High efficiency, site-specific excision of a marker gene by the phage P1 cre-loxP system in the yellow fever mosquito, Aedes aegypti

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.

Optimizing CRE and PhiC31 mediated recombination in Aedes aegypti

Introduction: Genetic manipulation of Aedes aegypti is key to developing a deeper understanding of this insects' biology, vector-virus interactions and makes future genetic control strategies possible. Despite some advances, this process remains laborious and requires highly skilled researchers and specialist equipment. Methods: Here we present two improved methods for genetic manipulation in this species. Use of transgenic lines which express Cre recombinase and a plasmid-based method for expressing PhiC31 when injected into early embryos. Results: Use of transgenic lines which express Cre recombinase allowed, by simple crossing schemes, germline or somatic recombination of transgenes, which could be utilized for numerous genetic manipulations. PhiC31 integrase based methods for site-specific integration of genetic elements was also improved, by developing a plasmid which expresses PhiC31 when injected into early embryos, eliminating the need to use costly and unstable mRNA as is the current standard. Discussion: Here we have expanded the toolbox for synthetic biology in Ae. aegypti. These methods can be easily transferred into other mosquito and even insect species by identifying appropriate promoter sequences. This advances the ability to manipulate these insects for fundamental studies, and for more applied approaches for pest control.

Mosquito transgenesis for malaria control

Malaria is one of the deadliest diseases. Because of the ineffectiveness of current malaria-control methods, several novel mosquito vector-based control strategies have been proposed to supplement existing control strategies. Mosquito transgenesis and gene drive have emerged as promising tools for preventing the spread of malaria by either suppressing mosquito populations by self-destructing mosquitoes or replacing mosquito populations with disease-refractory populations. Here we review the development of mosquito transgenesis and its application for malaria control, highlighting the transgenic expression of antiparasitic effector genes, inactivation of host factor genes, and manipulation of miRNAs and lncRNAs. Overall, from a malaria-control perspective, mosquito transgenesis is not envisioned as a stand-alone approach; rather, its use is proposed as a complement to existing vector-control strategies.

Making gene drive biodegradable

Gene drive systems have long been sought to modify mosquito populations and thus combat malaria and dengue. Powerful gene drive systems have been developed in laboratory experiments, but may never be used in practice unless they can be shown to be acceptable through rigorous field-based testing. Such testing is complicated by the anticipated difficulty in removing gene drive transgenes from nature. Here, we consider the inclusion of self-elimination mechanisms into the design of homing-based gene drive transgenes. This approach not only caused the excision of the gene drive transgene, but also generates a transgene-free allele resistant to further action by the gene drive. Strikingly, our models suggest that this mechanism, acting at a modest rate (10%) as part of a single-component system, would be sufficient to cause the rapid reversion of even the most robust homing-based gene drive transgenes, without the need for further remediation. Modelling also suggests that unlike gene drive transgenes themselves, self-eliminating transgene approaches are expected to tolerate substantial rates of failure. Thus, self-elimination technology may permit rigorous field-based testing of gene drives by establishing strict time limits on the existence of gene drive transgenes in nature, rendering them essentially biodegradable.

This article is part of the theme issue ‘Novel control strategies for mosquito-borne diseases'.

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.

3,4-Dihydroxyphenylacetaldehyde synthase and cuticle formation in insects

Cuticle is the most important structure that protects mosquitoes and other insect species from adverse environmental conditions and infections of microorganism. The physiology and biochemistry of insect cuticle formation have been studied for many years and our understanding of cuticle formation and hardening has increased considerably. This is especially true for flexible cuticle. The recent discovery of a novel enzyme that catalyzes the production of 3,4-dihydroxyphenylacetaldehyde (DOPAL) in insects provides intriguing insights concerning the flexible cuticle formation in insects. For convenience, the enzyme that catalyzes the production DOPAL from l-dopa is named DOPAL synthase. In this mini-review, we summarize the biochemical pathways of cuticle formation and hardening in general and discuss DOPAL synthase-mediated protein crosslinking in insect flexible cuticle in particular.

Genetic technologies for disease vectors

The first genetic technologies for insect vectors of disease were introduced 20 years ago. As of today there are 12 classes of genetic technologies used as functional genomic tools for insect vectors of important diseases. Although the applications of genetic technologies in insect disease vectors have been conducted primarily in mosquitoes, other insect systems could benefit from current technologies. While the various technological platforms are likely to function in diverse arthropods, the delivery of these technologies to cells and tissues of interest is the major technical constraint that limits their widespread adoption. Increased community resources of various types would enhance the adoption of these technologies and potentially eliminate technical limitations.

