feeding habits have adapted to deal with different chemical challenges. Three major gene families are involved in xenobiotic detoxification in insects: the cytochromes P450 (CYPs), carboxyl/cholinesterases (CCEs), and glutathione transferases (GSTs). Here we perform a comparative analysis on the complement of these gene superfamilies across five hemipteran species; four heteropterans (the pentatomid plant feeders Nezara viridula and Halyomorpha halys; the hematophagous Cimex lectularius, Cimicidae, and Rhodnius prolixus, Reduviidae), and one Auchenorrhyncha plant feeder (Nilaparvata lugens).

Results

Our results point to an expansion of several enzyme families associated with xenobiotic detoxification in heteropterans with respect to other species and the existence of a dynamic evolution pattern including CYP3 clan, hormone and pheromone processing class in the CCE superfamily, and sigma class in GST superfamily. Other detoxification-related families are reduced in the hemipteran species analyzed here: reduction or even absence of epsilon class and reduced delta class in GST superfamily; absence of mitochondrial CYP12 family; absence of CYP9 family in CYP3 clan; and reduction or even absence of some dietary/detoxification groups of CCEs. Interestingly, the most polyphagous species analyzed here (H. halys) is also the one that presents the largest repertoire of detoxification enzymes. Gene cluster analysis suggests that this could be due to gene duplication events.

Conclusions

The evolutionary analysis performed here reveals characteristics that are both common and particular for heteropterans. The composition and organization of detoxification-related gene families could shed light on evolutionary forces that shaped their divergence. These families are important for both the detoxification of diet products and for conferring tolerance or resistance to synthetic insecticides. Furthermore, we present the first comprehensive analysis of detoxification gene superfamilies in N. viridula, an understudied species in spite of its economic relevance as a crop pest. The information obtained is of interest for basic insect science as well as for the control of harmful species and the management of insecticide resistance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08974-y.

Warm Blood Meal Increases Digestion Rate and Milk Protein Production to Maximize Reproductive Output for the Tsetse Fly, Glossina morsitans

The ingestion of blood represents a significant burden that immediately increases water, oxidative, and thermal stress, but provides a significant nutrient source to generate resources necessary for the development of progeny. Thermal stress has been assumed to solely be a negative byproduct that has to be alleviated to prevent stress. Here, we examined if the short thermal bouts incurred during a warm blood meal are beneficial to reproduction. To do so, we examined the duration of pregnancy and milk gland protein expression in the tsetse fly, Glossina morsitans, that consumed a warm or cool blood meal. We noted that an optimal temperature for blood ingestion yielded a reduction in the duration of pregnancy. This decline in the duration of pregnancy is due to increased rate of blood digestion when consuming warm blood. This increased digestion likely provided more energy that leads to increased expression of transcript for milk-associated proteins. The shorter duration of pregnancy is predicted to yield an increase in population growth compared to those that consume cool or above host temperatures. These studies provide evidence that consumption of a warm blood meal is likely beneficial for specific aspects of vector biology.

Ambivalent Roles of Oxidative Stress in Triangular Relationships among Arthropod Vectors, Pathogens and Hosts

Blood-feeding arthropods, particularly ticks and mosquitoes are considered the most important vectors of arthropod-borne diseases affecting humans and animals. While feeding on blood meals, arthropods are exposed to high levels of reactive oxygen species (ROS) since heme and other blood components can induce oxidative stress. Different ROS have important roles in interactions among the pathogens, vectors, and hosts. ROS influence various metabolic processes of the arthropods and some have detrimental effects. In this review, we investigate the various roles of ROS in these arthropods, including their innate immunity and the homeostasis of their microbiomes, that is, how ROS are utilized to maintain the balance between the natural microbiota and potential pathogens. We elucidate the mechanism of how ROS are utilized to fight off invading pathogens and how the arthropod-borne pathogens use the arthropods’ antioxidant mechanism to defend against these ROS attacks and their possible impact on their vector potentials or their ability to acquire and transmit pathogens. In addition, we describe the possible roles of ROS in chemical insecticide/acaricide activity and/or in the development of resistance. Overall, this underscores the importance of the antioxidant system as a potential target for the control of arthropod and arthropod-borne pathogens.

Effects of Wolbachia elimination and B-vitamin supplementation on bed bug development and reproduction

Obligate blood feeders, such as Cimex lectularius (common bed bug), have symbiotic associations with nutritional endosymbionts that produce B-vitamins. To quantify the symbiont’s contribution to host fitness in these obligate mutualisms, the symbiont must be eliminated and its absence rigorously confirmed. We developed and validated procedures for complete elimination of Wolbachia (Wb) in bed bugs and quantified development and reproduction in bed bugs with and without Wb and with and without B-vitamins supplementation. Aposymbiotic bed bugs had slower nymphal development, reduced adult survivorship, smaller adult size, fewer eggs per female, and lower hatch rate than bed bugs that harbored Wb. In aposymbiotic bed bugs that were fed B-vitamins-supplemented blood, nymph development time, adult survivorship and hatch rate recovered to control levels, but adult size and egg number only partially recovered. These results underscore the nutritional dependence of bed bugs on their Wb symbiont and suggest that Wb may provide additional nutritional benefits beyond the B-vitamin mix that we investigated.

The Parasite Load of Trypanosoma cruzi Modulates Feeding and Defecation Patterns of the Chagas Disease Vector Triatoma infestans

Trypanosoma cruzi is the causal agent of Chagas disease, a parasitic zoonosis transmitted mainly through the feces of triatomine insects. Triatoma infestans is the main triatomine vector of this disease in South America. Previous research has shown that T. cruzi infection modifies the behavior of triatomines. We evaluated, for the first time, the effect of parasite load on feeding and defecation behavior, which we quantified by using real-time PCR. The detection time of the host was shorter in infected individuals, and the number of bites increased, while the dejection time was reduced when compared with the non-infected group. A significant correlation between the parasite load and the behavioral changes registered in the infected triatomines was found. These results would indicate that the intensity of T. cruzi infection modulates the feeding and defecation behavior of T. infestans, increasing the vector competence of this triatomine vector.

