The Aedes aegypti genome: a comparative perspective

The updated genome of the Hungarian population of Aedes koreicus

Vector-borne diseases pose a potential risk to human and animal welfare, and understanding their spread requires genomic resources. The mosquito Aedes koreicus is an emerging vector that has been introduced into Europe more than 15 years ago but only a low quality, fragmented genome was available. In this study, we carried out additional sequencing and assembled and characterized the genome of the species to provide a background for understanding its evolution and biology. The updated genome was 1.1 Gbp long and consisted of 6099 contigs with an N50 value of 329,610 bp and a BUSCO score of 84%. We identified 22,580 genes that could be functionally annotated and paid particular attention to the identification of potential insecticide resistance genes. The assessment of the orthology of the genes indicates a high turnover at the terminal branches of the species tree of mosquitoes with complete genomes, which could contribute to the adaptation and evolutionary success of the species. These results could form the basis for numerous downstream analyzes to develop targets for the control of mosquito populations.

Neofunctionalization driven by positive selection led to the retention of the loqs2 gene encoding an Aedes specific dsRNA binding protein

BACKGROUND

Mosquito borne viruses, such as dengue, Zika, yellow fever and Chikungunya, cause millions of infections every year. These viruses are mostly transmitted by two urban-adapted mosquito species, Aedes aegypti and Aedes albopictus. Although mechanistic understanding remains largely unknown, Aedes mosquitoes may have unique adaptations that lower the impact of viral infection. Recently, we reported the identification of an Aedes specific double-stranded RNA binding protein (dsRBP), named Loqs2, that is involved in the control of infection by dengue and Zika viruses in mosquitoes. Preliminary analyses suggested that the loqs2 gene is a paralog of loquacious (loqs) and r2d2, two co-factors of the RNA interference (RNAi) pathway, a major antiviral mechanism in insects.

RESULTS

Here we analyzed the origin and evolution of loqs2. Our data suggest that loqs2 originated from two independent duplications of the first double-stranded RNA binding domain of loqs that occurred before the origin of the Aedes Stegomyia subgenus, around 31 million years ago. We show that the loqs2 gene is evolving under relaxed purifying selection at a faster pace than loqs, with evidence of neofunctionalization driven by positive selection. Accordingly, we observed that Loqs2 is localized mainly in the nucleus, different from R2D2 and both isoforms of Loqs that are cytoplasmic. In contrast to r2d2 and loqs, loqs2 expression is stage- and tissue-specific, restricted mostly to reproductive tissues in adult Ae. aegypti and Ae. albopictus. Transgenic mosquitoes engineered to express loqs2 ubiquitously undergo developmental arrest at larval stages that correlates with massive dysregulation of gene expression without major effects on microRNAs or other endogenous small RNAs, classically associated with RNA interference.

CONCLUSIONS

Our results uncover the peculiar origin and neofunctionalization of loqs2 driven by positive selection. This study shows an example of unique adaptations in Aedes mosquitoes that could ultimately help explain their effectiveness as virus vectors.

Voltage-gated proton channels in polyneopteran insects

Voltage‐gated proton channels (H_V1) are expressed in eukaryotes, including basal hexapods and polyneopteran insects. However, currently, there is little known about H_V1 channels in insects. A characteristic aspartate (Asp) that functions as the proton selectivity filter (SF) and the RxWRxxR voltage‐sensor motif are conserved structural elements in H_V1 channels. By analysing Transcriptome Shotgun Assembly (TSA) databases, we found 33 polyneopteran species meeting these structural requirements. Unexpectedly, an unusual natural variation Asp to glutamate (Glu) at SF was found in Phasmatodea and Mantophasmatodea. Additionally, we analysed the expression and function of H_V1 in the phasmatodean stick insect Extatosoma tiaratum (Et). EtH_V1 is strongly expressed in nervous tissue and shows pronounced inward proton conduction. This is the first study of a natural occurring Glu within the SF of a functional H_V1 and might be instrumental in uncovering the physiological function of H_V1 in insects.

