Assessment of Aedes albopictus reference genes for quantitative PCR at different stages of development

Stability and suitability of housekeeping genes in phlebotomine sand flies

We investigated gene expression patterns in Lutzomyia and Phlebotomus sand fly vectors of leishmaniases. Using quantitative PCR, we assessed the expression stability of potential endogenous control genes commonly used in dipterans. We analyzed Lutzomyia longipalpis and Phlebotomus papatasi samples from L3 and L4 larval stages, adult sand flies of different sexes, diets, dsRNA injection, and Leishmania infection. Six genes were evaluated: actin, α-tubulin, GAPDH, 60 S ribosomal proteins L8 and L32 (RiboL8 and RiboL32), and elongation factor 1-α (EF1-α). EF1-α was among the most stably expressed along with RiboL8 in L. longipalpis larvae and RiboL32 in adults. In P. papatasi, EF1-α and RiboL32 were the top in larvae, while EF1-α and actin were the most stable in adults. RiboL8 and actin were the most stable genes in dissected tissues and infected guts. Additionally, five primer pairs designed for L. longipalpis or P. papatasi were effective in PCR with Lutzomyia migonei, Phlebotomus duboscqi, Phlebotomus perniciosus, and Sergentomyia schwetzi cDNA. Furthermore, L. longipalpis RiboL32 and P. papatasi α-tubulin primers were suitable for qPCR with cDNA from the other four species. Our research provides tools to enhance relative gene expression studies in sand flies, facilitating the selection of endogenous control for qPCR.

Light pollution disrupts circadian clock gene expression in two mosquito vectors during their overwintering dormancy

Artificial light at night (ALAN) is an increasingly important form of environmental disturbance as it alters Light:Dark cycles that regulate daily and seasonal changes in physiology and phenology. The Northern house mosquito (Culex pipiens) and the tiger mosquito (Aedes albopictus) enter an overwintering dormancy known as diapause that is cued by short days. These two species differ in diapause strategy: Cx. pipiens diapause as adult females while Ae. albopictus enter a maternally-programmed, egg diapause. Previous studies found that ALAN inhibits diapause in both species, but the mechanism is unknown. As the circadian clock is implicated in the regulation of diapause in many insects, we examined whether exposure to ALAN altered the daily expression of core circadian cloc genes (cycle, Clock, period, timeless, cryptochrome 1, cryptochrome 2, and Par domain protein 1) in these two species when reared under short-day, diapause-inducing conditions. We found that exposure to ALAN altered the abundance of several clock genes in adult females of both species, but that clock gene rhythmicity was maintained for most genes. ALAN also had little effect on clock gene abundance in mature oocytes that were dissected from female Ae. albopictus that were reared under short day conditions. Our findings indicate that ALAN may inhibit diapause initiation through the circadian clock in two medically-important mosquitoes.

Reference Genes for Expression Analysis Using RT-qPCR in Cnaphalocrocis medinalis (Lepidoptera: Pyralidae)

Cnaphalocrocis medinalis is a destructive migratory rice pest. Although many studies have investigated its behavioral and physiological responses to environmental changes and migration-inducing factors, little is known about its molecular mechanisms. This study was conducted to select suitable RT-qPCR reference genes to facilitate future gene expression studies. Here, thirteen candidate housekeeping genes (EF1α, AK, EF1β, GAPDH, PGK, RPL13, RPL18, RPS3, 18S rRNA, TBP1, TBP2, ACT, and UCCR) were selected to evaluate their stabilities under different conditions using the ∆CT method; the geNorm, NormFinder, BestKeeper algorithms; and the online tool RefFinder. The results showed that the most stable reference genes were EF1β, PGK, and RPL18, related to developmental stages; RPS3 and RPL18 in larval tissues; EF1β and PGK in larvae feeding on different rice varieties; EF1α, EF1β, and PGK in larvae temperature treatments; PGK and RPL13, related to different adult ages; PGK, EF1α, and ACT, related to adult nutritional conditions; RPL18 and PGK, related to adult mating status; and, RPS3 and PGK, related to different adult take-off characteristics. Our results reveal reference genes that apply to various experimental conditions and will greatly improve the reliability of RT-qPCR analysis for the further study of gene function in this pest.