Atypical strategies for cuticle pigmentation in the blood-feeding hemipteran Rhodnius prolixus

Pigmentation in insects has been linked to mate selection and predator evasion, thus representing an important aspect for natural selection. Insect body color is classically associated to the activity of tyrosine pathway enzymes, and eye color to pigment synthesis through the tryptophan and guanine pathways, and their transport by ABC proteins. Among the hemiptera, the genetic basis for pigmentation in kissing bugs such as Rhodnius prolixus, that transmit Chagas disease to humans, has not been addressed. Here we report the functional analysis of R. prolixus eye and cuticle pigmentation genes. Consistent with data for most insect clades, we show that knockdown for yellow results in a yellow cuticle, while scarlet and cinnabar knockdowns display red eyes as well as cuticle phenotypes. In addition, tyrosine pathway aaNATpreto knockdown resulted in a striking dark cuticle that displays no color pattern or UV reflectance. In contrast, knockdown of ebony and tan, that encode NBAD branch tyrosine pathway enzymes, did not generate the expected dark and light brown phenotypes, respectively, as reported for other insects. We hypothesize that R. prolixus, which requires tyrosine pathway enzymes for detoxification from the blood diet, evolved an unusual strategy for cuticle pigmentation based on the preferential use of a color erasing function of the aaNATpreto tyrosine pathway branch. We also show that genes classically involved in the generation and transport of eye pigments regulate red body color in R. prolixus. This is the first systematic approach to identify the genes responsible for the generation of color in a blood-feeding hemiptera, providing potential visible markers for future transgenesis.

Ecological Contacts and Host Specificity Promote Replacement of Nutritional Endosymbionts in Ticks

Symbiosis with vitamin-provisioning microbes is essential for the nutrition of animals with some specialized feeding habits. While coevolution favors the interdependence between symbiotic partners, their associations are not necessarily stable: Recently acquired symbionts can replace ancestral symbionts. In this study, we demonstrate successful replacement by Francisella-like endosymbionts (-LE), a group of B-vitamin-provisioning endosymbionts, across tick communities driven by horizontal transfers. Using a broad collection of Francisella-LE-infected tick species, we determined the diversity of Francisella-LE haplotypes through a multi-locus strain typing approach and further characterized their phylogenetic relationships and their association with biological traits of their tick hosts. The patterns observed showed that Francisella-LE commonly transfer through similar ecological networks and geographic distributions shared among different tick species and, in certain cases, through preferential shuffling across congeneric tick species. Altogether, these findings reveal the importance of geographic, ecological, and phylogenetic proximity in shaping the replacement pattern in which new nutritional symbioses are initiated.

Beyond the eye: Kynurenine pathway impairment causes midgut homeostasis dysfunction and survival and reproductive costs in blood-feeding mosquitoes

Insect ommochrome biosynthesis pathways metabolize tryptophan to generate eye-color pigments and wild-type alleles of pathway genes are useful phenotypic markers in transgenesis studies. Pleiotropic effects of mutations in some genes exert a load on both survival and reproductive success in blood-feeding species. Here, we investigated the challenges imposed on mosquitoes by the increase of tryptophan metabolites resulting from blood meal digestion and the impact of disruptions of the ommochrome biosynthesis pathway. Female mosquitoes with spontaneous and induced mutations in the orthologs of the genes encoding kynurenine hydroxylase in Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus exhibited impaired survival and reproductive phenotypes that varied in type and severity among the species. A compromised midgut permeability barrier function was also observed in An. stephensi. Surprisingly, mutant mosquitoes displayed an increase in microbiota compared to controls that was not accompanied by a general induction of immune genes. Antibiotic treatment rescued some deleterious traits implicating a role for the kynurenine pathway (KP) in midgut homeostasis. Supplemental xanthurenic acid, a KP end-product, rescued lethality and limited microbiota proliferation in Ae. aegypti. These data implicate the KP in the regulation of the host/microbiota interface. These pleiotropic effects on mosquito physiology are important in the development of genetic strategies targeting vector mosquitoes.

On the use of inhibitors of 4-hydroxyphenylpyruvate dioxygenase as a vector-selective insecticide in the control of mosquitoes

BACKGROUND

Blood-sucking insects incorporate many times their body weight of blood in a single meal. Because proteins are the major component of vertebrate blood, its digestion in the gut generates extremely high concentrations of free amino acids. Previous reports showed that the tyrosine degradation pathway plays an essential role in adapting these animals to blood feeding. Inhibition of 4-hydroxyphenylpyruvate dioxygenase (HPPD), the rate-limiting step of tyrosine degradation, results in the death of insects after a blood meal. Therefore, it has been suggested that compounds that block the catabolism of tyrosine could act selectively on blood-feeding insects. Here, we evaluated the toxicity against mosquitoes of three HPPD inhibitors currently used as herbicides and in human health.

RESULTS

Of the compounds tested, nitisinone (NTBC) proved to be more potent than mesotrione (MES) and isoxaflutole (IFT) in Aedes aegypti. NTBC was lethal to Ae. aegypti in artificial feeding assays [median lethal dose (LD₅₀ ): 4.53 μm] and in topical application (LD₅₀ : 0.012 nmol/mosquito). NTBC was also lethal to Ae. aegypti populations that were resistant to neurotoxic insecticides, and to other mosquito species (Anopheles and Culex).

