Germline transformation of Drosophila virilis mediated by the transposable element hobo

Highly efficient integration and expression of piggyBac-derived cassettes in the honeybee (Apis mellifera)

Honeybees (Apis mellifera), which are important pollinators of plants, display remarkable individual behaviors that collectively contribute to the organization of a complex society. Advances in dissecting the complex processes of honeybee behavior have been limited in the recent past due to a lack of genetic manipulation tools. These tools are difficult to apply in honeybees because the unit of reproduction is the colony, and many interesting phenotypes are developmentally specified at later stages. Here, we report highly efficient integration and expression of piggyBac-derived cassettes in the honeybee. We demonstrate that 27 and 20% of queens stably transmitted two different expression cassettes to their offspring, which is a 6- to 30-fold increase in efficiency compared with those generally reported in other insect species. This high efficiency implies that an average beekeeping facility with a limited number of colonies can apply this tool. We demonstrated that the cassette stably and efficiently expressed marker genes in progeny under either an artificial or an endogenous promoter. This evidence of efficient expression encourages the use of this system to inhibit gene functions through RNAi in specific tissues and developmental stages by using various promoters. We also showed that the transgenic marker could be used to select transgenic offspring to be employed to facilitate the building of transgenic colonies via the haploid males. We present here the first to our knowledge genetic engineering tool that will efficiently allow for the systematic detection and better understanding of processes underlying the biology of honeybees.

DNA sequence requirements for hobo transposable element transposition in Drosophila melanogaster

We have conducted a structure and functional analysis of the hobo transposable element of Drosophila melanogaster. A minimum of 141 bp of the left (L) end and 65 bp of the right (R) end of the hobo were shown to contain sequences sufficient for transposition. Both ends of hobo contain multiple copies of the motifs GGGTG and GTGGC and we show that the frequency of hobo transposition increases as a function of the copy number of these motifs. The R end of hobo contains a unique 12 bp internal inverted repeat that is identical to the hobo terminal inverted repeats. We show that this internal inverted repeat suppresses transposition activity in a hobo element containing an intact L end and only 475 bp of the R end. In addition to establishing cis-sequences requirements for transposition, we analyzed trans-sequence effects of the hobo transposase. We show a hobo transposase lacking the first 49 amino acids catalyzed hobo transposition at a higher frequency than the full-length transposase suggesting that, similar to the related Ac transposase, residues at the amino end of the transposase reduce transposition. Finally, we compared target site sequences of hobo with those of the related Hermes element and found both transposons have strong preferences for the same insertion sites.

A new versatile gene-trap vector for insect transgenics

A new piggyBac-based gene-trap vector, pB-GT1, was constructed. pB-GT1 contains three marker genes, dsRed, Gal4, and EGFP. dsRed is under the control of the constitutive 3xP3 promoter, which induces dsRed expression wherever the vector is inserted in the host genome. The Gal4 sequence has no promoter but is preceded by the splice acceptor site so that it can be transcribed as a transcript fused with the host exon 5' to the insertion site. EGFP is driven by the constitutive ie+hr promoter but lacks a poly(A)(+) signal sequence, and thus the EGFP expression is detectable only when its transcript is fused with the host exon 3' downstream of the insertion. By the microinjection of the vector into fertilized eggs, we obtained transgenic Drosophila with a single copy of pB-GT1, which was inserted into the first intron of the ovo gene. The female flies of this transgenic line are sterile, indicating that the insertion inactivated the ovo gene, generating a new allele of this locus, ovo(pB-GT1). RT-PCR analysis demonstrated that an ovo-Gal4-fusion transcript is produced in ovo(pB-GT1) flies. The fact that UAS-EGFP reporter expression was detected in ovo(pB-GT1) germ cells in a pattern similar to that reported for wild-type ovo indicates that functional Gal4 is expressed via pB-GT1, recapitulating the endogenous expression pattern of the trapped gene. pB-GT1 is thus useful in insect genomics for the efficient assignment of functions of individual genes.

