Post-integration stabilization of a transposon vector by terminal sequence deletion in Drosophila melanogaster

Improvement on the genetic engineering of an invasive agricultural pest insect, the cherry vinegar fly, Drosophila suzukii

Background

The invasive fly Drosophila suzukii has become an established fruit pest in Europe, the USA, and South America with no effective and safe pest management. Genetic engineering enables the development of transgene-based novel genetic control strategies against insect pests and disease vectors. This, however, requires the establishment of reliable germline transformation techniques. Previous studies have shown that D. suzukii is amenable to transgenesis using the transposon-based vectors piggyBac and Minos, site-specific recombination (lox/Cre), and CRISPR/Cas9 genome editing.

Results

We experienced differences in the usability of piggyBac-based germline transformation in different strains of D. suzukii: we obtained no transgenic lines in a US strain, a single rare transgenic line in an Italian strain, but observed a reliable transformation rate of 2.5 to 11% in a strain from the French Alps. This difference in efficiency was confirmed by comparative examination of these three strains. In addition, we used an attP landing site line to successfully established φC31-integrase-mediated plasmid integration at a rate of 10% and generated landing site lines with two attP sequences to effectively perform φC31-Recombinase Mediated Cassette Exchange (φC31-RMCE) with 11% efficiency. Moreover, we isolated and used the endogenous regulatory regions of Ds nanos to express φC31 integrase maternally to generate self-docking lines for φC31-RMCE. Besides, we isolated the promoter/enhancer of Ds serendipity α to drive the heterologous tetracycline-controlled transactivator (tTA) during early embryonic development and generated a testes-specific tTA driver line using the endogenous beta-2-tubulin (β2t) promoter/enhancer.

Conclusion

Our results provide evidence that the D. suzukii strain AM derived from the French Alps is more suitable for piggyBac germline transformation than other strains. We demonstrated the feasibility of using φC31-RMCE in the cherry vinegar fly and generated a set of lines that can be used for highly efficient integration of larger constructs. The φC31-based integration will facilitate modification and stabilization of previously generated transgenic lines that carry at least one attP site in the transgene construction. An early embryo-specific and a spermatogenesis-specific driver line were generated for future use of the binary expression system tet-off to engineer tissue- and stage-specific effector gene expression for genetic pest control strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-020-00940-5.

Unpredictable recombination of PB transposon in Silkworm: a potential risk

The piggyBac (PB) transposon is the most widely used vector for generating transgenic silkworms. The stability of the PB transposon in the receptor is a serious concern that requires attention because of biosafety concerns. In this study, we found that the transgene silkworm developed loss of reporter gene traits. To further investigate the regularity, we traced the genes and traits of this silkworm. After successful alteration of the silkworm genome with the MASP1 gene (named red-eyed silkworm; RES), silkworm individuals with lost reporter genes were found after long-term transgenerational breeding and were designated as the white-eyed silkworm (WES). PCR amplification indicated that exogenous genes had been lost in the WES. Testing was conducted on the PB transposons, and the left arm (L arm) did not exist; however, the right arm (R arm) was preserved. Amino acid analysis showed that the amino acid content of the WES changed versus the common silkworm and RES. These results indicate that the migration of PB transposons in Bombyx mori does occur and is unpredictable. This is because the silkworm genome contains multiple PB-like sequences that might influence the genetic stability of transgenic lines. When using PB transposons as a transgene vector, it is necessary to fully evaluate and take necessary measures to prevent its re-migration in the recipient organism. Further experiments are needed if we want to clarify the regularity of the retransposition phenomenon and the direct and clear association with similar sequences of transposons.

An Anopheles stephensi Promoter-Trap: Augmenting Genome Annotation and Functional Genomics

The piggyBac transposon was modified to generate gene trap constructs, which were then incorporated into the genome of the Asian malaria vector, Anopheles stephensi and remobilized through genetic crosses using a piggyBac transposase expressing line. A total of 620 remobilization events were documented, and 73 were further characterized at the DNA level to identify patterns in insertion site preferences, remobilization frequencies, and remobilization patterns. Overall, the use of the tetameric AmCyan reporter as the fusion peptide displayed a preference for insertion into the 5'-end of transcripts. Notably 183 - 44882 bp upstream of the An. stephensi v1.0 ab initio gene models, which demonstrated that the promoter regions for the genes of An. stephensi are further upstream of the 5'-proximal regions of the genes in the ab inito models than may be otherwise predicted. RNA-Seq transcript coverage supported the insertion of the splice acceptor gene trap element into 5'-UTR introns for nearly half of all insertions identified. The use of a gene trap element that prefers insertion into the 5'-end of genes supports the use of this technology for the random generation of knock-out mutants, as well as the experimental confirmation of 5'-UTR introns in An. stephensi.

