Identification of two cationic amino acid transporters required for nutritional signaling during mosquito reproduction

Identification of antennal alternative splicing by combining genome and full-length transcriptome analysis in Bactrocera dorsalis

Alternative splicing is an essential post-transcriptional regulatory mechanism that diversifies gene function by generating multiple protein isoforms from a single gene and act as a crucial role in insect environmental adaptation. Olfaction, a key sense for insect adaptation, relies heavily on the antennae, which are the primary olfactory organs expressing most of the olfactory genes. Despite the extensive annotation of olfactory genes within insect antennal tissues facilitated by high-throughput sequencing technology advancements, systematic analyses of alternative splicing are still relatively less. In this study, we focused on the oriental fruit fly (Bactrocera dorsalis), a significant pest of fruit crops. We performed a detailed analysis of alternative splicing in its antennae by utilizing the full-length transcriptome of its antennal tissue and the insect's genome. The results revealed 8600 non-redundant full-length transcripts identified in the oriental fruit fly antennal full-length transcriptome, spanning 4,145 gene loci. Over 40% of these loci exhibited multiple isoforms. Among these, 161 genes showed sex-biased isoform switching, involving seven different types of alternative splicing. Notably, events involving alternative transcription start sites (ATSS) and alternative transcription termination sites (ATTS) were the most common. Of all the genes undergoing ATSS and ATTS alternative splicing between male and female, 32 genes were alternatively spliced in protein coding regions, potentially affecting protein function. These genes were categorized based on the length of the sex-biased isoforms, with the highest difference in isoform fraction (dIF) associated with the ATSS type, including genes such as BdorABCA13, BdorCAT2, and BdorTSN3. Additionally, transcription factor binding sites for doublesex were identified upstream of both BdorABCA13 and BdorCAT2. Besides being expressed in the antennal tissues, BdorABCA13 and BdorCAT2 are also expressed in the mouthparts, legs, and genitalia of both female and male adults, suggesting their functional diversity. This study reveals alternative splicing events in the antennae of Bactrophora dorsalis from two aspects: odorant receptor genes and other types of genes expressed in the antennae. This study not only provides a research foundation for understanding the regulation of gene function by alternative splicing in the oriental fruit fly but also offers new insights for utilizing olfaction-based behavioral manipulation techniques to manage this pest.

Ecdysone-controlled nuclear receptor ERR regulates metabolic homeostasis in the disease vector mosquito Aedes aegypti

Hematophagous mosquitoes require vertebrate blood for their reproductive cycles, making them effective vectors for transmitting dangerous human diseases. Thus, high-intensity metabolism is needed to support reproductive events of female mosquitoes. However, the regulatory mechanism linking metabolism and reproduction in mosquitoes remains largely unclear. In this study, we found that the expression of estrogen-related receptor (ERR), a nuclear receptor, is activated by the direct binding of 20-hydroxyecdysone (20E) and ecdysone receptor (EcR) to the ecdysone response element (EcRE) in the ERR promoter region during the gonadotropic cycle of Aedes aegypti (named AaERR). RNA interference (RNAi) of AaERR in female mosquitoes led to delayed development of ovaries. mRNA abundance of genes encoding key enzymes involved in carbohydrate metabolism (CM)-glucose-6-phosphate isomerase (GPI) and pyruvate kinase (PYK)-was significantly decreased in AaERR knockdown mosquitoes, while the levels of metabolites, such as glycogen, glucose, and trehalose, were elevated. The expression of fatty acid synthase (FAS) was notably downregulated, and lipid accumulation was reduced in response to AaERR depletion. Dual luciferase reporter assays and electrophoretic mobility shift assays (EMSA) determined that AaERR directly activated the expression of metabolic genes, such as GPI, PYK, and FAS, by binding to the corresponding AaERR-responsive motif in the promoter region of these genes. Our results have revealed an important role of AaERR in the regulation of metabolism during mosquito reproduction and offer a novel target for mosquito control.

RNAi-Mediated Knockdown of Acidic Ribosomal Stalk Protein P1 Arrests Egg Development in Adult Female Yellow Fever Mosquitoes, Aedes aegypti

After taking a blood meal, the fat body of the adult female yellow fever mosquito, Aedes aegypti, switches from a previtellogenic state of arrest to an active state of synthesizing large quantities of yolk protein precursors (YPPs) that are crucial for egg development. The synthesis of YPPs is regulated at both the transcriptional and translational levels. Previously, we identified the cytoplasmic protein general control nonderepressible 1 (GCN1) as a part of the translational regulatory pathway for YPP synthesis. In the current study, we used the C-terminal end of GCN1 to screen for protein-protein interactions and identified 60S acidic ribosomal protein P1 (P1). An expression analysis and RNAi-mediated knockdown of P1 was performed to further investigate the role of P1 in mosquito reproduction. We showed that in unfed (absence of a blood meal) adult A. aegypti mosquitoes, P1 was expressed ubiquitously in the mosquito organs and tissues tested. We also showed that the RNAi-mediated knockdown of P1 in unfed adult female mosquitoes resulted in a strong, transient knockdown with observable phenotypic changes in ovary length and egg deposition. Our results suggest that 60S acidic ribosomal protein P1 is necessary for mosquito reproduction and is a promising target for mosquito population control.

A scoping review on the use of different blood sources and components in the artificial membrane feeding system and its effects on blood-feeding and fecundity rate of Aedes aegypti

In some laboratories, mosquitoes' direct blood-feeding on live animals has been replaced with various membrane blood-feeding systems. The selection of blood meal sources used in membrane feeding is crucial in vector mass rearing as it influences the mosquitoes' development and reproductive fitness. Therefore, this scoping review aimed to evaluate the existing literature on the use of different blood sources and components in artificial membrane feeding systems and their effects on blood-feeding and the fecundity rate of Ae. aegypti. A literature review search was conducted by using PubMed, Scopus, and Web of Science databases according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA-ScR). The EndNote version 20 software was used to import all searched articles. Relevant information was retrieved for analysis into a Microsoft Excel Spreadsheet. A total of 104 full-text articles were assessed for eligibility criteria, whereby the articles should include the comparison between different types of blood source by using the membrane feeding systems. Only 16 articles were finally included in the analysis. Several studies had reported that human blood was superior in blood-feeding Ae. aegypti as compared to sheep blood which resulted in lower fecundity due to accumulation of free fatty acids (FFA) in the cuticles. In contrast, cattle whole blood and pig whole blood showed no significant differences in the blood-feeding and fecundity rate as compared to human blood. This review also indicated that bovine whole blood and pig whole blood enhanced Ae. aegypti's vitellogenesis and egg production as compared to plasma and blood cells. In addition, human blood of up to 10 days after the expiration date could still be used to establish Ae. aegypti colonies with good blood-feeding rates and number of eggs produced. Thus, future studies must consider the importance of selecting suitable blood sources and components for membrane blood feeding especially in mosquito colonisation and control measure studies.

Functional Analysis of Amino Acid Transporter Genes ACYPI000536 and ACYPI004320 in Acyrthosiphon pisum

In recent years, pea aphids have become major pests of alfalfa. Our previous study found that "Gannong 5" is a highly aphid-resistant alfalfa variety and that "Lie Renhe" is a susceptible one. The average field susceptibility index of "Gannong 5" was 31.31, and the average field susceptibility index of "Lie Renhe" was 80.34. The uptake and balance of amino acids in insects are usually dependent on amino acid transporters. RT-qPCR was used to detect the relative expression levels of seven amino acid transporter differential genes in the different instar pea aphids fed on resistant and susceptible alfalfa varieties after 24 h, and two key genes were selected. When pea aphids fed on "Gannong 5", the expression of ACYPI004320 was significantly higher than that in pea aphids fed on "Lie Renhe"; however, the expression of ACYPI000536 was significantly lower than that in pea aphids fed on "Lie Renhe". Afterward, the RNA interference with pea aphid ACYPI000536 and ACYPI004320 genes was performed using a plant-mediated method, and gene function was verified via liquid chromatography-mass spectrometry and pea aphid sensitivity to aphid-resistant and susceptible alfalfa varieties. The results showed that the down-regulation of the ACYPI000536 gene expression led to an increase in the histidine and lysine contents in pea aphids, which, in turn, led to an increase in mortality when pea aphids fed on the susceptible variety "Lie Renhe". The down-regulation of the ACYPI004320 gene expression led to an increase in phenylalanine content in pea aphids, which, in turn, led to a decrease in mortality when pea aphids fed on the resistant variety "Gannong 5".

