The lepidopteran transposon vector, piggyBac, mediates germ-line transformation in the Mediterranean fruit fly

A new Minos vector for eye-specific expression of white+ marker in Ceratitis capitata and in distantly related dipteran species

The genetic transformation of insects by transposable elements is based on the use of selectable genetic markers required to identify transgenic individuals. Conserved regulatory sequences can be used to develop single constructs capable of adequate expression of a marker, across a range of different species. We present evidence that the Drosophila GBS regulatory element (Glass-binding site), derived from the Rh1 rhodopsin gene, is able to drive in vivo eye-specific expression of a Ccwhite+ transgene in the Mediterranean fruitfly Ceratitis capitata. The Ceratitis lineage diverged from that of Drosophila approximately 120 Myr ago. As the GBS regulatory sequence seems to be partially conserved in the more distantly related dipteran species Anopheles gambiae (250 Myr), we propose that the GBS may be widely useful for driving eye-specific expression in a wide range of dipteran species.

Evaluation of the activities of the medfly and Drosophila hsp70 promoters in vivo in germ-line transformed medflies

The promoter of the hsp70 gene of Drosophila melanogaster has been widely used for the expression of foreign genes in other insects. It has been generally assumed that because this gene is highly conserved, its promoter will function efficiently in other species. We report the results of a quantitative comparison of the activities of the medfly and D. melanogaster hsp70 promoters in vivo in transformed medflies. We constructed transformed lines containing the lacZ reporter gene under the control of the two promoters by using Minos-mediated germ-line transformation. The activity of each promoter was evaluated in 15 transformed lines by beta-galactosidase quantitative assays. The heat-inducible activity of the medfly promoter was found several times higher than the respective activity of the heterologous D. melanogaster promoter. These results were confirmed by northern blot analysis and indicate that the D. melanogaster promoter does not work efficiently in medfly. The -263/+105 medfly promoter region that was used in this study was found able to drive heat shock expression of the lacZ reporter gene in all stages of medfly, except early embryonic stages, in a similar fashion to the endogenous hsp70 genes. However the heat inducible RNA levels driven from this promoter region were significantly lower than the endogenous hsp70 RNA levels, suggesting that additional upstream and/or downstream sequences to the -263/+105 region may be necessary for optimum function of the medfly hsp70 promoter in vivo.

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.

Germ line transformation of the olive fly Bactrocera oleae using a versatile transgenesis marker

The olive fruit fly (olive fly) Bactrocera oleae (Dacus), recently introduced in North America, is the most destructive pest of olives worldwide. The lack of an efficient gene transfer technology for olive fly has hampered molecular analysis, as well as development of genetic techniques for its control. We have developed a Minos-based transposon vector carrying a self-activating cassette which overexpresses the enhanced green fluorescent protein (EGFP). Efficient transposase-mediated integration of one to multiple copies of this vector was achieved in the germ line of B. oleae by coinjecting the vector along with in vitro synthesized Minos transposase mRNA into preblastoderm embryos. The self-activating gene construct combined with transposase mRNA present a system with potential for transgenesis of very diverse species.

Formation and loss of large, unstable tandem arrays of the piggyBac transposable element in the yellow fever mosquito, Aedes aegypti

The Class II transposable element, piggyBac, was used to transform the yellow fever mosquito, Aedes aegypti. In two transformed lines only 15-30% of progeny inherited the transgene, with these individuals displaying mosaic expression of the EGFP marker gene. Southern analyses, gene amplification of genomic DNA, and plasmid rescue experiments provided evidence that these lines contained a high copy number of piggyBac transformation constructs and that much of this DNA consisted of both donor and helper plasmids. A detailed analysis of one line showed that the majority of piggyBac sequences were unit-length donor or helper plasmids arranged in a large tandem array that could be lost en masse in a single generation. Despite the presence of a transposase source and many intact donor elements, no conservative (cut and paste) transposition of piggyBac was observed in these lines. These results reveal one possible outcome of uncontrolled and/or unexpected recombination in this mosquito, and support the conclusion that further investigation is necessary before transposable elements such as piggyBac can be used as genetic drive mechanisms to move pathogen-resistance genes into mosquito populations.

piggyBac internal sequences are necessary for efficient transformation of target genomes

A previously reported piggyBac minimal sequence cartridge, which is capable of efficient transposition in embryo interplasmid transposition assays, failed to produce transformants at a significant frequency in Drosophila melanogaster compared with full-length or less extensive internal deletion constructs. We have re-examined the importance of these internal domain (ID) sequences for germline transformation using a PCR strategy that effectively adds increasing lengths of ID sequences to each terminus. A series of these piggyBac ID synthetic deletion plasmids containing the 3xP3-ECFP marker gene are compared for germline transformation of D. melanogaster. Our analyses identify a minimal sequence configuration that is sufficient for movement of piggyBac vectored sequences from plasmids into the insect genome. Southern hybridizations confirm the presence of the piggyBac transposon sequences, and insertion site analyses confirm these integrations target TTAA sites. The results verify that ID sequences adjacent to the 5' and 3' terminal repeat domains are crucial for effective germline transformation with piggyBac even though they are not required for excision or interplasmid transposition. Using this information we reconstructed an inverted repeat cartridge, ITR1.1k, and a minimal piggyBac transposon vector, pXL-BacII-ECFP, each of which contains these identified ID sequences in addition to the terminal repeat configuration previously described as essential for mobility. We confirm in independent experiments that these new minimal constructs yield transformation frequencies similar to the control piggyBac vector. Sequencing analyses of our constructs verify the position and the source of a point mutation within the 3' internal repeat sequence of our vectors that has no apparent effect on transformation efficiency.

In vivo gene transfer into the adult honeybee brain by using electroporation

The honeybee, Apis mellifera L., is a social insect and they show wide variety of exquisite social behaviors to maintain colony activity. To enable the elucidation of those social behaviors at a molecular level and gene function in the nervous system, we developed an in vivo method to perform gene transfer in the adult brain of living honeybee by electroporation. When green fluorescent protein-expressing plasmid was transferred to the brain with this system, green fluorescence was observed near the anode location. The expression of transfected genes was confirmed at both transcriptional and translational levels by reverse transcription-polymerase chain reaction and immunoblot analyses. This system will facilitate the analysis of gene function and the regulatory mechanisms of gene networks in the nervous system and provide clues to clarify the relation between those genes and the complex behaviors of the honeybee.