Evolutionary superscaffolding and chromosome anchoring to improve Anopheles genome assemblies

Background

New sequencing technologies have lowered financial barriers to whole genome sequencing, but resulting assemblies are often fragmented and far from ‘finished’. Updating multi-scaffold drafts to chromosome-level status can be achieved through experimental mapping or re-sequencing efforts. Avoiding the costs associated with such approaches, comparative genomic analysis of gene order conservation (synteny) to predict scaffold neighbours (adjacencies) offers a potentially useful complementary method for improving draft assemblies.

Results

We evaluated and employed 3 gene synteny-based methods applied to 21 Anopheles mosquito assemblies to produce consensus sets of scaffold adjacencies. For subsets of the assemblies, we integrated these with additional supporting data to confirm and complement the synteny-based adjacencies: 6 with physical mapping data that anchor scaffolds to chromosome locations, 13 with paired-end RNA sequencing (RNAseq) data, and 3 with new assemblies based on re-scaffolding or long-read data. Our combined analyses produced 20 new superscaffolded assemblies with improved contiguities: 7 for which assignments of non-anchored scaffolds to chromosome arms span more than 75% of the assemblies, and a further 7 with chromosome anchoring including an 88% anchored Anopheles arabiensis assembly and, respectively, 73% and 84% anchored assemblies with comprehensively updated cytogenetic photomaps for Anopheles funestus and Anopheles stephensi.

Conclusions

Experimental data from probe mapping, RNAseq, or long-read technologies, where available, all contribute to successful upgrading of draft assemblies. Our evaluations show that gene synteny-based computational methods represent a valuable alternative or complementary approach. Our improved Anopheles reference assemblies highlight the utility of applying comparative genomics approaches to improve community genomic resources.

Hsp70/J-protein machinery from Glossina morsitans morsitans, vector of African trypanosomiasis

Tsetse flies (Glossina spp.) are the sole vectors of the protozoan parasites of the genus Trypanosoma, the causative agents of African Trypanosomiasis. Species of Glossina differ in vector competence and Glossina morsitans morsitans is associated with transmission of Trypanosoma brucei rhodesiense, which causes an acute and often fatal form of African Trypanosomiasis. Heat shock proteins are evolutionarily conserved proteins that play critical roles in proteostasis. The activity of heat shock protein 70 (Hsp70) is regulated by interactions with its J-protein (Hsp40) co-chaperones. Inhibition of these interactions are emerging as potential therapeutic targets. The assembly and annotation of the G. m. morsitans genome provided a platform to identify and characterize the Hsp70s and J-proteins, and carry out an evolutionary comparison to its well-studied eukaryotic counterparts, Drosophila melanogaster and Homo sapiens, as well as Stomoxys calcitrans, a comparator species. In our study, we identified 9 putative Hsp70 proteins and 37 putative J-proteins in G. m. morsitans. Phylogenetic analyses revealed three evolutionarily distinct groups of Hsp70s, with a closer relationship to orthologues from its blood-feeding dipteran relative Stomoxys calcitrans. G. m. morsitans also lacked the high number of heat inducible Hsp70s found in D. melanogaster. The potential localisations, functions, domain organisations and Hsp70/J-protein partnerships were also identified. A greater understanding of the heat shock 70 (Hsp70) and J-protein (Hsp40) families in G. m. morsitans could enhance our understanding of the cell biology of the tsetse fly.

Diet-Induced Nutritional Stress and Pathogen Interference in Wolbachia-Infected Aedes aegypti