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.

Cchobo, a hobo-related sequence in Ceratitis capitata

A hobo-related sequence, Cchobo, with high similarity to the Drosophila melanogaster HFL1 and hobo108 elements was isolated from the medfly. Thirteen PCR-derived clones, which share 97.9-100% DNA identity, were sequenced, seven of which do not show frame-shift or stop codon mutations in their conceptual translations. The consensus sequence has 99.7% DNA identity with the D. melanogaster hobo element HFLI. In a phylogenetic analysis with other hobo-related elements, Cchobo clusters with the HFL1 and hobo108 elements from D. melanogaster and hobo-related elements from D. simulans, D. mauritiana and Mamestra brassicae. These elements may have undergone horizontal transfer in the recent past. The genomic distribution of Cchobo was studied by FISH to mitotic and polytene chromosomes, which revealed that Cchobo is distributed within both the heterochromatin and euchromatin. Intra- and interstrain polymorphisms were detected both at euchromatic and heterochromatic sites. These findings suggest that active copies of the element may be present in the medfly genome.

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.

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.

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.

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.

Evolution of yellow gene regulation and pigmentation in Drosophila

BACKGROUND

Changes in developmental gene expression are central to phenotypic evolution, but the genetic mechanisms underlying these changes are not well understood. Interspecific differences in gene expression can arise from evolutionary changes in cis-regulatory DNA and/or in the expression of trans-acting regulatory proteins, but few case studies have distinguished between these mechanisms. Here, we compare the regulation of the yellow gene, which is required for melanization, among distantly related Drosophila species with different pigment patterns and determine the phenotypic effects of divergent Yellow expression.

RESULTS

Yellow expression has diverged among D. melanogaster, D. subobscura, and D. virilis and, in all cases, correlates with the distribution of black melanin. Species-specific Yellow expression patterns were retained in D. melanogaster transformants carrying the D. subobscura and D. virilis yellow genes, indicating that sequence evolution within the yellow gene underlies the divergence of Yellow expression. Evolutionary changes in the activity of orthologous cis-regulatory elements are responsible for differences in abdominal Yellow expression; however, cis-regulatory element evolution is not the sole cause of divergent Yellow expression patterns. Transformation of the D. melanogaster yellow gene into D. virilis altered its expression pattern, indicating that trans-acting factors that regulate the D. melanogaster yellow gene have also diverged between these two species. Finally, we found that the phenotypic effects of evolutionary changes in Yellow expression depend on epistatic interactions with other genes.

CONCLUSIONS

Evolutionary changes in Yellow expression correlate with divergent melanin patterns and are a result of evolution in both cis- and trans-regulation. These changes were likely necessary for the divergence of pigmentation, but evolutionary changes in other genes were also required.

Construction of a mariner-based transposon for epitope-tagging and genomic targeting

Mobile genetic elements are often employed for constructing gene fusions or to perform mutagenesis. mariner transposons are well-suited to such applications because of their low site specificity, in vitro activity, and exceptionally broad host range. This report describes a mariner-based method for rapidly creating a large number of insertion mutants that can be converted to in-frame epitope fusions in a single step. First, a mariner-based vector is used to deliver a FLP recombinase substrate randomly into a target molecule. Expression of the FLP recombinase is then induced to catalyse the excision of sequences flanked by FLP recombinase target recognition sites, leaving behind a triple-FLAG epitope. The reversibility of the excision event provides opportunities for using genomic targeting methods easily to create transcriptional or translational fusions to genes of interest.

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.

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.

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.

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.