CONCLUSION

HPPD inhibitors, particularly NTBC, represent promising new drugs for mosquito control. Because they affect only blood-feeding organisms, they represent a safer and more environmentally friendly alternative to conventional neurotoxic insecticides. © 2021 Society of Chemical Industry.

Microbiomes of Blood-Feeding Arthropods: Genes Coding for Essential Nutrients and Relation to Vector Fitness and Pathogenic Infections. A Review

BACKGROUND

Blood-feeding arthropods support a diverse array of symbiotic microbes, some of which facilitate host growth and development whereas others are detrimental to vector-borne pathogens. We found a common core constituency among the microbiota of 16 different arthropod blood-sucking disease vectors, including Bacillaceae, Rickettsiaceae, Anaplasmataceae, Sphingomonadaceae, Enterobacteriaceae, Pseudomonadaceae, Moraxellaceae and Staphylococcaceae. By comparing 21 genomes of common bacterial symbionts in blood-feeding vectors versus non-blooding insects, we found that certain enteric bacteria benefit their hosts by upregulating numerous genes coding for essential nutrients. Bacteria of blood-sucking vectors expressed significantly more genes (p < 0.001) coding for these essential nutrients than those of non-blooding insects. Moreover, compared to endosymbionts, the genomes of enteric bacteria also contained significantly more genes (p < 0.001) that code for the synthesis of essential amino acids and proteins that detoxify reactive oxygen species. In contrast, microbes in non-blood-feeding insects expressed few gene families coding for these nutrient categories. We also discuss specific midgut bacteria essential for the normal development of pathogens (e.g., Leishmania) versus others that were detrimental (e.g., bacterial toxins in mosquitoes lethal to Plasmodium spp.).

Aedes aegypti Infection With Trypanosomatid Strigomonas culicis Alters Midgut Redox Metabolism and Reduces Mosquito Reproductive Fitness

Aedes aegypti mosquitoes transmit arboviruses of important global health impact, and their intestinal microbiota can influence vector competence by stimulating the innate immune system. Midgut epithelial cells also produce toxic reactive oxygen species (ROS) by dual oxidases (DUOXs) that are essential players in insect immunity. Strigomonas culicis is a monoxenous trypanosomatid that naturally inhabits mosquitoes; it hosts an endosymbiotic bacterium that completes essential biosynthetic pathways of the parasite and influences its oxidative metabolism. Our group previously showed that S. culicis hydrogen peroxide (H₂O₂)-resistant (WTR) strain is more infectious to A. aegypti mosquitoes than the wild-type (WT) strain. Here, we investigated the influence of both strains on the midgut oxidative environment and the effect of infection on mosquito fitness and immunity. WT stimulated the production of superoxide by mitochondrial metabolism of midgut epithelial cells after 4 days post-infection, while WTR exacerbated H₂O₂ production mediated by increased DUOX activity and impairment of antioxidant system. The infection with both strains also disrupted the fecundity and fertility of the females, with a greater impact on reproductive fitness of WTR-infected mosquitoes. The presence of these parasites induced specific transcriptional modulation of immune-related genes, such as attacin and defensin A during WTR infection (11.8- and 6.4-fold, respectively) and defensin C in WT infection (7.1-fold). Thus, we propose that A. aegypti oxidative response starts in early infection time and does not affect the survival of the H₂O₂-resistant strain, which has a more efficient antioxidant system. Our data provide new biological aspects of A. aegypti–S. culicis relationship that can be used later in alternative vector control strategies.

Lipid transport proteins in malaria, from Plasmodium parasites to their hosts

Eukaryotic unicellular pathogens from the genus Plasmodium are the etiological agents of malaria, a disease that persists over a wide range of vertebrate species, including humans. During its dynamic lifecycle, survival in the different hosts depends on the parasite's ability to establish a suitable environmental milieu. To achieve this, specific host processes are exploited to support optimal growth, including extensive modifications to the infected host cell. These modifications include the formation of novel membranous structures, which are induced by the parasite. Consequently, to maintain a finely tuned and dynamic lipid environment, the organisation and distribution of lipids to different cell sites likely requires specialised lipid transfer proteins (LTPs). Indeed, several parasite and host-derived LTPs have been identified and shown to be essential at specific stages. Here we describe the roles of LTPs in parasite development and adaptation to its host including how the latest studies are profiting from the improved genetic, lipidomic and imaging toolkits available to study Plasmodium parasites. Lastly, a list of predicted Plasmodium LTPs is provided to encourage research in this field.

Paratransgenic manipulation of a tsetse microRNA alters the physiological homeostasis of the fly's midgut environment

Tsetse flies are vectors of parasitic African trypanosomes, the etiological agents of human and animal African trypanosomoses. Current disease control methods include fly-repelling pesticides, fly trapping, and chemotherapeutic treatment of infected people and animals. Inhibiting tsetse's ability to transmit trypanosomes by strengthening the fly's natural barriers can serve as an alternative approach to reduce disease. The peritrophic matrix (PM) is a chitinous and proteinaceous barrier that lines the insect midgut and serves as a protective barrier that inhibits infection with pathogens. African trypanosomes must cross tsetse's PM in order to establish an infection in the fly, and PM structural integrity negatively correlates with trypanosome infection outcomes. Bloodstream form trypanosomes shed variant surface glycoproteins (VSG) into tsetse's gut lumen early during the infection establishment, and free VSG molecules are internalized by the fly's PM-producing cardia. This process results in a reduction in the expression of a tsetse microRNA (miR275) and a sequential molecular cascade that compromises PM integrity. miRNAs are small non-coding RNAs that are critical in regulating many physiological processes. In the present study, we investigated the role(s) of tsetse miR275 by developing a paratransgenic expression system that employs tsetse's facultative bacterial endosymbiont, Sodalis glossinidius, to express tandem antagomir-275 repeats (or miR275 sponges). This system induces a constitutive, 40% reduction in miR275 transcript abundance in the fly's midgut and results in obstructed blood digestion (gut weights increased by 52%), a significant increase (p-value < 0.0001) in fly survival following infection with an entomopathogenic bacteria, and a 78% increase in trypanosome infection prevalence. RNA sequencing of cardia and midgut tissues from paratransgenic tsetse confirmed that miR275 regulates processes related to the expression of PM-associated proteins and digestive enzymes as well as genes that encode abundant secretory proteins. Our study demonstrates that paratransgenesis can be employed to study microRNA regulated pathways in arthropods that house symbiotic bacteria.

