I element distribution in mitotic heterochromatin of Drosophila melanogaster reactive strains: identification of a specific site which is correlated with the reactivity levels

The I factor is a Drosophila melanogaster LINE-like element that efficiently transposes in the genetic system of I-R hybrid dysgenesis. It has been suggested that some of the I-related sequences located in the heterochromatin of D. melanogaster are involved in the regulation of I factor activity. In this work we have performed fluorescent in situ hybridization (FISH) mapping of I element sequences in mitotic heterochromatin of nine differentially reactive D. melanogaster strains. The results of our analysis showed that a single hybridization site mapping to region h28 of the distal heterochromatin of the X chromosome is present in three strains with low or intermediate levels of reactivity, while it is undetectable in six highly reactive strains. Together, these observations suggest a negative correlation between I sequences located at h28 and the level of reactivity. To this regard, it is intriguing that flamenco and COM, two loci that regulate the activity of D. melanogaster endogenous retroviruses also map to the distal heterochromatin of the X chromosome. Our data represent the first experimental evidence in favour of a silencing effect exerted by naturally occurring I element sequences located in pericentromeric heterochromatin.

Drosophila germline invasion by the endogenous retrovirus gypsy: involvement of the viral env gene

The endogenous retrovirus gypsy is expressed at high levels in mutant flamenco female flies. Gypsy viral particles extracted from such flies can infect naive flamenco individuals raised in the presence of these extracts mixed into their food. This results in the integration of new proviruses into the germline genome. These proviruses can then increase their copy number by (1) expression in the flamenco female somatic cells, (2) transfer into the oocyte and (3) integration into the genome of the progeny. Surprisingly, unlike the infection observed in the feeding experiments, this strategy of endogenous proviral multiplication does not seem to involve the expression of the viral env gene.

Trans-complementation of an endonuclease-defective tagged I element as a tool for the study of retrotransposition in Drosophila melanogaster

I factors are non-LTR retrotransposons of Drosophila melanogaster that transpose at high frequency in the germline of females resulting from appropriate crosses, allowing in vivo studies of the retrotransposition process. Reverse transcription of a full-length RNA intermediate is thought to occur at the site of integration, using a 3' hydroxyl group generated by endonucleolytic cleavage of the genomic DNA to prime synthesis of the first cDNA strand. This target-primed reverse transcription (TPRT) process is mediated by endonuclease and reverse transcriptase activities encoded by the element. We have designed a molecularly tagged, endonuclease-defective I element that can be mobilised with high efficiency by constructs that express the product of the I factor ORF2 in trans. This indicates that the endonuclease activity required for retrotransposition of the I factor can be provided in trans. Using this system, we show that the endonuclease domain of the R1Bm retrotransposon from Bombyx mori cannot functionally replace that of the I factor.

Characterization of the flamenco region of the Drosophila melanogaster genome

The flamenco gene, located at 20A1-3 in the beta-heterochromatin of the Drosophila X chromosome, is a major regulator of the gypsy/mdg4 endogenous retrovirus. As a first step to characterize this gene, approximately 100 kb of genomic DNA flanking a P-element-induced mutation of flamenco was isolated. This DNA is located in a sequencing gap of the Celera Genomics project, i.e., one of those parts of the genome in which the "shotgun" sequence could not be assembled, probably because it contains long stretches of repetitive DNA, especially on the proximal side of the P insertion point. Deficiency mapping indicated that sequences required for the normal flamenco function are located >130 kb proximal to the insertion site. The distal part of the cloned DNA does, nevertheless, contain several unique sequences, including at least four different transcription units. Dip1, the closest one to the P-element insertion point, might be a good candidate for a gypsy regulator, since it putatively encodes a nuclear protein containing two double-stranded RNA-binding domains. However, transgenes containing dip1 genomic DNA were not able to rescue flamenco mutant flies. The possible nature of the missing flamenco sequences is discussed.