Recent advances in transgenic arthropod technology

The ability to insert foreign genes into arthropod genomes has led to a diverse set of potential applications for transgenic arthropods, many of which are designed to advance public health or improve agricultural production. New techniques for expressing foreign genes in arthropods have now been successfully used in at least 18 different genera. However, advances in field biology are lagging far behind those in the laboratory, and considerable work is needed before deployment in nature can be a reality. A mechanism to drive the gene of interest though a natural population must be developed and thoroughly evaluated before any field release, but progress in this area has been limited. Likewise, serious consideration of potential risks associated with deployment in nature has been lacking. This review gives an overview of the most promising techniques for expressing foreign genes in arthropods, considers the potential risks associated with their deployment, and highlights the areas of research that are most urgently needed for the field to advance out of the laboratory and into practice.

piggyBac transformation of the New World screwworm, Cochliomyia hominivorax, produces multiple distinct mutant strains

Sterile insect technique (SIT) programs are designed to eradicate pest species by releasing mass-reared, sterile insects into an infested area. The first major implementation of SIT was the New World Screwworm Eradication Program, which successfully eliminated the New World screwworm (NWS), Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), from the Continental US, Mexico and much of Central America. Ionizing radiation is currently used for sterilization, but transgenic insect techniques could replace this method, providing a safer, more cost-effective alternative. Genetic transformation methods have been demonstrated in NWS, and verified by Southern blot hybridization, PCR and sequencing of element insertion junctions. A lethal insertional mutation and enhancer detection-like phenotypic expression variations are presented and discussed. In addition to supporting the eradication efforts, transformation methods offer potential means to identify genes and examine gene function in NWS.

Medfly promoters relevant to the sterile insect technique

This review summarizes structural and functional studies on medfly promoters and regulatory elements that can be used for driving sex-specific, conditional and constitutive gene expression in this species. Sex-specific and conditional promoters are important for generating transgenic sexing strains that could increase the performance of the Sterile Insect Technique while strong constitutive promoters are necessary for developing sensitive transgenic marker systems. The review focuses on the functional analysis of the promoters of two male-specific and heat shock medfly genes. A special emphasis is put on the potential utility of these promoters for developing transgenic sexing strains.

Insect transgenesis and its potential role in agriculture and human health

The ability to genetically engineer insects other than Drosophila melanogaster has further extended modern genetic techniques into important insect pest species ranging from fruit fly pests of horticulture to mosquito vectors of human disease. In only a relatively short period of time, a range of transgenes have been inserted into more than 10 insect pest species. Genetic transformation of these pest species has proven to be a very important laboratory tool in analyzing gene function and effects on phenotype however the full extension of this technology into the field is yet to be realized. Here we briefly review the development of transgenic technology in pest insect species and discuss the challenges that remain in this applied area of insect genetics and entomology.

Medfly transposable elements: diversity, evolution, genomic impact and possible applications

The medfly genome has been shown to contain a rich assortment of transposable elements from the mariner, Tc1, hAT and gypsy/Ty3 families. These elements display different levels of diversity, abundance and distribution in the genome. The presence of actively transposing elements in the medfly genome is revealed by hybrid dysgenesis phenomena, insertion site polymorphisms and other genetic instabilities. The medfly has been a target of transformation studies involving the exogenous elements Minos, Hermes and piggyBac from three families. The presence of active endogenous homologous elements can have important implications for the stability of such transgenic lines. The potential applications of endogenous elements for medfly population analysis and control are discussed.

Bactrocera tryoni and closely related pest tephritids--molecular analysis and prospects for transgenic control strategies

Bactrocera tryoni is a serious pest of horticulture in eastern Australia. Here we review molecular data relevant to pest status and development of a transformation system for this species. The development of transformation vectors for non-drosophilid insects has opened the door to the possibility of improving the sterile insect technique (SIT), by genetically engineering factory strains of pest insects to produce male-only broods. Transposition assays indicate that all five of the vectors currently used for transformation in non-drosophilid species have the potential to be useful as transformation vectors in B. tryoni. Evidence of cross mobilization of hobo by an endogenous Homer element emphasises the necessity to understand the endogenous transposons within a species. The sex-specific doublesex and yolk protein genes have been characterized with a view to engineering a female-specific lethal gene or modifying gene expression through RNA interference (RNAi). Data are presented which indicate the potential of RNAi to modify the sex ratio of resultant broods. An understanding of how pest status is determined and maintained is being addressed through the characterization of genes of the circadian clock that enable the fly to adapt to environmental cues. Such an understanding will be useful in the future to the effective delivery of sophisticated pest control measures.

Understanding and improving transgene stability and expression in insects for SIT and conditional lethal release programs

Genetically transformed insect pests provide significant opportunities to create strains for improved sterile insect technique and new strategies based on conditional lethality. A major concern for programs that rely on the release of transgenic insects is the stability of the transgene, and maintenance of consistent expression of genes of interest within the transgene. Transgene instability would influence the integrity of the transformant strain upon which the effectiveness of the biological control program depends. Loss or intra-genomic transgene movement would result in strain attributes important to the program being lost or diminished, and the mass-release of such insects could significantly exacerbate the insect pest problem. Instability resulting in intra-genomic movement may also be a prelude to inter-genomic transgene movement between species resulting in ecological risks. This is less of a concern for short-term releases, where transgenic insects are not expected to survive in the environment beyond two or three generations. Transgene movement may occur, however, into infectious agents during mass-rearing, and the potential for movement after release is a possibility for programs using many millions of insects. The primary methods of addressing potential transgene instability relate to an understanding of the vector system used for gene transfer, the potential for its mobilization by the same or a related vector system, and methods required to identify transformants and determine if unexpected transgene movement has occurred. Methods also exist for preventing transposon-mediated mobilization, by deleting or rearranging vector sequences required for transposition using recombination systems. Stability of transgene expression is also a critical concern, especially in terms of potential epigenetic interactions with host genomes resulting in gene silencing that have been observed in plants and fungi, and it must be determined if this or related phenomena can occur in insects.