Coxiella burnetii and Related Tick Endosymbionts Evolved from Pathogenic Ancestors

Both symbiotic and pathogenic bacteria in the family Coxiellaceae cause morbidity and mortality in humans and animals. For instance, Coxiella-like endosymbionts (CLEs) improve the reproductive success of ticks—a major disease vector, while Coxiella burnetii causes human Q fever, and uncharacterized coxiellae infect both animals and humans. To better understand the evolution of pathogenesis and symbiosis in this group of intracellular bacteria, we sequenced the genome of a CLE present in the soft tick Ornithodoros amblus (CLEOA) and compared it to the genomes of other bacteria in the order Legionellales. Our analyses confirmed that CLEOA is more closely related to C. burnetii, the human pathogen, than to CLEs in hard ticks, and showed that most clades of CLEs contain both endosymbionts and pathogens, indicating that several CLE lineages have evolved independently from pathogenic Coxiella. We also determined that the last common ancestorof CLEOA and C. burnetii was equipped to infect macrophages and that even though horizontal gene transfer (HGT) contributed significantly to the evolution of C. burnetii, most acquisition events occurred primarily in ancestors predating the CLEOA–C. burnetii divergence. These discoveries clarify the evolution of C. burnetii, which previously was assumed to have emerged when an avirulent tick endosymbiont recently gained virulence factors via HGT. Finally, we identified several metabolic pathways, including heme biosynthesis, that are likely critical to the intracellular growth of the human pathogen but not the tick symbiont, and show that the use of heme analog is a promising approach to controlling C. burnetii infections.

Early Post-Prandial Regulation of Protein Expression in the Midgut of Chagas Disease Vector Rhodnius prolixus Highlights New Potential Targets for Vector Control Strategy

Chagas disease is a vector-borne parasitic disease caused by the flagellated protozoan Trypanosoma cruzi and transmitted to humans by a large group of bloodsucking triatomine bugs. Triatomine insects, such as Rhodnius prolixus, ingest a huge amount of blood in a single meal. Their midgut represents an important interface for triatomine–trypanosome interactions. Furthermore, the development of parasites and their vectorial transmission are closely linked to the blood feeding and digestion; thus, an understanding of their physiology is essential for the development of new strategies to control triatomines. In this study, we used label-free quantitative proteomics to identify and analyze the early effect of blood feeding on protein expression in the midgut of Rhodnius prolixus. We both identified and quantified 124 proteins in the anterior midgut (AM) and 40 in the posterior midgut (PM), which vary significantly 6 h after feeding. The detailed analysis of these proteins revealed their predominant involvement in the primary function of hematophagy, including proteases, proteases inhibitors, amino acids metabolism, primary metabolites processing, and protein folding. Interestingly, our proteomics data show a potential role of the AM in protein digestion. Moreover, proteins related to detoxification processes and innate immunity, which are largely accepted to be triggered by blood ingestion, were mildly modulated. Surprisingly, one third of blood-regulated proteins in the AM have unknown function. This work contributes to the improvement of knowledge on the digestive physiology of triatomines in the early hours post-feeding. It provides key information for selecting new putative targets for the development of triatomine control tools and their potential role in the vector competence, which could be applied to other vector species.

Soft ticks perform evaporative cooling during blood-feeding

Feeding on the blood of warm-blooded vertebrates is associated to thermal stress in haematophagous arthropods. It has been demonstrated that blood-sucking insects protect their physiological integrity either by synthesising heat-shock proteins or by means of thermoregulatory mechanisms. In this work, we describe the first thermoregulatory mechanism in a tick species, Ornithodoros rostratus. By performing real-time infrared thermography during feeding on mice we found that this acarian eliminates big amounts of fluid (urine) through their coxal glands; this fluid quickly spreads over the cuticular surface and its evaporation cools-down the body of the tick. The spread of the fluid is possible thanks to capillary diffusion through the sculptured exoskeleton of Ornithodoros. We discuss our findings in the frame of the adaptive strategies to cope with the thermal stress experienced by blood-sucking arthropods at each feeding event on warm-blooded hosts.

"Urate and NOX5 Control Blood Digestion in the Hematophagous Insect Rhodnius prolixus"

Low levels of reactive oxygen species (ROS) are now recognized as essential players in cell signaling. Here, we studied the role of two conserved enzymes involved in redox regulation that play a critical role in the control of ROS in the digestive physiology of a blood-sucking insect, the kissing bug Rhodnius prolixus. RNAi-mediated silencing of RpNOX5 and RpXDH induced early mortality in adult females after a blood meal. Recently, a role for RpNOX5 in gut motility was reported, and here, we show that midgut peristalsis is also under the control of RpXDH. Together with impaired peristalsis, silencing either genes impaired egg production and hemoglobin digestion, and decreased hemolymph urate titers. Ultrastructurally, the silencing of RpNOX5 or RpXDH affected midgut cells, changing the cells of blood-fed insects to a phenotype resembling the cells of unfed insects, suggesting that these genes work together in the control of blood digestion. Injection of either allopurinol (an XDH inhibitor) or uricase recapitulated the gene silencing effects, suggesting that urate itself is involved in the control of blood digestion. The silencing of each of these genes influenced the expression of the other gene in a complex way both in the unfed state and after a blood meal, revealing signaling crosstalk between them that influences redox metabolism and nitrogen excretion and plays a central role in the control of digestive physiology.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Cytochrome c Oxidase at Full Thrust: Regulation and Biological Consequences to Flying Insects

