Challenges of Robust RNAi-Mediated Gene Silencing in Aedes Mosquitoes

In this study, we report the complexities and challenges associated with achieving robust RNA interference (RNAi)-mediated gene knockdown in the mosquitoes Aedes aegypti and Aedes albopictus, a pivotal approach for genetic analysis and vector control. Despite RNAi's potential for species-specific gene targeting, our independent efforts to establish oral delivery of RNAi for identifying genes critical for mosquito development and fitness encountered significant challenges, failing to reproduce previously reported potent RNAi effects. We independently evaluated a range of RNAi-inducing molecules (siRNAs, shRNAs, and dsRNAs) and administration methods (oral delivery, immersion, and microinjection) in three different laboratories. We also tested various mosquito strains and utilized microorganisms for RNA delivery. Our results reveal a pronounced inconsistency in RNAi efficacy, characterized by minimal effects on larval survival and gene expression levels in most instances despite strong published effects for the tested targets. One or multiple factors, including RNase activity in the gut, the cellular internalization and processing of RNA molecules, and the systemic dissemination of the RNAi signal, could be involved in this variability, all of which are barely understood in mosquitoes. The challenges identified in this study highlight the necessity for additional research into the underlying mechanisms of mosquito RNAi to develop more robust RNAi-based methodologies. Our findings emphasize the intricacies of RNAi application in mosquitoes, which present a substantial barrier to its utilization in genetic control strategies.

Deep orange gene editing triggers temperature-sensitive lethal phenotypes in Ceratitis capitata

BACKGROUND

The Mediterranean fruit fly, Ceratitis capitata, is a significant agricultural pest managed through area-wide integrated pest management (AW-IPM) including a sterile insect technique (SIT) component. Male-only releases increase the efficiency and cost-effectiveness of SIT programs, which can be achieved through the development of genetic sexing strains (GSS). The most successful GSS developed to date is the C. capitata VIENNA 8 GSS, constructed using classical genetic approaches and an irradiation-induced translocation with two selectable markers: the white pupae (wp) and temperature-sensitive lethal (tsl) genes. However, currently used methods for selecting suitable markers and inducing translocations are stochastic and non-specific, resulting in a laborious and time-consuming process. Recent efforts have focused on identifying the gene(s) and the causal mutation(s) for suitable phenotypes, such as wp and tsl, which could be used as selectable markers for developing a generic approach for constructing GSS. The wp gene was recently identified, and efforts have been initiated to identify the tsl gene. This study investigates Ceratitis capitata deep orange (Ccdor) as a tsl candidate gene and its potential to induce tsl phenotypes.

RESULTS

An integrated approach based on cytogenetics, genomics, bioinformatics, and gene editing was used to characterize the Ccdor. Its location was confirmed on the right arm of chromosome 5 in the putative tsl genomic region. Knock-out of Ccdor using CRISPR/Cas9-NHEJ and targeting the fourth exon resulted in lethality at mid- and late-pupal stage, while the successful application of CRISPR HDR introducing a point mutation on the sixth exon resulted in the establishment of the desired strain and two additional strains (dor 12del and dor 51dup), all of them expressing tsl phenotypes and presenting no (or minimal) fitness cost when reared at 25 °C. One of the strains exhibited complete lethality when embryos were exposed at 36 °C.

CONCLUSIONS

Gene editing of the deep orange gene in Ceratitis capitata resulted in the establishment of temperature-sensitive lethal mutant strains. The induced mutations did not significantly affect the rearing efficiency of the strains. As deep orange is a highly conserved gene, these data suggest that it can be considered a target for the development of tsl mutations which could potentially be used to develop novel genetic sexing strains in insect pests and disease vectors.

Improved piggyBac Transformation with Capped Transposase mRNA in Pest Insects

Creating transgenic insects is a key technology in insect genetics and molecular biology. A widely used instrument in insect transgenesis is the piggyBac transposase, resulting in essentially random genomic integrations. In contrast, site-specific recombinases allow the targeted integration of the transgene construct into a specific genomic target site. Both strategies, however, often face limitations due to low transgenesis efficiencies. We aimed to enhance transgenesis efficiencies by utilizing capped mRNA as a source of transposase or recombinase instead of a helper plasmid. A systematic comparison of transgenesis efficiencies in Aedes mosquitoes, as models for hard-to-transform insects, showed that suppling piggyBac transposase as mRNA increased the average transformation efficiency in Aedes aegypti from less than 5% with the plasmid source to about 50% with mRNA. Similar high activity was observed in Ae. albopictus with pBac mRNA. No efficiency differences between plasmid and mRNA were observed in recombination experiments. Furthermore, a hyperactive version of piggyBac transposase delivered as a plasmid did not improve the transformation efficiency in Ae. aegypti or the agricultural pest Drosophila suzukii. We believe that the use of mRNA has strong potential for enhancing piggyBac transformation efficiencies in other mosquitoes and important agricultural pests, such as tephritids.

Broad anti-pathogen potential of DEAD box RNA helicase eIF4A-targeting rocaglates

Inhibition of eukaryotic initiation factor 4A has been proposed as a strategy to fight pathogens. Rocaglates exhibit the highest specificities among eIF4A inhibitors, but their anti-pathogenic potential has not been comprehensively assessed across eukaryotes. In silico analysis of the substitution patterns of six eIF4A1 aa residues critical to rocaglate binding, uncovered 35 variants. Molecular docking of eIF4A:RNA:rocaglate complexes, and in vitro thermal shift assays with select recombinantly expressed eIF4A variants, revealed that sensitivity correlated with low inferred binding energies and high melting temperature shifts. In vitro testing with silvestrol validated predicted resistance in Caenorhabditis elegans and Leishmania amazonensis and predicted sensitivity in Aedes sp., Schistosoma mansoni, Trypanosoma brucei, Plasmodium falciparum, and Toxoplasma gondii. Our analysis further revealed the possibility of targeting important insect, plant, animal, and human pathogens with rocaglates. Finally, our findings might help design novel synthetic rocaglate derivatives or alternative eIF4A inhibitors to fight pathogens.

An Optimized/Scale Up-Ready Protocol for Extraction of Bacterially Produced dsRNA at Good Yield and Low Costs

Double-stranded RNA (dsRNA) can trigger RNA interference (RNAi) and lead to directed silencing of specific genes. This natural defense mechanism and RNA-based products have been explored for their potential as a sustainable and ecofriendly alternative for pest control of species of agricultural importance and disease vectors. Yet, further research, development of new products and possible applications require a cost-efficient production of dsRNA. In vivo transcription of dsRNA in bacterial cells has been widely used as a versatile and inducible system for production of dsRNA combined with a purification step required to extract the dsRNA. Here, we optimized an acidic phenol-based protocol for extraction of bacterially produced dsRNA at low cost and good yield. In this protocol, bacterial cells are efficiently lysed, with no viable bacterial cells present in the downstream steps of the purification. Furthermore, we performed a comparative dsRNA quality and yield assessment of our optimized protocol and other protocols available in the literature and confirmed the cost-efficiency of our optimized protocol by comparing the cost of extraction and yields of each extraction method.