Knockdown of the Sodium/Potassium ATPase Subunit Beta 2 Reduces Egg Production in the Dengue Vector, Aedes aegypti

The Na+/K+ ATPase (NKA) is present in the cellular membrane of most eukaryotic cells. It utilizes energy released by ATP hydrolysis to pump sodium ions out of the cell and potassium ions into the cell, which establishes and controls ion gradients. Functional NKA pumps consist of three subunits, alpha, beta, and FXYD. The alpha subunit serves as the catalytic subunit while the beta and FXYD subunits regulate the proper folding and localization, and ion affinity of the alpha subunit, respectively. Here we demonstrate that knockdown of NKA beta subunit 2 mRNA (nkaβ2) reduces fecundity in female Ae. aegypti. We determined the expression pattern of nkaβ2 in several adult mosquito organs using qRT-PCR. We performed RNAi-mediated knockdown of nkaβ2 and assayed for lethality, and effects on female fecundity. Tissue expression levels of nkaβ2 mRNA were highest in the ovaries with the fat body, midgut and thorax having similar expression levels, while Malpighian tubules had significantly lower expression. Survival curves recorded post dsRNA injection showed a non-significant decrease in survival of nkaβ2 dsRNA-injected mosquitoes compared to GFP dsRNA-injected mosquitoes. We observed a significant reduction in the number of eggs laid by nkaβ2 dsRNA-injected mosquitoes compared to control mosquitoes. These results, coupled with the tissue expression profile of nkaβ2, indicate that this subunit plays a role in normal female Ae. aegypti fecundity. Additional research needs to be conducted to determine the exact role played by NKAβ2 in mosquito post-blood meal nutrient sensing, transport, yolk precursor protein (YPP) synthesis and yolk deposition.

General control nonderepressible 1 interacts with cationic amino acid transporter 1 and affects Aedes aegypti fecundity

BACKGROUND

The amino acid transporter protein cationic amino acid transporter 1 (CAT1) is part of the nutrient sensor in the fat body of mosquitoes. A member of the SLC7 family of cationic amino acid transporters, it is paramount for the detection of elevated amino acid levels in the mosquito hemolymph after a blood meal and the subsequent changes in gene expression in the fat body.

METHODS

We performed a re-annotation of Aedes aegypti cationic amino acid transporters (CATs) and selected the C-terminal tail of CAT1 to perform a yeast two-hybrid screen to identify putative interactors of this protein. One interesting interacting protein we identified was general control nonderepressible 1 (GCN1). We determined the expression pattern of GCN1 in several adult organs and structures using qRT-PCR and western blots. Finally, we knocked down GCN1 using double-stranded RNA and identified changes in downstream signaling intermediates and the effects of knockdown on vitellogenesis and fecundity.

RESULTS

In a screen for Ae. aegypti CAT1-interacting proteins we identified GCN1 as a putative interactor. GCN1 is highly expressed in the ovaries and fat body of the mosquito. We provide evidence that eukaryotic translation initiation factor 2 subunit alpha (eIF2α) phosphorylation changed during vitellogenesis and that RNA interference knockdown of GCN1 in whole mosquitoes reduced egg clutch sizes of treated mosquitoes relative to controls.

CONCLUSIONS

Aedes aegypti CAT1 and GCN1 are likely interacting partners and GCN1 is likely necessary for proper egg development. Our data suggest that GCN1 is part of a nutrient sensor mechanism in various mosquito tissues involved in vitellogenesis.

Genome-Wide Analysis of the Amino Acid Auxin Permease (AAAP) Gene Family and Identification of an AAAP Gene Associated with the Growth and Reproduction of the Brown Planthopper, Nilaparvata lugens (Stål)

Amino acids play a vital role in several biological processes in organisms and are mainly acquired through diet by most insects. The amino acid auxin permease (AAAP) transporter family is an important amino acid transporter gene family in insects for the transportation of amino acids into and out of cells across the plasma membrane. Here, we identified 21 putative AAAP family members in the genome of the brown planthopper (BPH), Nilaparvata lugens, a devastating pest that feeds only on the phloem sap of rice plants. Molecular characteristic analysis indicated large variations in protein features and amino acid sequences among the predicted AAAP family members in BPH. Phylogenetic analysis clustered these AAAP transporters into three subgroups, with the members in the same group sharing a similar pattern of conserved motif distribution. Through ortholog gene recognition and spatiotemporal gene expression analysis, the AAAP gene NlAAAP07, which was predicted to regulate BPH larval growth and female fecundity, was identified. RNA interference (RNAi)-mediated suppression of NlAAAP07 significantly postponed the duration of 3rd instar nymphs developing into adults from 7.4 days to 9.0 days, and decreased the oviposition amount and egg hatching rate of females by 30.7% and 11.0%, respectively. Our results provide a foundation for further functional analysis of AAAP transporters in BPH.

Transcriptional Profiles of Diploid Mutant Apis mellifera Embryos after Knockout of csd by CRISPR/Cas9

Simple Summary

In honey bees, males are haploid while females are diploid, leading to a fundamental difference in genetic materials between the sexes. In order to better control the comparison of gene expression between males and females, diploid mutant males were generated by knocking out the sex-determining gene, complementary sex determiner (csd), in fertilized embryos. The diploid mutant drones had male external morphological features, as well as male gonads. RNA sequencing was performed on the diploid mutant embryos and one-day-old larvae. The transcriptome analysis showed that several female-biased genes, such as worker-enriched antennal (Wat), vitellogenin (Vg), and some venom-related genes, were down-regulated in the diploid mutant males. In contrast, some male-biased genes, like takeout and apolipophorin-III-like protein (A4), were up-regulated. Moreover, the co-expression gene networks suggested that csd might interact very closely with fruitless (fru), feminizer (fem) might have connections with hexamerin 70c (hex70c), and transformer-2 (tra2) might play roles with troponin T (TpnT). Foundational information about the differences in the gene expression caused by sex differentiation was provided in this study. It is believed that this study will pave the ground for further research on the different mechanisms between males and females in honey bees.

Abstract

In honey bees, complementary sex determiner (csd) is the primary signal of sex determination. Its allelic composition is heterozygous in females, and hemizygous or homozygous in males. To explore the transcriptome differences after sex differentiation between males and females, with genetic differences excluded, csd in fertilized embryos was knocked out by CRISPR/Cas9. The diploid mutant males at 24 h, 48 h, 72 h, and 96 h after egg laying (AEL) and the mock-treated females derived from the same fertilized queen were investigated through RNA-seq. Mutations were detected in the target sequence in diploid mutants. The diploid mutant drones had typical male morphological characteristics and gonads. Transcriptome analysis showed that several female-biased genes, such as worker-enriched antennal (Wat), vitellogenin (Vg), and some venom-related genes, were down-regulated in the diploid mutant males. In contrast, some male-biased genes, such as takeout and apolipophorin-III-like protein (A4), had higher expressions in the diploid mutant males. Weighted gene co-expression network analysis (WGCNA) indicated that there might be interactions between csd and fruitless (fru), feminizer (fem) and hexamerin 70c (hex70c), transformer-2 (tra2) and troponin T (TpnT). The information provided by this study will benefit further research on the sex dimorphism and development of honey bees and other insects in Hymenoptera.

Genome-Wide Identification and Characterization of Amino Acid Polyamine Organocation Transporter Family Genes Reveal Their Role in Fecundity Regulation in a Brown Planthopper Species (Nilaparvata lugens)

The brown planthopper (BPH), Nilaparvata lugens Stål (Hemiptera:Delphacidae), is one of the most destructive pests of rice worldwide. As a sap-feeding insect, the BPH is incapable of synthesizing several amino acids which are essential for normal growth and development. Therefore, the insects have to acquire these amino acids from dietary sources or their endosymbionts, in which amino acid transporters (AATs) play a crucial role by enabling the movement of amino acids into and out of insect cells. In this study, a common amino acid transporter gene family of amino acid/polyamine/organocation (APC) was identified in BPHs and analyzed. Based on a homology search and conserved functional domain recognition, 20 putative APC transporters were identified in the BPH genome. Molecular trait analysis showed that the verified BPH APC family members were highly variable in protein features, conserved motif distribution patterns, and exon/intron organization. Phylogenetic analysis of five hemipteran species revealed an evolutionary pattern of interfamily conservation and lineage-specific expansion of this gene family. Moreover, stage- and tissue-specific expression analysis revealed diverse expression patterns in the 20 BPH APC transporter genes. Lastly, a potential BPH fecundity regulatory gene of NlAPC09 was identified and shown to participate in the fecundity regulation through the use of quantitative polymerase chain reaction (qPCR) and RNA inference experiments. Our results provide a basis for further functional investigations of APC transporters in BPH.

20-Hydroxyecdysone (20E) signaling as a promising target for the chemical control of malaria vectors

With the rapid development and spread of resistance to insecticides among anopheline malaria vectors, the efficacy of current World Health Organization (WHO)-approved insecticides targeting these vectors is under threat. This has led to the development of novel interventions, including improved and enhanced insecticide formulations with new targets or synergists or with added sterilants and/or antimalarials, among others. To date, several studies in mosquitoes have revealed that the 20-hydroxyecdysone (20E) signaling pathway regulates both vector abundance and competence, two parameters that influence malaria transmission. Therefore, insecticides which target 20E signaling (e.g. methoxyfenozide and halofenozide) may be an asset for malaria vector control. While such insecticides are already commercially available for lepidopteran and coleopteran pests, they still need to be approved by the WHO for malaria vector control programs. Until recently, chemicals targeting 20E signaling were considered to be insect growth regulators, and their effect was mostly studied against immature mosquito stages. However, in the last few years, promising results have been obtained by applying methoxyfenozide or halofenozide (two compounds that boost 20E signaling) to Anopheles populations at different phases of their life-cycle. In addition, preliminary studies suggest that methoxyfenozide resistance is unstable, causing the insects substantial fitness costs, thereby potentially circumventing one of the biggest challenges faced by current vector control efforts. In this review, we first describe the 20E signaling pathway in mosquitoes and then summarize the mechanisms whereby 20E signaling regulates the physiological processes associated with vector competence and vector abundance. Finally, we discuss the potential of using chemicals targeting 20E signaling to control malaria vectors.