The pathogen interference phenotype greatly restricts infection with dengue virus (DENV) and other pathogens in Wolbachia-infected Aedes aegypti, and is a vital component of Wolbachia-based mosquito control. Critically, the phenotype's causal mechanism is complex and poorly understood, with recent evidence suggesting that the cause may be species specific. To better understand this important phenotype, we investigated the role of diet-induced nutritional stress on interference against DENV and the avian malarial parasite Plasmodium gallinaceum in Wolbachia-infected Ae. aegypti, and on physiological processes linked to the phenotype. Wolbachia-infected mosquitoes were fed one of four different concentrations of sucrose, and then challenged with either P. gallinaceum or DENV. Interference against P. gallinaceum was significantly weakened by the change in diet however there was no effect on DENV interference. Immune gene expression and H2O2 levels have previously been linked to pathogen interference. These traits were assayed for mosquitoes on each diet using RT-qPCR and the Amplex Red Hydrogen Peroxide/Peroxidase Assay Kit, and it was observed that the change in diet did not significantly affect immune expression, but low carbohydrate levels led to a loss of ROS induction in Wolbachia-infected mosquitoes. Our data suggest that host nutrition may not influence DENV interference for Wolbachia-infected mosquitoes, but Plasmodium interference may be linked to both nutrition and oxidative stress. This pathogen-specific response to nutritional change highlights the complex nature of interactions between Wolbachia and pathogens in mosquitoes.

A maturing understanding of the composition of the insect gene repertoire

Recent insect genome sequencing initiatives have dramatically accelerated the accumulation of genomics data resources sampling species from different lineages to explore the incredible diversity of insect biology. These efforts have built a comprehensive catalogue of the insect gene repertoire, which is expanded with each newly-sequenced genome and continually refined using knowledge from cross-species comparisons and new sources of evidence. Since the sequencing of the very first insect genomes, comparative analyses have identified shared (homologous) and equivalent (orthologous) genes, as well as subsets of genes that appear to be unique. With the number of available insect genomes fast approaching one hundred, a maturing understanding of the composition of the insect gene repertoire broadly partitions it into an expected core of universally-present orthologues and a diverse array of lineage-specific and species-specific genes. While homology and orthology help to build evolutionarily-informed functional hypotheses for many genes from these newly-sequenced genomes, experimental interrogations are required to test such hypotheses and to probe the functions of genes for which homology offers no clues. Such taxonomically-restricted genes may represent the current contents of an evolutionary melting pot, out of which novel adaptations have emerged to make insects the most successful group of animals on Earth.

Cytochrome P450-encoding genes from the Heliconius genome as candidates for cyanogenesis

Cytochrome P450s are important both in the metabolism of xenobiotics and the production of compounds such as cyanogenic glucosides, which insects use in their defence. In the present study, we use transcriptomic and genomic information to isolate and name P450-encoding genes from the butterfly Heliconius melpomene. We classify each of the putative genes into its appropriate superfamily and compare the distribution of P450s across sequenced insects. We also identify homologues of two P450s known to be involved in cyanogenesis in the six-spot Burnet moth, Zygaena filipendulae. Classification of Heliconius P450s should be an important step in the dissection of their role in the exploitation of their host plant, the passion vine Passiflora.

Antagonistic relationships between intron content and codon usage bias of genes in three mosquito species: functional and evolutionary implications

Genome biology of mosquitoes holds potential in developing knowledge-based control strategies against vectorborne diseases such as malaria, dengue, West Nile, and others. Although the genomes of three major vector mosquitoes have been sequenced, attempts to elucidate the relationship between intron and codon usage bias across species in phylogenetic contexts are limited. In this study, we investigated the relationship between intron content and codon bias of orthologous genes among three vector mosquito species. We found an antagonistic relationship between codon usage bias and the intron number of genes in each mosquito species. The pattern is further evident among the intronless and the intron-containing orthologous genes associated with either low or high codon bias among the three species. Furthermore, the covariance between codon bias and intron number has a directional component associated with the species phylogeny when compared with other nonmosquito insects. By applying a maximum likelihood-based continuous regression method, we show that codon bias and intron content of genes vary among the insects in a phylogeny-dependent manner, but with no evidence of adaptive radiation or species-specific adaptation. We discuss the functional and evolutionary significance of antagonistic relationships between intron content and codon bias.

Gene amplification, ABC transporters and cytochrome P450s: unraveling the molecular basis of pyrethroid resistance in the dengue vector, Aedes aegypti

Background

Pyrethroid insecticides are widely utilized in dengue control. However, the major vector, Aedes aegypti, is becoming increasingly resistant to these insecticides and this is impacting on the efficacy of control measures. The near complete transcriptome of two pyrethroid resistant populations from the Caribbean was examined to explore the molecular basis of this resistance.