Flight dispersal represents a key aspect of the evolutionary and ecological success of insects, allowing escape from predators, mating, and colonization of new niches. The huge energy demand posed by flight activity is essentially met by oxidative phosphorylation (OXPHOS) in flight muscle mitochondria. In insects, mitochondrial ATP supply and oxidant production are regulated by several factors, including the energy demand exerted by changes in adenylate balance. Indeed, adenylate directly regulates OXPHOS by targeting both chemiosmotic ATP production and the activities of specific mitochondrial enzymes. In several organisms, cytochrome c oxidase (COX) is regulated at transcriptional, post-translational, and allosteric levels, impacting mitochondrial energy metabolism, and redox balance. This review will present the concepts on how COX function contributes to flying insect biology, focusing on the existing examples in the literature where its structure and activity are regulated not only by physiological and environmental factors but also how changes in its activity impacts insect biology. We also performed in silico sequence analyses and determined the structure models of three COX subunits (IV, VIa, and VIc) from different insect species to compare with mammalian orthologs. We observed that the sequences and structure models of COXIV, COXVIa, and COXVIc were quite similar to their mammalian counterparts. Remarkably, specific substitutions to phosphomimetic amino acids at critical phosphorylation sites emerge as hallmarks on insect COX sequences, suggesting a new regulatory mechanism of COX activity. Therefore, by providing a physiological and bioenergetic framework of COX regulation in such metabolically extreme models, we hope to expand the knowledge of this critical enzyme complex and the potential consequences for insect dispersal.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

Reproductive output and larval survival of American dog ticks (Dermacentor variabilis) from a population at the northern distributional limit

Female reproductive output and larval survival were determined for American dog ticks, Dermacentor variabilis (Say), from a recently established population near the northern distributional limit in Saskatchewan (Canada). Oviposition took 10-21 days at 25 °C and 95% relative humidity (RH). Temperature and relative humidity had a marked effect on egg development time and larval survival. Unfed larvae survived more than 100 days at 32 °C (with 95% RH) and 25 and 5 °C (with ≥ 85% RH). However, survival times declined markedly at lower relative humidities. In addition, 95% of the larvae placed in field enclosures survived for 140 days over winter during which they were exposed to sub-zero temperatures and 95-100% RH, while covered with snow. The median survival times (LT₅₀) of unfed larvae submerged underwater was 68 days. These results show that D. variabilis larvae in populations near the periphery of the northern distributional limit are adapted to cope with sub-zero temperatures in winter, and can survive in the temporary pools of water created by the spring snow melt.

The Phlebotomus papatasi systemic transcriptional response to trypanosomatid-contaminated blood does not differ from the non-infected blood meal

Background

Leishmaniasis, caused by parasites of the genus Leishmania, is a disease that affects up to 8 million people worldwide. Parasites are transmitted to human and animal hosts through the bite of an infected sand fly. Novel strategies for disease control require a better understanding of the key step for transmission, namely the establishment of infection inside the fly.

Methods

The aim of this work was to identify sand fly systemic transcriptomic signatures associated with Leishmania infection. We used next generation sequencing to describe the transcriptome of whole Phlebotomus papatasi sand flies when fed with blood alone (control) or with blood containing one of three trypanosomatids: Leishmania major, L. donovani and Herpetomonas muscarum, the latter being a parasite not transmitted to humans.

Results

Of the trypanosomatids studied, only L. major was able to successfully establish an infection in the host P. papatasi. However, the transcriptional signatures observed after each parasite-contaminated blood meal were not specific to success or failure of a specific infection and they did not differ from each other. The transcriptional signatures were also indistinguishable after a non-contaminated blood meal.

Conclusions

The results imply that sand flies perceive Leishmania as just one feature of their microbiome landscape and that any strategy to tackle transmission should focus on the response towards the blood meal rather than parasite establishment. Alternatively, Leishmania could suppress host responses. These results will generate new thinking around the concept of stopping transmission by controlling the parasite inside the insect.

Repurposing the orphan drug nitisinone to control the transmission of African trypanosomiasis

Tsetse transmit African trypanosomiasis, which is a disease fatal to both humans and animals. A vaccine to protect against this disease does not exist so transmission control relies on eliminating tsetse populations. Although neurotoxic insecticides are the gold standard for insect control, they negatively impact the environment and reduce populations of insect pollinator species. Here we present a promising, environment-friendly alternative to current insecticides that targets the insect tyrosine metabolism pathway. A bloodmeal contains high levels of tyrosine, which is toxic to haematophagous insects if it is not degraded and eliminated. RNA interference (RNAi) of either the first two enzymes in the tyrosine degradation pathway (tyrosine aminotransferase (TAT) and 4-hydroxyphenylpyruvate dioxygenase (HPPD)) was lethal to tsetse. Furthermore, nitisinone (NTBC), an FDA-approved tyrosine catabolism inhibitor, killed tsetse regardless if the drug was orally or topically applied. However, oral administration of NTBC to bumblebees did not affect their survival. Using a novel mathematical model, we show that NTBC could reduce the transmission of African trypanosomiasis in sub-Saharan Africa, thus accelerating current disease elimination programmes.