Wolbachia's Deleterious Impact on Aedes aegypti Egg Development: The Potential Role of Nutritional Parasitism

Simple Summary

Mosquito-borne viral diseases such as dengue, Zika and chikungunya cause a significant global health burden and are currently increasing in outbreak frequency and geographical reach. Wolbachia pipientis, an endosymbiotic bacterium, offers a solution to this. When Wolbachia is introduced into the main mosquito vector of these viruses, Aedes aegypti, it alters the mosquito’s reproductive biology, as well as reducing the ability of the mosquitoes to transmit viruses. These traits can be leveraged to reduce virus transmission within a community by mass releasing Wolbachia-infected mosquitoes. However, Wolbachia has some negative effects on Aedes aegypti fitness, particularly egg longevity, and the reason behind this remains ambiguous. Insect fitness is very important for the success for Wolbachia-biocontrol strategies as they rely on the released insects being competitive with the wild mosquito population. This review summarises the fitness effects of Wolbachia on Aedes aegypti and investigates the possible contribution of Wolbachia as a nutritional parasite in lowering host fitness. It proposes the next stages of research that can be conducted to address nutritional parasitism to aid in the expansion of Wolbachia-based disease management programs worldwide.

Abstract

The artificial introduction of the endosymbiotic bacterium, Wolbachia pipientis, into Aedes (Ae.) aegypti mosquitoes reduces the ability of mosquitoes to transmit human pathogenic viruses and is now being developed as a biocontrol tool. Successful introgression of Wolbachia-carrying Ae. aegypti into native mosquito populations at field sites in Australia, Indonesia and Malaysia has been associated with reduced disease prevalence in the treated community. In separate field programs, Wolbachia is also being used as a mosquito population suppression tool, where the release of male only Wolbachia-infected Ae. aegypti prevents the native mosquito population from producing viable eggs, subsequently suppressing the wild population. While these technologies show great promise, they require mass rearing of mosquitoes for implementation on a scale that has not previously been done. In addition, Wolbachia induces some negative fitness effects on Ae. aegypti. While these fitness effects differ depending on the Wolbachia strain present, one of the most consistent and significant impacts is the shortened longevity and viability of eggs. This review examines the body of evidence behind Wolbachia’s negative effect on eggs, assesses nutritional parasitism as a key cause and considers how these impacts could be overcome to achieve efficient large-scale rearing of these mosquitoes.

Frizzled 2 Functions in the Regulation of TOR-Mediated Embryonic Development and Fecundity in Cyrtorhinus lividipennis Reuter

The mirid bug, Cyrtorhinus lividipennis Reuter, is an important predator of rice planthoppers in Asia. In a previous study, C. lividipennis fed on gramineous weeds with brown planthopper (BPH) eggs had reduced development compared to those fed on rice with BPH eggs. In the current study, the concentrations of selected amino acids (AAs) were higher in rice than five gramineous species, which might explain the enhanced growth of C. lividipennis on rice. When C. lividipennis was fed on AA-deprived artificial diets, the Wnt/β-catenin pathway was inhibited. Furthermore, C. lividipennis females silenced for expression of Frizzled 2 (Fz2) showed a significant reduction in the Wnt/β-catenin and target of rapamycin (TOR) pathways. Silencing Fz2 led to decreased expression of the vitellogenin gene (Vg), lower Vg accumulation in oocytes, reduced soluble protein in ovaries and fat bodies, reduced titers of juvenile hormone, prolonged preoviposition periods, and lower predation capacity, body weight, and egg numbers as controlled to controls. Fz2 silencing resulted in undeveloped ovaries and the inhibition of oocyte growth in the ovarioles, resulting in decreased numbers of offspring and reduced hatching rates. The silencing of Fz2 also resulted in aberrant embryos with undeveloped eyespots and organs, suggesting that Fz2 is an essential gene for embryonic development, oogenesis, and egg maturation. In summary, this study established a potential link between Wnt and TOR pathways, which interact synergistically to regulate C. lividipennis reproduction in response to AA signals. These results provide valuable new information that can be applied to large-scale rearing of C. lividipennis predators, which is important for reducing planthopper damage in rice fields.

Transcriptome profiling of maternal stress-induced wing dimorphism in pea aphids

Wing dimorphism, that is, wingless and winged forms, can be induced by maternal stress signals and is an adaptive response of aphids to environmental changes. Here, we investigated the ecological and molecular effects of three kinds of stress, namely crowding, predation, and aphid alarm pheromone, on wing dimorphism. These three stressors induced high proportion of up to 60% of winged morphs in offspring. Transcriptome analysis of stress-treated female aphids revealed different changes in maternal gene expression induced by the three stressors. Crowding elicited widespread changes in the expression of genes involved in nutrient accumulation and energy mobilization. Distinct from crowding, predation caused dramatic expression changes in cuticle protein (CP) genes. Twenty-three CP genes that belong to CP RR2 subfamily and are highly expressed in legs and embryos were greatly repressed by the presence of ladybird. By contrast, application of alarm pheromone, E-β-farnesene, caused slight changes in gene expression. The three factors shared a responsive gene, cuticle protein 43. This study reveals the adaptive response of aphids to environmental stresses and provides a rich resource on genome-wide expression genes for exploring molecular mechanisms of ecological adaptation in aphids.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.55b2b15.

Two of the three Transformer-2 genes are required for ovarian development in Aedes albopictus

In Drosophila melanogaster, transformer-2 (tra2) plays an essential role in the sex-specific splicing of doublesex (dsx) and fruitless (fru), two key transcription factor genes that program sexual differentiation and regulate sexual behavior. In the present study, the sequences and expression profiles of three tra2 (Aalbtra2) genes in the Asian tiger mosquito, Aedes albopictus (Ae. albopictus) were characterized. Phylogenetic analysis revealed that these paralogs resulted from two duplication events. The first occurred in the common ancestor of Culicidae, giving rise to the tra2-α and tra2-β clades that are found across divergent mosquito genera, including Aedes, Culex, and Anopheles. The second occurred within the tra2-α clade, giving rise to tra2-γ in Ae. albopictus. In addition to the conserved RNA recognition motif (RRM), arginine-rich/serine-rich regions (RS domains) and a linker region, a glycine-rich region located between the RRM and RS2 was observed in Tra2-α and Tra2-γ of Ae. albopictus that has not yet been described in the Tra2 proteins of dipteran insects. Quantitative real-time PCR detected relatively high levels of transcripts from all three tra2 paralogs in 0-2 h embryos, suggesting maternal deposition of these transcripts. All three Aalbtra2 genes were highly expressed in the ovary, while Aalbtra2-β was also highly expressed in the testis. RNAi-mediated knockdown of any or all Aalbtra2 genes did not result in an obvious switch of the sex-specificity in dsx and fru splicing in the whole-body samples. However, knockdown of transcripts from all three tra2 genes significantly reduced the female isoform of dsx mRNA and increased the male isoform of the dsx mRNA in both the ovary and the fat body in adult females. Furthermore, knockdown of either Aalbtra2-α or Aalbtra2-γ or all three Aalbtra2 led to a decrease in ovariole number and ovary size after a blood meal. Taken together, these results indicate that two of the three tra2 genes affect female ovarian development.

Blood feeding activates the vitellogenic stage of oogenesis in the mosquito Aedes aegypti through inhibition of glycogen synthase kinase 3 by the insulin and TOR pathways

Most mosquitoes, including Aedes aegypti, only produce eggs after blood feeding on a vertebrate host. Oogenesis in A. aegypti consists of a pre-vitellogenic stage before blood feeding and a vitellogenic stage after blood feeding. Primary egg chambers remain developmentally arrested during the pre-vitellogenic stage but complete oogenesis to form mature eggs during the vitellogenic stage. In contrast, the signaling factors that maintain primary egg chambers in pre-vitellogenic arrest or that activate vitellogenic growth are largely unclear. Prior studies showed that A. aegypti females release insulin-like peptide 3 (ILP3) and ovary ecdysteroidogenic hormone (OEH) from brain neurosecretory cells after blood feeding. Here, we report that primary egg chambers exit pre-vitellogenic arrest by 8 h post-blood meal as evidenced by proliferation of follicle cells, endoreplication of nurse cells, and formation of cytoophidia. Ex vivo assays showed that ILP3 and OEH stimulate primary egg chambers to exit pre-vitellogenic arrest in the presence of nutrients but not in their absence. Characterization of associated pathways indicated that activation of insulin/insulin growth factor signaling (IIS) by ILP3 or OEH inactivated glycogen synthase kinase 3 (GSK3) via phosphorylation by phosphorylated Akt. GSK3 inactivation correlated with accumulation of the basic helix-loop-helix transcription factor Max and primary egg chambers exiting pre-vitellogenic arrest. Direct inhibition of GSK3 by CHIR-99021 also stimulated Myc/Max accumulation and primary egg chambers exiting pre-vitellogenic arrest. Collectively, our results identify GSK3 as a key factor in regulating the pre- and vitellogenic stages of oogenesis in A. aegypti.