Site-specific cassette exchange systems in the Aedes aegypti mosquito and the Plutella xylostella moth

Genetically engineered insects are being evaluated as potential tools to decrease the economic and public health burden of mosquitoes and agricultural pest insects. Here we describe a new tool for the reliable and targeted genome manipulation of pest insects for research and field release using recombinase mediated cassette exchange (RMCE) mechanisms. We successfully demonstrated the established ΦC31-RMCE method in the yellow fever mosquito, Aedes aegypti, which is the first report of RMCE in mosquitoes. A new variant of this RMCE system, called iRMCE, combines the ΦC31-att integration system and Cre or FLP-mediated excision to remove extraneous sequences introduced as part of the site-specific integration process. Complete iRMCE was achieved in two important insect pests, Aedes aegypti and the diamondback moth, Plutella xylostella, demonstrating the transferability of the system across a wide phylogenetic range of insect pests.

In vivo site-specific integration of transgene in silkworm via PhiC31 integrase-mediated cassette exchange

Current techniques for genetic engineering of the silkworm Bombyx mori genome utilize transposable elements, which result in positional effects and insertional mutagenesis through random insertion of exogenous DNA. New methods for introducing transgenes at specific positions are therefore needed to overcome the limitations of transposon-based strategies. Although site-specific recombination systems have proven powerful tools for genome manipulation in many organisms, their use has not yet been well established for the integration of transgenes in the silkworm. We describe a method for integrating target genes at pre-defined chromosomal sites in the silkworm via phiC31/att site-specific recombination system-mediated cassette exchange. Successful recombinase-mediated cassette exchange (RMCE) was observed in the two transgenic target strains with an estimated transformation efficiency of 3.84-7.01%. Our results suggest that RMCE events between chromosomal attP/attP target sites and incoming attB/attB sites were more frequent than those in the reciprocal direction. This is the first report of in vivo RMCE via phiC31 integrase in the silkworm, and thus represents a key step toward establishing genome manipulation technologies in silkworms and other lepidopteran species.

Next-generation site-directed transgenesis in the malaria vector mosquito Anopheles gambiae: self-docking strains expressing germline-specific phiC31 integrase

Diseases transmitted by mosquitoes have a devastating impact on global health and the situation is complicated due to difficulties with both existing control measures and the impact of climate change. Genetically modified mosquitoes that are refractory to disease transmission are seen as having great potential in the delivery of novel control strategies. The Streptomyces phage phiC31 integrase system has been successfully adapted for site-directed transgene integration in a range of insects, thus overcoming many limitations due to size constraints and random integration associated with transposon-mediated transformation. Using this technology, we previously published the first site-directed transformation of Anopheles gambiae, the principal vector of human malaria. Mosquitoes were initially engineered to incorporate the phiC31 docking site at a defined genomic location. A second phase of genetic modification then achieved site-directed integration of an anti-malarial effector gene. In the current publication we report improved efficiency and utility of the phiC31 integrase system following the generation of Anopheles gambiae self-docking strains. Four independent strains, with docking sites at known locations on three different chromosome arms, were engineered to express integrase under control of the regulatory regions of the nanos gene from Anopheles gambiae. The resulting protein accumulates in the posterior oocyte to provide integrase activity at the site of germline development. Two self-docking strains, exhibiting significantly different levels of integrase expression, were assessed for site-directed transgene integration and found to demonstrate greatly improved survival and efficiency of transformation. In the fight against malaria, it is imperative to establish a broad repertoire of both anti-malarial effector genes and tissue-specific promoters to regulate their expression, enabling those offering maximum effect with minimum fitness cost to be identified. The improved technology we describe here will facilitate comparative studies of effector transgenes, allowing informed choices to be made that potentially lead to transmission blockade.

[Progress in Cre/lox site-specific recombination system in higher eukaryotes]

Cre/lox system derived from P1 bacteriaphage can quickly and effectively achieve gene insertion, deletion, replacement, and inversion by means of site-specific recombination. As one of the most important tools for gene targeting at present, Cre/lox system has been widely used in Arabidopsis thaliana, Oryza sativa L., Mus musculus, Drosophila melanogaster, Danio rerio, and other higher eukaryotic organisms. This review roundly described the basic profile of Cre/lox system, and its application in higher eukaryotes. In addition, we also discussed the main problems and developmental trend of the Cre/lox system in this review, which can be a good reference for using Cre/lox system to realize the gene manipulations of the different high eukaryotic organisms.