New and highly efficient method for silkworm transgenesis usingAutographa californica nucleopolyhedrovirus andpiggyBac transposable elements

We have developed a new method for the transgenesis of the silkworm, Bombyx mori. This method couples the use of recombinant baculoviruses with the use of the piggyBac transposable element. One recombinant AcNPV, designated the helper virus, is designed to express the piggyBac transposase under the control of the Drosophila hsp70 promoter. Another recombinant AcNPV encoded the gene to be incorporated into the silkworm genome, in this case a green fluorescent protein (GFP) gene, under the control of B. mori actin A3 promoter and franked by the piggyBac inverted terminal repeats. Preblastoderm eggs were inoculated with a fine needle coated with a mixture of these two recombinant baculoviruses. Most of the inoculated larvae hatched and a high proportion of the newly hatched G0 larvae expressed the GFP marker. Transgenesis was confirmed by Southern blot analysis of G1 insects, sequencing the insertion site junctions isolated by inverse PCR, and the marker segregated in Mendelian fashion, as evidenced by the appearance of green fluorescence in G2 insects. Thus, transgenic silkworms were easily and efficiently obtained using this new method.

Targeted Gene Expression Using the GAL4/UAS System in the Silkworm Bombyx mori

The silkworm Bombyx mori is one of the most well-studied insects in terms of both genetics and physiology and is recognized as the model lepidopteran insect. To develop an efficient system for analyzing gene function in the silkworm, we investigated the feasibility of using the GAL4/UAS system in conjunction with piggyBac vector-mediated germ-line transformation for targeted gene expression. To drive the GAL4 gene, we used two endogenous promoters that originated from the B. mori actin A3 (BmA3) and fibroin light-chain (FiL) genes and the artificial promoter 3xP3. GFP was used as the reporter. In initial tests of the function of the GAL4/UAS system, we generated transgenic animals that carried the UAS-GFP construct plus either BmA3-GAL4 or 3xP3-GAL4. GFP fluorescence was observed in the tissues of GFP-positive animals, in which both promoters drove GAL4 gene expression. Animals that possessed only the GAL4 gene or UAS-GFP construct did not show GFP fluorescence. In addition, as a further test of the ability of the GAL4/UAS system to drive tissue-specific expression we constructed FiL-GAL4 lines with 3xP3-CFP as the transformation marker. FiL-GAL4 × UAS-GFP crosses showed GFP expression in the posterior silk gland, in which the endogenous FiL gene is normally expressed. These results show that the GAL4/UAS system is applicable to B. mori and emphasize the potential of this system for controlled analyses of B. mori gene function.

piggyBac-mediated germline transformation in the beetle Tribolium castaneum

The lepidopteran transposable element piggyBac can mediate germline insertions in at least four insect orders. It therefore shows promise as a broad-spectrum transformation vector, but applications such as enhancer trapping and transposon-tag mutagenesis are still lacking. We created, cloned, sequenced and genetically mapped a set of piggyBac insertions in the red flour beetle, Tribolium castaneum. Transpositions were precise, and specifically targeted the canonical TTAA recognition sequence. We detected several novel reporter-expression domains, indicating that piggyBac could be used to identify enhancer regions. We also demonstrated that a primary insertion of a non-autonomous element can be efficiently remobilized to non-homologous chromosomes by injection of an immobile helper element into embryos harbouring the primary insertion. These developments suggest potential for more sophisticated methods of piggyBac-mediated genome manipulation.

Post-integration behavior of a Mos1 mariner gene vector in Aedes aegypti

The post-integration behavior of insect gene vectors will determine the types of applications for which they can be used. Transposon mutagenesis, enhancer trapping, and the use of transposable elements as genetic drive systems in insects requires transposable elements with high rates of remobilization in the presence of transposase. We investigated the post-integration behavior of the Mos1 mariner element in transgenic Aedes aegypti by examining both germ-line and somatic transpositions of a non-autonomous element in the presence of Mos1 transposase. Somatic transpositions were occasionally detected while germ-line transposition was only rarely observed. Only a single germ-line transposition event was recovered after screening 14,000 progeny. The observed patterns of transposition suggest that Mos1 movement takes place between the S phase and anaphase. The data reported here indicate that Mos1 will be a useful vector in Ae. aegypti for applications requiring a very high degree of vector stability but will have limited use in the construction of genetic drive, enhancer trap, or transposon tagging systems in this species.

United States Department of Agriculture-Agricultural Research Service: advances in the molecular genetic analysis of insects and their application to pest management

USDA-ARS scientists have made important contributions to the molecular genetic analysis of agriculturally important insects, and have been in the forefront of using this information for the development of new pest management strategies. Advances have been made in the identification and analysis of genetic systems involved in insect development, reproduction and behavior which enable the identification of new targets for control, as well as the development of highly specific insecticidal products. Other studies have been on the leading edge of developing gene transfer technology to better elucidate these biological processes though functional genomics and to develop new transgenic strains for biological control. Important contributions have also been made to the development and use of molecular markers and methodologies to identify and track insect populations. The use of molecular genetic technology and strategies will become increasingly important to pest management as genomic sequencing information becomes available from important pest insects, their targets and other associated organisms.