Fat body-specific vitellogenin expression regulates host-seeking behaviour in the mosquito Aedes albopictus

The high vector competence of mosquitoes is intrinsically linked to their reproductive strategy because females need a vertebrate blood meal to develop large batches of eggs. However, the molecular mechanisms and pathways regulating mosquito host-seeking behaviour are largely unknown. Here, we test whether host-seeking behaviour may be linked to the female's energy reserves, with low energy levels triggering the search for a nutrient-rich blood meal. Our results demonstrate that sugar feeding delays host-seeking behaviour in the invasive tiger mosquito Aedes albopictus, but the levels of energy reserves do not correlate with changes in host-seeking behaviour. Using tissue-specific gene expression analyses, we show for the first time, to our knowledge, that sugar feeding alone induces a transient up-regulation of several vitellogenesis-related genes in the female fat body, resembling the transcriptional response after a blood meal. Specifically, high expression levels of a vitellogenin gene (Vg-2) correlated with the lowest host-seeking activity of sugar-fed females. Knocking down the Vg-2 gene via RNA interference (RNAi) restored host-seeking behaviour in these females, firmly establishing that Vg-2 gene expression has a pivotal role in regulating host-seeking behaviour in young Ae. albopictus females. The identification of a molecular mechanism regulating host-seeking behaviour in mosquitoes could pave the way for novel vector control strategies aiming to reduce the biting activity of mosquitoes. From an evolutionary perspective, this is the first demonstration of vitellogenin genes controlling feeding-related behaviours in nonsocial insects, while vitellogenins are known to regulate caste-specific foraging and brood-care behaviours in eusocial insects. Hence, this work confirms the key role of vitellogenin in controlling feeding-related behaviours in distantly related insect orders, suggesting that this function could be more ubiquitous than previously thought.

Fresh-blood-free diet for rearing malaria mosquito vectors

Mosquito breeding depends on the supply of fresh vertebrate blood, a major bottleneck for large-scale production of Anopheles spp. Feeding alternatives to fresh blood are thus a priority for research, outdoor large-cage trials and control interventions. Several artificial meal compositions were tested and Anopheles oogenesis, egg laying and development into the next generation of adult mosquitoes were followed. We identified blood-substitute-diets that supported ovarian development, egg maturation and fertility as well as, low progeny larval mortality, and normal development of offspring into adult mosquitoes. The formulated diet is an effective artificial meal, free of fresh blood that mimics a vertebrate blood meal and represents an important advance for the sustainability of Anopheles mosquito rearing in captivity.

Artificial blood feeding for Culicidae colony maintenance in laboratories: does the blood source condition matter?

Culicidae colonization in laboratory is paramount to conduct studies aiming at a better understanding of mosquitoes' capacity to transmit pathogens that cause deadly diseases. Colonization requires female blood feeding, a necessary step for maturation of female's oocytes. Direct blood feeding on anesthetized mammals implies in a number of disadvantages when compared to artificial blood feeding. Consequently, laboratories worldwide have been trying to -feed female mosquitoes artificially in order to replace direct feeding. In this study, we compared the effects of direct blood feeding and artificial blood feeding on important life traits of three Culicidae species. Artificial feeding was performed using citrated or defibrinated sheep blood and citrated or defibrinated rabbit blood. Direct feeding was performed using anesthetized guinea pigs as the blood source and the experiment control. Results indicated that artificial feeding using sheep blood was not good enough to justify its use in the maintenance of laboratory colonies of Culicidae. However, artificial feeding using rabbit blood maintained a recovery rate always very close to the control, especially when blood was citrated. We concluded that artificial feeding using citrated rabbit blood can substitute direct feeding on mammals reducing the use of animals, eliminating the need to maintain a bioterium in the laboratory and reducing costs in scientific researches involving Culicidae vectors.

Differential accumulation of leucine and methionine in red and green pea aphids leads to different fecundity in response to nitrogen fertilization

BACKGROUND

Nitrogen fertilization affects plants directly and herbivorous insects indirectly. Although insect species and even genotypes are known to differ in their responses to nitrogen fertilization, the physiological and molecular mechanisms remain unclear. This study assessed the fecundity and related regulatory signaling pathways in the green and red morphs of pea aphid (Acyrthosiphon pisum) feeding on Medicago truncatula with and without nitrogen fertilization.

RESULTS

Nitrogen fertilization significantly increased foliar amino acid concentrations and consequently increased the concentrations of several individual essential amino acids in body tissue of the green morph. The increased concentration of Leu, Ile, Met and Val was consistent with enhanced biosynthesis of these amino acids in the endosymbiont Buchnera. Under nitrogen fertilization, Leu and Met accumulated in the green morph enhanced the target of rapamycin (TOR) signaling pathway, which consequently increased fecundity by promoting vitellogenin synthesis. In the red morph, however, nitrogen fertilization did not change the concentration of essential amino acids, TOR signaling or fecundity.

CONCLUSION

Specific amino acids accumulation and the nutrient transduction pathway in pea aphids are responsible for genotype-specific fecundity in response to nitrogen fertilization, which could be used as potential target for pest control. © 2018 Society of Chemical Industry.

Diverse Host and Restriction Factors Regulate Mosquito-Pathogen Interactions

Mosquitoes transmit diseases that seriously impact global human health. Despite extensive knowledge of the life cycles of mosquito-borne parasites and viruses within their hosts, control strategies have proven insufficient to halt their spread. An understanding of the relationships established between such pathogens and the host tissues they inhabit is therefore paramount for the development of new strategies that specifically target these interactions, to prevent the pathogens' maturation and transmission. Here we present an updated account of the antagonists and host factors that affect the development of Plasmodium, the parasite causing malaria, and mosquito-borne viruses, such as dengue virus and Zika virus, within their mosquito vectors, and we discuss the similarities and differences between Plasmodium and viral systems, looking toward the elucidation of new targets for disease control.

Molecular characterization and functional analysis of components of the TOR pathway of the salmon louse, Lepeophtheirus salmonis (Krøyer, 1838)

The salmon louse Lepeophtheirus salmonis (Copepods, Caligida) is a marine ectoparasite infecting salmonid fishes in the northern hemisphere. At present, salmon lice infections are the most severe disease problem in the salmon farming industry causing significant economic losses. Due to development of resistance towards available chemotherapeutants, it is clear that new chemotherapeutants or non-chemical control methods are essential to manage the parasite in the future. The TOR signaling pathway is present in all metazoans and is a major regulator of cellular activity according to nutrient availability. In this study, we identified the TOR pathway genes in salmon louse; LsTSC1, LsTSC2, LsRheb, LsTOR, LsRaptor and LsRictor. RNA interference mediated gene silencing was performed to elucidate the functional role of each member of the pathway. Our results show that interference of the TOR signaling pathway either directly or indirectly inhibits many biological processes including egg maturation. In addition, the effect of gene knock-down results in more comprehensive physiological defects when targeting TORC1 and the upstream regulator Rheb. This is the first report on the TOR pathway in the salmon louse and that our research contributes to the basic knowledge of the parasite that could lead to development of novel treatment methods.

The non-canonical Notch signaling is essential for the control of fertility in Aedes aegypti

The Notch signaling pathway is a highly evolutionarily-conserved cell-cell signaling pathway that regulates many events during development. It plays a pivotal role in the regulation of fundamental cellular processes, such as cell proliferation, stem cell maintenance, and differentiation during embryonic and adult development. However, functions of Notch signaling in Aedes aegypti, the major mosquito vector for dengue, are largely unknown. In this study, we identified a unique feature of A. aegypti Notch (AaNotch) in the control of the sterile-like phenotype in female mosquitoes. Silencing AaNotch with a reverse genetic approach significantly reduced the fecundity and fertility of the mosquito. Silencing AaNotch also resulted in the prevention of micropyle formation, which led to impaired fertilization. In addition, JNK phosphorylation (a signaling molecule in the non-canonical Notch signaling pathway) was inhibited in the absence of AaNotch. Furthermore, treatment with a JNK inhibitor in the mosquito resulted in impaired fecundity and fertility. Taken together, our results demonstrate that non-canonical Notch signaling is essential for controlling fertility in the A. aegypti mosquito.

Regulatory Pathways Controlling Female Insect Reproduction

The synthesis of vitellogenin and its uptake by maturing oocytes during egg maturation are essential for successful female reproduction. These events are regulated by the juvenile hormones and ecdysteroids and by the nutritional signaling pathway regulated by neuropeptides. Juvenile hormones act as gonadotropins, regulating vitellogenesis in most insects, but ecdysteroids control this process in Diptera and some Hymenoptera and Lepidoptera. The complex crosstalk between the juvenile hormones, ecdysteroids, and nutritional signaling pathways differs distinctly depending on the reproductive strategies adopted by various insects. Molecular studies within the past decade have revealed much about the relationships among, and the role of, these pathways with respect to regulation of insect reproduction. Here, we review the role of juvenile hormones, ecdysteroids, and nutritional signaling, along with that of microRNAs, in regulating female insect reproduction at the molecular level.