Enhancing the stability and ecological safety of mass-reared transgenic strains for field release by redundant conditional lethality systems

The genetic manipulation of agriculturally important insects now allows the development of genetic sexing and male sterility systems for more highly efficient biologically-based population control programs, most notably the Sterile Insect Technique (SIT), for both plant and animal insect pests. Tetracycline-suppressible (Tet-off) conditional lethal systems may function together so that transgenic strains will be viable and fertile on a tetracycline-containing diet, but female-lethal and male sterile in tetracycline-free conditions. This would allow their most efficacious use in a unified system for sterile male-only production for SIT. A critical consideration for the field release of such transgenic insect strains, however, is a determination of the frequency and genetic basis of lethality revertant survival. This will provide knowledge essential to evaluating the genetic stability of the lethality system, its environmental safety, and provide the basis for modifications ensuring optimal efficacy. For Tet-off lethal survival determinations, development of large-scale screening protocols should also allow the testing of these modifications, and test the ability of other conditional lethal systems to fully suppress propagation of rare Tet-off survivors. If a dominant temperature sensitive (DTS) pupal lethality system proves efficient for secondary lethality in Drosophila, it may provide the safeguard needed to support the release of sexing/sterility strains, and potentially, the release of unisex lethality strains as a form of genetic male sterility. Should the DTS Prosβ2(1) mutation prove effective for redundant lethality, its high level of structural and functional conservation should allow host-specific cognates to be created for a wide range of insect species.

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.

An efficient strategy for producing a stable, replaceable, highly efficient transgene expression system in silkworm, Bombyx mori

We developed an efficient strategy that combines a method for the post-integration elimination of all transposon sequences, a site-specific recombination system, and an optimized fibroin H-chain expression system to produce a stable, replaceable, highly efficient transgene expression system in the silkworm (Bombyx mori) that overcomes the disadvantages of random insertion and post-integration instability of transposons. Here, we generated four different transgenic silkworm strains, and of one the transgenic strains, designated TS1-RgG2, with up to 16% (w/w) of the target protein in the cocoons, was selected. The subsequent elimination of all the transposon sequences from TS1-RgG2 was completed by the heat-shock-induced expression of the transposase in vivo. The resulting transgenic silkworm strain was designated TS3-g2 and contained only the attP-flanked optimized fibroin H-chain expression cassette in its genome. A phiC31/att-system-based recombinase-mediated cassette exchange (RMCE) method could be used to integrate other genes of interest into the same genome locus between the attP sites in TS3-g2. Controlling for position effects with phiC31-mediated RMCE will also allow the optimization of exogenous protein expression and fine gene function analyses in the silkworm. The strategy developed here is also applicable to other lepidopteran insects, to improve the ecological safety of transgenic strains in biocontrol programs.

Remobilizing deleted piggyBac vector post-integration for transgene stability in silkworm

Deletion of transposable elements post-genomic integration holds great promise for stability of the transgene in the host genome and has an essential role for the practical application of transgenic animals. In this study, a modified piggyBac vector that mediated deletion of the transposon sequence post-integration for transgene stability in the economically important silkworm Bombyx mori was constructed. The piggyBac vector architecture contains inversed terminal repeat sequences L1, L2 and R1, which can form L1/R1 and L2/R1 types of transposition cassettes. hsp70-PIG as the piggyBac transposase expression cassette for initial transposition, further remobilization and transgene stabilization test was transiently expressed in a helper vector or integrated into the modified vector to produce a transgenic silkworm. Shortening L2 increased the transformation frequency of L1/R1 into the silkworm genome compared to L2/R1. After the integration of L1/R1 into the genome, the remobilization of L2/R1 impaired the transposon structure and the resulting transgene linked with an impaired transposon was stable in the genome even in the presence of exogenously introduced transposase, whereas those flanked by the intact transposon were highly mobile in the genome. Our results demonstrated the feasibility of post-integration deletion of transposable elements to guarantee true transgene stabilization in silkworm. We suggest that the modified vector will be a useful resource for studies of transgenic silkworms and other piggyBac-transformed organisms.