Germline transformation of the sawfly, Athalia rosae (Hymenoptera: Symphyta), mediated by a piggyBac-derived vector

A piggyBac construct carrying two green fluorescent protein (GFP)-coding sequences one driven by Bombyx mori actin gene promoter and the other by Drosophila melanogaster heat-shock protein 70 (hsp70) promoter were injected together with a nonautonomous helper plasmid containing an active piggyBac transposase gene into the posterior end of mature unfertilized eggs dissected from the ovaries of Athalia rosae (Hymenoptera: Symphyta). These injected eggs, which developed as haploid male embryos upon artificial activation, were cultured to adulthood. Of 278 injected eggs, 61 grew to G(0) haploid adult males. These G(0) haploid males were individually mated to diploid females. The progeny embryos (G(1) generation) were examined for GFP expression. Four GFP-positive embryos (from three independent G(0) matings) were obtained. Two eclosed as diploid adult G(1) females. Mature unfertilized eggs dissected from the GFP-positive G(1) diploid females were activated artificially, and the resultant embryos were examined for GFP expression, separated and cultured to adulthood (G(2) generation). The G(2) haploid embryos segregated to GFP-positive and -negative individuals. By mating the G(2) adult haploid males individually to diploid females, stocks were established in which the piggyBac construct was stably integrated into the genome, as evidenced by GFP expression and Southern blot hybridization. The piggyBac transposition occurred at its canonical target TTAA sequence. These results, which demonstrate the first successful stable transposon-mediated germline transformation in Hymenoptera, will expand the usefulness of the piggyBac vector.

Developmental profiles and ecdysone regulation of the mRNAs for two ecdysone receptor isoforms in the Mediterranean fruit fly Ceratitis capitata

Using 5' RACE with specific primers for the ecdysone receptor B1 isoform of the Mediterranean fruit fly (medfly), Ceratitis capitata, we isolated a cDNA clone encoding the specific region of the medfly ecdysone receptor A isoform (CcEcR-A). The CcEcR-A-specific region was very similar to the EcR-A-specific region of Drosophila melanogaster and less similar to the EcR-A-specific regions of Lepidoptera. The developmental expression of both CcEcR-A and CcEcR-B1 mRNAs was studied in whole animals, salivary glands and ovaries by RT-PCR, using isoform-specific primers. Both CcEcR mRNAs are present in very early embryos, decrease to very low levels during the first hours of embryogenesis and are highly expressed in all consequent embryonic stages. During metamorphosis both isoforms are present showing two peaks; the first at the larval-prepupal transition and the second during the second half of prepupal development. These peaks are correlated with the two puffing cycles and the two major 20-hydroxyecdysone (20E) increases that occur during medfly metamorphosis. CcEcR-B1 mRNA was directly induced in larval salivary glands in vitro by 20E, even at very low concentrations of the hormone, while CcEcR-A mRNA was slightly induced only by high 20E concentrations and in the absence of a protein synthesis inhibitor. During oogenesis, the CcEcR mRNAs were expressed synchronously, peaking at the beginning of both previtellogenic and vitellogenic phases.

Germline transformants spreading out to many insect species

The past 5 years have witnessed significant advances in our ability to introduce genes into the genomes of insects of medical and agricultural importance. A number of transposable elements now exist that are proving to be sufficiently robust to allow genetic transformation of species within three orders of insects. In particular all of these transposable elements can be used genetically to transform mosquitoes. These developments, together with the use of suitable genes as genetic markers, have enabled several genes and promoters to be transferred between insect species and their effects on the phenotype of the transgenic insect determined. Within a very short period of time, insights into the function of insect promoters in homologous and heterologous insect species are being gained. Furthermore, strategies aimed at ameliorating the harmful effects of pest insects, such as their ability to vector human pathogens, are now being tested in the pest insects themselves. We review the progress that has been made in the development of transgenic technology in pest insect species and conclude that the repertoire of transposable element-based genetic tools, long available to Drosophila geneticists, can now be applied to other insect species. In addition, it is likely that these developments will lead to the generation of pest insects that display a significantly reduced ability to transmit pathogens in the near future.

Extrachromosomal transposition of the transposable element Minos in embryos of the cricket Gryllus bimaculatus

Effective germline transformation of insects has been shown to depend on the right choice of transposon system and selection marker. In this study the promoter region of a Gryllus cytoplasmic actin (GbA3/4) gene was isolated and characterized, and was used to drive the expression of Minos transposase in embryos of the cricket Gryllus bimaculatus. Active Minos transposase was produced in these embryos as monitored through established transposon excision and interplasmid transposition assays. In contrast, Drosophila melanogaster hsp70 promoter, previously used to express Minos transposase in a number of insect species and insect cell lines, failed to produce any detectable Minos transposase activity, as recorded by using the very sensitive transposon excision assay. In addition, the GbA3/4 promoter was found to drive expression of enhanced green fluorescent protein (eGFP) predominantly in vitellophages of the developing Gryllus eggs when a plasmid carrying a GbA3/4 promoter-eGFP fusion gene was transiently injected into embryos. These results strongly support the use of Minos transposons marked with the GbA3/4 promoter-eGFP for the genetic transformation of this emerging model insect species.

Fluorescent transformation markers for insect transgenesis

The first effectively achieved germ-line transformations of non-drosophilid insects were based on mutant rescue of eye color phenotypes. However, for most insect species neither visible mutants nor corresponding cloned genes are available. Therefore, the development of broadly applicable and reliable transformation markers will be of great importance to fully exploit the enormous potential transgenic insect technology has to offer. Here we review transposon-mediated germ-line transformation approaches that employ green fluorescent protein (GFP) variants to identify successful gene transfer. Furthermore, we provide novel data on the use of DsRed as an additional red fluorescent transformation marker for insect transgenesis. In conclusion, fluorescent proteins controlled by suitable strong promoters possess ideal characteristics to serve as transformation markers for a wide range of insect species.

Genetic engineering in insects of agricultural importance

The past five years have witnessed the extension of genetic transformation techniques into 11 insect species covering four orders within the Insecta. While the robustness of these transformation systems can be improved, there is now a highly likely probability that transformation of a given insect species will ensue, provided transposable element-containing plasmid DNA can be effectively delivered to the embryo or some other life stage. These developments have shifted emphasis to concerns of transgene stability and the regulation of the rearing and release of these transgenic insects. They have also led to some elegant demonstrations of genetic sexing mechanisms in Drosophila melanogaster with the expectation that similar systems be extended into pest insect species. These developments and issues are discussed in this short review.