From first base: the sequence of the tip of the X chromosome of Drosophila melanogaster, a comparison of two sequencing strategies

We present the sequence of a contiguous 2.63 Mb of DNA extending from the tip of the X chromosome of Drosophila melanogaster. Within this sequence, we predict 277 protein coding genes, of which 94 had been sequenced already in the course of studying the biology of their gene products, and examples of 12 different transposable elements. We show that an interval between bands 3A2 and 3C2, believed in the 1970s to show a correlation between the number of bands on the polytene chromosomes and the 20 genes identified by conventional genetics, is predicted to contain 45 genes from its DNA sequence. We have determined the insertion sites of P-elements from 111 mutant lines, about half of which are in a position likely to affect the expression of novel predicted genes, thus representing a resource for subsequent functional genomic analysis. We compare the European Drosophila Genome Project sequence with the corresponding part of the independently assembled and annotated Joint Sequence determined through "shotgun" sequencing. Discounting differences in the distribution of known transposable elements between the strains sequenced in the two projects, we detected three major sequence differences, two of which are probably explained by errors in assembly; the origin of the third major difference is unclear. In addition there are eight sequence gaps within the Joint Sequence. At least six of these eight gaps are likely to be sites of transposable elements; the other two are complex. Of the 275 genes in common to both projects, 60% are identical within 1% of their predicted amino-acid sequence and 31% show minor differences such as in choice of translation initiation or termination codons; the remaining 9% show major differences in interpretation.

Identification of Waldo-A and Waldo-B, two closely related non-LTR retrotransposons in Drosophila

We have identified two novel, closely related subfamilies of non-long-terminal-repeat (non-LTR) retrotransposons in Drosophila melanogaster, the Waldo-A and Waldo-B subfamilies, that are in the same lineage as site-specific LTR retrotransposons of the R1 clade. Both contain potentially active copies with two large open reading frames, having coding capacities for a nucleoprotein as well as endonuclease and reverse transcriptase activities. Many copies are truncated at the 5' end, and most are surrounded by target site duplications of variable lengths. Elements of both subfamilies have a nonrandom distribution in the genome, often being inserted within or very close to (CA)(n) arrays. At the DNA level, the longest elements of Waldo-A and Waldo-B are 69% identical on their entire length, except for the 5' untranslated regions, which have a mosaic organization, suggesting that one arose from the other following new promoter acquisition. This event occurred before the speciation of the D. melanogaster subgroup of species, since both Waldo-A and Waldo-B coexist in other species of this subgroup.

New insights on homology-dependent silencing of I factor activity by transgenes containing ORF1 in Drosophila melanogaster

I factors in Drosophila melanogaster are non-LTR retrotransposons that transpose at very high frequencies in the germ line of females resulting from crosses between reactive females (devoid of active I factors) and inducer males (containing active I factors). Constructs containing I factor ORF1 under the control of the hsp70 promoter repress I factor activity. This repressor effect is maternally transmitted and increases with the transgene copy number. It is irrespective of either frame integrity or transcriptional orientation of ORF1, suggesting the involvement of a homology-dependent trans-silencing mechanism. A promoterless transgene displays no repression. The effect of constructs in which ORF1 is controlled by the hsp70 promoter does not depend upon heat-shock treatments. No effect of ORF1 is detected when it is controlled by the I factor promoter. We discuss the relevance of the described regulation to the repression of I factors in I strains.

Retrotransposition of the I factor, a non-long terminal repeat retrotransposon of Drosophila, generates tandem repeats at the 3' end

Non-long terminal repeat (LTR) retrotransposons or LINEs transpose by reverse transcription of an RNA intermediate and are thought to use the 3' hydroxyl of a chromosomal cleavage to initiate synthesis of the first strand of the cDNA. Many of them terminate in a poly(dA) sequence at the 3' end of the coding strand although some, like the I factor of Drosophila melanogaster, have 3' ends formed by repeats of the trinucleotide TAA. We report results showing that I factor transcripts end a few nucleotides downstream of the TAA repeats and that these extra nucleotides are not integrated into chromosomal DNA during retrotransposition. We also show that the TAA repeats are not required for transposition and that I elements containing mutations affecting the TAA sequences generate transposed copies ending with tandem repeats of various types. Our results suggest that during integration the 3' end of the I factor RNA template can pair with nucleotides at the target site and that tandem duplications are generated by the reverse transcriptase of the I factor in a manner that is reminiscent of the activity of the reverse transcriptases of telomerases. Reverse transcriptases of other non-LTR retrotransposons may function in a similar way.