Fat Body Organ Culture System in Aedes Aegypti, a Vector of Zika Virus

The insect fat body plays a central role in insect metabolism and nutrient storage, mirroring functions of the liver and fat tissue in vertebrates. Insect fat body tissue is usually distributed throughout the insect body. However, it is often concentrated in the abdomen and attached to the abdominal body wall.

The mosquito fat body is the sole source of yolk proteins, which are critical for egg production. Therefore, the in vitro culture of mosquito fat body tissues represents an important system for the study of mosquito physiology, metabolism, and, ultimately, egg production. The fat body culture process begins with the preparation of solutions and reagents, including amino acid stock solutions, Aedes physiological saline salt stock solution (APS), calcium stock solution, and fat body culture medium. The process continues with fat body dissection, followed by an experimental treatment. After treatment, a variety of different analyses can be performed, including RNA sequencing (RNA-Seq), qPCR, Western blots, proteomics, and metabolomics.

In our example experiment, we demonstrate the protocol through the excision and culture of fat bodies from the yellow fever mosquito, Aedes aegypti, a principal vector of arboviruses including dengue, chikungunya, and Zika. RNA from fat bodies cultured under a physiological condition known to upregulate yolk proteins versus the control were subject to RNA-Seq analysis to demonstrate the potential utility of this procedure for investigations of gene expression.

Amino acid metabolic signaling influences Aedes aegypti midgut microbiome variability

The mosquito midgut microbiota has been shown to influence vector competence for multiple human pathogens. The microbiota is highly variable in the field, and the sources of this variability are not well understood, which limits our ability to understand or predict its effects on pathogen transmission. In this work, we report significant variation in female adult midgut bacterial load between strains of A. aegypti which vary in their susceptibility to dengue virus. Composition of the midgut microbiome was similar overall between the strains, with 81-92% of reads coming from the same five bacterial families, though we did detect differences in the presence of some bacterial families including Flavobacteriaceae and Entobacteriaceae. We conducted transcriptomic analysis on the two mosquito strains that showed the greatest difference in bacterial load, and found that they differ in transcript abundance of many genes implicated in amino acid metabolism, in particular the branched chain amino acid degradation pathway. We then silenced this pathway by targeting multiple genes using RNA interference, which resulted in strain-specific bacterial proliferation, thereby eliminating the difference in midgut bacterial load between the strains. This suggests that the branched chain amino acid (BCAA) degradation pathway controls midgut bacterial load, though the mechanism underlying this remains unclear. Overall, our results indicate that amino acid metabolism can act to influence the midgut microbiota. Moreover, they suggest that genetic or physiological variation in BCAA degradation pathway activity may in part explain midgut microbiota variation in the field.

Genome-wide Identification and Expression Analysis of Amino Acid Transporters in the Whitefly, Bemisia tabaci (Gennadius)

The whitefly (Bemisia tabaci) is a cosmopolitan and devastating pest of agricultural crops and ornamentals. B. tabaci causes extensive damage by feeding on phloem and by transmitting plant viruses. Like many other organisms, insects depend on amino acid transporters (AATs) to transport amino acids into and out of its cells. We present a genome-wide and transcriptome-wide investigation of the following two families of AATs in B. tabaci biotype B: amino acid/auxin permease (AAAP) and amino acid/polyamine/organocation (APC). A total of 14 putative APCs and 25 putative AAAPs were identified, and a 10-paralog B. tabaci-specific expansion of AAAPs was found by maximum likelihood phylogeny. Detailed gene structure information revealed that 9 members of the B. tabaci-specific AAAP family expansion closely situated on a same scaffold. Expression profiling of the B. tabaci B APC and AAAP genes as affected by stage and plant host showed diverse expression patterns. The analysis of evolutionary rates indicated that purifying selection can explain the B. tabaci-specific AAAP expansion. RNA interference (RNAi)-mediated suppression of two AAAP genes (BtAAAP15 and BtAAAP21) significantly increased the mortality of B. tabaci B adults. The results provide a foundation for future functional analysis of APC and AAAP genes in B. tabaci.

RNA interference in mosquito: understanding immune responses, double-stranded RNA delivery systems and potential applications in vector control

RNA interference (RNAi) refers to the process of post-transcriptional silencing of cellular mRNA by the application of double-stranded RNA (dsRNA). RNAi strategies have been widely employed to regulate gene expression in plants and animals including insects. With the availability of the full genome sequences of major vector mosquitoes, RNAi has been increasingly used to conduct genetic studies of human pathogens in mosquito vectors and to study the evolution of insecticide resistance in mosquitoes. This review summarizes the recent progress in our understanding of mosquito-pathogen interactions using RNAi and various methods of dsRNA delivery in mosquitoes at different stages. We also discuss potential applications of this technology to develop novel tools for vector control.

Effect of RNAi-mediated knockdown of NlTOR gene on fertility of male Nilaparvata lugens

The target gene of rapamycin (TOR) is conserved from invertebrates to vertebrates, and plays critical roles in cell growth, nutrient sensing, lifespan and reproduction. In this paper, we employed RNA interference (RNAi) to study the function of TOR in male brown planthoppers (BPH), Nilaparvata lugens. Here we discovered that no offspring was produced when wildtype females BPH mated with NlTOR RNAi males. To understand the influence of NlTOR on male BPH infertility, we dissected the reproductive system of a NlTOR RNAi male, and found that the testes were normal and the seminal vesicles were full of sperm, while the accessory gland was poorly developed after knocking down NlTOR expression. Furthermore, transmission electron microscopy data showed defective structure of epithelial cells of the accessory gland, and the deferent duct was collapsed. Also, behavioral data revealed that wildtype virgin females refused to mate with NlTOR RNAi males, and the acoustic signals of dsNlTOR males differed from controls. Our results indicate that NlTOR plays an essential role in the development of the male BPH reproductive system and regulates mating behavior.

Hormone and receptor interplay in the regulation of mosquito lipid metabolism

Mosquitoes transmit devastating human diseases because they need vertebrate blood for egg development. Metabolism in female mosquitoes is tightly coupled with blood meal-mediated reproduction, which requires an extremely high level of energy consumption. Functional analysis has shown that major genes encoding for enzymes involved in lipid metabolism (LM) in the mosquito fat bodies are down-regulated at the end of the juvenile hormone (JH)-controlled posteclosion (PE) phase but exhibit significant elevation in their transcript levels during the post-blood meal phase (PBM), which is regulated mainly by 20-hydroxyecdysone (20E). Reductions in the transcript levels of genes encoding triacylglycerol (TAG) catabolism and β-oxidation enzymes were observed to correlate with a dramatic accumulation of lipids in the PE phase; in contrast, these transcripts were elevated significantly and lipid stores were diminished during the PBM phase. The RNAi depletion of Methoprene-tolerant (Met) and ecdysone receptor (EcR), receptors for JH and 20E, respectively, reversed the LM gene expression and the levels of lipid stores and metabolites, demonstrating the critical roles of these hormones in LM regulation. Hepatocyte nuclear factor 4 (HNF4) RNAi-silenced mosquitoes exhibited down-regulation of the gene transcripts encoding TAG catabolism and β-oxidation enzymes and an inability to use lipids effectively, as manifested by TAG accumulation. The luciferase reporter assay showed direct regulation of LM-related genes by HNF4. Moreover, HNF4 gene expression was down-regulated by Met and activated by EcR and Target of rapamycin, providing a link between nutritional and hormonal regulation of LM in female mosquitoes.

Incomplete removal of Wolbachia with tetracycline has two-edged reproductive effects in the thelytokous wasp Encarsia formosa (Hymenoptera: Aphelinidae)

Wolbachia pipientis are intracellular endosymbionts that induce parthenogenesis in the parasitoid Encarsia formosa. Previous studies that focused on effects of Wolbachia on the wasp usually used tetracycline to remove Wolbachia without concern for the joint influences of tetracycline and Wolbachia. Here we treated the wasps (F0 lines) with tetracycline to produce offspring (F1 lines) which were not fed tetracycline to avoid antibiotic influence. The quantitative data and fluorescence in situ hybridization showed that Wolbachia titers were reduced but not totally removed. The Wolbachia that infected the male offspring were unpredictably detected. Low dose tetracycline enhanced the fertility of 2-day-old F0 wasps after 24 h of treatment; however, compared with controls, the oocyte load of 3- to 6-day-old tetracycline-treated wasps decreased day by day, and tetracycline reduced the longevity of the wasps. The fecundity of controls was significantly higher than that of the treated F1-10 and F1-20 generations. Gene expression of vitellogenin reflected the same trend as that of wasp fecundities in both F0 and F1 lines. Moreover, female offspring proportions of F0 and F1 lines were related to the titer of infected Wolbachia, demonstrating that Wolbachia titer affected the sex determination of E. formosa.

RNA Interference for Mosquito and Mosquito-Borne Disease Control

RNA interference (RNAi) is a powerful tool to silence endogenous mosquito and mosquito-borne pathogen genes in vivo. As the number of studies utilizing RNAi in basic research grows, so too does the arsenal of physiological targets that can be developed into products that interrupt mosquito life cycles and behaviors and, thereby, relieve the burden of mosquitoes on human health and well-being. As this technology becomes more viable for use in beneficial and pest insect management in agricultural settings, it is exciting to consider its role in public health entomology. Existing and burgeoning strategies for insecticide delivery could be adapted to function as RNAi trigger delivery systems and thereby expedite transformation of RNAi from the lab to the field for mosquito control. Taken together, development of RNAi-based vector and pathogen management techniques & strategies are within reach. That said, tools for successful RNAi design, studies exploring RNAi in the context of vector control, and studies demonstrating field efficacy of RNAi trigger delivery have yet to be honed and/or developed for mosquito control.