Site-specific, TALENs-mediated transformation of Bombyx mori

Transposon-based genetic transformation has facilitated insect functional genomics and new strategies of pest management. However, there is a need for alternative, site-specific approaches to overcome limitations of random integration (and associated position-effects) and potential instability of inserted transgenes. Here we describe a transposon-free, site-specific genetic transformation system mediated by transcription activator-like effector nucleases (TALENs) in the silkworm, Bombyx mori, a lepidopteran model insect. We successfully established a site-specific transgenic system with comparable transformation efficiency to transposon-based genetic transformation through microinjection of TALENs mRNA targeting the BmBLOS2 locus and a linearizable donor plasmid encoding an expression cassette of the DsRed2 red fluorescent protein. This system provides a valuable approach for insect transgenesis and will enable future functional gene analysis and generate novel applications in agricultural and medical insect pest-management technologies.

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.

Establishment of transgenic lines for jumpstarter method using a composite transposon vector in the ladybird beetle, Harmonia axyridis

In this post-genomic era, genome-wide functional analysis is indispensable. The recent development of RNA interference techniques has enabled researchers to easily analyze gene function even in non-model organisms. On the other hand, little progress has been made in the identification and functional analyses of cis-regulatory elements in non-model organisms. In order to develop experimental platform for identification and analyses of cis-regulatory elements in a non-model organism, in this case, the ladybird beetle, Harmonia axyridis, we established transgenic transposon-tagged lines using a novel composite vector. This vector enables the generation of two types of insertion products (jumpstarter and mutator). The jumpstarter portion carries a transposase gene, while the mutator segment carries a reporter gene for detecting enhancers. The full-composite element is flanked by functional termini (required for movement); however, the mutator region has an extra terminus making it possible for the mutator to remobilize on its own, thus leaving an immobile jumpstarter element behind. Each insertion type is stable on its own, but once crossed, jumpstarters can remobilize mutators. After crossing a jumpstarter and mutator line, all tested G2 females gave rise to at least one new insertion line in the next generation. This jumping rate is equivalent to the P-element-mediated jumpstarter method in Drosophila. These established transgenic lines will offer us the ideal experimental materials for the effective screening and identification of enhancers in this species. In addition, this jumpstarter method has the potential to be as effective in other non-model insect species and thus applicable to any organism.

Not all GMOs are crop plants: non-plant GMO applications in agriculture

Since tools of modern biotechnology have become available, the most commonly applied and often discussed genetically modified organisms are genetically modified crop plants, although genetic engineering is also being used successfully in organisms other than plants, including bacteria, fungi, insects, and viruses. Many of these organisms, as with crop plants, are being engineered for applications in agriculture, to control plant insect pests or diseases. This paper reviews the genetically modified non-plant organisms that have been the subject of permit approvals for environmental release by the United States Department of Agriculture/Animal and Plant Health Inspection Service since the US began regulating genetically modified organisms. This is an indication of the breadth and progress of research in the area of non-plant genetically modified organisms. This review includes three examples of promising research on non-plant genetically modified organisms for application in agriculture: (1) insects for insect pest control using improved vector systems; (2) fungal pathogens of insects to control insect pests; and (3) virus for use as transient-expression vectors for disease control in plants.

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.

Post-integration silencing of piggyBac transposable elements in Aedes aegypti

The piggyBac transposon, originating in the genome of the Lepidoptera Trichoplusia ni, has a broad host range, making it useful for the development of a number of transposon-based functional genomic technologies including gene vectors, enhancer-, gene- and protein-traps. While capable of being used as a vector for the creation of transgenic insects and insect cell lines, piggyBac has very limited mobility once integrated into the genome of the yellow fever mosquito, Aedes aegypti. A transgenic Aedes aegypti cell line (AagPB8) was created containing three integrated piggyBac elements and the remobilization potential of the elements was tested. The integrated piggyBac elements in AagPB8 were transpositionally silent in the presence of functional transposase, which was shown to be capable of catalyzing the movement of plasmid-borne piggyBac elements in the same cells. The structural integrity of one of the integrated elements along with the quality of element-flanking DNA, which is known to influence transposition rates, were tested in D. melanogaster. The element was found to be structurally intact, capable of transposition and excision in the soma and germ-line of Drosophila melanogaster, and in a DNA sequence context highly conducive to element movement in Drosophila melanogaster. These data show that transpositional silencing of integrated piggyBac elements in the genome of Aedes aegypti appears to be a function of higher scale genome organization or perhaps epigenetic factors, and not due to structural defects or suboptimal integration sites.