Construction of a Vibrio cholerae vaccine candidate using transposon delivery and FLP recombinase-mediated excision

Recent efforts to develop a vaccine against the diarrheal disease cholera have focused on the use of live attenuated strains of the causative organism, Vibrio cholerae. The Ogawa lipopolysaccharide phenotype is expressed by many epidemic strains, and motility defects reduce the risk of reactive diarrhea in vaccine recipients. We therefore converted a motile Inaba(+) vaccine candidate, Peru-2, to a nonmotile Ogawa(+) phenotype using a mariner-based transposon carrying rfbT, the gene required for expression of the Ogawa phenotype. Analysis of 22 nonmotile Peru-2 mutants showed that two were Ogawa(+), and both of these strains had insertions in the flgE gene. It was possible to convert these strains to antibiotic sensitivity by introducing a recombinase that acts on sites flanking the antibiotic marker on the transposon. The resulting strains are competent for colonization in infant mice and may therefore be suitable as vaccine candidates for use either independently or in a combination with strains of different biotypes and serotypes.

Mobility assays confirm the broad host-range activity of the Minos transposable element and validate new transformation tools

Fast and reliable methods for assessing the mobility of the transposable element Minos have been developed. These methods are based on the detection of excision and insertion of Minos transposons from and into plasmids which are co-introduced into cells. Excision is detected by polymerase chain reaction (PCR) with appropriate primers. Transposition is assayed by marker rescue in Escherichia coli, using a transposon plasmid that carries a tetracycline resistance gene and a target plasmid carrying a gene that can be selected against in E. coli. Using both assays, Minos was shown to transpose in Drosophila melanogaster cells and embryos, and in cultured cells of a mosquito, Aedes aegypti, and a lepidopteran, Spodoptera frugiperda. In all cases, mobility was dependent on the presence of exogenously supplied transposase, and both excision and transposition were precise. The results indicate that Minos can transpose in heterologous insect species with comparable efficiencies and therefore has the potential to be used as a transgenesis vector for diverse species.

Evidence for recent invasion of the medaka fish genome by the Tol2 transposable element

Tol2 is a transposable element of the terminal-inverted-repeat class, residing in the genome of the medaka fish Oryzias latipes. The genus Oryzias contains more than 10 species for which phylogenetic relationships have previously been estimated. To infer the history of Tol2 in this genus we performed genomic Southern blots and PCR analyses of 10 of the species. It was revealed that Tol2 occurs in 2 of the 10 species (O. curvinotus and O. latipes) and that the length and the restriction map structure of Tol2 are identical in the two cases. Further, sequencing analysis revealed an extremely low level of divergence compared with that in a nuclear gene. These results suggest recent incorporation of Tol2 into one or both of the two species, implying horizontal transfer of Tol2 from one species to the other or into them both from a common source.

Toward Anopheles transformation: Minos element activity in anopheline cells and embryos

The ability of the Minos transposable element to function as a transformation vector in anopheline mosquitoes was assessed. Two recently established Anopheles gambiae cell lines were stably transformed by using marked Minos transposons in the presence of a helper plasmid expressing transposase. The markers were either the green fluorescent protein or the hygromycin B phosphotransferase gene driven by the Drosophila Hsp70 promoter. Cloning and sequencing of the integration sites demonstrated that insertions in the cell genome occurred through the action of Minos transposase. Furthermore, an interplasmid transposition assay established that Minos transposase is active in the cytoplasmic environment of Anopheles stephensi embryos: interplasmid transposition events isolated from injected preblastoderm embryos were identified as Minos transposase-mediated integrations, and no events were recorded in the absence of an active transposase. These results demonstrate that Minos vectors are suitable candidates for germ-line transformation of anopheline mosquitoes.

Germline transformation of the silkworm Bombyx mori L. using a piggyBac transposon-derived vector

We have developed a system for stable germline transformation in the silkworm Bombyx mori L. using piggyBac, a transposon discovered in the lepidopteran Trichoplusia ni. The transformation constructs consist of the piggyBac inverted terminal repeats flanking a fusion of the B. mori cytoplasmic actin gene BmA3 promoter and the green fluorescent protein (GFP). A nonautonomous helper plasmid encodes the piggyBac transposase. The reporter gene construct was coinjected into preblastoderm eggs of two strains of B. mori. Approximately 2% of the individuals in the G1 broods expressed GFP. DNA analyses of GFP-positive G1 silkworms revealed that multiple independent insertions occurred frequently. The transgene was stably transferred to the next generation through normal Mendelian inheritance. The presence of the inverted terminal repeats of piggyBac and the characteristic TTAA sequence at the borders of all the analyzed inserts confirmed that transformation resulted from precise transposition events. This efficient method of stable gene transfer in a lepidopteran insect opens the way for promising basic research and biotechnological applications.