Artificial Diets for Mosquitoes

Mosquito-borne diseases are responsible for more than a million human deaths every year. Modern mosquito control strategies such as sterile insect technique (SIT), release of insects carrying a dominant lethal (RIDL), population replacement strategies (PR), and Wolbachia-based strategies require the rearing of large numbers of mosquitoes in culture for continuous release over an extended period of time. Anautogenous mosquitoes require essential nutrients for egg production, which they obtain through the acquisition and digestion of a protein-rich blood meal. Therefore, mosquito mass production in laboratories and other facilities relies on vertebrate blood from live animal hosts. However, vertebrate blood is expensive to acquire and hard to store for longer times especially under field conditions. This review discusses older and recent studies that were aimed at the development of artificial diets for mosquitoes in order to replace vertebrate blood.

Serotonin signaling mediates protein valuation and aging

Research into how protein restriction improves organismal health and lengthens lifespan has largely focused on cell-autonomous processes. In certain instances, however, nutrient effects on lifespan are independent of consumption, leading us to test the hypothesis that central, cell non-autonomous processes are important protein restriction regulators. We characterized a transient feeding preference for dietary protein after modest starvation in the fruit fly, Drosophila melanogaster, and identified tryptophan hydroxylase (Trh), serotonin receptor 2a (5HT2a), and the solute carrier 7-family amino acid transporter, JhI-21, as required for this preference through their role in establishing protein value. Disruption of any one of these genes increased lifespan up to 90% independent of food intake suggesting the perceived value of dietary protein is a critical determinant of its effect on lifespan. Evolutionarily conserved neuromodulatory systems that define neural states of nutrient demand and reward are therefore sufficient to control aging and physiology independent of food consumption.

DOI:http://dx.doi.org/10.7554/eLife.16843.001

Limiting the amount of protein eaten, while still eating enough to avoid starving, has an unexpected effect: it can slow down aging and extend the lifespan in many animals from flies to mice. Previous work suggests that how an animal perceives food can also influence how fast the animal ages. For example, both flies and worms actually have shorter lifespans if their food intake is reduced when they can still “smell” food in their environment. However, the sensory cues that trigger changes in lifespan and the molecular mechanisms behind these effects are largely unknown.

Ro et al. therefore asked whether fruit flies recognize protein in their food, and if so, whether such a recognition system would influence how the flies age. Flies that had been deprived of food for a brief period tended to eat more protein than other flies that had not been starved. Ro et al. then revealed that serotonin, a brain chemical that can alter the activity of nerve cells, plays a key role in how fruit flies decide to feed specifically on foods that contain protein. Further experiments revealed also that flies age faster when they are allowed to interact with protein in their diet independently from other nutrients, despite eating the same amount. Disrupting any of several components involved in serotonin signaling protected the flies from this effect and led to them living almost twice as long under these conditions.

Ro et al. propose that the components of the recognition system work together to determine the reward associated with consuming protein by enhancing how much an animal values the protein in its food. As such, it is this protein reward or value – rather than just eating protein itself – that influences how quickly the fly ages. Further work is now needed to understand how the brain mechanisms that allow animals to perceive and evaluate food act to control lifespan and aging.

DOI:http://dx.doi.org/10.7554/eLife.16843.002

Interplay between insecticide-treated bed-nets and mosquito demography: implications for malaria control

Although malaria prevalence has witnessed a significant reduction within the past decade, malaria still constitutes a major health and economic problem, especially to low-income countries. Insecticide-treated nets (ITNs) remain one of the primary measures for preventing the malignant disease. Unfortunately, the success of ITN campaigns is hampered by improper use and natural decay in ITN-efficacy over time. Many models aimed at studying malaria transmission and control fail to account for this decay, as well as mosquito demography and feeding preferences exhibited by mosquitoes towards humans. Omitting these factors can misrepresent disease risk, while understanding their effects on malaria dynamics can inform control policy. We present a model for malaria dynamics that incorporates these factors, and a systematic analysis, including stability and sensitivity analyses of the model under different conditions. The model with constant ITN-efficacy exhibits a backward bifurcation emphasizing the need for sustained control measures until the basic reproduction number, R0, drops below a critical value at which control is feasible. The infectious and partially immune human populations and R0 are highly sensitive to the probability that a mosquito feeds successfully on a human, ITN coverage and the maximum biting rate of mosquitoes, irrespective of whether ITN-efficacy is constant or declines over time. This implies that ITNs play an important role in disease control. When ITN-efficacy wanes over time, we identify disease risks and corresponding ITN coverage, as well as feeding preference levels for which the disease can be controlled or eradicated. Our study leads to important insights that could assist in the design and implementation of better malaria control strategies. We conclude that ITNs that can retain their effectiveness for longer periods will be more appropriate in the fight against malaria and that making more ITNs available to highly endemic regions is necessary for malaria containment.

Substrate specificity and transport mechanism of amino-acid transceptor Slimfast from Aedes aegypti

Anautogenous mosquitoes depend on vertebrate blood as nutrient source for their eggs. A highly efficient set of membrane transporters mediates the massive movement of nutrient amino acids between mosquito tissues after a blood meal. Here we report the characterization of the amino-acid transporter Slimfast (Slif) from the yellow-fever mosquito Aedes aegypti using codon-optimized heterologous expression. Slif is a well-known component of the target-of-rapamycin signalling pathway and fat body nutrient sensor, but its substrate specificity and transport mechanism were unknown. We found that Slif transports essential cationic and neutral amino acids with preference for arginine. It has an unusual dual-affinity mechanism with only the high affinity being Na⁺ dependent. Tissue-specific expression and blood meal-dependent regulation of Slif are consistent with conveyance of essential amino acids from gut to fat body. Slif represents a novel transport system and type of transceptor for sensing and transporting essential amino acids during mosquito reproduction.

Anautogenous mosquitoes need to obtain essential amino acids from a blood meal for reproduction. Here, the authors examine the amino acid transporter Slimfast from the yellow-fever mosquito and describe both its specificity and mechanism of action.

Nutritional Control of Insect Reproduction

The amino acid-Target of Rapamycin (AA/TOR) and insulin pathways play a pivotal role in reproduction of female insects, serving as regulatory checkpoints that guarantee the sufficiency of nutrients for developing eggs. Being evolutionary older, the AA/TOR pathway functions as an initial nutritional sensor that not only activates nutritional responses in a tissue-specific manner, but is also involved in the control of insect insulin-like peptides (ILPs) secretion. Insulin and AA/TOR pathways also assert their nutritionally linked influence on reproductive events by contributing to the control of biosynthesis and secretion of juvenile hormone and ecdysone. This review covers the present status of our understanding of the contributions of AA/TOR and insulin pathways in insect reproduction.

Regulation of Gene Expression Patterns in Mosquito Reproduction

In multicellular organisms, development, growth and reproduction require coordinated expression of numerous functional and regulatory genes. Insects, in addition to being the most speciose animal group with enormous biological and economical significance, represent outstanding model organisms for studying regulation of synchronized gene expression due to their rapid development and reproduction. Disease-transmitting female mosquitoes have adapted uniquely for ingestion and utilization of the huge blood meal required for swift reproductive events to complete egg development within a 72-h period. We investigated the network of regulatory factors mediating sequential gene expression in the fat body, a multifunctional organ analogous to the vertebrate liver and adipose tissue, of the female Aedes aegypti mosquito. Transcriptomic and bioinformatics analyses revealed that ~7500 transcripts are differentially expressed in four sequential waves during the 72-h reproductive period. A combination of RNA-interference gene-silencing and in-vitro organ culture identified the major regulators for each of these waves. Amino acids (AAs) regulate the first wave of gene activation between 3 h and 12 h post-blood meal (PBM). During the second wave, between 12 h and 36 h, most genes are highly upregulated by a synergistic action of AAs, 20-hydroxyecdysone (20E) and the Ecdysone-Receptor (EcR). Between 36 h and 48 h, the third wave of gene activation—regulated mainly by HR3—occurs. Juvenile Hormone (JH) and its receptor Methoprene-Tolerant (Met) are major regulators for the final wave between 48 h and 72 h. Each of these key regulators also has repressive effects on one or more gene sets. Our study provides a better understanding of the complexity of the regulatory mechanisms related to temporal coordination of gene expression during reproduction. We have detected the novel function of 20E/EcR responsible for transcriptional repression. This study also reveals the previously unidentified large-scale effects of HR3 and JH/Met on transcriptional regulation during the termination of vitellogenesis and remodeling of the fat body.