Insect transgenesis: current applications and future prospects

The ability to manipulate the genomes of many insects has become a practical reality over the past 15 years. This has been led by the identification of several useful transposon vector systems that have allowed the identification and development of generalized, species-specific, and tissue-specific promoter systems for controlled expression of gene products upon introduction into insect genomes. Armed with these capabilities, researchers have made significant strides in both fundamental and applied transgenics in key model systems such as Bombyx mori, Tribolium casteneum, Aedes aegypti, and Anopheles stephensi. Limitations of transposon systems were identified, and alternative tools were developed, thus significantly increasing the potential for applied transgenics for control of both agricultural and medical insect pests. The next 10 years promise to be an exciting time of transitioning from the laboratory to the field, from basic research to applied control, during which the full potential of gene manipulation in insect systems will ultimately be realized.

Comparison of transgene expression in Aedes aegypti generated by mariner Mos1 transposition and ΦC31 site-directed recombination

Transgenic mosquitoes generated by transposable elements (TEs) often poorly express transgenes owing to position effects. To avoid these effects, the ΦC31 site-directed recombination system was used to insert transgenes into a locus favourable for gene expression in Aedes aegypti. We describe phenotypes of mariner Mos1 TE and ΦC31 transgenic mosquitoes expressing the enhanced green fluorescent protein (EGFP) reporter in midguts of blood-fed females. Mosquitoes of nine TE-generated lines [estimated transformation frequency (TF): 9.3%] clearly expressed the eye-specific selection marker but only 2/9 lines robustly expressed the EGFP reporter. The piggyBac TE-generated ΦC31 docking strain, attP26, supported recombination with attB site containing donors at an estimated TF of 1.7-4.9%. Using a codon-optimized ΦC31 integrase mutant instead of the 'wild-type' enzyme did not affect TF. Site-directed recombination of line attP26 with an attB-containing donor expressing EGFP from the Ae. aegypti carboxypeptidase promoter produced one transgenic line with blood-fed females expressing the reporter in midgut tissue. Docking strain attP26 also supported robust expression of Flock House virus B2 from the Ae. aegypti polyubiquitin promoter. Our data confirm that eye-specific selection marker expression alone is not a reliable indicator for robust gene-of-interest expression in Ae. aegypti and that the ΦC31 system can ensure predictable transgene expression in this mosquito species.

Validation of novel promoter sequences derived from two endogenous ubiquitin genes in transgenic Aedes aegypti

To date, only a limited number of promoter sequences have been described to drive transgene expression in the disease vector Aedes aegypti. We sought to increase this repertoire by characterizing the ability of upstream sequences derived from the Ae. aegypti Ub(L40) and polyubiquitin genes to drive the expression of marker proteins. Both genomic fragments were able to drive robust expression of luciferase in cultured mosquito cells. Following Mos1-transformation, the Ub(L40) promoter drove strong expression of a fluorescent marker in early larvae and in ovaries, while the polyubiquitin promoter drove robust EGFP expression in all stages of development, including constitutive expression throughout the midgut. These promoter fragments provide two new expression profiles for future Ae. aegypti genetic experiments.

Homing endonucleases catalyze double-stranded DNA breaks and somatic transgene excision in Aedes aegypti

Aedes aegypti is a major vector of arthropod-borne viruses such as yellow fever virus and dengue viruses. Efforts to discern the function of genes involved in important behaviours, such as vector competence and host seeking through reverse genetics, would greatly benefit from the ability to generate targeted gene disruptions. Homing endonucleases are selfish elements which catalyze double-stranded DNA (dsDNA) breaks in a sequence-specific manner. In this report we demonstrate that the homing endonucleases I-PpoI, I-SceI, I-CreI and I-AniI are all able to induce dsDNA breaks in adult female Ae. aegypti chromosomes as well as catalyze the somatic excision of a transgene. These experiments provide evidence that homing endonucleases can be used to manipulate the genome of this important disease vector.

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.

Transgenesis of schistosomes: approaches employing mobile genetic elements

Draft genome sequences for Schistosoma mansoni and Schistosoma japonicum are now available. However, the identity and importance of most schistosome genes have yet to be determined. Recently, progress has been made towards the genetic manipulation and transgenesis of schistosomes. Both loss-of-function and gain-of-function approaches appear to be feasible in schistosomes based on findings described in the past 5 years. This review focuses on reports of schistosome transgenesis, specifically those dealing with the transformation of schistosomes with exogenous mobile genetic elements and/or their endogenous relatives for the genetic manipulation of schistosomes. Transgenesis mediated by mobile genetic elements offers a potentially tractable route to introduce foreign genes to schistosomes, a means to determine the importance of schistosome genes, including those that could be targeted in novel interventions and the potential to undertake large-scale forward genetics by insertional mutagenesis.