Principal Findings

Two previously described target site mutations, 1016I and 1534C were detected in pyrethroid resistant populations from Grand Cayman and Cuba. In addition between two and five per cent of the Ae. aegypti transcriptome was differentially expressed in the resistant populations compared to a laboratory susceptible population. Approximately 20 per cent of the genes over-expressed in resistant mosquitoes were up-regulated in both Caribbean populations (107 genes). Genes with putative monooxygenase activity were significantly over represented in the up-regulated subset, including five CYP9 P450 genes. Quantitative PCR was used to confirm the higher transcript levels of multiple cytochrome P450 genes from the CYP9J family and an ATP binding cassette transporter. Over expression of two genes, CYP9J26 and ABCB4, is due, at least in part, to gene amplification.

Significance

These results, and those from other studies, strongly suggest that increases in the amount of the CYP9J cytochrome P450s are an important mechanism of pyrethroid resistance in Ae. aegypti. The genetic redundancy resulting from the expansion of this gene family makes it unlikely that a single gene or mutation responsible for pyrethroid resistance will be identified in this mosquito species. However, the results from this study do pave the way for the development of new pyrethroid synergists and improved resistance diagnostics. The role of copy number polymorphisms in detoxification and transporter genes in providing protection against insecticide exposure requires further investigation.

Dengue is the most rapidly spreading arboviral infection of humans and each year there are 50–100 million cases of dengue fever. There is no vaccine or drug to prevent dengue infection so control of the mosquitoes that transmit this virus is the only option to reduce transmission. Removing mosquito habitats close to human homes can be effective but in reality most dengue control programmes rely on a small number of chemical insecticides. Therefore, when the mosquito vectors develop resistance to the available insecticides, dengue control is jeopardized. In this study we examined the causes of resistance to the insecticide class most commonly used in mosquito control, the pyrethroids. We found that a group of genes, which have been implicated in detoxifying these insecticides in other populations of dengue vectors, were highly over expressed in both these Caribbean populations and we investigated the molecular basis of this increased expression. The next steps, which will be a considerable challenge, are to utilize this information to develop effective means of restoring insecticide susceptibility in dengue vectors.

Strain Variation in the Transcriptome of the Dengue Fever Vector, Aedes aegypti

Studies of transcriptome dynamics provide a basis for understanding functional elements of the genome and the complexity of gene regulation. The dengue vector mosquito, Aedes aegypti, exhibits great adaptability to diverse ecological conditions, is phenotypically polymorphic, and shows variation in vectorial capacity to arboviruses. Previous genome sequencing showed richness in repetitive DNA and transposable elements that can contribute to genome plasticity. Population genetic studies revealed a varying degree of worldwide genetic polymorphism. However, the extent of functional genetic polymorphism across strains is unknown. The transcriptomes of three Ae. aegypti strains, Chetumal (CTM), Rexville D-Puerto Rico (Rex-D) and Liverpool (LVP), were compared. CTM is more susceptible than Rex- D to infection by dengue virus serotype 2. A total of 4188 transcripts exhibit either no or small variation (<2-fold) among sugar-fed samples of the three strains and between sugar- and blood-fed samples within each strain, corresponding most likely to genes encoding products necessary for vital functions. Transcripts enriched in blood-fed mosquitoes encode proteins associated with catalytic activities, molecular transport, metabolism of lipids, carbohydrates and amino acids, and functions related to blood digestion and the progression of the gonotropic cycle. Significant qualitative and quantitative differences were found in individual transcripts among strains including differential representation of paralogous gene products. The majority of immunity-associated transcripts decreased in accumulation after a bloodmeal and the results are discussed in relation to the different susceptibility of CTM and Rex-D mosquitoes to DENV2 infection.