A Functional Comparison of the 3xP3 Promoter by Recombinase-Mediated Cassette Exchange in Drosophila and a Tephritid Fly, Anastrepha suspensa

Transposable elements are widely used as vectors for integrating transgenes into the genome of insects. However, the random nature of transposon vector integrations often results in mutations and makes transgene expression subject to variable genomic position effects. This makes reliable quantitative comparisons of different transgenes difficult and development of highly fit transgenic strains laborious. Tools for site-specific transgene targeting are essential for functional genomic comparisons and to develop the most advanced transgenic insect strains for applied use. Here we describe a recombinase-mediated cassette exchange gene targeting system based on Cre/loxP that is highly efficient in Drosophila, and for the first time in a non-drosophilid, the tephritid fly, Anastrepha suspensa This system allowed a comparison of the Drosophila constitutive polyubiquitin promoter and the artificial 3xP3 tissue-specific promoter in the same genomic context within each species, showing that the widely used 3xP3 promoter is apparently nonfunctional in the tephritid fly.

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.

Highly efficient and specific genome editing in silkworm using custom TALENs

Establishment of efficient genome editing tools is essential for fundamental research, genetic engineering, and gene therapy. Successful construction and application of transcription activator-like effector nucleases (TALENs) in several organisms herald an exciting new era for genome editing. We describe the production of two active TALENs and their successful application in the targeted mutagenesis of silkworm, Bombyx mori, whose genetic manipulation methods are parallel to those of Drosophila and other insects. We will also show that the simultaneous expression of two pairs of TALENs generates heritable large chromosomal deletion. Our results demonstrate that (i) TALENs can be used in silkworm and (ii) heritable large chromosomal deletions can be induced by two pairs of TALENs in whole organisms. The generation and the high frequency of TALENs-induced targeted mutagenesis in silkworm will promote the genetic modification of silkworm and other insect species.

FLP recombinase-mediated site-specific recombination in silkworm, Bombyx mori

A comprehensive understanding of gene function and the production of site-specific genetically modified mutants are two major goals of genetic engineering in the post-genomic era. Although site-specific recombination systems have been powerful tools for genome manipulation of many organisms, they have not yet been established for use in the manipulation of the silkworm Bombyx mori genome. In this study, we achieved site-specific excision of a target gene at predefined chromosomal sites in the silkworm using a FLP/FRT site-specific recombination system. We first constructed two stable transgenic target silkworm strains that both contain a single copy of the transgene construct comprising a target gene expression cassette flanked by FRT sites. Using pre-blastoderm microinjection of a FLP recombinase helper expression vector, 32 G3 site-specific recombinant transgenic individuals were isolated from five of 143 broods. The average frequency of FLP recombinase-mediated site-specific excision in the two target strains genome was approximately 3.5%. This study shows that it is feasible to achieve site-specific recombination in silkworms using the FLP/FRT system. We conclude that the FLP/FRT system is a useful tool for genome manipulation in the silkworm. Furthermore, this is the first reported use of the FLP/FRT system for the genetic manipulation of a lepidopteran genome and thus provides a useful reference for the establishment of genome manipulation technologies in other lepidopteran species.

Strategy for enhanced transgenic strain development for embryonic conditional lethality in Anastrepha suspensa

Here the first reproductive sterility system for the tephritid fruit fly pest, Anastrepha suspensa, is presented, based on lethality primarily limited to embryos heterozygous for a conditional lethal transgene combination. This tetracycline (Tet)-suppressible system uses a driver construct having the promoter from the newly isolated embryo-specific A. suspensa serendipity α gene linked to the Tet-transactivator. This was used to drive expression of a phosphomutated variant of the pro-apoptotic cell death gene, hid, from A. ludens, that was isolated, based on its identity to A. suspensa hid. The Alhid(Ala2) variant was shown to have the highest cell death activity in an in vitro A. suspensa cell death assay compared to the orthologous genes Ashid, Dmhid, and the variant Dmhid(Ala5). These cell death assays also allowed a determination of the most-efficient driver-effector cassette combinations for use in A. suspensa transformants, resulting in two hybrid strains exhibiting 100% lethality. One strain was 96% lethal in embryos in the absence of tetracycline, with none surviving past the first larval instar, which is critical for pests that are most damaging in late-larval stages. We demonstrate that the isolation and in vitro validation of species-specific promoters and lethal effector genes can greatly improve the efficiency of creating high-performance conditional lethality strains that may be extended to other insect pest species.