A comparative analysis of Aedes albopictus and Aedes aegypti subjected to diapause-inducing conditions reveals conserved and divergent aspects associated with diapause, as well as novel genes associated with its onset

Aedes albopictus and Aedes aegypti are mosquito species that are distributed worldwide and transmit diverse arboviruses of medical importance, such as those causing yellow fever, dengue, chikungunya and Zika. A. albopictus embryos may remain viable for long periods in the environment due to their ability to become dormant through quiescence or diapause, a feature that contributes to their dispersion and hinders control actions. Diapause incidence can vary among natural populations of A. albopictus, but metabolic and genetic parameters associated with its induction still need to be better defined. The present study aimed to investigate the effect of exposure to diapause-inducing conditions on several biological parameters in different populations of A. albopictus (from tropical and temperate areas) and the diapause-refractory A. aegypti (tropical and subtropical populations). As expected, only the A. albopictus populations exhibited diapause, but with a lower incidence for the population from a tropical area. Exposure to diapause-inducing conditions, however, led to a sharp reduction in fecundity for both A. albopictus and A. aegypti tropical populations, with no effect on fertility (>90%). It also led to a prolonged period as pupae for the progeny of all induced groups, with a further delay for those from temperate climates. In all those induced groups, the lipid contents in eggs and adult females were higher than in the non-induced controls, with the highest values observed for both A. albopictus groups. Three genes were selected to have their expression profile investigated: cathepsin, idgf4, and pepck. Upon exposure to diapause-inducing conditions, all three genes were upregulated in the A. albopictus embryos from the tropical region, but only idgf4 was upregulated in the temperate climate embryos. This represents a new gene associated with diapause that can be used as a target to evaluate and prevent embryonic dormancy, a possible new vector control strategy for mosquito species from temperate areas, such as A. albopictus.

A species-specific lncRNA modulates the reproductive ability of the asian tiger mosquito

Long non-coding RNA (lncRNA) research has emerged as an independent scientific field in recent years. Despite their association with critical cellular and metabolic processes in plenty of organisms, lncRNAs are still a largely unexplored area in mosquito research. We propose that they could serve as exceptional tools for pest management due to unique features they possess. These include low inter-species sequence conservation and high tissue specificity. In the present study, we investigated the role of ovary-specific lncRNAs in the reproductive ability of the Asian tiger mosquito, Aedes albopictus. Through the analysis of transcriptomic data, we identified several lncRNAs that were differentially expressed upon blood feeding; we called these genes Norma (NOn-coding RNA in Mosquito ovAries). We observed that silencing some of these Normas resulted in significant impact on mosquito fecundity and fertility. We further focused on Norma3 whose silencing resulted in 43% oviposition reduction, in smaller ovaries and 53% hatching reduction of the laid eggs, compared to anti-GFP controls. Moreover, a significant downregulation of 2 mucins withing a neighboring (∼100 Kb) mucin cluster was observed in smaller anti-Norma3 ovaries, indicating a potential mechanism of in-cis regulation between Norma3 and the mucins. Our work constitutes the first experimental proof-of-evidence connecting lncRNAs with mosquito reproduction and opens a novel path for pest management.

A salivary factor binds a cuticular protein and modulates biting by inducing morphological changes in the mosquito labrum

The mosquito proboscis is an efficient microelectromechanical system, which allows the insect to feed on vertebrate blood quickly and painlessly. Its efficiency is further enhanced by the insect saliva, although through unclear mechanisms. Here, we describe the initial trigger of an unprecedented feedback signaling pathway in Aedes mosquitoes affecting feeding behavior. We identified LIPS proteins in the saliva of Aedes mosquitoes that promote feeding in the vertebrate skin. LIPS show a new all-helical protein fold constituted by two domains. The N-terminal domain interacts with a cuticular protein (Cp19) located at the tip of the mosquito labrum. Upon interaction, the morphology of the labral cuticle changes, and this modification is most likely sensed by proprioceptive neurons. Our study identifies an additional role of mosquito saliva and underlines that the external cuticle is a possible site of key molecular interactions affecting the insect biology and its vector competence.