Autophagy and its role in regeneration and remodeling within invertebrate

Background

Acting as a cellular cleaner by packaging and transporting defective proteins and organelles to lysosomes for breakdown, autophagic process is involved in the regulation of cell remodeling after cell damage or cell death in both vertebrate and invertebrate. In human, limitations on the regenerative capacity of specific tissues and organs make it difficult to recover from diseases. Comprehensive understanding on its mechanism within invertebrate have strong potential provide helpful information for challenging these diseases.

Method

In this study, recent findings on the autophagy function in three invertebrates including planarian, hydra and leech with remarkable regenerative ability were summarized. Furthermore, molecular phylogenetic analyses of DjATGs and HvATGs were performed on these three invertebrates compared to that of Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, Mus musculus and Homo sapiens.

Results

In comparison with Scerevisiae, C elegans, D melanogaster, M musculus and human, our analysis exhibits the following characteristics of autophagy and its function in regeneration within invertebrate. Phylogenetical analysis of ATGs revealed that most autophagy-related genes (ATGs) were highly similar to their homologs in other species, which indicates that autophagy is a highly conservative biological function in both vertebrate and invertebrate. Structurally, almost all the core amino acids necessary for the function of ATG8 in mammal were observed in invertebrate HvATG8s and DjATG8s. For instance, ubiquitin-like domain as a signature structure in each ATG8, was observed in all ATG8s in three invertebrates. Basically, autophagy plays a key role in the regulation of regeneration in planarian. DjATG8-2 and DjATG8-3 associated with mTOR signaling pathway are sophisticated in the invertebrate tissue/organ regeneration. Furthermore, autophagy is involved in the pathway of neutralization of toxic molecules input from blood digestion in the leech.

Conclusions

The recent investigations on autophagy in invertebrate including planarian, hydra and leech suggest that autophagy is evolutionally conserved from yeast to mammals. The fundamental role of its biological function in the invertebrate contributing to the regeneration and maintenance of cellular homeostasis in these three organisms could make tremendous information to confront life threatening diseases in human including cancers and cardiac disorders.

Anopheles metabolic proteins in malaria transmission, prevention and control: a review

The increasing resistance to currently available insecticides in the malaria vector, Anopheles mosquitoes, hampers their use as an effective vector control strategy for the prevention of malaria transmission. Therefore, there is need for new insecticides and/or alternative vector control strategies, the development of which relies on the identification of possible targets in Anopheles. Some known and promising targets for the prevention or control of malaria transmission exist among Anopheles metabolic proteins. This review aims to elucidate the current and potential contribution of Anopheles metabolic proteins to malaria transmission and control. Highlighted are the roles of metabolic proteins as insecticide targets, in blood digestion and immune response as well as their contribution to insecticide resistance and Plasmodium parasite development. Furthermore, strategies by which these metabolic proteins can be utilized for vector control are described. Inhibitors of Anopheles metabolic proteins that are designed based on target specificity can yield insecticides with no significant toxicity to non-target species. These metabolic modulators combined with each other or with synergists, sterilants, and transmission-blocking agents in a single product, can yield potent malaria intervention strategies. These combinations can provide multiple means of controlling the vector. Also, they can help to slow down the development of insecticide resistance. Moreover, some metabolic proteins can be modulated for mosquito population replacement or suppression strategies, which will significantly help to curb malaria transmission.

Convergence of Nutritional Symbioses in Obligate Blood Feeders

Symbiosis with intracellular or gut bacteria is essential for the nutrition of animals with an obligate blood-feeding habit. Divergent bacterial lineages have independently evolved functional interactions with obligate blood feeders, but all converge to an analogous biochemical feature: the provisioning of B vitamins. Although symbionts and blood feeders coevolved interdependently for millions of years we stress that their associations are not necessarily stable. Ancestral symbionts can be replaced by recently acquired bacteria with similar biochemical features, a dynamic that emerges through a combination of phylogenetic and ecological constraints. Specifically, we highlight the lateral transfer of a streamlined biotin (B₇ vitamin) operon, and conjecture that its extensive spread across bacterial lineages may drive the emergence of novel nutritional symbioses with blood feeders.

Expansions and contractions in gene families of independently-evolved blood-feeding insects

BACKGROUND

The blood-feeding behavior evolved multiple times in Insecta lineages and it represents an excellent opportunity to study patterns of convergent molecular evolution regarding this habit. In insects the expansion of some gene families is linked with blood-feeding behavior, but a wide study comparing the evolution of these gene families among different lineages is still missing. Here we gathered genomic data from six independently-evolved hematophagous lineages, aiming to identify convergent expansions and/or contractions of gene families in hematophagous lineages of insects.

RESULTS

We found four rapidly evolving gene families shared by at least two hematophagous independently-evolved lineages, including a heat-shock and a chemosensory protein. On the expression of these four rapidly evolving gene families we found more genes expressed in mated individuals compared with virgin individuals in rapidly-expanded families and more genes expressed in non-blood-feeding individuals compared with blood-feeding individuals in rapidly-contracted families.

CONCLUSION

Our results reveal a new set of candidate genes to be explored in further analysis to help the development of new strategies to deal with blood-feeding vectors and also presents a new perspective to study the evolution of hematophagy identifying convergent molecular patterns.