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.

A new powerful method for site-specific transgene stabilization based on chromosomal double-strand break repair

Transgenic insects are a promising tool in sterile insect techniques and population replacement strategies. Such transgenic insects can be created using nonautonomous transposons, which cannot be transferred without a transposase source. In biocontrol procedures where large numbers of insects are released, there is increased risk of transgene remobilization caused by external transposase sources that can alter the characteristics of the transgenic organisms lead horizontal transgene transfer to other species. Here we describe a novel, effective method for transgene stabilization based on the introduction of directed double-strand breaks (DSB) into a genome-integrated sequence and their subsequent repair by the single-strand annealing (SSA) pathway. Due to the construct's organization, the repair pathway is predictable, such that all transposon and marker sequences can be deleted, while preserving integration of exogenous DNA in the genome. The exceptional conservation of DNA repair pathways makes this method suitable for a broad range of organisms.

Site-specific integration and expression of an anti-malarial gene in transgenic Anopheles gambiae significantly reduces Plasmodium infections

Diseases transmitted by mosquitoes have a devastating impact on global health and this is worsening due to difficulties with existing control measures and climate change. Genetically modified mosquitoes that are refractory to disease transmission are seen as having great potential in the delivery of novel control strategies. Historically the genetic modification of insects has relied upon transposable elements which have many limitations despite their successful use. To circumvent these limitations the Streptomyces phage phiC31 integrase system has been successfully adapted for site-specific transgene integration in insects. Here, we present the first site-specific transformation of Anopheles gambiae, the principal vector of human malaria. Mosquitoes were initially engineered to incorporate the phiC31 targeting site at a defined genomic location. A second phase of genetic modification then achieved site-specific integration of Vida3, a synthetic anti-malarial gene. Expression of Vida3, specifically in the midgut of bloodfed females, offered consistent and significant protection against Plasmodium yoelii nigeriensis, reducing average parasite intensity by 85%. Similar protection was observed against Plasmodium falciparum in some experiments, although protection was inconsistent. In the fight against malaria, it is imperative to establish a broad repertoire of both anti-malarial effector genes and tissue-specific promoters for their expression, enabling those offering maximum effect with minimum fitness cost to be identified. In the future, this technology will allow effective comparisons and informed choices to be made, potentially leading to complete transmission blockade.

Recombination technologies for enhanced transgene stability in bioengineered insects

Transposon-based vectors currently provide the most suitable gene transfer systems for insect germ-line transformation and are used for molecular improvement of the Sterile Insect Technique. However, the long time stability of genome-integrated transposon constructs depends on the absence of transposase activity that could remobilize the transposon-embedded transgenes. To achieve transgene stability transposon vectors are usually non-autonomous, lacking a functional transposase gene, and chosen so that endogenous or related transposon activities are not present in the host. Nevertheless, the non-autonomous transposon-embedded transgenes could become unstable by the unintended presence of a mobilizing transposase that may have been undetected or subsequently entered the host species by horizontal gene transfer. Since the field release of transgenic insects will present environmental concerns relating to large populations and high mobility, it will be important to ensure that transgene constructs are stably integrated for maintaining strain integrity and eliminating the possibility for unintentional transfer into the genome of another organism. Here we review efficient methods to delete or rearrange terminal repeat sequences of transposons necessary for their mobility, subsequent to their initial genomic integration. These procedures should prevent transposase-mediated remobilization of the transgenes, ensuring their genomic stability.