The Dengue Virus Nonstructural Protein 1 (NS1) Interacts with the Putative Epigenetic Regulator DIDO1 to Promote Flavivirus Replication in Mosquito Cells

Dengue virus (DENV) NS1 is a multifunctional protein essential for viral replication. To gain insights into NS1 functions in mosquito cells, the protein interactome of DENV NS1 in C6/36 cells was investigated using a proximity biotinylation system and mass spectrometry. A total of 817 mosquito targets were identified as protein-protein interacting with DENV NS1. Approximately 14% of them coincide with interactomes previously obtained in vertebrate cells, including the oligosaccharide transferase complex, the chaperonin containing TCP-1, vesicle localization, and ribosomal proteins. Notably, other protein pathways not previously reported in vertebrate cells, such as epigenetic regulation and RNA silencing, were also found in the NS1 interactome in mosquito cells. Due to the novel and strong interactions observed for NS1 and the epigenetic regulator DIDO1 (Death-Inducer Obliterator 1), the role of DIDO1 in viral replication was further explored. Interactions between NS1 and DIDO1 were corroborated in infected mosquito cells, by colocalization and proximity ligation assays. Silencing DIDO1 expression results in a significant reduction in DENV and ZIKV replication and progeny production. Comparison of transcription analysis of mock or DENV infected cells silenced for DIDO1 revealed variations in multiple gene expression pathways, including pathways associated with DENV infection such as RNA surveillance, IMD, and Toll. These results suggest that DIDO1 is a host factor involved in the negative modulation of the antiviral response necessary for flavivirus replication in mosquito cells. Our findings uncover novel mechanisms of NS1 to promote DENV and ZIKV replication, and add to the understanding of NS1 as a multifunctional protein. IMPORTANCE Dengue is the most important mosquito-borne viral disease to humans. Dengue virus NS1 is a multifunctional protein essential for replication and modulation of innate immunity. To gain insights into NS1 functions, the protein interactome of dengue virus NS1 in Aedes albopictus cells was investigated using a proximity biotinylation system and mass spectrometry. Several protein pathways, not previously observed in vertebrate cells, such as transcription and epigenetic regulation, were found as part of the NS1 interactome in mosquito cells. Among those, DIDO1 was found to be a necessary host factor for dengue and Zika virus replication in mosquito cells. Transcription analysis of infected mosquito cells silenced for DIDO1 revealed alterations of the IMD and Toll pathways, part of the antiviral response in mosquitoes. The results suggest that DIDO1 is a host factor involved in modulation of the antiviral response and necessary for flavivirus replication.

Temperate Conditions Limit Zika Virus Genome Replication

Zika virus is a mosquito-borne flavivirus known to cause severe birth defects and neuroimmunological disorders. We have previously demonstrated that mosquito transmission of Zika virus decreases with temperature. While transmission was optimized at 29°C, it was limited at cool temperatures (<22°C) due to poor virus establishment in the mosquitoes. Temperature is one of the strongest drivers of vector-borne disease transmission due to its profound effect on ectothermic mosquito vectors, viruses, and their interaction. Although there is substantial evidence of temperature effects on arbovirus replication and dissemination inside mosquitoes, little is known about whether temperature affects virus replication directly or indirectly through mosquito physiology. In order to determine the mechanisms behind temperature-induced changes in Zika virus transmission potential, we investigated different steps of the virus replication cycle in mosquito cells (C6/36) at optimal (28°C) and cool (20°C) temperatures. We found that the cool temperature did not alter Zika virus entry or translation, but it affected genome replication and reduced the amount of double-stranded RNA replication intermediates. If replication complexes were first formed at 28°C and the cells were subsequently shifted to 20°C, the late steps in the virus replication cycle were efficiently completed. These data suggest that cool temperature decreases the efficiency of Zika virus genome replication in mosquito cells. This phenotype was observed in the Asian lineage of Zika virus, while the African lineage Zika virus was less restricted at 20°C. IMPORTANCE With half of the human population at risk, arboviral diseases represent a substantial global health burden. Zika virus, previously known to cause sporadic infections in humans, emerged in the Americas in 2015 and quickly spread worldwide. There was an urgent need to better understand the disease pathogenesis and develop therapeutics and vaccines, as well as to understand, predict, and control virus transmission. In order to efficiently predict the seasonality and geography for Zika virus transmission, we need a deeper understanding of the host-pathogen interactions and how they can be altered by environmental factors such as temperature. Identifying the step in the virus replication cycle that is inhibited under cool conditions can have implications in modeling the temperature suitability for arbovirus transmission as global environmental patterns change. Understanding the link between pathogen replication and environmental conditions can potentially be exploited to develop new vector control strategies in the future.