In addition to being vectors of devastating human diseases, mosquitoes represent outstanding model organisms for studying regulatory mechanisms of differential gene expression due to their rapid reproductive cycles. About 7500 transcripts are differentially expressed in four sequential waves during the 72-h reproductive period in the fat body, a critical reproductive organ. The major regulators for these waves of gene expression are the two very important insect hormones, 20-hydroxyecdysone (20E) and Juvenile hormone (JH), their respective receptors Ecdysone Receptor (EcR) and Methoprene-Tolerant (Met), amino acids and the orphan nuclear receptor HR3. These key regulators are responsible for activation and repression of co-regulated gene sets, at different time points, within the 72-h reproductive period. Importantly, this study, apart from providing an insight into the regulatory complexity involved in the temporal coordination of gene expression, also reveals the previously unidentified roles of 20E/EcR, JH/Met and HR3 during the 72-h period post blood meal.

Temporal Coordination of Carbohydrate Metabolism during Mosquito Reproduction

Hematophagous mosquitoes serve as vectors of multiple devastating human diseases, and many unique physiological features contribute to the incredible evolutionary success of these insects. These functions place high-energy demands on a reproducing female mosquito, and carbohydrate metabolism (CM) must be synchronized with these needs. Functional analysis of metabolic gene profiling showed that major CM pathways, including glycolysis, glycogen and sugar metabolism, and citrate cycle, are dramatically repressed at post eclosion (PE) stage in mosquito fat body followed by a sharply increase at post-blood meal (PBM) stage, which were also verified by Real-time RT-PCR. Consistent to the change of transcript and protein level of CM genes, the level of glycogen, glucose and trehalose and other secondary metabolites are also periodically accumulated and degraded during the reproductive cycle respectively. Levels of triacylglycerols (TAG), which represent another important energy storage form in the mosquito fat body, followed a similar tendency. On the other hand, ATP, which is generated by catabolism of these secondary metabolites, showed an opposite trend. Additionally, we used RNA interference studies for the juvenile hormone and ecdysone receptors, Met and EcR, coupled with transcriptomics and metabolomics analyses to show that these hormone receptors function as major regulatory switches coordinating CM with the differing energy requirements of the female mosquito throughout its reproductive cycle. Our study demonstrates how, by metabolic reprogramming, a multicellular organism adapts to drastic and rapid functional changes.

Mosquitoes transmit numerous devastating human diseases due to their obligatory hematophagy that is required for the efficient reproduction. Metabolism must be synchronized with high energetic needs of a female mosquito for host seeking, blood feeding and rapid egg development. Each reproductive cycle is divided into two phases that are sequentially governed by juvenile hormone (JH) and 20-hydroxyecdysone. During the pre-blood meal phase, the JH receptor Methoprene-tolerant (Met) controls carbohydrate metabolism (CM) pathways and its RNA interference (RNAi) silencing caused up-regulation of CM enzymes at the transcript and protein levels activating glycolytic flux and depletion of storage and circulating sugars. During the second, post blood meal phase, CM was regulated by the ecdysone receptor EcR and its RNAi silencing had a dramatic effect opposite to that of Met RNAi. Thus, we show that Met and EcR function as regulatory switches coordinating carbohydrate metabolism with energetic requirements of the female mosquito reproductive cycle.

Frizzled 2 is a key component in the regulation of TOR signaling-mediated egg production in the mosquito Aedes aegypti

The Wnt signaling pathway was first discovered as a key event in embryonic development and cell polarity in Drosophila. Recently, several reports have shown that Wnt stimulates translation and cell growth by activating the mTOR pathway in mammals. Previous studies have demonstrated that the Target of Rapamycin (TOR) pathway plays an important role in mosquito vitellogenesis. However, the interactions between these two pathways are poorly understood in the mosquito. In this study, we hypothesized that factors from the TOR and Wnt signaling pathways interacted synergistically in mosquito vitellogenesis. Our results showed that silencing Aedes aegypti Frizzled 2 (AaFz2), a transmembrane receptor of the Wnt signaling pathway, decreased the fecundity of mosquitoes. We showed that AaFz2 was highly expressed at the transcriptional and translational levels in the female mosquito 6 h after a blood meal, indicating amino acid-stimulated expression of AaFz2. Notably, the phosphorylation of S6K, a downstream target of the TOR pathway, and the expression of vitellogenin were inhibited in the absence of AaFz2. A direct link was found in this study between Wnt and TOR signaling in the regulation of mosquito reproduction.

Small mosquitoes, large implications: crowding and starvation affects gene expression and nutrient accumulation in Aedes aegypti

Background

Environmental factors such as temperature, nutrient availability, and larval density determine the outcome of postembryonic development in mosquitoes. Suboptimal temperatures, crowding, and starvation during the larval phase reduce adult mosquito size, nutrient stores and affect vectorial capacity.

Methods

In this study we compared adult female Aedes aegypti, Rockefeller strain, raised under standard laboratory conditions (Large) with those raised under crowded and nutritionally deprived conditions (Small). To compare the gene expression and nutritional state of the major energy storage and metabolic organ, the fat body, we performed transcriptomics using Illumina based RNA-seq and metabolomics using GC/MS on females before and 24 hours following blood feeding.

Results

Analysis of fat body gene expression between the experimental groups revealed a large number of significantly differentially expressed genes. Transcripts related to immunity, reproduction, autophagy, several metabolic pathways; including amino acid degradation and metabolism; and membrane transport were differentially expressed. Metabolite profiling identified 60 metabolites within the fat body to be significantly affected between small and large mosquitoes, with the majority of detected free amino acids at a higher level in small mosquitoes compared to large.

Conclusions

Gene expression and metabolites in the adult fat body reflect the individual post-embryonic developmental history of a mosquito larva. These changes affect nutritional storage and utilization, immunity, and reproduction. Therefore, it is apparent that changes in larval environment due to weather conditions, nutrition availability, vector control efforts, and other factors can affect adult vectorial capacity in the field.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0863-9) contains supplementary material, which is available to authorized users.

Four-way regulation of mosquito yolk protein precursor genes by juvenile hormone-, ecdysone-, nutrient-, and insulin-like peptide signaling pathways

Anautogenous mosquito females require a meal of vertebrate blood in order to initiate the production of yolk protein precursors by the fat body. Yolk protein precursor gene expression is tightly repressed in a state-of-arrest before blood meal-related signals activate it and expression levels rise rapidly. The best understood example of yolk protein precursor gene regulation is the vitellogenin-A gene (vg) of the yellow fever mosquito Aedes aegypti. Vg-A is regulated by (1) juvenile hormone signaling, (2) the ecdysone-signaling cascade, (3) the nutrient sensitive target-of-rapamycin signaling pathway, and (4) the insulin-like peptide (ILP) signaling pathway. A plethora of new studies have refined our understanding of the regulation of yolk protein precursor genes since the last review on this topic in 2005 (Attardo et al., 2005). This review summarizes the role of these four signaling pathways in the regulation of vg-A and focuses upon new findings regarding the interplay between them on an organismal level.

Dynamic recruitment of amino acid transporters to the insect/symbiont interface

Symbiosis is well known to influence bacterial symbiont genome evolution and has recently been shown to shape eukaryotic host genomes. Intriguing patterns of host genome evolution, including remarkable numbers of gene duplications, have been observed in the pea aphid, a sap-feeding insect that relies on a bacterial endosymbiont for amino acid provisioning. Previously, we proposed that gene duplication has been important for the evolution of symbiosis based on aphid-specific gene duplication in amino acid transporters (AATs), with some paralogs highly expressed in the cells housing symbionts (bacteriocytes). Here, we use a comparative approach to test the role of gene duplication in enabling recruitment of AATs to bacteriocytes. Using genomic and transcriptomic data, we annotate AATs from sap-feeding and non sap-feeding insects and find that, like aphids, AAT gene families have undergone independent large-scale gene duplications in three of four additional sap-feeding insects. RNA-seq differential expression data indicate that, like aphids, the sap-feeding citrus mealybug possesses several lineage-specific bacteriocyte-enriched paralogs. Further, differential expression data combined with quantitative PCR support independent evolution of bacteriocyte enrichment in sap-feeding insect AATs. Although these data indicate that gene duplication is not necessary to initiate host/symbiont amino acid exchange, they support a role for gene duplication in enabling AATs to mediate novel host/symbiont interactions broadly in the sap-feeding suborder Sternorrhyncha. In combination with recent studies on other symbiotic systems, gene duplication is emerging as a general pattern in host genome evolution.

Phenylalanine metabolism regulates reproduction and parasite melanization in the malaria mosquito

The blood meal of the female malaria mosquito is a pre-requisite to egg production and also represents the transmission route for the malaria parasite. The proper and rapid assimilation of proteins and nutrients in the blood meal creates a significant metabolic challenge for the mosquito. To better understand this process we generated a global profile of metabolite changes in response to blood meal of Anopheles gambiae, using Gas Chromatography-Mass Spectrometry (GC-MS). To disrupt a key pathway of amino acid metabolism we silenced the gene phenylalanine hydroxylase (PAH) involved in the conversion of the amino acid phenylalanine into tyrosine. We observed increased levels of phenylalanine and the potentially toxic metabolites phenylpyruvate and phenyllactate as well as a reduction in the amount of tyrosine available for melanin synthesis. This in turn resulted in a significant impairment of the melanotic encapsulation response against the rodent malaria parasite Plasmodium berghei. Furthermore silencing of PAH resulted in a significant impairment of mosquito fertility associated with reduction of laid eggs, retarded vitellogenesis and impaired melanisation of the chorion. Carbidopa, an inhibitor of the downstream enzyme DOPA decarboxylase that coverts DOPA into dopamine, produced similar effects on egg melanization and hatching rate suggesting that egg chorion maturation is mainly regulated via dopamine. This study sheds new light on the role of amino acid metabolism in regulating reproduction and immunity.