Evolution of the Gypsy endogenous retrovirus in the Drosophila melanogaster subgroup

We conducted a phylogenetic survey of the endogenous retrovirus Gypsy in the eight species of the Drosophila melanogaster subgroup. A 362-bp fragment from the integrase gene (int) was amplified, cloned, and sequenced. Phylogenetic relationships of the elements isolated from independent clones were compared with the host phylogeny. Our results indicate that two main lineages of Gypsy exist in the melanogaster subgroup and that vertical and horizontal transmission have played a crucial role in the evolution of this insect endogenous retrovirus.

From sequence to chromosome: the tip of the X chromosome of D. melanogaster

One of the rewards of having a Drosophila melanogaster whole-genome sequence will be the potential to understand the molecular bases for structural features of chromosomes that have been a long-standing puzzle. Analysis of 2.6 megabases of sequence from the tip of the X chromosome of Drosophila identifies 273 genes. Cloned DNAs from the characteristic bulbous structure at the tip of the X chromosome in the region of the broad complex display an unusual pattern of in situ hybridization. Sequence analysis revealed that this region comprises 154 kilobases of DNA flanked by 1.2-kilobases of inverted repeats, each composed of a 350-base pair satellite related element. Thus, some aspects of chromosome structure appear to be revealed directly within the DNA sequence itself.

Proviral amplification of the Gypsy endogenous retrovirus of Drosophila melanogaster involves env-independent invasion of the female germline

Gypsy is an infectious endogenous retrovirus of Drosophila melanogaster. The gypsy proviruses replicate very efficiently in the genome of the progeny of females homozygous for permissive alleles of the flamenco gene. This replicative transposition is correlated with derepression of gypsy expression, specifically in the somatic cells of the ovaries of the permissive mothers. The determinism of this amplification was studied further by making chimeric mothers containing different permissive/restrictive and somatic/germinal lineages. We show here that the derepression of active proviruses in the permissive soma is necessary and sufficient to induce proviral insertions in the progeny, even if the F1 flies derive from restrictive germ cells devoid of active proviruses. Therefore, gypsy endogenous multiplication results from the transfer of some gypsy-encoded genetic material from the soma towards the germen of the mother and its subsequent insertion into the chromosomes of the progeny. This transfer, however, is not likely to result from retroviral infection of the germline. Indeed, we also show here that the insertion of a tagged gypsy element, mutant for the env gene, occurs at high frequency, independently of the production of gypsy Env proteins by any transcomplementing helper. The possible role of the env gene for horizontal transfer to new hosts is discussed.

High-frequency retrotransposition of a marked I factor in Drosophila melanogaster correlates with a dynamic expression pattern of the ORF1 protein in the cytoplasm of oocytes

To study the expression of the I factor, a non-long-terminal-repeat retrotransposon responsible for I-R hybrid dysgenesis in Drosophila melanogaster, we have tagged the ORF1 protein (ORF1p) by inserting the HA epitope in its N-terminal region. In transgenic flies, this modification is compatible with a high rate of autonomous transposition and allows direct estimation of the transposition frequency. I factor transposes in the germline of females (SF) that are daughters from crosses between I strain males (which contain active copies of the I factor) and R strain females (which do not). We analyzed the expression pattern of ORF1p by indirect immunofluorescence. Its expression correlates with retrotransposition. During oogenesis ORF1p appears unexpectedly as a cytoplasmic product, which accumulates with a specific pattern into the oocyte. A comparison of the expression patterns under conditions that modify the transposing activity of the element clarifies some aspects of I-factor functioning in the transposition process.

Morphological and molecular characterization of new Drosophila cell lines established from a strain permissive for gypsy transposition

The gypsy element of Drosophila melanogaster is the first retrovirus identified in invertebrates. Its transposition is controlled by a host gene called flamenco (flam): restrictive alleles of this gene maintain the retrovirus in a repressed state while permissive alleles allow high levels of transposition. To develop a cell system to study the gypsy element, we established four independent cell lines derived from the Drosophila strain SS, which contains a permissive allele of flamenco, and which is devoid of transposing copies of gypsy. The ultrastructural analysis of three SS cell lines revealed some remarkable characteristics, such as many nuclear virus-like particles, cytoplasmic dense particles, and massive cisternae filled with a fibrous material of unknown origin. Gypsy intragenomic distribution has been compared between the three cell lines and the original SS fly strain, and revealed in two of the cell lines an increase in copy number of a restriction fragment usually present in active gypsy elements. This multiplication seems to have occurred during the passage to the cell culture. Availability of SS cell lines should assist studies of gypsy transposition and infectivity and might be useful to produce high amounts of gypsy viral particles. These new lines already allowed us to show that the Envelope-like products of gypsy can be expressed as membrane proteins.