Genome-wide identification of Aedes albopictus long noncoding RNAs and their association with dengue and Zika virus infection

The Asian tiger mosquito, Aedes albopictus (Ae. albopictus), is an important vector that transmits arboviruses such as dengue (DENV), Zika (ZIKV) and Chikungunya virus (CHIKV). Long noncoding RNAs (lncRNAs) are known to regulate various biological processes. Knowledge on Ae. albopictus lncRNAs and their functional role in virus-host interactions are still limited. Here, we identified and characterized the lncRNAs in the genome of an arbovirus vector, Ae. albopictus, and evaluated their potential involvement in DENV and ZIKV infection. We used 148 public datasets, and identified a total of 10, 867 novel lncRNA transcripts, of which 5,809, 4,139, and 919 were intergenic, intronic and antisense respectively. The Ae. albopictus lncRNAs shared many characteristics with other species such as short length, low GC content, and low sequence conservation. RNA-sequencing of Ae. albopictus cells infected with DENV and ZIKV showed that the expression of lncRNAs was altered upon virus infection. Target prediction analysis revealed that Ae. albopictus lncRNAs may regulate the expression of genes involved in immunity and other metabolic and cellular processes. To verify the role of lncRNAs in virus infection, we generated mutations in lncRNA loci using CRISPR-Cas9, and discovered that two lncRNA loci mutations, namely XLOC_029733 (novel lncRNA transcript id: lncRNA_27639.2) and LOC115270134 (known lncRNA transcript id: XR_003899061.1) resulted in enhancement of DENV and ZIKV replication. The results presented here provide an important foundation for future studies of lncRNAs and their relationship with virus infection in Ae. albopictus.

Ae. albopictus is an important vector of arboviruses such as dengue and Zika viruses. Studies on virus-host interaction at gene expression and molecular level are crucial especially in devising methods to inhibit virus replication in Aedes mosquitoes. Previous reports have shown that, besides protein-coding genes, noncoding RNAs such as lncRNAs are also involved in virus-host interaction. In this study, we report a comprehensive catalog of novel lncRNA transcripts in the genome of Ae. albopictus. We also show that the expression of lncRNAs was altered upon infection with dengue and Zika. Additionally, depletion of certain lncRNAs resulted in increased replication of dengue and Zika; hence, suggesting potential association of lncRNAs in virus infection. Results of this study provide a new avenue to the investigation of mosquito-virus interactions, especially in the aspect of noncoding genes.

Selection of Reference Genes for Quantitative Real-Time PCR in Aquatica leii (Coleoptera: Lampyridae) Under Five Different Experimental Conditions

Aquatic fireflies are important indicators of the quality of freshwater environments and key models for research on insect adaptation to freshwater environments. For these investigations, gene expression analyses using quantitative real-time PCR are heavily dependent on reliable reference genes. In this study, based on a transcriptome assembly and annotation for the aquatic firefly Aquatica leii at the adult and larval stages, 10 candidate reference genes (α-tubulin, β-tubulin, β-actin, EF1A, SDHA, UBQ, GST, GAPDH, RPS31, and RPL13A) were identified for analyses of expression stability. Quantitative real-time PCR analyses for each candidate reference genes in A. leii was conducted for four developmental stages, four adult tissue types, two adult sexes, and two ecological stressors [adults exposed to five temperatures and larvae exposed to four concentrations of benzo(a)pyrene]. Results were evaluated by three independent algorithms (geNorm, NormFinder, and BestKeeper) and one comparative algorithm (RefFinder). The expression stability of candidate reference genes in A. leii differed under various conditions. Reference genes with the most stable expressions levels in different tissues, temperatures, sexes, developmental stages, and concentrations of benzo(a)pyrene were α-tubulin, GST, β-actin, β-tubulin, and α-tubulin, respectively. Furthermore, the optimal normalization factors (NFs) for the quantification of the expression levels of target genes by quantitative real-time PCR analyses of A. leii were identified for each experimental group. In particular, NF = 2 for different tissues (α-tubulin + β-tubulin), different sexes (β-actin + EF1A), and larvae exposed to different concentrations of benzo(a)pyrene (α-tubulin + EF1A); NF = 3 for developmental stages (GST + GAPDH + SDHA) and adults exposed to different temperatures (β-tubulin + EFA + GST). In addition, we surveyed the expression profiles of two target genes (CYP3A and CSP8) in larvae exposed to different concentrations of benzo(a)pyrene and in different adult tissues. The results further validated the reliability of the reference genes. The optimal reference genes for various experimental conditions identified in these analyses provide a useful tool for ecological studies of aquatic fireflies.