Germline transformation of Drosophila melanogaster with the piggyBac transposon vector

Germline transformation of Drosophila melanogaster was attempted with the piggyBac gene-transfer system from the cabbage looper moth, Trichoplusia ni. Using a self-regulated transposase helper and a white marked vector, a transformation frequency of 1-3% per fertile G0 was obtained, similar to that previously achieved in the medfly. Use of an hsp70-regulated helper increased this frequency more than eight-fold. Transformation with a vector marked with white and green fluorescent protein (GFP) under polyubiquitin-nuclear localizing sequence regulation yielded seventy G1 transformants which all expressed GFP, but only twenty-seven of these expressed eye pigmentation that would have allowed their selection based on white+ expression. PiggyBac transformation in two distantly related dipteran species and efficient expression of the gfp marker supports the potential use of this system in other dipterans, and perhaps insects in general.

Hyperactive transposase mutants of the Himar1 mariner transposon

Mariner-family transposable elements are active in a wide variety of organisms and are becoming increasingly important genetic tools in species lacking sophisticated genetics. The Himar1 element, isolated from the horn fly, Haematobia irritans, is active in Escherichia coli when expressed appropriately. We used this fact to devise a genetic screen for hyperactive mutants of Himar1 transposase that enhance overall transposition from approximately 4- to 50-fold as measured in an E. coli assay. Purified mutant transposases retain their hyperactivity, although to a lesser degree, in an in vitro transposition assay. Mutants like those described herein should enable sophisticated analysis of the biochemistry of mariner transposition and should improve the use of these elements as genetic tools, both in vivo and in vitro.

In vivo transposition of mariner-based elements in enteric bacteria and mycobacteria

mariner family transposons are widespread among eukaryotic organisms. These transposons are apparently horizontally transmitted among diverse eukaryotes and can also transpose in vitro in the absence of added cofactors. Here we show that transposons derived from the mariner element Himar1 can efficiently transpose in bacteria in vivo. We have developed simple transposition systems by using minitransposons, made up of short inverted repeats flanking antibiotic resistance markers. These elements can efficiently transpose after expression of transposase from an appropriate bacterial promoter. We found that transposition of mariner-based elements in Escherichia coli produces diverse insertion mutations in either a targeted plasmid or a chromosomal gene. With Himar1-derived transposons we were able to isolate phage-resistant mutants of both E. coli and Mycobacterium smegmatis. mariner-based transposons will provide valuable tools for mutagenesis and genetic manipulation of bacteria that currently lack well developed genetic systems.

Precise excision and transposition of piggyBac in pink bollworm embryos

Transposable elements such as P, hobo, Hermes, mariner and Minos have been successfully harnessed as gene vectors to achieve the transformation of several dipteran species including Drosophila melanogaster, Ceratitis capitata and Aedes aegypti. Plasmid-based excision and transposition assays have been useful indicators of an element's ability to be mobilized in vivo and thus potentially serve as a transforming vector. We report that the transposable element piggyBac is capable of precise excision and transposition in the pink bollworm (Pectinophora gossypiella), a worldwide pest of cultivated cotton. Combined with a suitable marker gene, the piggyBac element may serve as a vector for germline transformation in this and (potentially) other lepidopteran species.