Transgene expression from the Tribolium castaneum Polyubiquitin promoter

The highly conserved Ubiquitin proteins are expressed from genes with strong, constitutively active promoters in many species, making these promoters attractive candidates for use in driving transgene expression. Here we report the cloning and characterization of the Tribolium castaneum Polyubiquitin (TcPUb) gene. We placed the TcPUb promoter upstream of the coding region of the T. castaneum eye-colour gene Tc vermilion (Tcv) and injected this construct into embryos from a Tcv-deficient strain. Transient expression of Tcv during embryogenesis resulted in complete rescue of the larval mutant phenotype. We then incorporated the TcPUb-Tcv chimera into a piggyBac donor. Resulting germline transformants were easily recognized by rescue of eye pigmentation, illustrating the potential of the TcPUb promoter for use in driving transgene expression.

Use of the piggyBac transposon for germ-line transformation of insects

Germ-line transformation of insects is now possible with four independent transposable element vector systems. Among these, the TTAA-insertion site specific transposon, piggyBac, discovered in Trichoplusia ni, is one of the most widely used. Transformations have been achieved in a wide variety of dipterans, lepidopterans, and a coleopteran, and for many species, piggyBac transposition was first tested by plasmid-based mobility assays in cell lines and embryos. All plasmid and genomic insertions are consistent with the duplication of a TTAA insertion site, and most germ-line integrations appear to be stable, though this is largely based on stable marker phenotypes. Of the vector systems presently in use for non-drosophilids, piggyBac is the only one not currently associated with a superfamily of transposable elements, though other elements exist that share its TTAA insertion site specificity. While functional piggyBac elements have only been isolated from T. ni, nearly identical elements have been discovered in a dipteran species, Bactrocera dorsalis, and closely related elements exist in another moth species, Spodoptera frugiperda. It appears that piggyBac has recently traversed insect orders by horizontal transmission, possibly mediated by a baculovirus or other viral system. This interspecies movement has important implications for the practical use of piggyBac to create transgenic insect strains for field release.

Germ-line transformation of the South American malaria vector, Anopheles albimanus, with a piggyBac/EGFP transposon vector is routine and highly efficient

Stable and efficient germ-line transformation was achieved in the South American malaria vector, Anopheles albimanus, using a piggyBac vector marked with an enhanced green fluorescent protein gene regulated by the Drosophila melanogaster polyubiquitin promoter. Transgenic mosquitoes were identified from four independent experiments at frequencies ranging from 20 to 43% per fertile G0. Fluorescence was observable throughout the body of larvae and pupae, and abdominal segments of adults. Transgenic lines analysed by Southern hybridization had one to six germ-line integrations, with most lines having three or more integrations. Hybridized transposon vector fragments and insertion site sequences were consistent with precise piggyBac-mediated integrations, although this was not verified for all lines. The piggyBac/PUbnlsEGFP vector appears to be a robust transformation system for this anopheline species, in contrast to the use of a piggyBac vector in An. gambiae. Further tests are needed to determine if differences in anopheline transformation efficiency are due to the marker systems or to organismal or cellular factors specific to the species.

High frequencies of Minos transposon mobilization are obtained in insects by using in vitro synthesized mRNA as a source of transposase

One of the most frequently encountered problems in transposon-mediated transgenesis is low transformation frequency, often resulting from difficulty in expressing from injected plasmid DNA constructs adequate levels of transposase in embryos. Capped RNA corresponding to the spliced transcript of the Minos transposable element has been synthesized in vitro and shown to be an effective source of transposase protein for Minos transposon mobilization. Transposase produced by this mRNA is shown to catalyze excision of a Minos transposon from plasmid DNA in Medfly embryos. When injected into Drosophila or Medfly embryos, transposase mRNA leads to a several-fold increase in transformation efficiencies compared with injected plasmids expressing transposase. Also, frequent mobilization of a Minos transposon from the X chromosome into autosomes was demonstrated after injections of Minos transposase mRNA into pre-blastoderm Drosophila embryos. The high rates of transposition achieved with transposase mRNA suggest that this is a powerful system for genetic applications in Drosophila and other insects.

Transposable element-mediated transgenesis in insects beyond Drosophila

In the last few years, cases of transformation involving insects other than Dipterans have been reported. Although transgenics have been created only in a few species, transposable element vectors may be successfully developed in most insect forms in the near future. The major remaining problems revolving round transformation in wide-ranging species of insects are mainly related to methods of DNA delivery. Transposable element-mediated gene transfer in non-Drosophila insects is reviewed. In addition, the current status of honeybee transformation will be explained as an example of an insect transgenic system that faces substantial obstacles to the creation of germ-line transformants.

Germ line transformation of the yellow fever mosquito, Aedes aegypti, mediated by transpositional insertion of a piggyBac vector

Mosquito-vectored diseases such as yellow fever and dengue fever continue to have a substantial impact on human populations world-wide. Novel strategies for control of these mosquito vectored diseases can arise through the development of reliable systems for genetic manipulation of the insect vector. A piggyBac vector marked with the Drosophila melanogaster cinnabar (cn) gene was used to transform the white-eyed khw strain of Aedes aegypti. Microinjection of preblastoderm embryos resulted in four families of cinnabar transformed insects. An overall transformation frequency of 4%, with a range of 0% to as high as 13% for individual experiments, was achieved when using a heat-shock induced transposase providing helper plasmid. Southern hybridizations indicated multiple insertion events in three of four transgenic lines, while the presence of duplicated target TTAA sites at either ends of individual insertions confirmed characteristic piggyBac transposition events in these three transgenic lines. The transgenic phenotype has remained stable for more than twenty generations. The transformations effected using the piggyBac element establish the potential of this element as a germ-line transformation vector for Aedine mosquitoes.

Unexpected stability of mariner transgenes in Drosophila

A number of mariner transformation vectors based on the mauritiana subfamily of transposable elements were introduced into the genome of Drosophila melanogaster and examined for their ability to be mobilized by the mariner transposase. Simple insertion vectors were constructed from single mariner elements into which exogenous DNA ranging in size from 1.3 to 4.5 kb had been inserted; composite vectors were constructed with partial or complete duplications of mariner flanking the exogenous DNA. All of the simple insertion vectors showed levels of somatic and germline excision that were at least 100-fold lower than the baseline level of uninterrupted mariner elements. Although composite vectors with inverted duplications were unable to be mobilized at detectable frequencies, vectors with large direct duplications of mariner could be mobilized. A vector consisting of two virtually complete elements flanking exogenous DNA yielded a frequency of somatic eye-color mosaicism of approximately 10% and a frequency of germline excision of 0.04%. These values are far smaller than those observed for uninterrupted elements. The results imply that efficient mobilization of mariner in vivo requires the presence and proper spacing of sequences internal to the element as well as the inverted repeats.