Genomic and cytogenetic analysis of the Ceratitis capitata temperature-sensitive lethal region

Genetic sexing strains (GSS) are an important tool in support of sterile insect technique (SIT) applications against insect pests and disease vectors. The yet unknown temperature-sensitive lethal (tsl) gene and the recently identified white pupae (wp) gene have been used as selectable markers in the most successful GSS developed so far, the Ceratitis capitata (medfly) VIENNA 8 GSS. The molecular identification of the tsl gene may open the way for its use as a marker for the development of GSS in other insect pests and disease vectors of SIT importance. Prior studies have already shown that the tsl gene is located on the right arm of chromosome 5, between the wp and Zw loci (tsl genomic region). In the present study, we used genomic, transcriptomic, bioinformatic, and cytogenetic approaches to characterize and analyze this genomic region in wild-type and tsl mutant medfly strains. Our results suggested the presence of 561 genes, with 322 of them carrying SNPs and/or insertion-deletion (indel) mutations in the tsl genomic region. Furthermore, comparative transcriptomic analysis indicated the presence of 32 differentially expressed genes, and bioinformatic analysis revealed the presence of 33 orthologs with a described heat-sensitive phenotype of Drosophila melanogaster in this region. These data can be used in functional genetic studies to identify the tsl gene(s) and the causal mutation(s) responsible for the temperature-sensitive lethal phenotype in medfly, and potentially additional genes causing a similar phenotype.

Effects of antibiotics on the in vitro expression of tetracycline-off constructs and the performance of Drosophila suzukii female-killing strains

Genetic control strategies such as the Release of Insects Carrying a Dominant Lethal (RIDL) gene and Transgenic Embryonic Sexing System (TESS) have been demonstrated in the laboratory and/or deployed in the field. These strategies are based on tetracycline-off (Tet-off) systems which are regulated by antibiotics such as Tet and doxycycline (Dox). Here, we generated several Tet-off constructs carrying a reporter gene cassette mediated by a 2A peptide. Different concentrations (0.1, 10, 100, 500, and 1,000 μg/mL) and types (Tet or Dox) of antibiotics were used to evaluate their effects on the expression of the Tet-off constructs in the Drosophila S2 cells. One or both of the two concentrations, 100 and 250 μg/mL, of Tet or Dox were used to check the influence on the performances of a Drosophila suzukii wild-type strain and female-killing (FK) strains employing TESS. Specifically, the Tet-off construct for these FK strains contains a Drosophila suzukii nullo promoter to regulate the tetracycline transactivator gene and a sex-specifically spliced pro-apoptotic gene hid ^Ala4 to eliminate females. The results suggested that the in vitro expression of the Tet-off constructs was controlled by antibiotics in a dose-dependent manner. ELISA experiments were carried out identifying Tet at 34.8 ng/g in adult females that fed on food supplemented with Tet at 100 μg/mL. However, such method did not detect Tet in the eggs produced by antibiotic-treated flies. Additionally, feeding Tet to the parents showed negative impact on the fly development but not the survival in the next generation. Importantly, we demonstrated that under certain antibiotic treatments females could survive in the FK strains with different transgene activities. For the strain V229_M4f1 which showed moderate transgene activity, feeding Dox to fathers or mothers suppressed the female lethality in the next generation and feeding Tet or Dox to mothers generated long-lived female survivors. For the strain V229_M8f2 which showed weak transgene activity, feeding Tet to mothers delayed the female lethality for one generation. Therefore, for genetic control strategies employing the Tet-off system, the parental and transgenerational effects of antibiotics on the engineered lethality and insect fitness must be carefully evaluated for a safe and efficient control program.

Temperature Sensitivity of Wild-Type, Mutant and Genetic Sexing Strains of Ceratitis capitata

Area-wide integrated pest management (AW-IPM) programmes with a sterile insect technique component (SIT) are used to control populations of insect pests worldwide, including the Mediterranean fruit fly, Ceratitis capitata. SIT consists of the mass rearing, radiation-induced sterilization, handling, and release of sterile insects over the target area. Although SIT can be performed by using both sterile males and females, male-only releases significantly increase the efficiency and cost-effectiveness of SIT applications. Male-only releases can be achieved by using genetic sexing strains (GSS). The medfly VIENNA 8 GSS is based on two selectable markers, the white pupae (wp) gene, and the temperature-sensitive lethal (tsl) genes. The latter allows the elimination of females by exposing embryos to elevated temperatures. This study assessed the temperature sensitivity of twenty-seven medfly strains through a TSLT. Our results indicated significant differences among the strains regarding egg hatching as well as pupal and adult recovery rates due to the presence or absence of the tsl mutation and/or the genetic background of the strains. Our findings are discussed in the context of SIT applications, the importance of the tsl gene for developing genetic sexing strains, and climate change.

CRISPR-mediated mutagenesis of the odorant receptor co-receptor (Orco) gene disrupts olfaction-mediated behaviors in Bactrocera dorsalis

Olfaction plays an essential role in insect behavior such as host location, foraging, mating, and oviposition. The odorant receptor co-receptor (Orco) is an obligatory odorant receptor and indispensable in odor perception. Here, we characterized the Orco gene from the oriental fruit fly, Bactrocera dorsalis (Hendel), a notorious agriculture pest. The olfactory deficiency mutants were generated by editing the BdorOrco gene using the CRISPR/Cas9 system. Electroantennograms (EAG) and olfactory preference assays confirmed that BdorOrco^-/- mutant flies had reduced perception of methyl eugenol, β-caryophyllene, and ethyl acetate. Oviposition bioassays showed that the eggs laid by BdorOrco^-/- females mediated by benzothiazole and 1-octen-3-ol were significantly decreased. In addition, BdorOrco^-/- mutant flies took a significantly longer time to locate the food source compared with wild type (WT) flies. Altogether, our data indicated that Orco is essential for multiple physiological processes in B. dorsalis, and it expands our understanding of the function of insect Orco.

Radiation dose-rate is a neglected critical parameter in dose-response of insects

Reproductive sterility is the basis of the sterile insect technique (SIT) and essential for its success in the field. Numerous factors that influence dose–response in insects have been identified. However, historically the radiation dose administered has been considered a constant. Efforts aiming to standardize protocols for mosquito irradiation found that, despite carefully controlling many variable factors, there was still an unknown element responsible for differences in expected sterility levels of insects irradiated with the same dose and handling protocols. Thus, together with previous inconclusive investigations, the question arose whether dose really equals dose in terms of biological response, no matter the rate at which the dose is administered. Interestingly, the dose rate effects studied in human nuclear medicine indicated that dose rate could alter dose–response in mammalian cells. Here, we conducted experiments to better understand the interaction of dose and dose rate to assess the effects in irradiated mosquitoes. Our findings suggest that not only does dose rate alter irradiation-induced effects, but that the interaction is not linear and may change with dose. We speculate that the recombination of reactive oxygen species (ROS) in treatments with moderate to high dose rates might minimize indirect radiation-induced effects in mosquitoes and decrease sterility levels, unless dose along with its direct effects is increased. Together with further studies to identify an optimum match of dose and dose rate, these results could assist in the development of improved methods for the production of high-quality sterile mosquitoes to enhance the efficiency of SIT programs.