Copy number control of a transposable element, the I factor, a LINE-like element in Drosophila

The I factor is a LINE-like transposable element in Drosophila. Most strains of Drosophila melanogaster, inducer strains, contain 10-15 copies of the I factor per haploid genome located in the euchromatic regions of the chromosome arms. These are not present in a few strains known as reactive strains. I factors transpose at low frequency in inducer strains but at high frequency in the female progeny of crosses between reactive and inducer flies. We have found that the activity of the I factor promoter is sensitive to the number of copies of the first 186 nucleotides of the I factor sequence, which constitutes the 5'-untranslated region. The activity of the I factor decreases as the copy number of this sequence increases.

Potentially active copies of the gypsy retroelement are confined to the Y chromosome of some strains of Drosophila melanogaster possibly as the result of the female-specific effect of the flamenco gene

Gypsy is an endogenous retrovirus present in the genome of Drosophila melanogaster. This element is mobilized only in the progeny of females which contain active gypsy elements and which are homozygous for permissive alleles of a host gene called flamenco (flam). Some data strongly suggest that gypsy elements bearing a diagnostic HindIII site in the central region of the retrovirus body represent a subfamily that appears to be much more active than elements devoid of this site. We have taken advantage of this structural difference to assess by the Southern blotting technique the genomic distribution of active gypsy elements. In some of the laboratory Drosophila stocks tested, active gypsy elements were found to be restricted to the Y chromosome. Further analyses of 14 strains tested for the permissive vs. restrictive status of their flamenco alleles suggest that the presence of permissive alleles of flam in a stock tends to be associated with the confinement of active gypsy elements to the Y chromosome. This might be the result of the female-specific effect of flamenco on gypsy activity.

About the origin of retroviruses and the co-evolution of the gypsy retrovirus with the Drosophila flamenco host gene

The gypsy element of Drosophila melanogaster is the first retrovirus identified so far in invertebrates. According to phylogenetic data, gypsy belongs to the same group as the Ty3 class of LTR-retrotransposons, which suggests that retroviruses evolved from this kind of retroelements before the radiation of vertebrates. There are other invertebrate retroelements that are also likely to be endogenous retroviruses because they share with gypsy some structural and functional retroviral-like characteristics. Gypsy is controlled by a Drosophila gene called flamenco, the restrictive alleles of which maintain the retrovirus in a repressed state. In permissive strains, functional gypsy elements transpose at high frequency and produce infective particles. Defective gypsy proviruses located in pericentromeric heterochromatin of all strains seem to be very old components of the genome of Drosophila melanogaster, which indicates that gypsy invaded this species, or an ancestor, a long time ago. At that time, Drosophila melanogaster presumably contained permissive alleles of the flamenco gene. One can imagine that the species survived to the increase of genetic load caused by the retroviral invasion because restrictive alleles of flamenco were selected. The characterization of a retrovirus in Drosophila, one of the most advanced model organisms for molecular genetics, provides us with an exceptional clue to study how a species can resist a retroviral invasion.

A genetically marked I element in Drosophila melanogaster can be mobilized when ORF2 is provided in trans

I factors in Drosophila melanogaster are non-LTR retrotransposons similar to mammalian LINEs. They transpose at very high frequencies in the germ line of SF females resulting from crosses between reactive females, devoid of active I factors, and inducer males, containing active I factors. The vermilion marked IviP2 element was designed to allow easy phenotypical screening for retrotransposition events. It is deleted in ORF2 and therefore cannot produce reverse transcriptase. IviP2 can be mobilized at very low frequencies by actively transposing I factors in the germ line of SF females. This paper shows that IviP2 can be mobilized more efficiently in the germ line of strongly reactive females in the absence of active I factors, when it is trans-complemented by the product of ORF2 synthesized from the hsp70 heat-shock promoter. This represents a promising step toward the use of marked I elements to study retrotransposition and as tools for mutagenesis.