Survey for positively selected coding regions in the genome of the hematophagous tsetse fly Glossina morsitans identifies candidate genes associated with feeding habits and embryonic development

Tsetse flies are responsible for the transmission of Trypanossoma sp. to vertebrate animals in Africa causing huge health issues and economic loss. The availability of the genome sequence of Glossina morsitans enabled the discovery of several genes related to medically important phenotypes and novel physiological features. However, a genome-wide scan for coding regions that underwent positive selection is still missing, which is surprising given the evolution of traits associated with the hematophagy in this lineage. In this study, we employed an experimental design that controlled for the rate of false positives and we performed a scan of 3,318 G. morsitans genes. We found 145 genes with significant historical signal of positive selection. These genes were categorized into 18 functional classes after careful manual annotation. Based on their attributed functions, we identified candidate genes related with feeding habits and embryonic development. When our results were contrasted with gene expression data, we confirmed that most genes that underwent adaptive molecular evolution were frequently expressed in organs associated with key physiological evolutionary innovations in the G. morsitans lineage, namely, the salivary gland, the midgut, fat body tissue, and in the spermatophore.

The Influence of Environmental Cues on the Development of Trypanosoma cruzi in Triatominae Vector

Trypanosoma cruzi, a hemoflagellate parasite, is the etiological agent of Chagas disease that affects about 6-7 million people worldwide, mostly in Latin America. The parasite life cycle is complex and alternates between an invertebrate host-Triatominae vector-and a mammalian host. The parasite adaptation to the several microenvironments through which it transits is critical to success in establishing infection. Moreover, environmental cues also play an important role on the parasite development, and it can modulate the infection. In the present study, we discussed how the temperature oscillations and the nutritional state of the invertebrate host can affect the parasite development, multiplication, and the differentiation process of epimastigote forms into metacyclic trypomastigotes, called metacyclogenesis. The impact of oxidative imbalance and osmotic stresses on the parasite-vector relationship are also discussed.

How are arbovirus vectors able to tolerate infection?

One of the defining features of mosquito vectors of arboviruses such as Dengue and Zika is their ability to tolerate high levels of virus proliferation without suffering significant pathology. This adaptation is central to vector competence and disease spread. The molecular mechanisms, pathways, cellular and metabolic adaptations responsible for mosquito disease tolerance are still largely unknown and may represent effective ways to control mosquito populations and prevent arboviral diseases. In this review article, we describe the key link between disease tolerance and pathogen transmission, and how vector control methods may benefit by focusing efforts on dissecting the mechanisms underlying mosquito tolerance of arboviral infections. We briefly review recent work investigating tolerance mechanisms in other insects, describe the state of the art regarding the mechanisms of disease tolerance in mosquitos, and highlight the emerging role of gut microbiota in mosquito immunity and disease tolerance.

Hc-hrg-2, a glutathione transferase gene, regulates heme homeostasis in the blood-feeding parasitic nematode Haemonchus contortus

BACKGROUND

Haemonchus contortus, a blood-feeding parasite, is constantly surrounded by large quantities of heme released from the catabolism of host red blood cells. To cope with the toxicity of free heme, H. contortus needs to uptake and detoxify the heme, a process believed to be paramount for parasite survival.

METHODS

A heme-responsive gene Hc-hrg-2 was identified which is the homologue of Ce-hrg-2. The transcriptional levels in all developmental stages and heme-responsive ability of Hc-hrg-2 were analyzed by qRT-PCR. Immunofluorescence analysis and cell transfections were performed to analyze the expression pattern of Hc-HGR-2. Statistical analyses were performed with GraghPad Prism 6.0 using Student's t-test.

RESULTS

To investigate the heme homeostasis of H. contortus, we first identified a heme-responsive gene Hc-hrg-2, a homolog of Ce-hrg-2 that is involved in heme transport in the hypodermis of Caenorhabditis elegans. Using qRT-PCR, we showed that Hc-hrg-2 mRNA was expressed throughout all life-cycle stages of H. contortus with the highest level in the third-stage larvae (L3s). Notably, transcription of Hc-hrg-2 in the exsheathed L3s was significantly upregulated in the presence of high concentration of heme. We found that Hc-HRG-2 protein was mainly located in the hypodermal tissues of adult H. contortus in vivo and the endoplasmic reticulum in the transfected mammalian cells. Our in vitro assay demonstrated that Hc-HRG-2 is a heme-binding protein with glutathione S-transferase activity and heme had a significant effect on its enzymatic activity when a model substrate 1-chloro-2, 4-dinitrobenzene (CDNB) was used.

CONCLUSIONS

Hc-hrg-2 is a heme-responsive gene and engaged in heme homeostasis regulation in hypodermal tissues during the free-living stages of H. contortus.

Experimental population modification of the malaria vector mosquito, Anopheles stephensi

Small laboratory cage trials of non-drive and gene-drive strains of the Asian malaria vector mosquito, Anopheles stephensi, were used to investigate release ratios and other strain properties for their impact on transgene spread during simulated population modification. We evaluated the effects of transgenes on survival, male contributions to next-generation populations, female reproductive success and the impact of accumulation of gene drive-resistant genomic target sites resulting from nonhomologous end-joining (NHEJ) mutagenesis during Cas9, guide RNA-mediated cleavage. Experiments with a non-drive, autosomally-linked malaria-resistance gene cassette showed ‘full introduction’ (100% of the insects have at least one copy of the transgene) within 8 weeks (≤ 3 generations) following weekly releases of 10:1 transgenic:wild-type males in an overlapping generation trial design. Male release ratios of 1:1 resulted in cages where mosquitoes with at least one copy of the transgene fluctuated around 50%. In comparison, two of three cages in which the malaria-resistance genes were linked to a gene-drive system in an overlapping generation, single 1:1 release reached full introduction in 6–8 generations with a third cage at ~80% within the same time. Release ratios of 0.1:1 failed to establish the transgenes. A non-overlapping generation, single-release trial of the same gene-drive strain resulted in two of three cages reaching 100% introduction within 6–12 generations following a 1:1 transgenic:wild-type male release. Two of three cages with 0.33:1 transgenic:wild-type male single releases achieved full introduction in 13–16 generations. All populations exhibiting full introduction went extinct within three generations due to a significant load on females having disruptions of both copies of the target gene, kynurenine hydroxylase. While repeated releases of high-ratio (10:1) non-drive constructs could achieve full introduction, results from the 1:1 release ratios across all experimental designs favor the use of gene drive, both for efficiency and anticipated cost of the control programs.