Identification and Evaluation of Reference Genes for Normalization of Gene Expression in Developmental Stages, Sexes, and Tissues of Diaphania caesalis (Lepidoptera, Pyralidae)

Diaphania caesalis (Walker) is an important boring insect mainly distributed in subtropical and tropical areas and attacked tropical woody grain crops, such as starchy plants of Artocarpus. Quantitative real-time polymerase chain reaction (qRT-PCR) is a powerful approach for investigating target genes expression profiles at the transcriptional level. However, the identification and selection of internal reference genes, which is often overlooked, is the most vital step before the analysis of target gene expression by qRT-PCR. So far, the reliable internal reference genes under a certain condition of D. caesalis have not been investigated. Therefore, this study evaluated the expression stability of eight candidate reference genes including ACT, β-TUB, GAPDH, G6PDH, RPS3a, RPL13a, EF1α, and EIF4A in different developmental stages, tissues and sexes using geNorm, NormFinder and BestKeeper algorithms. To verify the stability of the recommended internal reference genes, the expression levels of DcaeOBP5 were analyzed under different treatment conditions. The results indicated that ACT, RPL13a, β-TUB, RPS3a, and EF1α were identified as the most stable reference genes for further studies on target gene expression involving different developmental stages of D. caesalis. And ACT and EIF4A were recommended as stable reference genes for different tissues. Furthermore, ACT, EF1α, and RPS3a were ranked as the best reference genes in different sexes based on three algorithms. Our research represents the critical first step to normalize qRT-PCR data and ensure the accuracy of expression of target genes involved in phylogenetic and physiological mechanism at the transcriptional level in D. caesalia.

RNA interference-mediated knockdown of 3, 4-dihydroxyphenylacetaldehyde synthase affects larval development and adult survival in the mosquito Aedes aegypti

BACKGROUND

The cuticle is an indispensable structure that protects the mosquito against adverse environmental conditions and prevents pathogen entry. While most cuticles are hard and rigid, some parts of cuticle are soft and flexible to allow movement and blood-feeding. It has been reported that 3, 4-dihydroxyphenylacetaldehyde (DOPAL) synthase is associated with flexible cuticle formation in Aedes aegypti. However, the molecular function of DOPAL synthase in the ontogenesis of mosquito remains largely unknown. In this study, we characterized gene expression profiles of DOPAL synthase and investigated its functions in larvae and female adults of Aedes agypti by RNAi.

RESULTS

Our results suggest that the expression of DOPAL synthase is different during development and the transcriptional level reached its peak at the female white pupal stage, and DOPAL synthase was more highly expressed in the cuticle and midgut than other tissues in the adult. The development process from larva to pupa was slowed down strikingly by feeding the first-instar larvae with chitosan/DOPAL synthase dsRNA nanoparticles. A qRT-PCR analysis confirmed that the dsRNA-mediated transcription of the DOPAL synthase was reduced > 50% in fourth-instar larvae. Meanwhile, larval molt was abnormal during development. Transmission electron microscopy results indicated that the formation of endocuticle and exocuticle was blocked. In addition, we detected that the dsDOPAL synthase RNA caused significant mortality when injected into the female adult mosquitoes.

CONCLUSIONS

Our findings demonstrate that DOPAL synthase plays a critical role in mosquito larval development and adult survival and suggest that DOPAL synthase could be a good candidate gene in RNAi intervention strategies in mosquito control.

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.