Mosquito genomics: progress and challenges

The whole-genome sequencing of mosquitoes has facilitated our understanding of fundamental biological processes at their basic molecular levels and holds potential for application to mosquito control and prevention of mosquito-borne disease transmission. Draft genome sequences are available for Anopheles gambiae, Aedes aegypti, and Culex quinquefasciatus. Collectively, these represent the major vectors of African malaria, dengue fever and yellow fever viruses, and lymphatic filariasis, respectively. Rapid advances in genome technologies have revealed detailed information on genome architecture as well as phenotype-specific transcriptomics and proteomics. These resources allow for detailed comparative analyses within and across populations as well as species. Next-generation sequencing technologies will likely promote a proliferation of genome sequences for additional mosquito species as well as for individual insects. Here we review the current status of genome research in mosquitoes and identify potential areas for further investigations.

Infectious dengue-1 virus entry into mosquito C6/36 cells

The entry of dengue virus-1 (DENV-1) strain Hawaii into mosquito C6/36 cells was analyzed using a variety of biochemical inhibitors together with electron microscopy. The treatment with ammonium chloride, chlorpromazine, dansylcadaverine and dynasore inhibited virus yields, determined by infectivity titrations, whereas nystatin and methyl-β-cyclodextrin did not have any effect. The effect of the clathrin and dynamin inhibitors on DENV-1 entry was corroborated by detection of internalized virions using immunofluorescence staining. Furthermore, electron micrographs showed the incoming virions attached to electron-dense invaginations of the plasma membrane and within coated vesicles that resembled clathrin-coated pits and vesicles, respectively. The susceptibility to clathrin and dynamin inhibitors of clinical isolates from recent outbreaks was comparable to that shown by the cell culture-adapted reference strain. Similarly, DENV-3 strain H87 and DENV-4 strain 8124 were also inhibited in the presence of ammonium chloride, chlorpromazine and dynasore, allowing conclude that the infectious entry of DENV serotypes to mosquito cells occurs by low pH-dependent clathrin-mediated endocytosis.

Coadaptation of isoacceptor tRNA genes and codon usage bias for translation efficiency in Aedes aegypti and Anopheles gambiae

The transfer RNAs (tRNAs) are essential components of translational machinery. We determined that tRNA isoacceptors (tRNAs with different anticodons but incorporating the same amino acid in protein synthesis) show differential copy number abundance, genomic distribution patterns and sequence evolution between Aedes aegypti and Anopheles gambiae mosquitoes. The tRNA-Ala genes are present in unusually high copy number in the Ae. aegypti genome but not in An. gambiae. Many of the tRNA-Ala genes of Ae. aegypti are flanked by a highly conserved sequence that is not observed in An. gambiae. The relative abundance of tRNA isoacceptor genes is correlated with preferred (or optimal) and nonpreferred (or rare) codons for ∼2-4% of the predicted protein coding genes in both species. The majority (∼74-85%) of these genes are related to pathways involved with translation, energy metabolism and carbohydrate metabolism. Our results suggest that these genes and the related pathways may be under translational selection in these mosquitoes.

Sequence-structure-function relations of the mosquito leucine-rich repeat immune proteins

Background

The discovery and characterisation of factors governing innate immune responses in insects has driven the elucidation of many immune system components in mammals and other organisms. Focusing on the immune system responses of the malaria mosquito, Anopheles gambiae, has uncovered an array of components and mechanisms involved in defence against pathogen infections. Two of these immune factors are LRIM1 and APL1C, which are leucine-rich repeat (LRR) containing proteins that activate complement-like defence responses against malaria parasites. In addition to their LRR domains, these leucine-rich repeat immune (LRIM) proteins share several structural features including signal peptides, patterns of cysteine residues, and coiled-coil domains.

Results

The identification and characterisation of genes related to LRIM1 and APL1C revealed putatively novel innate immune factors and furthered the understanding of their likely molecular functions. Genomic scans using the shared features of LRIM1 and APL1C identified more than 20 LRIM-like genes exhibiting all or most of their sequence features in each of three disease-vector mosquitoes with sequenced genomes: An. gambiae, Aedes aegypti, and Culex quinquefasciatus. Comparative sequence analyses revealed that this family of mosquito LRIM-like genes is characterised by a variable number of 6 to 14 LRRs of different lengths. The "Long" LRIM subfamily, with 10 or more LRRs, and the "Short" LRIMs, with 6 or 7 LRRs, also share the signal peptide, cysteine residue patterning, and coiled-coil sequence features of LRIM1 and APL1C. The "TM" LRIMs have a predicted C-terminal transmembrane region, and the "Coil-less" LRIMs exhibit the characteristic LRIM sequence signatures but lack the C-terminal coiled-coil domains.