Mitochondrial superoxide dismutase overexpression and low oxygen conditioning hormesis improve the performance of irradiated sterile males

The Sterile Insect Technique (SIT) is a successful autocidal control method that uses ionizing radiation to sterilize insects. However, irradiation in normal atmospheric conditions can be damaging for males, because irradiation generates substantial biological oxidative stress that, combined with domestication and mass-rearing conditions, may reduce sterile male sexual competitiveness and quality. In this study, biological oxidative stress and antioxidant capacity were experimentally manipulated in Anastrepha suspensa using a combination of low-oxygen conditions and transgenic overexpression of mitochondrial superoxide dismutase (SOD2) to evaluate their role in the sexual behavior and quality of irradiated males. Our results showed that SOD2 overexpression enhances irradiated insect quality and improves male competitiveness in leks. However, the improvements in mating performance were modest, as normoxia-irradiated SOD2 males exhibited only a 22% improvement in mating success compared to normoxia-irradiated wild type males. Additionally, SOD2 overexpression did not synergistically improve the mating success of males irradiated in either hypoxia or severe hypoxia. Short-term hypoxic and severe-hypoxic conditioning hormesis, per se, increased antioxidant capacity and enhanced sexual competitiveness of irradiated males relative to non-irradiated males in leks. Our study provides valuable new information that antioxidant enzymes, particularly SOD2, have potential to improve the quality and lekking performance of sterile males used in SIT programs.

A transgenic female killing system for the genetic control of Drosophila suzukii

The spotted wing Drosophila (Drosophila suzukii) is an invasive pest of soft-skinned fruit crops. It is rapidly transmitted in Europe and North America, causing widespread agricultural losses. Genetic control strategies such as the sterile insect technique (SIT) have been proposed as environment-friendly and species-restricted approaches for this pest. However, females are inefficient agents in SIT programs. Here we report a conditional female-killing (FK) strategy based on the tetracycline-off system. We assembled sixteen genetic constructs for testing in vitro and in vivo. Twenty-four independent transgenic strains of D. suzukii were generated and tested for female-specific lethality. The strongest FK effect in the absence of tetracycline was achieved by the construct containing D. suzukii nullo promoter for early gene expression, D. suzukii pro-apoptotic gene hidAla4 for lethality, and the transformer gene intron from the Mediterranean fruit fly Ceratitis capitata for female-specific splicing. One strain carrying this construct eliminated 100% of the female offspring during embryogenesis and produced only males. However, homozygous females from these FK strains were not viable on a tetracycline-supplemented diet, possibly due to the basal expression of hidAla4. Potential improvements to the gene constructs and the use of such FK strains in an SIT program are discussed.

White pupae phenotype of tephritids is caused by parallel mutations of a MFS transporter

Mass releases of sterilized male insects, in the frame of sterile insect technique programs, have helped suppress insect pest populations since the 1950s. In the major horticultural pests Bactrocera dorsalis, Ceratitis capitata, and Zeugodacus cucurbitae, a key phenotype white pupae (wp) has been used for decades to selectively remove females before releases, yet the gene responsible remained unknown. Here, we use classical and modern genetic approaches to identify and functionally characterize causal wp^- mutations in these distantly related fruit fly species. We find that the wp phenotype is produced by parallel mutations in a single, conserved gene. CRISPR/Cas9-mediated knockout of the wp gene leads to the rapid generation of white pupae strains in C. capitata and B. tryoni. The conserved phenotype and independent nature of wp^- mutations suggest this technique can provide a generic approach to produce sexing strains in other major medical and agricultural insect pests.

Characterization of the Drosophila suzukii β2-tubulin gene and the utilization of its promoter to monitor sex separation and insemination

The Drosophila melanogaster β2-tubulin gene (Dm-β2t) controls the function of microtubules in the testis and sperm, and has been evaluated for use in biocontrol strategies based on the sterile insect technique, including sexing and the induction of male sterility. The spotted-wing Drosophila (Drosophila suzukii) is native to eastern Asia but has spread globally as an invasive pest of fruit crops, so biocontrol strategies are urgently required for this species. We therefore isolated the β2tubulin ortholog Ds-β2t from the USA laboratory strain of D. suzukii and confirmed the presence of functional motifs by aligning orthologs from multiple insects. The developmental expression profile of Ds-β2t was determined by RT-PCR using gene-specific primers and was similar to that of Dm-β2t. We then isolated the Ds-β2t promoter and used it to generate transgenic strains expressing a testis-specific fluorescent protein starting from the thirdinstar larvae. Efficient sexing was achieved based on fluorescence detection, and the transgenic males showed a similar survival rate to wild-type males. Fluorescence imaging and PCR were also used to confirm the insemination of wild-type females by transgenic males. We therefore confirm that D. suzukii strains expressing fluorescent markers under the control of the Ds-β2t promoter can be used for sexing and the confirmation of mating, and we discuss the wider potential of the Ds-β2t promoter in the context of genetic control strategies for D. suzukii.

The hAT-family transposable element, hopper, from Bactrocera dorsalis is a functional vector for insect germline transformation

Background

The hopper hAT-family transposable element isolated from the Oriental fruit fly, Bactrocera dorsalis, is distantly related to both the Drosophila hobo element and the Activator element from maize. The original 3120 bp hopper^Bd-Kah element isolated from the Kahuku wild-type strain was highly degenerate and appeared to have a mutated transposase and terminal sequences, while a second 3131 bp element, hopper^Bd-we, isolated from a white eye mutant strain had an intact transposase reading frame and terminal sequences consistent with function.

Results

The hopper^Bd-we element was tested for function by its ability to mediate germline transformation in two dipteran species other than B. dorsalis. This was achieved by creating a binary vector/helper transformation system by linking the hopper^Bd-we transposase reading frame to a D. melanogaster hsp70 promoter for a heat-inducible transposase helper plasmid, and creating vectors marked with the D. melanogaster mini-white⁺ or polyubiquitin-regulated DsRed fluorescent protein markers.

Conclusions

Both vectors were successfully used to transform D. melanogaster, and the DsRed vector was also used to transform the Caribbean fruit fly, Anastrepha suspensa, indicating a wide range of hopper function in dipteran species and, potentially, non-dipteran species. This vector provides a new tool for insect genetic modification for both functional genomic analysis and the control of insect populations.

Identification and characterization of four Drosophila suzukii cellularization genes and their promoters

Background

The spotted-wing Drosophila (Drosophila suzukii) is a widespread invasive pest that causes severe economic damage to fruit crops. The early development of D. suzukii is similar to that of other Drosophilids, but the roles of individual genes must be confirmed experimentally. Cellularization genes coordinate the onset of cell division as soon as the invagination of membranes starts around the nuclei in the syncytial blastoderm. The promoters of these genes have been used in genetic pest-control systems to express transgenes that confer embryonic lethality. Such systems could be helpful in sterile insect technique applications to ensure that sterility (bi-sex embryonic lethality) or sexing (female-specific embryonic lethality) can be achieved during mass rearing. The activity of cellularization gene promoters during embryogenesis controls the timing and dose of the lethal gene product.

Results

Here, we report the isolation of the D. suzukii cellularization genes nullo, serendipity-α, bottleneck and slow-as-molasses from a laboratory strain. Conserved motifs were identified by comparing the encoded proteins with orthologs from other Drosophilids. Expression profiling confirmed that all four are zygotic genes that are strongly expressed at the early blastoderm stage. The 5′ flanking regions from these cellularization genes were isolated, incorporated into piggyBac vectors and compared in vitro for the promoter activities. The Dsnullo promoter showed the highest activity in the cell culture assays using D. melanogaster S2 cells.