Competition for amino acids between Wolbachia and the mosquito host, Aedes aegypti

The endosymbiont Wolbachia represents a promising method of dengue control, as it reduces the ability of the primary vector, the mosquito Aedes aegypti, to transmit viruses. When mosquitoes infected with the virulent Wolbachia strain wMelPop are fed non-human blood, there is a drastic reduction in mosquito fecundity and egg viability. Wolbachia has a reduced genome and is clearly dependent on its host for a wide range of nutritional needs. The fitness defects seen in wMelPop-infected A. aegypti could be explained by competition between the mosquito and the symbiont for essential blood meal nutrients, the profiles of which are suboptimal in non-human blood. Here, we examine cholesterol and amino acids as candidate molecules for competition, as they have critical roles in egg structural development and are known to vary between blood sources. We found that Wolbachia infection reduces total cholesterol levels in mosquitoes by 15-25%. We then showed that cholesterol supplementation of a rat blood meal did not improve fecundity or egg viability deficits. Conversely, amino acid supplementation of sucrose before and after a sheep blood meal led to statistically significant increases in fecundity of approximately 15-20 eggs per female and egg viability of 30-40%. This mosquito system provides the first empirical evidence of competition between Wolbachia and a host over amino acids and may suggest a general feature of Wolbachia-insect associations. These competitive processes could affect many aspects of host physiology and potentially mosquito fitness, a key concern for Wolbachia-based mosquito biocontrol.

Tyrosine pathway regulation is host-mediated in the pea aphid symbiosis during late embryonic and early larval development

BACKGROUND

Nutritional symbioses play a central role in insects' adaptation to specialized diets and in their evolutionary success. The obligatory symbiosis between the pea aphid, Acyrthosiphon pisum, and the bacterium, Buchnera aphidicola, is no exception as it enables this important agricultural pest insect to develop on a diet exclusively based on plant phloem sap. The symbiotic bacteria provide the host with essential amino acids lacking in its diet but necessary for the rapid embryonic growth seen in the parthenogenetic viviparous reproduction of aphids. The aphid furnishes, in exchange, non-essential amino acids and other important metabolites. Understanding the regulations acting on this integrated metabolic system during the development of this insect is essential in elucidating aphid biology.

RESULTS

We used a microarray-based approach to analyse gene expression in the late embryonic and the early larval stages of the pea aphid, characterizing, for the first time, the transcriptional profiles in these developmental phases. Our analyses allowed us to identify key genes in the phenylalanine, tyrosine and dopamine pathways and we identified ACYPI004243, one of the four genes encoding for the aspartate transaminase (E.C. 2.6.1.1), as specifically regulated during development. Indeed, the tyrosine biosynthetic pathway is crucial for the symbiotic metabolism as it is shared between the two partners, all the precursors being produced by B. aphidicola. Our microarray data are supported by HPLC amino acid analyses demonstrating an accumulation of tyrosine at the same developmental stages, with an up-regulation of the tyrosine biosynthetic genes. Tyrosine is also essential for the synthesis of cuticular proteins and it is an important precursor for cuticle maturation: together with the up-regulation of tyrosine biosynthesis, we observed an up-regulation of cuticular genes expression. We were also able to identify some amino acid transporter genes which are essential for the switch over to the late embryonic stages in pea aphid development.

CONCLUSIONS

Our data show that, in the development of A. pisum, a specific host gene set regulates the biosynthetic pathways of amino acids, demonstrating how the regulation of gene expression enables an insect to control the production of metabolites crucial for its own development and symbiotic metabolism.

Analysis of ovary-specific genes in relation to egg maturation and female nutritional condition in the mosquitoes Georgecraigius atropalpus and Aedes aegypti (Diptera: Culicidae)

Analysis of the reproductive physiology of anautogenous mosquitoes at the molecular level is complicated by the simultaneity of ovarian maturation and the digestion of a blood meal. In contrast to anautogenous mosquitoes, autogenous female mosquitoes can acquire greater nutrient stores as larvae and exhibit higher ovarian production of ecdysteroids at adult eclosion. These features essentially replace the role of a blood meal in provisioning the first batch of eggs and initiating egg development. To gain insight into the process of ovary maturation we first performed a transcript analysis of the obligatory autogenous mosquito Georgecraigius atropalpus (formerly Ochlerotatus atropalpus). We identified ESTs using suppressive subtractive hybridization (SSH) of transcripts from ovaries at critical times during oogenesis in the absence of blood digestion. Preliminary expression studies of genes such as apolipophorin III (APO) and oxysterol binding protein (OSBP) suggested these genes might be cued to female nutritional status. We then applied our findings to the medically important anautogenous mosquito Aedes aegypti. RNAi-based analyses of these genes in Ae. aegypti revealed a reduction in APO transcripts leads to reduced lipid levels in carcass and ovaries and that OSBP may play a role in overall lipid and sterol homeostasis. In addition to expanding our understanding of mosquito ovarian development, the continued use of a comparative approach between autogenous and anautogenous species may provide novel intervention points for the regulation of mosquito egg production.

SLC7 amino acid transporters of the yellow fever mosquito Aedes aegypti and their role in fat body TOR signaling and reproduction

BACKGROUND

An important function of the fat body in adult female mosquitoes is the conversion of blood meal derived amino acids (AA) into massive amounts of yolk protein precursors. A highly efficient transport mechanism for AAs across the plasma membrane of the fat body trophocytes is essential in order to deliver building blocks for the rapid synthesis of large amounts of these proteins. This mechanism consists in part of AA transporter proteins from the solute carrier family. These transporters have dual function; they function as transporters and participate in the nutrient signal transduction pathway that is activated in the fat body after a blood meal. In this study we focused on the solute carrier 7 family (SLC7), a family of AA transporters present in all metazoans that includes members with strong substrate specificity for cationic AAs.

METHODOLOGY/PRINCIPAL FINDINGS

We identified 11 putative SLC7 transporters in the genome sequence of Aedes aegypti. Phylogenetic analysis puts five of these in the cationic AA transporter subfamily (CAT) and six in the heterodimeric AA transporter (HAT) subfamily. All 11 A. aegypti SLC7 genes are expressed in adult females. Expression profiles are dynamic after a blood meal. We knocked down six fat body-expressed SLC7 transporters using RNAi and found that these 'knockdowns' reduced AA-induced TOR signaling. We also determined the effect these knockdowns had on the number of eggs deposited following a blood meal.

CONCLUSIONS/SIGNIFICANCE

Our analysis stresses the importance of SLC7 transporters in TOR signaling pathway and mosquito reproduction.

Distinct roles of isoforms of the heme-liganded nuclear receptor E75, an insect ortholog of the vertebrate Rev-erb, in mosquito reproduction

Mosquitoes are adapted to using vertebrate blood as a nutrient source to promote egg development and as a consequence serve as disease vectors. Blood-meal activated reproductive events in female mosquitoes are hormonally and nutritionally controlled with an insect steroid hormone 20-hydroxyecdysone (20E) playing a central role. The nuclear receptor E75 is an essential factor in the 20E genetic hierarchy, however functions of its three isoforms - E75A, E75B, and E75C - in mosquito reproduction are unclear. By means of specific RNA interference depletion of E75 isoforms, we identified their distinct roles in regulating the level and timing of expression of key genes involved in vitellogenesis in the fat body (an insect analog of vertebrate liver and adipose tissue) of the mosquito Aedes aegypti. Heme is required in a high level of expression of 20E-controlled genes in the fat body, and this heme action depends on E75. Thus, in mosquitoes, heme is an important signaling molecule, serving as a sensor of the availability of a protein meal for egg development. Disruption of this signaling pathway could be explored in the design of mosquito control approaches.

Nerveless and gutsy: intestinal nutrient sensing from invertebrates to humans

The increasingly recognized role of gastrointestinal signals in the regulation of food intake, insulin production and peripheral nutrient storage has prompted a surge of interest in studying how the gastrointestinal tract senses and responds to nutritional information. Identification of metabolically important intestinal nutrient sensors could provide potential new drug targets for the treatment of diabetes, obesity and gastrointestinal disorders. From a more fundamental perspective, the study of intestinal chemosensation is revealing novel, non-neuronal modes of communication involving differentiated epithelial cells. It is also identifying signalling mechanisms downstream of not only canonical receptors but also nutrient transporters, thereby supporting a chemosensory role for “transceptors” in the intestine. This review describes known and proposed mechanisms of intestinal carbohydrate, protein and lipid sensing, best characterized in mammalian systems. It also highlights the potential of invertebrate model systems such as C. elegans and Drosophila melanogaster by summarizing known examples of molecular evolutionary conservation. Recently developed genetic tools in Drosophila, an emerging model system for the study of physiology and metabolism, allow the temporal, spatial and high-throughput manipulation of putative intestinal sensors. Hence, fruit flies may prove particularly suited to the study of the link between intestinal nutrient sensing and metabolic homeostasis.