Germ-line transformation of the Australian sheep blowfly Lucilia cuprina

The Australian sheep blowfly, Lucilia cuprina, is the most important economic insect pest for the sheep industries in Australia and New Zealand. piggyBac-mediated germ-line transformation of L. cuprina was achieved with a helper plasmid that had the Drosophila melanogaster hsp70 promoter controlling expression of the transposase and a piggyBac vector with an EGFP marker gene. Two transformant lines were obtained, at a frequency of approximately 1-2% per fertile G0. One of these lines has a single copy of the transgene, the other most likely has four copies. This is the first report of germ-line transformation of L. cuprina and is an important step towards the generation of engineered strains that would be suitable for male-only release eradication/suppression programmes.

Germline transformation of the malaria vector, Anopheles gambiae, with the piggyBac transposable element

Germline transformation of the major African malaria vector, Anopheles gambiae, was achieved using the piggyBac transposable element marked with the enhanced green fluorescent protein (EGFP) injected into mosquito embryos. Two G1 generation male mosquitoes expressing EGFP were identified among 34 143 larvae screened. Genomic Southern data and sequencing of the piggyBac insertion boundaries showed that these two males arose from one piggyBac insertion event in the injected G0 embryos. Genetic cross data suggest that the insertion site of the element either resulted in, or is tightly linked to, a recessive lethal. This was demonstrated by a deficiency in the number of EGFP-expressing offspring from inbred crosses but expected ratios in outcrosses to non-transformed individuals and failure to establish a pure-breeding line. The insertion was weakly linked to the collarless locus on chromosome 2 and was shown by in situ hybridization to be located in division 28D of that chromosome. Particularly high levels of expression were observed uniformly in salivary glands and, in most individuals, in the anterior stomach. An improvement in the injection technique at the end of the studies resulted in increased G0 hatching, transient expression and EGFP-expression rates among G1 progeny.

Efficient transformation of the yellow fever mosquito Aedes aegypti using the piggyBac transposable element vector pBac[3xP3-EGFP afm]

We report efficient germ-line transformation in the yellow fever mosquito Aedes aegypti accomplished using the piggyBac transposable element vector pBac[3xP3-EGFP afm]. Two transgenic lines were established and characterized; each contained the Vg-Defensin A transgene with strong eye-specific expression of the enhanced green fluorescent protein (EGFP) marker gene regulated by the artificial 3xP3 promoter. Southern blot hybridization and inverse PCR analyses of genomic DNA demonstrated a precise piggyBac-mediated, single copy insertion of the pBac[3xP3-EGFP afm,Vg-DefA] transposon in each transgenic line. For each line, genetic analysis confirmed stability and integrity of the entire transposon construct in the mosquito genome through the G2-G6 generations. Successful establishment of homozygous transgenic lines indicated that in both cases a non-lethal integration of the transposon into the mosquito genome had occurred. The 3xP3-EGFP marker was tested in mosquitoes with different genetic backgrounds. In white-eyed transgenic mosquitoes, the strong eye-specific expression of GFP was observed throughout all stages of development, starting from newly hatched first instar larvae to adults. A similar level and pattern of fluorescence was observed in red-eyed mosquitoes that were generated by crossing the 3xP3-EGFP transformants with the kh(w) white-eye mosquitoes transformed with the Drosophila cinnabar gene. Importantly, the utility of the 3xP3-EGFP, as marker gene for transformation of wild type mosquitoes, was demonstrated by strong eye-specific GFP expression in larval and pupal stages of black-eyed hybrids of the 3xP3-EGFP white-eye transformants and the wild type Rockefeller/UGAL strain. Finally, analysis of the Vg-DefA transgene expression in transformants from two established lines demonstrated strong blood-meal activation and fat-body-specific expression regulated by the Vg 1.8-kb 5' upstream region.

Ecdysone-regulated chromosome puffing in the salivary glands of the Mediterranean fruit fly, Ceratitis capitata

The effect of ecdysone on the puffing activity of the polytene chromosomes of Ceratitis capitata has been studied in organ cultures of late-larval salivary glands. Culture of glands from 120-h-old larvae (puff stage 1) in the presence of ecdysone resulted in the initiation of the late-larval puffing cycle that is normally observed in 145-h-old larvae (puff stage 4). During a 7-h period in the presence of ecdysone, the puffing patterns of most loci resembled the in vivo patterns observed in the period between puff stages 4 and 10, indicating that the first puffing cycle can be initiated by the hormone and proceed almost to completion, in vitro. Culture of salivary glands in the presence of ecdysone and a protein-synthesis inhibitor, as well as ecdysone withdrawal and readdition experiments, indicated that most of the ecdysone-regulated puffs could be categorized into three classes: (i) the puffs that were suppressed immediately by ecdysone, even in the absence of protein synthesis; (ii) the puffs that were induced directly by ecdysone; and (iii) the puffs that were induced indirectly by ecdysone, that is, they were induced after a lag period of a few hours and required protein synthesis for their induction.

The nonA gene in Drosophila conveys species-specific behavioral characteristics

The molecular basis of species-specific differences in courtship behavior, a critical factor in preserving species boundaries, is poorly understood. Genetic analysis of all but the most closely related species is usually impossible, given the inviability of hybrids. We have therefore applied interspecific transformation of a single candidate behavioral locus, no-on-transient A (nonA), between Drosophila virilis and D. melanogaster, to investigate whether nonA, like the period gene, might encode species-specific behavioral information. Mutations in nonA can disrupt both visual behavior and the courtship song in D. melanogaster. The lovesong of nonA(diss) mutant males superficially resembles that of D. virilis, a species that diverged from D. melanogaster 40-60 mya. Transformation of the cloned D. virilis nonA gene into D. melanogaster hosts carrying a synthetic deletion of the nonA locus restored normal visual function (the phenotype most sensitive to nonA mutation). However, the courtship song of transformant males showed several features characteristic of the corresponding D. virilis signal, indicating that nonA can act as a reservoir for species-specific information. This candidate gene approach, together with interspecific transformation, can therefore provide a direct avenue to explore potential speciation genes in genetically and molecularly tractable organisms such as Drosophila.