Conclusions

The similarities in the gene coding and 5′ flanking sequence as well as in the expression pattern of the four cellularization genes between D. melanogaster and D. suzukii, suggest that conserved functions may be involved in both species. The high expression level at the early blastoderm stage of the four cellularization genes were confirmed, thus their promoters can be considered in embryonic lethality systems. While the Dsnullo promoter could be a suitable candidate, all reported promoters here are subject to further in vivo analyses before constructing potential pest control systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00939-y.

Evaluation of Hydrogen Peroxide Fumigation and Heat Treatment for Standard Emergency Arthropod Inactivation in BSL-3 Insectaries

Climate change and global movements of people and goods have accelerated the spread of invasive species, including insects that vector infectious diseases, which threaten the health of more than half of the world’s population. Increasing research efforts to control these diseases include the study of vector – pathogen interactions, involving the handling of pathogen-infected vector insects under biosafety level (BSL) 2 and 3 conditions. Like microbiology BSL-3 laboratories, BSL-3 insectaries are usually subjected to fixed-term or emergency room decontamination using recognized methods such as hydrogen peroxide (H₂O₂) or formaldehyde fumigation. While these procedures have been standardized and approved for the inactivation of diverse pathogens on surfaces, to date, there are no current standards for effective room-wide inactivation of insects in BSL-3 facilities in case of an emergency such as the accidental release of a large number of infected vectors. As H₂O₂ is often used for standard room decontamination in BSL-3 facilities, we evaluated H₂O₂ fumigation as a potential standard method for the safe, room-wide deactivation of insects in BSL-3 insectaries in comparison to heat treatment. To account for physiological diversity in vector insect species, six species from three different orders were tested. For the H₂O₂ fumigation we observed a strong but also varying resilience across all species. Lethal exposure time for the tested dipterans ranged from nine to more than 24 h. Furthermore, the coleopteran, Tribolium castaneum, did not respond to continuous H₂O₂ exposure for 48 h under standard room decontamination conditions. In contrast, temperatures of 50°C effectively killed all the tested species within 2 to 10 min. The response to lower temperatures (40–48°C) again showed a strong variation between species. In summary, results suggest that H₂O₂ fumigation, especially in cases where a gas generator is part of the laboratory equipment, may be used for the inactivation of selected species but is not suitable as a general emergency insect inactivation method under normal room decontamination conditions. In contrast, heat treatment at 48 to 50°C has the potential to be developed as an approved standard procedure for the effective inactivation of insects in BSL-3 facilities.

CRISPR/Cas9 mediated disruption of the white gene leads to pigmentation deficiency and copulation failure in Drosophila suzukii

The Spotted-wing Drosophila (Drosophila suzukii) is a devastating invasive pest of fruit crops. In D. melanogaster, the white (w) gene was associated with pigmentation and mating behavior, which are also important aspects to understand the invasion biology as well as to develop control strategies for D. suzukii. Here, we show that the generation of D. suzukii white-eyed mutants by CRISPR/Cas9 mutagenesis of the w gene resulted in the complete failure of copulation when w^- males were individually paired with w^- females in small circular arenas (diameter 0.7 cm) for 24 h. Further analysis showed that the mating defect was associated with w^- males and could not be rectified by two years of inbreeding by crossing sibling w^- females with w⁺ males, dim red illumination, male-female sexual training, changing to large arenas (diameter 3.5 cm), or different sex ratios. Profound pigmentation deficiency was detected in the compound eyes, ocelli, Malpighian tubules and testis sheaths in the w^- flies. Specifically, testis imaging showed that w^- males failed to deposit any pigments into pigment cells of the testis sheath, and produced smaller sperms and less seminal fluid compared to those from wildtype males. Together these observations suggest that the w gene plays an essential role in the regulation of sexual behavior and reproduction in D. suzukii. The similarities and differences in w gene function between D. suzukii and D. melanogaster in the context of pigmentation and mating behavior are discussed.

Genetic breakdown of a Tet-off conditional lethality system for insect population control

Genetically modified conditional lethal strains have been created to improve the control of insect pest populations damaging to human health and agriculture. However, understanding the potential for the genetic breakdown of lethality systems by rare spontaneous mutations, or selection for inherent suppressors, is critical since field release studies are in progress. This knowledge gap was addressed in a Drosophila tetracycline-suppressible embryonic lethality system by analyzing the frequency and structure of primary-site spontaneous mutations and second-site suppressors resulting in heritable survivors from 1.2 million zygotes. Here we report that F₁ survivors due to primary-site deletions and indels occur at a 5.8 × 10⁻⁶ frequency, while survival due to second-site maternal-effect suppressors occur at a ~10⁻⁵ frequency. Survivors due to inherent lethal effector suppressors could result in a resistant field population, and we suggest that this risk may be mitigated by the use of dual redundant, albeit functionally unrelated, lethality systems.

Insect population control using conditional lethal systems could break down due to spontaneous mutations that render the system ineffective. Here the authors analyse the structure and frequency of such mutations in Drosophila and suggest the use of dual lethality systems to mitigate their survival.

The role of oxygen depletion and subsequent radioprotective effects during irradiation of mosquito pupae in water

BACKGROUND

Radiation induced sterility is the basis of the Sterile Insect Technique, by which a target insect pest population is suppressed by releasing artificially reared sterile males of the pest species in overflooding numbers over a target site. In order for the sterile males to be of high biological quality, effective standard irradiation protocols are required. Following studies investigating the effects of mosquito pupae irradiation in water versus in air, there is a need to investigate the oxy-regulatory behavior of mosquito pupae in water to better understand the consequences of irradiation in hypoxic versus normoxic conditions.

METHODS

Pupae of Aedes aegypti, Ae. albopictus, and Anopheles arabiensis were submerged in water inside air-tight 2 ml glass vials at a density of 100 pupae/ml and the dissolved oxygen (DO) levels in the water were measured and plotted over time. In addition, male pupae of Ae. aegypti (aged 40-44 h), Ae. albopictus (aged 40-44 h) and An. arabiensis (aged 20-24 h) were irradiated in a gammacell220 at increasing doses in either hypoxic (water with < 0.5% O₂ content) or normoxic (in air) conditions. The males were then mated to virgin females and resulting eggs were checked for induced sterility.

RESULTS

All three species depleted the water of DO to levels under 0.5% within 30 minutes, with An. arabiensis consuming oxygen the fastest at under 10 minutes. Following irradiation, the protective effect of hypoxia was observed across species and doses (P < 0.0001), increasing at higher doses. This effect was most pronounced in An. arabiensis.

CONCLUSIONS

The consumption of dissolved oxygen by pupae submerged in water was significantly different between species, indicating that their oxy-regulatory capacity seems to have possibly evolved according to their preferred breeding site characteristics. This needs to be considered when sterilizing male mosquitoes at pupal stage in water. Depending on species, their DO consumption rates and their density, irradiation doses needed to achieve full sterility may vary significantly. Further assessments are required to ascertain optimal conditions in terms of ambient atmosphere during pupal irradiation to produce competitive sterile males, and temperature and density dependent effects are expected.