High efficiency site-specific genetic engineering of the mosquito genome

Current techniques for the genetic engineering of insect genomes utilize transposable genetic elements, which are inefficient, have limited carrying capacity and give rise to position effects and insertional mutagenesis. As an alternative, we investigated two site-specific integration mechanisms in the yellow fever mosquito, Aedes aegypti. One was a modified CRE/lox system from phage P1 and the other a viral integrase system from Streptomyces phage phi C31. The modified CRE/lox system consistently failed to produce stable germline transformants but the phi C31 system was highly successful, increasing integration efficiency by up to 7.9-fold. The ability to efficiently target transgenes to specific chromosomal locations and the potential to integrate very large transgenes has broad applicability to research on many medically and economically important species.

Site-specific genomic targeting in Drosophila

Transcriptional regulation of transgenes depends upon genomic localization in higher eukaryotes. For the applied use of transgenic organisms as producers of pharmaceutically relevant proteins or as pest population control agents, a method to make transgene expression predictable is highly desirable. A targeting method that allows precise cassette replacement comprising solely genes of interest (without extraneous donor vector sequences) would be highly advantageous for insects and other multicellular organisms. In this report, we describe a method for transgene targeting to predefined chromosomal sites in Drosophila by using a transposon vector that, once integrated in the germ line, acts as an acceptor site for donor vectors. To make recombinational insertions irreversible, a FLP recombinase-mediated cassette exchange strategy was used, and to enhance donor-target pairing, a homing sequence from the linotte locus was used. Site-specific recombinants were screened by interconvertible eye fluorescence marker phenotypes yielding, on average, targeted insertions at a frequency of 23%. The cassette exchange system provides for repetitive integrations into the same locus, allowing comparative analysis of true transgenic alleles. Furthermore, this method was used to stabilize a targeted transgene by the postintegration excision of putatively mobile transposon sequences. The genomic targeting and stabilization strategy described for Drosophila should be applicable to other insects, specifically for the goals of optimizing heterologous protein expression and enhancing ecological safety of transgenic strains intended for release in biocontrol programs.

Utility of the FLP-FRT recombination system for genetic manipulation of rice

To develop an FLP-FRT recombination system- (derived from 2 mu plasmid of Saccharomyces cerevisiae) based marker gene removal application for rice, we introduced the gene for FLP recombinase, under the control of the maize ubiquitin-1 promoter, into the rice genome. FLP activity was monitored in callus and regenerated plants by an assay based on the deletion of the FRT-flanked DNA fragment, leading to the activation of the beta-glucuronidase gene. FLP activity was detected both in the callus and leaves of some of the transgenic lines. Based on our comparison of the recombination efficiency of the FLP-FRT system expressed in the transgenic lines with that of the widely used Cre-lox system (derived from bacteriophage P1), we suggest that the FLP-FRT system is a useful tool for the genetic manipulation of rice.

Innate immunity in the malaria vector Anopheles gambiae:comparative and functional genomics

The resurgence of malaria is at least partly attributed to the absence of an effective vaccine, parasite resistance to antimalarial drugs and resistance to insecticides of the anopheline mosquito vectors. Novel strategies are needed to combat the disease on three fronts: protection (vaccines),prophylaxis/treatment (antimalarial drugs) and transmission blocking. The latter entails either killing the mosquitoes (insecticides), preventing mosquito biting (bednets and repellents), blocking parasite development in the vector (transmission blocking vaccines), genetic manipulation or chemical incapacitation of the vector. During the past decade, mosquito research has been energized by several breakthroughs, including the successful transformation of anopheline vectors, analysis of gene function by RNAi,genome-wide expression profiling using DNA microarrays and, most importantly,sequencing of the Anopheles gambiae genome. These breakthroughs helped unravel some of the mechanisms underlying the dynamic interactions between the parasite and the vector and shed light on the mosquito innate immune system as a set of potential targets to block parasite development. In this context, putative pattern recognition receptors of the mosquito that act as positive and negative regulators of parasite development have been identified recently. Characterizing these molecules and others of similar function, and identifying their ligands on the parasite surface, will provide clues on the nature of the interactions that define an efficient parasite–vector system and open up unprecedented opportunities to control the vectorial capacity of anopheline mosquitoes.