Conclusions

The evolutionary plasticity of the LRIM LRR domains may provide templates for diverse recognition properties, while their coiled-coil domains could be involved in the formation of LRIM protein complexes or mediate interactions with other immune proteins. The conserved LRIM cysteine residue patterns are likely to be important for structural fold stability and the formation of protein complexes. These sequence-structure-function relations of mosquito LRIMs will serve to guide the experimental elucidation of their molecular roles in mosquito immunity.

Comparative Genomic Hybridization (CGH) reveals a neo-X chromosome and biased gene movement in stalk-eyed flies (genus Teleopsis)

Chromosomal location has a significant effect on the evolutionary dynamics of genes involved in sexual dimorphism, impacting both the pattern of sex-specific gene expression and the rate of duplication and protein evolution for these genes. For nearly all non-model organisms, however, knowledge of chromosomal gene content is minimal and difficult to obtain on a genomic scale. In this study, we utilized Comparative Genomic Hybridization (CGH), using probes designed from EST sequence, to identify genes located on the X chromosome of four species in the stalk-eyed fly genus Teleopsis. Analysis of log(2) ratio values of female-to-male hybridization intensities from the CGH microarrays for over 3,400 genes reveals a strongly bimodal distribution that clearly differentiates autosomal from X-linked genes for all four species. Genotyping of 33 and linkage mapping of 28 of these genes in Teleopsis dalmanni indicate the CGH results correctly identified chromosomal location in all cases. Syntenic comparison with Drosophila indicates that 90% of the X-linked genes in Teleopsis are homologous to genes located on chromosome 2L in Drosophila melanogaster, suggesting the formation of a nearly complete neo-X chromosome from Muller element B in the dipteran lineage leading to Teleopsis. Analysis of gene movement both relative to Drosophila and within Teleopsis indicates that gene movement is significantly associated with 1) rates of protein evolution, 2) the pattern of gene duplication, and 3) the evolution of eyespan sexual dimorphism. Overall, this study reveals that diopsids are a critical group for understanding the evolution of sex chromosomes within Diptera. In addition, we demonstrate that CGH is a useful technique for identifying chromosomal sex-linkage and should be applicable to other organisms with EST or partial genomic information.

Differential ammonia metabolism in Aedes aegypti fat body and midgut tissues

In order to understand at the tissue level how Aedes aegypti copes with toxic ammonia concentrations that result from the rapid metabolism of blood meal proteins, we investigated the incorporation of (15)N from (15)NH(4)Cl into amino acids using an in vitro tissue culture system. Fat body or midgut tissues from female mosquitoes were incubated in an Aedes saline solution supplemented with glucose and (15)NH(4)Cl for 10-40min. The media were then mixed with deuterium-labeled amino acids, dried and derivatized. The (15)N-labeled and unlabeled amino acids in each sample were quantified by mass spectrometry techniques. The results demonstrate that both tissues efficiently incorporate ammonia into amino acids, however, the specific metabolic pathways are distinct. In the fat body, the (15)N from (15)NH(4)Cl is first incorporated into the amide side chain of Gln and then into the amino group of Gln, Glu, Ala and Pro. This process mainly occurs via the glutamine synthetase (GS) and glutamate synthase (GltS) pathway. In contrast, (15)N in midgut is first incorporated into the amino group of Glu and Ala, and then into the amide side chain of Gln. Interestingly, our data show that the GS/GltS pathway is not functional in the midgut. Instead, midgut cells detoxify ammonia by glutamate dehydrogenase, alanine aminotransferase and GS. These data provide new insights into ammonia metabolism in A. aegypti mosquitoes.