Bicistronic expression and differential localization of proteins in insect cells and Drosophila suzukii using picornaviral 2A peptides

Polycistronic expression systems in insects can be used for applications such as recombinant protein production in cells, enhanced transgenesis methods, and the development of novel pest-control strategies based on the sterile insect technique (SIT). Here we tested the performance of four picornaviral 2A self-cleaving peptides (TaV-2A, DrosCV-2A, FMDV 2A1/31 and FMDV 2A1/32) for the co-expression and differential subcellular targeting of two fluorescent marker proteins in cell lines (Anastrepha suspensa AsE01 and Drosophila melanogaster S2 cells). We found that all four 2A peptides showed comparable activity in cell lines, leading to the production of independent upstream and downstream proteins that were directed to the nucleus or membrane by a C-terminal nuclear localization signal (NLS) on the upstream protein and a poly-lysine/CAAX membrane anchor on the downstream protein. TaV-2A and DrosCV-2A were inserted into piggyBac constructs to create transgenic D. suzukii strains, confirming efficient ribosomal skipping in vivo. Interestingly, we found that the EGFP-CAAX protein was distributed homogeneously in the membrane whereas the DsRed-CAAX protein formed clumps and aggregates that induced extensive membrane blebbing. Accordingly, only flies expressing the DsRed-NLS and EGFP-CAAX proteins could be bred to homozygosity whereas expression of EGFP-NLS and DsRed-CAAX was lethal in the homozygous state. Our results therefore demonstrate that the 2A constructs and two novel targeting motifs are functional in D. suzukii, and that the combination of EGFP-NLS and DsRed-CAAX shows dosage-dependent lethality. These molecular elements could be further used to improve expression systems in insects and generate novel pest control strains.

Overexpression of an antioxidant enzyme improves male mating performance after stress in a lek-mating fruit fly

In many species, courtship displays are reliable signals of male quality, and current hypotheses suggest that these displays allow females to choose males with high cellular function. Environmental stressors generate excess reactive oxygen species (ROS) that impair cellular function, and thus antioxidant pathways that remove ROS are probably critical for preserving complex sexual behaviours. Here, we test the hypothesis that enhanced antioxidant activity in mitochondria preserves mating performance following oxidative stress. Using a transgenic approach, we directly manipulated mitochondrial antioxidant activity in the Caribbean fruit fly, Anastrepha suspensa, a lek-mating species with elaborate sexual displays and intense sexual selection that is also a model for sterile insect technique programmes. We generated seven transgenic lines that overexpress mitochondrial superoxide dismutase (MnSOD). Radiation is a severe oxidative stressor used to induce sterility for sterile insect programmes. After radiation treatment, two lines with intermediate MnSOD overexpression showed enhanced mating performance relative to wild-type males. These improvements in mating corresponded with reduced oxidative damage to lipids, demonstrating that MnSOD overexpression protects flies from oxidative stress at the cellular level. For lines with improved mating performance, overexpression also preserved locomotor activity, as indicated by a laboratory climbing assay. Our results show a clear link between oxidative stress, antioxidant capacity and male performance. Our work has implications for fundamentally understanding the role of antioxidants in sexual selection, and shows promise for using transgenic approaches to enhance the field performance of insects released for area-wide pest management strategies and improving performance of biological control agents in general.

Genomic targeting by recombinase-mediated cassette exchange in the spotted wing drosophila, Drosophila suzukii

Drosophila suzukii is a significant pest of stone and small fruits. The genome of this species has been sequenced and manipulated by transposon-mediated transformation and CRISPR/Cas9 gene editing. These technologies open a variety of possibilities for functional genomics and genetic modifications that might improve biologically based population control strategies. Both of these approaches, however, would benefit from genome targeting that would avoid position effects and insertional mutations associated with random transposon vector insertions, and the limited DNA fragment insertion size allowed by gene editing. Here, we describe an efficient recombinase-mediated cassette exchange (RMCE) system for D. suzukii in which heterospecific lox recombination sites were integrated into the genome by transposon-mediated transformation and subsequently targeted for double recombination by a donor vector in the presence of Cre recombinase. Three loxN/lox2272 landing site lines have previously been created in D. suzukii, and quantitative PCR determined that polyubiquitin-regulated enhanced green fluorescent protein expression is least susceptible to position effect suppression in the 443_M26m1 line. We presume that RMCE target sites may also be inserted more specifically into the genome by homology-directed repair gene editing, thereby avoiding position effects and mutations, while eliminating restrictions on the size of donor constructs for subsequent insertion.

High-throughput analysis of insecticides on malaria vectors using liquid chromatography tandem mass spectrometry

BACKGROUND

Different setups and protocols have been developed for investigating insecticide effects on Anopheles (An.) mosquitoes, vectors of malaria. However, chemical uptake resulting from their tarsal contact with insecticide-treated material has seldom been investigated. To address the challenges encountered in the interpretation of bioassay data, a high throughput method for chemical analysis on malaria vectors was developed and validated for five selected insecticides including alpha-cypermethrin (aCYP), deltamethrin (DM), etofenprox (EPX), permethrin (PM), pirimiphos-methyl (PPM).

METHODS

The method includes a single chemical extraction step via an ultrasound probe on mosquito samples and analysis via liquid chromatography coupled to high-resolution tandem mass spectrometry (UHPLC-MS/MS). The protocol was established for two malaria vector species, Anopheles gambiae senso stricto (s.s.) and An. stephensi, both males and females. Recovery rates ranged from 70 to 100% without any influence of sex or species. The method was efficiently applied to female An. gambiae s.s. of the KISUMU1 reference strain, after susceptibility tests using the World Health Organization's standard protocol.

RESULTS

Susceptibility tests revealed 13.4-18.4 minutes knockdown times for 50% mosquitoes during exposure to EPX and pyrethroids. The mortality rates 24 hours post-exposure to insecticides were mostly 99-100%, except in two PM and three PPM assays suggesting possible or confirmed resistance to these insecticides. The mean insecticide uptake in dead mosquitoes ranged from 23 pg (aCYP) to 1812 pg (EPX) per specimen. However, the mean uptake in survivors to PM and PPM was reduced by at least 25%, suggesting that acute doses were not achieved in these specimens during bioassays.

CONCLUSIONS

The developed and validated UHPLC-MS/MS method could be used to address some limitations of bioassays or to assess the penetration of insecticides in mosquito matrix with reference to cuticle thickness and other insecticide resistance mechanisms.

Molecular tools to create new strains for mosquito sexing and vector control

Vector control programs based on population reduction by matings with mass-released sterile insects require the release of only male mosquitoes, as the release of females, even if sterile, would increase the number of biting and potentially disease-transmitting individuals. While small-scale releases demonstrated the applicability of sterile males releases to control the yellow fever mosquito Aedes aegypti, large-scale programs for mosquitoes are currently prevented by the lack of efficient sexing systems in any of the vector species.Different approaches of sexing are pursued, including classical genetic and mechanical methods of sex separation. Another strategy is the development of transgenic sexing systems. Such systems already exist in other insect pests. Genome modification tools could be used to apply similar strategies to mosquitoes. Three major tools to modify mosquito genomes are currently used: transposable elements, site-specific recombination systems, and genome editing via TALEN or CRISPR/Cas. All three can serve the purpose of developing sexing systems and vector control strains in mosquitoes in two ways: first, via their use in basic research. A better understanding of mosquito biology, including the sex-determining pathways and the involved genes can greatly facilitate the development of sexing strains. Moreover, basic research can help to identify other regulatory elements and genes potentially useful for the construction of transgenic sexing systems. Second, these genome modification tools can be used to apply the gained knowledge to build and test mosquito sexing strains for vector control.