Key aspects of the primary sex determination mechanism are conserved across the genus Drosophila

In D. melanogaster, a set of ‘X:A numerator genes’, which includes sisterlessA (sisA), determines sex by controlling the transcription of Sex-lethal (Sxl). We characterized sisA from D. pseudoobscura and D. virilis and studied the timing of sisA and Sxl expression with single cell-cycle resolution in D. virilis, both to guide structure-function studies of sisA and to help understand sex determination evolution. We found that D. virilis sisA shares 58% amino acid identity with its melanogaster ortholog. The identities confirm sisA as an atypical bZIP transcription factor. Although virilis sisA can substitute for melanogaster sisA, the protein is not fully functional in a heterologous context. The putative sisA regulatory sequence CAGGTAG is a potential ‘numerator box,’ since it is shared with the other strong X:A numerator gene, sisB, and its target, SxlPe. Temporal and spatial features of sisA and SxlPe expression are strikingly conserved, including rapid onset and cessation of transcription in somatic nuclei, early cessation of sisA transcription in budding pole cells and persistent high-level sisA expression in yolk nuclei. Expression of sisA and Sxl is as tightly coupled in virilis as it is in melanogaster. Taken together, these data indicate that the same primary sex determination mechanism exists throughout the genus Drosophila.

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

The piggyBac (IFP2) short inverted terminal repeat transposable element from the cabbage looper Trichoplusia ni was tested for gene transfer vector function as part of a bipartite vector-helper system in the Mediterranean fruit fly Ceratitis capitata. A piggyBac vector marked with the medfly white gene was tested with a normally regulated piggyBac transposase helper at two different concentrations in a white eye host strain. Both experiments yielded transformants at an approximate frequency of 3-5%, with a total of six lines isolated having pigmented eyes with various levels of coloration. G1 transformant siblings from each line shared at least one common integration, with several sublines having an additional second integration. For the first transformant line isolated, two integrations were determined to be stable for 15 generations. For five of the lines, a piggyBac-mediated transposition was verified by sequencing the insertion site junctions isolated by inverse PCR that identified a characteristic piggyBac TTAA target site duplication. The efficient and stable transformation of the medfly with a lepidopteran vector represents transposon function over a relatively large evolutionary distance and suggests that the piggyBac system will be functional in a broad range of insects.

Modern thoughts on an ancyent marinere: function, evolution, regulation

The mariner/Tc1 superfamily of transposable elements is one of the most diverse and widespread Class II transposable elements. Within the larger assemblage, the mariner-like elements (MLEs) and the Tc1-like elements (TLEs) are distinct families differing characteristically in the composition of the "D,D(35)E" cation-binding domain. Based on levels of sequence similarity, the elements in each family can be subdivided further into several smaller subfamilies. MLEs and TLEs both have an extraordinarily wide host range. They are abundant in insect genomes and other invertebrates and are found even in some vertebrate species including, in the case of mariner, humans, in which one element on chromosome 17p has been implicated as a hotspot of recombination. In spite of the extraordinary evolutionary success of the elements, virtually nothing is known about their mode of regulation within genomes. There is abundant evidence that the elements are disseminated to naive host genomes by horizontal transmission, and there is a substantial base of evidence for inference about the subsequent population dynamics. Studies of engineered mariner elements and induced mutations in the transposase have identified two mechanisms that may be operative in mariner regulation. One mechanism is overproduction inhibition, in which excessive wild-type transposase reduces the rate of excision of a target element. A second mechanism is dominant-negative complementation, in which certain mutant transposase proteins antagonize the activity of the wild-type transposase. The latter process may help explain why the vast majority of MLEs in nature undergo "vertical inactivation" by multiple mutations and, eventually, stochastic loss. There is also evidence that mariner/Tc1 elements can be mobilized in hybrid dysgenesis; in particular, certain dysgenic crosses in Drosophila virilis result in mobilization of a TLE designated Paris as well as the mobilization of other unrelated transposable elements.