Developing transgenic Anopheles mosquitoes for the sterile insect technique

In the last 10 years the availability of the genome sequence of Anopheles gambiae and the development of a transgenic technology for several species of Anopheles mosquitoes have, in combination, helped in enabling us to gain several insights into the biology of these mosquitoes that is relevant to their capacity as vectors of the malaria parasite. While this information is anticipated to inform many novel vector control strategies, the technique most likely to benefit in the near future from the availability of a reliable transgenic technology is the sterile insect technique (SIT), which relies on releasing large numbers of sterile insects to compete for mates in the wild, leading to population suppression. Although SIT has been proven to work reliably for many insects, the construction of suitable strains, and induction of sterility, has until now been a laborious process, combining classical genetics with radiation-induced sterility. Using transgenesis to create strains of Anopheles suitable for SIT could potentially offer several advantages over current approaches, in that the basic design of transgenic constructs designed for other insects should be rapidly transferable to mosquitoes, and induction of sterility as a product of the transgenic modification could obviate the requirement for radiation and its associated deleterious effects. In this paper the progress of different transgenic approaches in constructing tools for SIT will be reviewed.

Site-specific recombination for the modification of transgenic strains of the Mediterranean fruit fly Ceratitis capitata

Insect transgenesis is mainly based on the random genomic integration of DNA fragments embedded into non-autonomous transposable elements. Once a random insertion into a specific location of the genome has been identified as particularly useful with respect to transgene expression, the ability to make the insertion homozygous, and lack of fitness costs, it may be advantageous to use that location for further modification. Here we describe an efficient method for the modification of previously inserted transgenes by the use of the site-specific integration system from phage phiC31 in a tephritid pest species, the Mediterranean fruit fly Ceratitis capitata. First, suitable transgenic strains with randomly integrated attP landing sites within transposon-based vectors were identified by molecular and functional characterization. Second, donor plasmids containing an attB site, with additional markers, and transposon ends were integrated into attP sites by phiC31 integrase-mediated recombination. Third, transposase-encoding 'jumpstarter' strains were created and mated to transgenic strains resulting in the postintegrational excision of transposon ends, which left stably integrated transgene insertions that could not be remobilized. This three-step integration and stabilization system will allow the combination of several transgene-encoded advantageous traits at evaluated genomic positions to generate optimized strains for pest control that minimize environmental concerns.

Development and utilization of transgenic New World screwworm, Cochliomyia hominivorax

The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), was the first insect to be effectively controlled using the sterile insect technique (SIT). Recent efforts to improve SIT control of this species have centred on the development of genetically transformed strains using the piggyBac transposon vector system. Eight transgenic strains were produced incorporating an enhanced green fluorescent protein (EGFP) marker gene under polyubiquitin regulation that has the potential for use as a genetic marking system for released males. The transgenic strains were genetically and phenotypically characterized, including for life fitness parameters and mating competitiveness. These characteristics were unique for each strain and thus some strains were deemed suitable for incorporation into SIT eradication programmes. The strain with the best attributes is designated 'CLAY'. Four of the strains, including CLAY, have been successfully cryopreserved so that their original characteristics should be unchanged when further evaluation is required. With the demonstration of efficient germ-line transformation in NWS, allowing production of fit and competitive transformants, it is now possible to consider further transgenic strain development to improve SIT that are currently being tested in other dipteran species. This includes strains that allow genetic marking with fluorescent proteins, genetic sexing by female lethality, male-specific fluorescent sorting and male sterility by testis-specific lethality. The SIT may also be improved upon by new strategies resulting in lethality of offspring of released insects using conditional lethal systems based upon temperature-dependent or dietary tetracycline regulation of lethal gene expression. Both the creation of new NWS transgenic strains and the ecological safety of their release will be enhanced by new vector systems that allow specific genomic targeting of vector constructs and their subsequent immobilization, ensuring transgene and strain stability.

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.

Perspectives on the state of insect transgenics

Genetic transformation is a critical component to the fundamental genetic analysis of insect species and holds great promise for establishing strains that improve population control and behavior for practical application. This is especially so for insects that are disease vectors, many of which are currently subject to genomic sequence analysis, and intensive population control measures that must be improved for better efficacy and cost-effectiveness. Transposon-mediated germ-line transformation has been the ultimate goal for most fundamental and practical studies, and impressive strides have been made in recent development of transgene vector and marker systems for several mosquito species. This has resulted in rapid advances in functional genomic sequence analysis and new strategies for biological control based on conditional lethality. Importantly, advances have also been made in our ability to use these systems more effectively in terms of enhanced stability and targeting to specific genomic loci. Nevertheless, not all insects are currently amenable to germ-line transformation techniques, and thus advances in transient somatic expression and paratransgenesis have also been critical, if not preferable for some applications. Of particular importance is how this technology will be used for practical application. Early ideas for population replacement of indigenous pests with innocuous transgenic siblings by transposon-vector spread, may require reevaluation in terms of our current knowledge of the behavior of transposons currently available for transformation. The effective implementation of any control program using released transgenics, will also benefit from broadening the perspective of these control measures as being more mainstream than exotic.