Evaluation of horizontal gene transfer risk between the Mediterranean fruit fly Ceratitis capitata (Tephritidae) and its parasitoid Fopius ceratitivorus (Braconidae)

The transgenic strain of the Mediterranean fruit fly (medfly), Ceratitis capitata (Wied.) VIENNA 8 1260, developed from the classical genetic sexing strain VIENNA 8, has two molecular markers that exhibit red fluorescence in the body and green fluorescence in testicles and sperm. These traits offer a precise tool to discriminate between mass-reared sterile males and wild fertile males, and they could potentially increase the effectiveness of control programs for this pest. To assess the risk of horizontal transfer of the fluorescence transgenes in natural ecosystems, we used the VIENNA 8 1260 strain and the medfly parasitoid Fopius ceratitivorus. The fluorescence signal and the inheritance of the fluorescence gene markers were monitored for over 16 generations (about two years) in both species using fluorescence microscopy and a PCR-based assay. The PCR analysis was performed in four independent laboratories. Both fluorescence microscopy and PCR analysis indicated that no horizontal gene transfer of the DsRed transgene occurred during 16 generations of medfly parasitoid rearing under experimental conditions.

Highly efficient genome editing by homology-directed repair using Cas9 protein in Ceratitis capitata

The Mediterranean fruit fly Ceratitis capitata is a highly polyphagous and invasive insect pest, causing enormous economic damage in horticultural systems. A successful and environment-friendly control strategy is the sterile insect technique (SIT) that reduces pest populations through infertile matings with mass-released, sterilized insects. However, the SIT is not readily applicable to each pest species. While transgenic approaches hold great promise to improve critical aspects of the SIT to transfer it to new species, they are suspect to strict or even prohibitive legislation regarding the release of genetically modified (GM) organisms. In contrast, specific mutations created via CRISPR-Cas genome editing are not regulated as GM in the US, and might thus allow creating optimal strains for SIT. Here, we describe highly efficient homology-directed repair genome editing in C. capitata by injecting pre-assembled CRISPR-Cas9 ribonucleoprotein complexes using different guide RNAs and a short single-stranded oligodeoxynucleotide donor to convert an enhanced green fluorescent protein in C. capitata into a blue fluorescent protein. Six out of seven fertile and individually backcrossed G0 individuals generated 57-90% knock-in rate within their total offspring and 70-96% knock-in rate within their phenotypically mutant offspring. Based on the achieved efficiency, this approach could also be used to introduce mutations which do not produce a screenable phenotype and identify positive mutants with a reasonable workload. Furthermore, CRISPR-Cas HDR would allow to recreate mutations formerly identified in classical mutagenesis screens and to transfer them to related species to establish new (SIT-like) pest control systems. Considering the potential that CRISPR-induced alterations in organisms could be classified as non-GM in additional countries, such new strains could potentially be used for pest control applications without the need to struggle with GMO directives.

Cre/lox-Recombinase-Mediated Cassette Exchange for Reversible Site-Specific Genomic Targeting of the Disease Vector, Aedes aegypti

Site-specific genome modification (SSM) is an important tool for mosquito functional genomics and comparative gene expression studies, which contribute to a better understanding of mosquito biology and are thus a key to finding new strategies to eliminate vector-borne diseases. Moreover, it allows for the creation of advanced transgenic strains for vector control programs. SSM circumvents the drawbacks of transposon-mediated transgenesis, where random transgene integration into the host genome results in insertional mutagenesis and variable position effects. We applied the Cre/lox recombinase-mediated cassette exchange (RMCE) system to Aedes aegypti, the vector of dengue, chikungunya, and Zika viruses. In this context we created four target site lines for RMCE and evaluated their fitness costs. Cre-RMCE is functional in a two-step mechanism and with good efficiency in Ae. aegypti. The advantages of Cre-RMCE over existing site-specific modification systems for Ae. aegypti, phiC31-RMCE and CRISPR, originate in the preservation of the recombination sites, which 1) allows successive modifications and rapid expansion or adaptation of existing systems by repeated targeting of the same site; and 2) provides reversibility, thus allowing the excision of undesired sequences. Thereby, Cre-RMCE complements existing genomic modification tools, adding flexibility and versatility to vector genome targeting.

The whole genome sequence of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), reveals insights into the biology and adaptive evolution of a highly invasive pest species

Background

The Mediterranean fruit fly (medfly), Ceratitis capitata, is a major destructive insect pest due to its broad host range, which includes hundreds of fruits and vegetables. It exhibits a unique ability to invade and adapt to ecological niches throughout tropical and subtropical regions of the world, though medfly infestations have been prevented and controlled by the sterile insect technique (SIT) as part of integrated pest management programs (IPMs). The genetic analysis and manipulation of medfly has been subject to intensive study in an effort to improve SIT efficacy and other aspects of IPM control.

Results

The 479 Mb medfly genome is sequenced from adult flies from lines inbred for 20 generations. A high-quality assembly is achieved having a contig N50 of 45.7 kb and scaffold N50 of 4.06 Mb. In-depth curation of more than 1800 messenger RNAs shows specific gene expansions that can be related to invasiveness and host adaptation, including gene families for chemoreception, toxin and insecticide metabolism, cuticle proteins, opsins, and aquaporins. We identify genes relevant to IPM control, including those required to improve SIT.

Conclusions

The medfly genome sequence provides critical insights into the biology of one of the most serious and widespread agricultural pests. This knowledge should significantly advance the means of controlling the size and invasive potential of medfly populations. Its close relationship to Drosophila, and other insect species important to agriculture and human health, will further comparative functional and structural studies of insect genomes that should broaden our understanding of gene family evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-1049-2) contains supplementary material, which is available to authorized users.

Tetracycline-suppressible female lethality and sterility in the Mexican fruit fly, Anastrepha ludens

The sterile insect technique (SIT) involves the mass release of sterile males to suppress insect pest populations. SIT has been improved for larval pests by the development of strains for female-specific tetracycline-suppressible (Tet-off) embryonic lethal systems for male-only populations. Here we describe the extension of this approach to the Mexican fruit fly, Anastrepha ludens, using a Tet-off driver construct with the Tet-transactivator (tTA) under embryo-specific Anastrepha suspensa serendipity α (As-sry-α) promoter regulation. In the absence of tetracycline, tTA acts upon a Tet-response element linked to the pro-apoptotic cell death gene lethal effector, head involuation defective (hid), from A. ludens (Alhid(Ala2) ) that contains a sex-specific intron splicing cassette, resulting in female-specific expression of the lethal effector. Parental adults double-homozygous for the driver/effector vectors were expected to yield male-only progeny when reared on Tet-free diet, but a complete lack of oviposited eggs resulted for each of the three strains tested. Ovary dissection revealed nonvitellogenic oocytes in all strains that was reversible by feeding females tetracycline for 5 days after eclosion, resulting in male-only adults in one strain. Presumably the sry-α promoter exhibits prezygotic maternal expression as well as zygotic embryonic expression in A. ludens, resulting in a Tet-off sterility effect in addition to female-specific lethality.