Trans-kingdom transposition of the Drosophila element mariner within the protozoan Leishmania

Transposable elements of the mariner/Tc1 family are postulated to have spread by horizontal transfer and be relatively independent of host-specific factors. This was tested by introducing the Drosophila mauritiana element mariner into the human parasite Leishmania major, a trypanosomatid protozoan belonging to one of the most ancient eukaryotic lineages. Transposition in Leishmania was efficient, occurring in more than 20 percent of random transfectants, and proceeded by the same mechanism as in Drosophila. Insertional inactivation of a specific gene was obtained, and a modified mariner element was used to select for gene fusions, establishing mariner as a powerful genetic tool for Leishmania and other organisms. These experiments demonstrate the evolutionary range of mariner transposition in vivo and underscore the ability of this ubiquitous DNA to parasitize the eukaryotic genome.

A Drosophila melanogaster hobo-white(+) vector mediates low frequency gene transfer in D. virilis with full interspecific white(+) complementation

Transformation of a Drosophila virilis white mutant host strain was attempted using a hobo vector containing the D. melanogaster mini-white(+) cassette (H[w(+), hawN]) and an unmodified or heat shock regulated hobo transposase helper. Two transformant lines were recovered with the unmodified helper (HFL1), one containing only the white(+) marked vector, and a sibling line containing the vector as well as an HFL1 helper integration. An approximate total transformation frequency of 1% is deduced. A high frequency of wing and eye morphology mutants were also observed, suggesting that hobo may have mobilized a related element in D. virilis. The data reaffirms a relatively low transformation vector activity for the hobo transposon in D. virilis; however, nearly full interspecific expression white(+) marker supports its possible function in other species as well.

The Hermes element from Musca domestica can transpose in four families of cyclorrhaphan flies

Transgenic insect technology will provide opportunities to explore the basic biology of a broad range of insect species in ways that will prove insightful and important. It is also a technology that will provide opportunities to manipulate the genotypes of insects of practical significance to the health and welfare of humans. The Hermes transposable element from the housefly, Musca domestica, is a short inverted repeat-type element related to hobo from Drosophila melanogaster, Ac from Zea mays, and Tam3 from Antirrhinum majus. It has potential to become a versatile and efficient broad host-range insect transformation vector. The ability of Hermes to transpose when introduced into five species of diptera from four divergent families was tested using an in vivo, interplasmid transpositional recombination assay. Hermes was capable of transposing in all species tested, demonstrating that Hermes has a broad host-range. In addition, the rates of transposition were sufficiently high in all species tested to suggest that Hermes will be an efficient gene transfer vector in a wide range of insect species. The Hermes element also revealed a pattern of integration into the target substrate that permitted factors determining integration site selection to be identified. Primary nucleotide sequence of the integration site played a role as did proximity to preferred integration sites and the nucleosomal organization of the target.

Transposable elements and gene transformation in non-drosophilid insects

This review summarizes recent data on the development of non-drosophilid insect transformation systems. The discussion focuses on one particular approach to developing transformation systems that relies on the use of short inverted repeat-type transposable elements analogous to that employed for Drosophila melanogaster transformation. Representatives from four families of short inverted repeat-type transposable elements have been shown to either act as non-drosophilid gene vectors or to have the ability to transpose accurately when introduced into non-host insect cells. Minos, a member of the Tcl family of elements isolated originally from D. hydei has been successfully used as a germline transformation vector in the Medfly, Ceratitis capitata. Hermes, a member of the hAT family of elements isolated originally from Musca domestica has been successfully used as a gene transformation vector in D. melanogaster and has a host range that appears to include culicids. hobo, another member of the hAT family of elements isolated from D. melanogaster also has a broad host range that includes tephritid fruitflies. mariner(Mos), a member of the mariner family of elements isolated from D. mauritiana can transpose in calliphorids. Finally, piggyBac/IFP2, a member of the TTAA-specific family of elements isolated from Trichoplusia ni can transpose when introduced into Spodoptera frugiperda cells. Although routine transformation of insects other than D. melanogaster is not possible it is clear that the raw materials for the development of efficient transformation systems are now available.