Highly similar piggyBac elements in Bactrocera that share a common lineage with elements in noctuid moths

The piggyBac IFP2 transposable element, originally discovered in a Trichoplusia ni cell line, also exists as nearly identical elements in other noctuid lepidopterans, and in several species of the tephritid genus Bactrocera. To further define the distribution of piggyBacs in Bactrocera, and compare their relationship to sequences found in Lepidoptera, a survey by PCR amplification was performed in a range of Bactrocera species. Highly similar piggyBac sequences were found in all B. dorsalis complex species tested, as well as in species in the B. zonata and B. frauenfeldi complexes. All nucleotide sequences had > 94% identity to corresponding sequences in the T. ni IFP2 element, and > 88% identity among the sequences. Conserved primers did not amplify any distantly related sequences that have been found by computational searches in a wider range of insect and non-insect species. Notably, 55 nucleotide substitutions relative to IFP2 were common to all the Bactrocera sequences, 44 of which exist in piggyBacs previously sequenced from moths, with 17 resulting in amino acid substitutions. These piggyBac elements, that apparently traversed orders by horizontal transfer, probably arose from a lineage separate from IFP2 and the other known elements in T. ni. Implications for the presence of nearly identical piggyBacs, in widely distributed insects, to the applied use of piggyBac vectors are discussed.

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.

Germ-line transformation of the Mexican fruit fly

Germ-line transformation of a major agricultural pest, the Mexican fruit fly (Anastrepha ludens Loew, Mexfly), was achieved using composite piggyBac transposable elements marked with green, yellow and red fluorescent proteins (CopGreen, PhiYFP and J-Red). We also investigated the possibility of generating transposon-free insertions, in order to address potential concerns relating to proposed field use of transgenic Mexfly. We describe a highly efficient method for transforming Mexfly, compare efficiency of piggyBac terminal sequences for transformation and also describe the derivation of a transposon-free insertion line. The development of an efficient transformation system for Mexfly holds great promise for improved applications of the sterile insect technique, a major component of the present control measures for this economically important pest species.

Post-integration stability of piggyBac in Aedes aegypti

The post-integration activity of piggyBac transposable element gene vectors in Aedes aegypti mosquitoes was tested under a variety of conditions. The embryos from five independent transgenic lines of Ae. aegypti, each with a single integrated non-autonomous piggyBac transposable element gene vector, were injected with plasmids containing the piggyBac transposase open-reading frame under the regulatory control of the Drosophila melanogaster hsp70 promoter. No evidence for somatic remobilization was detected in the subsequent adults whereas somatic remobilization was readily detected when similar lines of transgenic D. melanogaster were injected with the same piggyBac transposase-expressing plasmid. Ae. aegypti heterozygotes of piggyBac reporter-containing transgenes and piggyBac transposase-expressing transgenes showed no evidence of somatic and germ-line remobilization based on phenotypic and molecular detection methods. The post-integration mobility properties of piggyBac in Ae. aegypti enhance the utility of this gene vector for certain applications, particularly those where any level of vector remobilization is unacceptable.

Post-integration behavior of a Minos transposon in the malaria mosquito Anopheles stephensi

Transposable elements represent important tools to perform functional studies in insects. In Drosophila melanogaster, the remobilization properties of transposable elements have been utilized for enhancer-trapping and insertional mutagenesis experiments, which have considerably helped in the functional characterization of the fruitfly genome. In Anopheles mosquitoes, the sole vectors of human malaria, as well as in other mosquito vectors of disease, the use of transposons has also been advocated to achieve the spread of anti-parasitic genes throughout field populations. Here we report on the post-integration behavior of the Minos transposon in both the germ-line and somatic tissues of Anopheles mosquitoes. Transgenic An. stephensi lines developed using the piggyBac transposon and expressing the Minos transposase were tested for their ability to remobilize an X-linked Minos element. Germ-line remobilization events were not detected, while somatic excisions and transpositions were consistently recovered. The analysis of these events showed that Minos activity in Anopheles cells is characterized by unconventional functionality of the transposon. In the two cases analyzed, re-integration of the transposon occurred onto the same X chromosome, suggesting a tendency for local hopping of Minos in the mosquito genome. This is the first report of the post-integration behavior of a transposable element in a human malaria vector.