Male-specific Y-linked transgene markers to enhance biologically-based control of the Mexican fruit fly, Anastrepha ludens (Diptera: Tephritidae)

BACKGROUND

Reliable marking systems are critical to the prospective field release of transgenic insect strains. This is to unambiguously distinguish released insects from wild insects in the field that are collected in field traps, and tissue-specific markers, such as those that are sperm-specific, have particular uses such as identifying wild females that have mated with released males. For tephritid fruit flies such as the Mexican fruit fly, Anastrepha ludens, polyubiquitin-regulated fluorescent protein body markers allow transgenic fly identification, and fluorescent protein genes regulated by the spermatocyte-specific β2-tubulin promoter effectively mark sperm. For sterile male release programs, both marking systems can be made male-specific by linkage to the Y chromosome.

RESULTS

An A. ludens wild type strain was genetically transformed with a piggyBac vector, pBXL{PUbnlsEGFP, Asβ2tub-DsRed.T3}, having the polyubiquitin-regulated EGFP body marker, and the β2-tubulin-regulated DsRed.T3 sperm-specific marker. Autosomal insertion lines effectively expressed both markers, but a single Y-linked insertion (YEGFP strain) expressed only PUbnlsEGFP. This insertion was remobilized by transposase helper injection, which resulted in three new autosomal insertion lines that expressed both markers. This indicated that the original Y-linked Asβ2tub-DsRed.T3 marker was functional, but specifically suppressed on the Y chromosome. The PUbnlsEGFP marker remained effective however, and the YEGFP strain was used to create a sexing strain by translocating the wild type allele of the black pupae (bp+) gene onto the Y, which was then introduced into the bp- mutant strain. This allows the mechanical separation of mutant female black pupae from male brown pupae, that can be identified as adults by EGFP fluorescence.

CONCLUSIONS

A Y-linked insertion of the pBXL{PUbnlsEGFP, Asβ2tub-DsRed.T3} transformation vector in A. ludens resulted in male-specific expression of the EGFP fluorescent protein marker, and was integrated into a black pupae translocation sexing strain (T(YEGFP/bp+), allowing the identification of male adults when used in sterile male release programs for population control. A unique observation was that expression of the Asβ2tub-DsRed.T3 sperm-specific marker, which was functional in autosomal insertions, was specifically suppressed in the Y-linked insertion. This may relate to the Y chromosomal regulation of male-specific germ-line genes in Drosophila.

Fitness cost implications of PhiC31-mediated site-specific integrations in target-site strains of the Mexican fruit fly, Anastrepha ludens (Diptera: Tephritidae)

Site-specific recombination technologies are powerful new tools for the manipulation of genomic DNA in insects that can improve transgenesis strategies such as targeting transgene insertions, allowing transgene cassette exchange and DNA mobilization for transgene stabilization. However, understanding the fitness cost implications of these manipulations for transgenic strain applications is critical. In this study independent piggyBac-mediated attP target-sites marked with DsRed were created in several genomic positions in the Mexican fruit fly, Anastrepha ludens. Two of these strains, one having an autosomal (attP_F7) and the other a Y-linked (attP_2-M6y) integration, exhibited fitness parameters (dynamic demography and sexual competitiveness) similar to wild type flies. These strains were thus selected for targeted insertion using, for the first time in mexfly, the phiC31-integrase recombination system to insert an additional EGFP-marked transgene to determine its effect on host strain fitness. Fitness tests showed that the integration event in the int_2-M6y recombinant strain had no significant effect, while the int_F7 recombinant strain exhibited significantly lower fitness relative to the original attP_F7 target-site host strain. These results indicate that while targeted transgene integrations can be achieved without an additional fitness cost, at some genomic positions insertion of additional DNA into a previously integrated transgene can have a significant negative effect. Thus, for targeted transgene insertions fitness costs must be evaluated both previous to and subsequent to new site-specific insertions in the target-site strain.

Towards mosquito sterile insect technique programmes: exploring genetic, molecular, mechanical and behavioural methods of sex separation in mosquitoes

When considering a mosquito release programme, one of the first issues to be addressed is how to eliminate/separate the females. The greatest number of options might eventually be available for those who can use transgenic mosquitoes, but the inherent characteristics of the target species may also provide possibilities for interim measures until more efficient methods can be developed. Differences in intrinsic size, in behaviour and in development rate between females and males are often available and useful for sexing. Efficient species-specific systems for eliminating females at the embryo stage have been developed, but most have since been discarded due to lack of use. Ideal systems specifically kill female embryos using some treatment that can be manipulated during production. Such killing systems are far more efficient than using intrinsic sexual differences, but they systems require selectable genetic markers and sex-linkage created by rare random chromosomal rearrangements. While intrinsic sexual differences should not be considered as long-term candidates for the development of robust and efficient sexing approaches, in the absence of these, the accessibility and integration of less efficient systems can provide a stop-gap measure that allows rapid start up with a minimum of investment. The International Atomic Energy Agency is funding over a 5 year period (2013-2018) a new Coordinated Research Project on "Exploring Genetic, Molecular, Mechanical and Behavioural Methods of Sex Separation in Mosquitoes" to network researchers and to address the critical need of genetic sexing strains for the implementation of the sterile insect technique (using radiation-sterilised or transgenic male mosquitoes) and for insect incompatibility technique programmes against disease-transmitting mosquitoes.

An EST database of the Caribbean fruit fly, Anastrepha suspensa (Diptera: Tephritidae)

Invasive tephritid fruit flies are a great threat to agriculture worldwide and warrant serious pest control measures. Molecular strategies that promote embryonic lethality in these agricultural pests are limited by the small amount of nucleotide sequence data available for tephritids. To increase the dataset for sequence mining, we generated an EST database by 454 sequencing of the caribfly, Anastrepha suspensa, a model tephritid pest. This database yielded 95,803 assembled sequences with 24% identified as independent transcripts. The percentage of caribfly sequences with hits to the closely related tephritid, Rhagoletis pomonella, transcriptome was higher (28%) than to Drosophila proteins/genes (18%) in NCBI. The database contained genes specifically expressed in embryos, genes involved in the cell death, sex-determination, and RNAi pathways, and transposable elements and microsatellites. This study significantly expands the nucleotide data available for caribflies and will be a valuable resource for gene isolation and genomic studies in tephritid insects.

Transgenic sexing system for Ceratitis capitata (Diptera: Tephritidae) based on female-specific embryonic lethality

Fruit fly pest species have been successfully controlled and managed via the Sterile Insect Technique (SIT), a control strategy that uses infertile matings of sterile males to wild females to reduce pest populations. Biological efficiency in the field is higher if only sterile males are released in SIT programs and production costs are also reduced. Sexing strains developed in the Mediterranean fruit fly Ceratitis capitata (medfly) through classical genetics are immensely beneficial to medfly SIT programs but exhibit reduced fertility and fitness. Moreover, transfer of such classical genetic systems to other tephritid species is difficult. Transgenic approaches can overcome this limitation of classical genetic sexing strains (GSSs), but had resulted so far in transgenic sexing strains (TSSs) with dominant lethality at late larval and pupal stages. Here we present a transgene-based female-specific lethality system for early embryonic sexing in medfly. The system utilizes the sex-specifically spliced transformer intron to restrict ectopic mRNA translation of the pro-apoptotic gene hid(Ala5) to females only. The expression of this lethal effector gene is driven by a tetracycline-repressible transactivator gene tTA that is under the control of promoters/enhancers of early-acting cellularization genes. Despite observed position effects on the sex-specific splicing, we could effectively establish this early-acting transgenic sexing system in the medfly C. capitata. After satisfactory performance in large scale tests, TSSs based on this system will offer cost-effective sexing once introduced into SIT programs. Moreover, this approach is straight forward to be developed also for other insect pest and vector species.