Isolation and characterization of the Xanthine dehydrogenase gene of the Mediterranean fruit fly, Ceratitis capitata

Xanthine dehydrogenase (XDH) is a member of the molybdenum hydroxylase family of enzymes catalyzing the oxidation of hypoxanthine and xanthine to uric acid. The enzyme is also required for the production of one of the major Drosophila eye pigments, drosopterin. The XDH gene has been isolated in many species representing a broad cross section of the major groups of living organisms, including the cDNA encoding XDH from the Mediterranean fruit fly Ceratitis capitata (CcXDH) described here. CcXDH is closely related to other insect XDHs and is able to rescue the phenotype of the Drosophila melanogaster XDH mutant, rosy, in germline transformation experiments. A previously identified medfly mutant, termed rosy, whose phenotype is suggestive of a disruption in XDH function, has been examined for possible mutations in the XDH gene. However, we find no direct evidence that a mutation in the CcXDH gene or that a reduction in the CcXDH enzyme activity is present in rosy medflies. Conclusive studies of the nature of the medfly rosy mutant will require rescue by germline transformation of mutant medflies.

Gene targeting in mosquito cells: a demonstration of 'knockout' technology in extrachromosomal gene arrays

BACKGROUND

Gene targeting would offer a number of advantages over current transposon-based strategies for insect transformation. These include freedom from both position effects associated with quasi-random integration and concerns over transgene instability mediated by endogenous transposases, independence from phylogenetic restrictions on transposon mobility and the ability to generate gene knockouts.

RESULTS

We describe here our initial investigations of gene targeting in the mosquito. The target site was a hygromycin resistance gene, stably maintained as part of an extrachromosomal array. Using a promoter-trap strategy to enrich for targeted events, a neomycin resistance gene was integrated into the target site. This resulted in knockout of hygromycin resistance concurrent with the expression of high levels of neomycin resistance from the resident promoter. PCR amplification of the targeted site generated a product that was specific to the targeted cell line and consistent with precise integration of the neomycin resistance gene into the 5' end of the hygromycin resistance gene. Sequencing of the PCR product and Southern analysis of cellular DNA subsequently confirmed this molecular structure.

CONCLUSIONS

These experiments provide the first demonstration of gene targeting in mosquito tissue and show that mosquito cells possess the necessary machinery to bring about precise integration of exogenous sequences through homologous recombination. Further development of these procedures and their extension to chromosomally located targets hold much promise for the exploitation of gene targeting in a wide range of medically and economically important insect species.

High amplification of a densovirus-derived vector in larval and adult tissues of Drosophila

The Lepidopteran densovirus-derived vector, pJlacZDeltaNS3, is a defective virus genome with an insertion of lacZ DNA in the viral structural protein coding sequence, and a deletion of the sequence coding the non-structural polypeptide NS3. pJlacZDeltaNS3 was injected into Drosophila eggs and the maintenance of the viral genome was monitored by expression of beta-galactosidase and by Southern blot hybridizations. Intense beta-galactosidase activity was observed in many somatic tissues of third-instar larvae and adult flies, in more than 60% of the injected animals. DNA analyses showed that staining in adult tissues correlated with the amplification of the vector. Together, these results suggest the occurrence of early events of integration of the vector into the Drosophila host genome.

Expression and function of the Drosophila melanogaster ADH in male Ceratitis capitata adults: a potential strategy for medfly genetic sexing based on gene-transfer technology

The aim of development of a Mediterranean fruit fly Ceratitis capitata genetic sexing strain derives from the large scale SIT programmes being carried out to control this pest. Toward this direction, we present here the male-specific expression of the Drosophila melanogaster alcohol dehydrogenase (ADH) in medfly transgenic adults generated by Minos-mediated germ line transformation. This expression pattern is obtained by using a promoter fragment of the male-specific gene MSSP-alpha2 of the medfly. We show that the heterologous enzyme is functional in the medfly oxidizing both ethanol and 2-propanol. Although leading to an approximately twofold increase of total ADH activity in male compared to female transgenic adults, these expression levels are not enough for performing genetic sexing when high doses of environmental alcohol are applied. This could be achieved either by further enhancement of the transgene expression or by generating an Adh- line to host the Minos insertions.

The chorion genes of the medfly. II. DNA sequence evolution of the autosomal chorion genes s18, s15, s19 and s16 in Diptera

We present a total of approximately 15 kb of DNA sequences, encompassing four chorion genes Ccs18, Ccs15, Ccs19, Cc16 and their flanking DNA in the medfly C. capitata. Comparison of coding regions, introns and intergenic sequences in five Dipteran species, D. melanogaster, D. subobscura, D. virilis, D. grimshawi and C. capitata documented an extensive divergence in introns and coding regions, but few well conserved elements in the proximal 5' flanking regions in all species. These elements are related to conserved regulatory features of three of the genes, including tissue- and temporal regulation. In the fourth, gene s15, significant alterations in the 5' flanking region may be responsible for its changed temporal regulation in C. capitata. One long intergenic sequence, located in the distal 5' flanking region of gene s18, is homologous to ACE3, a major amplification control element and contains an 80-bp A/T-rich sequence, known to stimulate strong binding of the origin recognition complex (ORC) in D. melanogaster. Analysis of the nucleotide composition of all chorion genes in C. capitata and D. melanogaster showed that C. capitata exhibit less biased representation of synonymous codons than does D. melanogaster.