A Moloney murine leukemia virus-based retroviral vector pseudotyped by the insect retroviral gypsy envelope can infect Drosophila cells

The gypsy element of Drosophila melanogaster is the first retrovirus identified so far in invertebrates. Previous data suggest that gypsy ENV-like ORF3 mediates viral infectivity. We have produced in the 293GP/LNhsp701ucL.3 human cell line a Moloney murine leukemia virus-based retroviral vector pseudotyped by the gypsy ENV-like protein. We have shown by immunostaining that the gypsy envelope protein is produced in 293GP/LNhsp701ucL.3 cells and that vector particles collected from these cells can infect Drosophila cells. Our results provide direct evidence that the infectious property of gypsy is due to its ORF3 gene product.

Expression of the Drosophila retrovirus gypsy as ultrastructurally detectable particles in the ovaries of flies carrying a permissive flamenco allele

The endogenous retrovirus gypsy is controlled by the Drosophila gene flamenco (flam). New insertions of gypsy occur in any individual Drosophila if its mother is homozygous for the flam1 permissive allele and contains functional gypsy proviruses. The ovaries of flam1 females also contain high amounts of gypsy RNAs. Unexpectedly however, gypsy derepression does not occur in the flam1 female germ-line proper but in the somatic follicular epithelium of the ovary. Since extracts from these females are able to efficiently infect the germ-line of a strain devoid of active gypsy proviruses, we assume that a similar kind of germ-line infection, which would occur inside the flam1 females themselves, could be required for gypsy insertions to occur in their progeny. This hypothesis was confirmed by electron microscopy observations showing that non-enveloped intracytoplasmic particles containing gypsy RNAs accumulate in the apical region of the flam1 follicle cells, close to specific membrane domains to which the gypsy envelope proteins are targeted, whereas both are absent in the flam+ controls. Low amounts of similar virus-like particles were also observed in flam1 oocytes, but it is not yet known whether they entered passively or as a result of membrane fusion. This is the first report of the beginning of a retrovirus cycle in invertebrates and these observations should be taken into account when explaining the maternal effect of the flamenco gene on the multiplication of gypsy proviruses.

The relationship between the flamenco gene and gypsy in Drosophila: how to tame a retrovirus

For a long time, retroviruses have been considered to be restricted to vertebrates. However, the genome of insects contains elements like gypsy in Drosophila melanogaster that are strikingly similar to vertebrate proviruses of retroviruses, which were considered to be transposable elements. Recent results indicate that gypsy has infective properties and is therefore a retrovirus, the first to be identified in invertebrates. It is normally repressed by a host gene called flamenco, which apparently controls the transposition and infective properties of gypsy. This provides an exceptional experimental model to investigate the genetic relationships between retroviruses and their hosts.

A genetically tagged, defective I element can be complemented by actively transposing I factors in the germline of I-R dysgenic females in Drosophila melanogaster

Non-LTR retrotransposons, also known as LINEs, transpose by reverse transcription of an RNA intermediate. Their mechanism of transposition is apparently different from that of retrotransposons and similar to that of proviruses of retroviruses. The I factor is responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. Inducer strains contain several functional I factors whereas reactive strains do not. Transposition of I factors can be experimentally induced: they are stable in inducer strains, but transpose at high frequency in the germline of females, known as SF females, produced by crossing reactive females and inducer males. We have constructed an I element, called IviP2, marked with the vermilion gene, the coding sequence of which was interrupted by an intron. Splicing of the intron can only occur in the transcript initiated from the I element promoter. Transposed copies expressing a wild-type vermilion phenotype were recovered in the germline of SF females in which I factors were actively transposing. This indicates that trans-complementation of a defective I element, deficient for the second open reading frame, by functional I factors can occur in the germline of dysgenic females.

IR hybrid dysgenesis increases the frequency of recombination in Drosophila melanogaster

The I factor is a LINE-like transposable element responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. Inducer strains of this species contain several I factors whereas reactive strains do not. I factors are stable in inducer strains, but transpose at high frequency in the germ-line of females, known as SF females, produced by crossing reactive females and inducer males. Various abnormalities occur in SF females, most of which result from this high rate of transposition. We report here that recombination is increased in the germ-line of these females. This is a new characteristic of the I-R system of hybrid dysgenesis that might also be associated with transposition of the I factor.