A transgenic embryonic sexing system for Anastrepha suspensa (Diptera: Tephritidae)

The Sterile Insect Technique (SIT) is a highly successful biologically-based strategy to control pest insect populations that relies on the large-scale release of sterilized males to render females in the field non-reproductive. For medfly, a mutant-based sexing system is available as well as a transgenic system where a tetracycline-suppressible (Tet-off) toxic molecule is female-specifically produced. However, the former classical genetic system took many years to refine, and the latter system results in female death by a poorly understood mechanism, primarily in the pupal stage after rearing costs have been incurred. Here we describe a Tet-off transgenic embryonic sexing system (TESS) for Anastrepha suspensa that uses a driver construct having the promoter from the embryo-specific A. suspensa serendipity α gene, linked to the Tet-transactivator. This was used to drive the expression of a phospho-mutated variant of the pro-apoptotic cell death gene, Alhid, from Anastrepha ludens. The system uses a sex-specific intron splicing cassette linked to a cell death gene lethal effector. Progeny from TESS strains heterozygous for the transgene combination were 80-100% males, whereas four double homozygous TESS strains had 100% male-only progeny, with female death limited primarily to embryogenesis. In a large-scale test, more than 30,000 eggs from two strains resulted in 100% male-only progeny. The transgenic sexing approach described here is highly effective and cost-efficient by eliminating most, if not all, female insects early in embryogenesis using a well-characterized apoptotic mechanism.

Male only progeny in Anastrepha suspensa by RNAi-induced sex reversion of chromosomal females

In Tephritidae sex determination is established by orthologs to the Drosophila melanogaster transformer and transformer-2 genes, though the primary signals for sex determination differ. The presence of the Y chromosome in the tephritid species is critical for male differentiation, while the ratio of X chromosomes to autosome ploidy is critical in drosophilids. Here the isolation, expression and function of tra and tra-2 orthologs are described for the agriculturally important tephritid, Anastrepha suspensa, and their possible use in genetically modified organisms for biologically-based pest management. The Astra and Astra-2 genes are highly conserved in structure, regulation and function with respect to those known from other tephritid species. Sex-specific transcripts for Astra were detected, one in females and three in males, whereas Astra-2 had a single common transcript found in both sexes. To test the function of these genes, Astra and Astra-2 dsRNA was injected into A. suspensa embryos from a transgenic strain having a Y-linked DsRed marker integration, allowing XY males to be distinguished from XX phenotypic males. Nearly all XX embryos developed into fully masculinized phenotypic male adults with no apparent female morphology. Upon dissection abnormal hypertrophic gonads were revealed in XX pseudomales but not in the XY males. Our findings suggest that Astra and Astra-2 are both necessary for female development, and that the potential exists for producing a male-only population when either gene alone, or both genes simultaneously, are knocked-down.

Pro-apoptotic cell death genes, hid and reaper, from the tephritid pest species, Anastrepha suspensa

Pro-apoptotic proteins from the reaper, hid, grim (RHG) family are primary regulators of programmed cell death in Drosophila due to their antagonistic effect on inhibitor of apoptosis (IAP) proteins, thereby releasing IAP-inhibition of caspases that effect apoptosis. Using a degenerate PCR approach to conserved domains from the 12 Drosophila species, we have identified the first reaper and hid orthologs from a tephritid, the Caribfly Anastrepha suspensa. As-hid is the first identified non-drosophilid homolog of hid, and As-rpr is the second non-drosophilid rpr homolog. Both genes share more than 50% amino acid sequence identity with their Drosophila homologs, suggesting that insect pro-apoptotic peptides may be more conserved than previously anticipated. Importantly, both genes encode the conserved IBM and GH3 motifs that are key for IAP-inhibition and mitochondrial localization. Functional verification of both genes as cell death effectors was demonstrated by cell death assays in A. suspensa embryonic cell culture, as well as in heterologous Drosophila melanogaster S2 cells. Notably, heterologous cell death activity was found to be higher for Anastrepha genes than their Drosophila counterparts. In common with the Drosophila cognates, As-hid and As-rpr negatively regulated the Drosophila inhibitor of apoptosis (DIAP1) gene to promote apoptosis, and both genes when used together effected increased cell death activity, indicating a co-operative function for As-hid and As-rpr. We show that these tephritid cell death genes are functional and potent as cell death effectors, and could be used to design improved transgenic lethality systems for insect population control.

Conditional embryonic lethality to improve the sterile insect technique in Ceratitis capitata (Diptera: Tephritidae)

BACKGROUND

The sterile insect technique (SIT) is an environment-friendly method used in area-wide pest management of the Mediterranean fruit fly Ceratitis capitata (Wiedemann; Diptera: Tephritidae). Ionizing radiation used to generate reproductive sterility in the mass-reared populations before release leads to reduction of competitiveness.

RESULTS

Here, we present a first alternative reproductive sterility system for medfly based on transgenic embryonic lethality. This system is dependent on newly isolated medfly promoter/enhancer elements of cellularization-specifically-expressed genes. These elements act differently in expression strength and their ability to drive lethal effector gene activation. Moreover, position effects strongly influence the efficiency of the system. Out of 60 combinations of driver and effector construct integrations, several lines resulted in larval and pupal lethality with one line showing complete embryonic lethality. This line was highly competitive to wildtype medfly in laboratory and field cage tests.

CONCLUSION

The high competitiveness of the transgenic lines and the achieved 100% embryonic lethality causing reproductive sterility without the need of irradiation can improve the efficacy of operational medfly SIT programs.

Plasticity in mRNA expression and localization of orthodenticle within higher Diptera

orthodenticle (otd) genes are found throughout the animal kingdom and encode well-studied homeodomain transcription factors that share conserved functions in cephalization, head segmentation, brain patterning, and the differentiation of photoreceptors. Otd proteins have been proposed as ancestral key players in anterior determination despite a high level of variation in gene expression at early developmental stages: otd is expressed strictly zygotically in the dipteran Drosophila melanogaster, while otd1 mRNA is contributed maternally to the embryo in the coleopteran Tribolium castaneum and maternal otd1 mRNA is localized to the anterior and posterior pole of the oocyte in the hymopteran Nasonia vitripennis. Here we demonstrate that such changes in otd mRNA expression and localization do not need to represent large phylogenetic distances but can occur even within closely related taxa. We show maternal otd expression in the medfly Ceratitis capitata and maternally localized otd mRNA in the caribfly Anastrepha suspensa, two cyclorrhaphan species closely related to Drosophila. This indicates considerable plasticity in expression and mRNA localization of key developmental genes even within short evolutionary distances.