Hermes-mediated germ-line transformation of the Mediterranean fruit fly Ceratitis capitata

We report the use of the Hermes transposable element for germ-line transformation of the Mediterranean fruit fly, Ceratitis capitata. Hermes was able to genetically transform this insect at an estimated frequency between 0.6 and 1.1%, which is comparable to the transformation frequencies obtained for this species when using other transposable elements. Hermes integrates into the medfly genome by a cut-and-paste mechanism and the sequences integrated into the genome are delimited by the terminal nucleotides of the Hermes inverted terminal repeats. Integration resulted in the generation of 8 bp target site duplications, the sequences of which conformed to the target site duplications generated by hAT element transposition in insects. The Hermes element is one additional genetic tool that can be deployed in manipulating and characterizing the medfly genome.

Genetic transformation of the housefly Musca domestica with the lepidopteran derived transposon piggyBac

The piggyBac transposable element was successfully used for stable genetic transformation of the housefly Musca domestica. The construct contains the EGFP marker under the control of Pax-6 binding sites, which can drive eye-specific expression in insect species as distantly related as Drosophila melanogaster and Tribolium castaneum [Berghammer, A.J., Klingler, M. and Wimmer, E.A. (1999) Nature 402: 370-371]. We obtained seven independent integration events among 41 fertile G0 Musca flies. Most of the transformed lines contained two or more chromosomal insertions of the EGFP marker which were stably inherited over more than 15 generations. piggyBac-mediated transposition was verified by identifying the characteristic TTAA duplication at the insertion sites. This first report of stable transmission of a genetic marker in Musca confirms the use of this vector-marker system for effective gene transfer in a broad range of insect species.

Genomic organization and characterization of the white locus of the Mediterranean fruitfly, Ceratitis capitata

An approximately 14-kb region of genomic DNA encoding the wild-type white eye (w+) color gene from the medfly, Ceratitis capitata has been cloned and characterized at the molecular level. Comparison of the intron-exon organization of this locus among several dipteran insects reveals distinct organizational patterns that are consistent with the phylogenetic relationships of these flies and the dendrogram of the predicted primary amino acid sequence of the white loci. An examination of w+ expression during medfly development has been carried out, displaying overall similarity to corresponding studies for white gene homologues in Drosophila melanogaster and other insects. Interestingly, we have detected two phenotypically neutral allelic forms of the locus that have arisen as the result of an apparently novel insertion or deletion event located in the large first intron of the medfly white locus. Cloning and sequencing of two mutant white alleles, w1 and w2, from the we,wp and M245 strains, respectively, indicate that the mutant conditions in these strains are the result of independent events--a frameshift mutation in exon 6 for w1 and a deletion including a large part of exon 2 in the case of w2.

A current perspective on insect gene transformation

The genetic transformation of non-drosophilid insects is now possible with several systems, with germ-line transformation reported in published and unpublished accounts for about 12 species using four different transposon vectors. For some of these species, transformation can now be considered routine. Other vector systems include viruses and bacterial symbionts that have demonstrated utility in species and applications requiring transient expression, and for some, the potential exists for genomic integration. Many of these findings are quite recent, presenting a dramatic turning point in our ability to study and manipulate agriculturally and medically important insects. This review discusses these findings from the perspective of all the contributions that has made this technology a reality, the research that has yet to be done for its safe and efficient use in a broader range of species, and an overview of the available methodology to effectively utilize these systems.

Transformation of the Caribbean fruit fly, Anastrepha suspensa, with a piggyBac vector marked with polyubiquitin-regulated GFP

Germ-line transformation was achieved in the Caribbean fruit fly, Anastrepha suspensa, using a piggyBac vector marked with an enhanced green fluorescent protein gene regulated by the Drosophila melanogaster polyubiquitin promoter. Four transgenic G(0) lines were selected exhibiting unambiguous GFP expression. Southern hybridization indicated the presence of one to four integrations in each of the transgenic lines with two integrations verified as piggyBac-mediated by sequencing their insertion sites. Fluorescence was detectable throughout development, and in adults was most intense from the thoracic flight muscle. Although adult cuticle quenched fluorescence, GFP was routinely detectable in the thorax. A quantitative spectrofluorometric assay was developed for GFP fluorescence that indicated differing levels of fluorescence among the transgenic lines, suggesting some level of position effect variegation/suppression. These results are encouraging for the use of this marker system in insect species not amenable to mutation-based visible markers. Together with the piggyBac vector, a transformation system is presented that has the potential to be universally applicable in insect species.

Genetic transformation systems in insects

The past 5 years have witnessed the emergence of techniques that permit the stable genetic transformation of a number of non-drosophilid insect species. These transposable-element-based strategies, together with virus-based techniques that allow the expression of genes to be quickly examined in insects, provide insect scientists with a first generation of genetic tools that can begin to be harnessed to further increase our understanding of gene function and regulation in insects. We review and compare the characteristics of these gene transfer systems and conclude that, although significant progress has been made, these systems still do not meet the requirements of robust genetic tools. We also review risk assessment issues arising from the generation and probable release of genetically engineered insects.

Methods for marking insects: current techniques and future prospects

Tracking the movement of insects in their natural habitat is essential for understanding their basic biology, demography, and ethology. A wide variety of markers have been used to assess insect population dynamics, dispersal, territoriality, feeding behavior, trophic-level interactions, and other ecological interactions. The ideal marker should persist without inhibiting the insect's "normal" biology. Furthermore, the marker should be environmentally safe, cost-effective, and easy to use. In this article, we review the current state of knowledge regarding insect marking, document the advantages and limitations of each marking technique, and discuss advances made in marking insects over the past decade.

Sperm-mediated transformation of the honey bee, Apis mellifera

Our primary objective was to identify techniques to transform the genome of the honey bee (Apis mellifera) with foreign DNA constructs. The strategy we adopted was to linearize foreign DNA and introduce it with sperm during the instrumental insemination of virgin queen honey bees. We analysed extracts from larvae within the same cohort and isolated the predicted fragment by means of PCR amplification of genomic DNA. Larvae that carried the construct also expressed the introduced DNA. We propagated several transgenic lines for up to three generations, which demonstrates its heritability. Once carried by a queen, the construct can be detected in that queen's larvae over several months. However, there was no evidence of integration of the construct, at least as determined by genomic Southern analysis. Nevertheless, this demonstrates the general viability of the technique for introduction of DNA, and it should be augmented by further use of transposable elements that enhance integration.