Evolutionary insights into insecticide resistance gene families of Anopheles gambiae

Insecticide resistance (IR) is one of the major obstacles in insect pests and insect borne disease control strategies, the mechanism of which is known to be genetically controlled. Three major gene families (CYP, GST and COE) have been identified encoding various proteins to metabolize endogenous as well as exogenous compounds that are responsible for IR mechanisms in insects. Understanding evolutionary patterns of genes of such important functions could lead to important understanding, based on which, further studies to control various insect borne infectious diseases could be initiated. We herein utilized the whole genome sequence information of the malaria vector Anopheles gambiae and inferred evolutionary pattern of the three known IR gene families (CYP, GST and COE). The pattern of conservation of IR genes across 38 other taxa was determined to infer evolutionary pattern of these gene families. Chromosomal distribution of IR genes was ascertained and each individual gene of IR gene families was also mapped on the chromosomal arms of An. gambiae. Differential distributional and quantitative aspects of introns in each gene were determined and genetic architecture of genes from all three gene families was compared to draw differential evolution of IR gene families. Further, phylogenetic relationships among genes of each of the three gene families were also inferred. These results in correlation with chromosomal location of each gene have provided valuable information about evolutionary history of IR gene families.

Identification and characterization of heat shock 70 genes in Aedes aegypti (Diptera: Culicidae)

Heat shock genes are highly evolutionarily conserved and are expressed to varying degrees in all organisms in response to stress. Heat shock 70 (hsp70) genes have been well characterized in a number of organisms, most notably Drosophila melanogaster, but not as yet for any of the major arthropod-borne viral mosquito vectors. To identify hsp70 genes in the yellow fever mosquito, Aedes aegypti (Diptera: Culicidae), basic local alignment searches of the Ae. aegypti genome were performed using D. melanogaster Hsp70 protein sequences as query. Two clusters of six previously unannotated AaHsp70 genes were identified and found to be organized into three pairs of nearly identical open reading frames, which mapped to two genomic scaffolds. Consistent with a designation as heat shock genes, no detectable level of expression of AaHsp70 genes was observed under normal rearing conditions (28 degrees C), with robust expression observed with a heat shock of 37-39 degrees C. Northern analysis showed heat-inducible expression of putative AaHsp70 genes at all life stages and in all tissues tested in a time- and temperature-dependent manner. Monitoring of AaHsp70 gene expression levels in field-caught Ae. aegypti may serve as a general marker for stress. In addition, promoter sequences from AaHsp70 genes may be used to control the expression of transgenes in an inducible manner.

Conserved cluster organization of insect Runx genes

Runx gene family transcription factors play important regulatory roles in metazoan development. The Drosophila genome contains four Runx genes, two of which are well studied (runt and lozenge) and two of which have not been explored in depth yet (CG42267 and CG34145). In the absence of paralog-specific orthologs in vertebrates, we investigated the evolutionary conservation of the Drosophila Runx genes in the genomes of mosquito, red flour beetle, honeybee, and distantly related Bilateria. All four Drosophila Runx genes are conserved in other endopterygote insects. Strikingly, their genetic linkage and genomic arrangement is also highly conserved despite past recombination events in the same genomic region. The preservation of Runx gene cluster organization for at least 300 million years resembles that of insect Hox and selected Wnt genes. We propose a model for the conserved cluster organization of developmental gene family paralogs based on differential coduplication of regulatory elements that act over long distances.

Gene expression studies in mosquitoes

Research on gene expression in mosquitoes is motivated by both basic and applied interests. Studies of genes involved in hematophagy, reproduction, olfaction, and immune responses reveal an exquisite confluence of biological adaptations that result in these highly-successful life forms. The requirement of female mosquitoes for a bloodmeal for propagation has been exploited by a wide diversity of viral, protozoan and metazoan pathogens as part of their life cycles. Identifying genes involved in host-seeking, blood feeding and digestion, reproduction, insecticide resistance and susceptibility/refractoriness to pathogen development is expected to provide the bases for the development of novel methods to control mosquito-borne diseases. Advances in mosquito transgenesis technologies, the availability of whole genome sequence information, mass sequencing and analyses of transcriptomes and RNAi techniques will assist development of these tools as well as deepen the understanding of the underlying genetic components for biological phenomena characteristic of these insect species.