The experimental introduction of manipulated genes into insect species has a long history in basic genetics. Recent advances in genome editing technologies have spurred considerable effort to exploit these methodologies to provide genetic solutions to some of the worst medical and agricultural problems caused by insects. Insect population suppression and population modification approaches have been proposed to control transmission of vector-borne diseases, including malaria. We used small cage trials to explore the efficacy of non-drive and gene-drive releases to deliver anti-malarial effector genes to a vector mosquito, Anopheles stephensi. We show that both approaches can work to introduce genes to high percentages, but as expected, the gene-drive approaches were more efficient in that they needed only a single release with a much lower number of released insects. The gene-drive females in our studies exhibited a significant load that resulted in some cage populations going to extinction. Furthermore, the accumulation of drive-resistant target genes prevented full introduction of the transgenes in those cages that did not go extinct. While none of the strains evaluated here are proposed for open release, these laboratory cage trials reveal features that can be used to improve next-generation gene-drive strains for population modification.

Non-canonical transcriptional regulation of heme oxygenase in Aedes aegypti

Heme oxygenase (HO) is a ubiquitous enzyme responsible for heme breakdown, which yields carbon monoxide (CO), biliverdin (BV) and ferrous ion. Here we show that the Aedes aegypti heme oxygenase gene (AeHO - AAEL008136) is expressed in different developmental stages and tissues. AeHO expression increases after a blood meal in the midgut, and its maximal transcription levels overlaps with the maximal rate of the further modified A. aegypti biglutaminyl-biliverdin (AeBV) pigment production. HO is a classical component of stress response in eukaryotic cells, being activated under oxidative stress or increased heme levels. Indeed, the final product of HO activity in the mosquito midgut, AeBV, exerts a protective antioxidant activity. AeHO, however, does not seem to be under a classical redox-sensitive transcriptional regulation, being unresponsive to heme itself, and even down regulated when insects face a pro-oxidant insult. In contrast, AeHO gene expression responds to nutrient sensing mechanisms, through the target of rapamycin (TOR) pathway. This unusual transcriptional control of AeHO, together with the antioxidant properties of AeBV, suggests that heme degradation by HO, in addition to its important role in protection of Aedes aegypti against heme exposure, also acts as a digestive feature, being an essential adaptation to blood feeding.

Thermoprotective adaptations are critical for arthropods feeding on warm-blooded hosts

Blood feeding in arthropods has evolved in multiple lineages. This feeding preference provides a source of ample proteins and lipids for egg production and survival, but ingestion of a large warm blood-meal can boost the arthropod's body temperature 15°-20°C within seconds to minutes. This represents one of, if not the most, rapid thermal change documented under a natural setting. Here, we describe mechanisms of thermoregulation and thermotolerance in arthropods during blood feeding. The ability to prevent blood-induced thermal damage is a fundamental physiological adaptation linked to the use of warm-blooded vertebrates as food sources. Specific functional and comparative studies have identified unique and divergent mechanisms that suppress or repair thermal stress during blood feeding. These mechanisms include countercurrent heat exchange, evaporative cooling, and upregulation of stress associated proteins.

Mosquito adaptations to hematophagia impact pathogen transmission

Mosquito-borne diseases such as Dengue fever, Chikungunya, and Malaria are critical threats to public health in many parts of the world. Female mosquitoes have evolved multiple adaptive mechanisms to hematophagy, including the ability to efficiently draw and digest blood, as well as the ability to eliminate excess fluids and toxic by-products of blood digestion. Pathogenic agents enter the mosquito digestive tract with the blood meal and need to travel through the midgut and into the hemocele in order to reach the salivary glands and infect a new host. Pathogens need to adjust to these hostile gut, hemocele, and salivary gland environments, and when possible influence the physiology and behavior of their hosts to enhance transmission.

Female bed bugs (Cimex lectularius L) anticipate the immunological consequences of traumatic insemination via feeding cues

We show that female insects can subtly change the management of their immune system contingent on infradian feeding cycles that act as cues to immune insult during mating. We experimentally reject the possibility that this is learned behavior, and show instead that it is dependent on the predictability of feeding which in turn is a cue for mating-induced infection. Although evidence exists for insect immune anticipation over life-time scales, this study links the temporal features of feeding to the insect’s mating behavior in the context of a system with infection caused by traumatic insemination. We predict similar mating ecology in other animals is likely to select for similar reproductive immune anticipation (RIA).

Not all encounters with pathogens are stochastic and insects can adjust their immune management in relation to cues associated with the likelihood of infection within a life cycle as well as across generations. In this study we show that female insects (bed bugs) up-regulate immune function in their copulatory organ in anticipation of mating by using feeding cues. Male bed bugs only mate with recently fed females and do so by traumatic insemination (TI). Consequently, there is a tight temporal correlation between female feeding and the likelihood of her being infected via TI. Females that received predictable access to food (and therefore predictable insemination and infection cycles) up-regulated induced immunity (generic antibacterial activity) in anticipation of feeding and mating. Females that received unpredictable (but the same mean periodicity) access to food did not. Females that anticipated mating-associated immune insult received measurable fitness benefits (survival and lifetime reproductive success) despite laying eggs at the same rate as females that were not able to predict these cycles. Given that mating is a time of increased likelihood of infection in many organisms, and is often associated with temporal cues such as courtship and/or feeding, we propose that anticipation of mating-associated infection in females may be more widespread than is currently evidenced.