Interplasmid transposition demonstrates piggyBac mobility in vertebrate species

The piggyBac transposon is an extremely versatile helper-dependent vector for gene transfer and germ line transformation in a wide range of invertebrate species. Analyses of genome sequencing databases have identified piggyBac homologues among several sequenced animal genomes, including the human genome. In this report we demonstrate that this insect transposon is capable of transposition in primate cells and embryos of the zebrafish, Danio rerio. piggyBac mobility was demonstrated using an interplasmid transposition assay that has consistently predicted the germ line transformation capabilities of this mobile element in several other species. Both transfected COS-7 primate cells and injected zebrafish embryos supported the helper-dependent movement of tagged piggyBac element between plasmids in the characteristic cut-and-paste, TTAA target-site specific manner. These results validate piggyBac as a valuable tool for genetic analysis of vertebrates.

Transposon-free insertions for insect genetic engineering

Methods involving the release of transgenic insects in the field hold great promise for controlling vector-borne diseases and agricultural pests. Insect transformation depends on nonautonomous transposable elements as gene vectors. The resulting insertions are stable in the absence of suitable transposase, however, such absence cannot always be guaranteed. We describe a method for post-integration elimination of all transposon sequences in the pest insect Medfly, Ceratitis capitata. The resulting insertions lack transposon sequences and are therefore impervious to transposase activity.

Highly conserved piggyBac elements in noctuid species of Lepidoptera

The piggyBac transposable element was originally discovered in a Trichoplusia ni cell line and nearly identical elements were subsequently discovered in the tephritid fly, Bactrocera dorsalis. This suggested the existence of piggyBac in additional insects and this study shows highly conserved, though not identical, piggyBac sequences in the noctuid species Heliocoverpa armigera, H. zea, and Spodoptera frugiperda, as well as new piggyBac sequences from the T. ni organismal genome. Genomic piggyBac elements could not be unambiguously identified in several other moth species indicating a discontinuous presence of piggyBac in the Lepidoptera. Most sequences have greater than 95% nucleotide identity to the original IFP2 piggyBac, except for a more diverged sequence in S. frugiperda, having approximately 78% identity. Variants of 1.3 and 0.8kb sequences found in both H. armigera and H. zea most likely became established by interbreeding, supporting the notion that the species are conspecific. None of the independent piggyBac sequences isolated from T. ni larval genomes are identical to IFP2, though all have an uninterrupted reading frame with the potential for encoding a functional transposase. The piggyBac sequences from T. ni and the Helicoverpa species, as well as those previously reported from B. dorsalis, all share three common nucleotide substitutions resulting in a single amino acid substitution in the transposase. This suggests that the original IFP2 piggyBac is a related variant of a predecessor element that became widespread. The existence of conserved piggyBac elements, some of which may have been transmitted horizontally between lepidopteran species, raises important considerations for the stability and practical use of piggyBac transformation vectors.

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.

An Anopheles transgenic sexing strain for vector control

Genetic manipulation of mosquito species that serve as vectors for human malaria is a prerequisite to the implementation of gene transfer technologies for the control of vector-borne diseases. Here we report on the development of transgenic sexing lines for the mosquito Anopheles stephensi, the principal vector of human malaria in Asia. Male mosquitoes, expressing enhanced green fluorescent protein (EGFP) under the control of the beta2-tubulin promoter, are identified by their fluorescent gonads in as early as their 3(rd) instar larval stage, and can be efficiently separated from females using both manual methods and automated sorting machines. Importantly, beta2-EGFP males are not impaired in their mating ability and viable fluorescent spermatozoa are also detected in spermathecae of wild-type females mated with transgenic males. The transgenic mosquito lines described here combine most of the features desired and required for a safe application of transgenic methodologies to malaria-control programs.

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.