Flamenco, a gene controlling the gypsy retrovirus of Drosophila melanogaster

Gypsy is an endogenous retrovirus of Drosophila melanogaster. It is stable and does not transpose with detectable frequencies in most Drosophila strains. However, we have characterized unstable strains, known as MG, in which it transposes at high frequency. These stocks contain more copies of gypsy than usual stocks. Transposition results in mutations in several genes such as ovo and cut. They are stable and are due to gypsy insertions. Integrations into the ovoD1 female sterile-dominant mutation result in a null allele of the gene and occurrence of fertile females. This phenomenon, known as the ovoD1 reversion assay, can be used to quantitate gypsy activity. We have shown that the properties of MG strains result from mutation of a host gene that we called flamenco (flam). It has a strict maternal effect on gypsy mobilization: transposition occurs at high frequency only in the germ line of the progeny of females homozygous for mutations of the gene. It is located at position 65.9 (20A1-3) on the X chromosome. The mutant allele present in MG strains is essentially recessive. Flamenco seems to control the infective properties of gypsy.

Gypsy transposition correlates with the production of a retroviral envelope-like protein under the tissue-specific control of the Drosophila flamenco gene

Gypsy displays striking similarities to vertebrate retroviruses, including the presence of a yet uncharacterized additional open reading frame (ORF3) and the recent evidence for infectivity. It is mobilized with high frequency in the germline of the progeny of females homozygous for the flamenco permissive mutation. We report the characterization of a gypsy subgenomic ORF3 RNA encoding typical retroviral envelope proteins. In females, env expression is strongly repressed by one copy of the non-permissive allele of flamenco. A less dramatic reduction in the accumulation of other transcripts and retrotranscripts is also observed. These effects correlate well with the inhibition of gypsy transposition in the progeny of these females, and are therefore likely to be responsible for this phenomenon. The effects of flamenco on gypsy expression are apparently restricted to the somatic follicle cells that surround the maternal germline. Moreover, permissive follicle cells display a typically polarized distribution of gypsy RNAs and envelope proteins, both being mainly accumulated at the apical pole, close to the oocyte. We propose a model suggesting that gypsy germinal transposition might occur only in individuals that have maternally inherited enveloped gypsy particles due to infection of the maternal germline by the soma.

Retroviruses in invertebrates: the gypsy retrotransposon is apparently an infectious retrovirus of Drosophila melanogaster

Retroviruses are commonly considered to be restricted to vertebrates. However, the genome of many eukaryotes contains mobile sequences known as retrotransposons with long terminal repeats (LTR retrotransposons) or viral retrotransposons, showing similarities with integrated proviruses of retroviruses, such as Ty elements in Saccharomyces cerevisiae, copia-like elements in Drosophila, and endogenous proviruses in vertebrates. The gypsy element of Drosophila melanogaster has LTRs and contains three open reading frames, one of which encodes potential products similar to gag-specific protease, reverse transcriptase, and endonuclease. It is more similar to typical retroviruses than to LTR retrotransposons. We report here experiments showing that gypsy can be transmitted by microinjecting egg plasma from embryos of a strain containing actively transposing gypsy elements into embryos of a strain originally devoid of transposing elements. Horizontal transfer is also observed when individuals of the "empty" stock are raised on medium containing ground pupae of the stock possessing transposing elements. These results suggest that gypsy is an infectious retrovirus and provide evidence that retroviruses also occur in invertebrates.

I factors in Drosophila melanogaster: transposition under control

I factors are responsible for the I-R system of hybrid dysgenesis in Drosophila melanogaster. They belong to the LINE class of mobile elements, which transpose via reverse transcription of a full-length RNA intermediate. I factors are active members of the I element family, which also contains defective I elements that are immobilized within peri-centromeric heterochromatin and represent very old components of the genome. Active I factors have recently invaded natural populations of Drosophila melanogaster, giving rise to inducer strains. Reactive strains, devoid of active I factors, derive from old laboratory stocks established before the invasion. Transposition of I factors is activated at very high frequencies in the germline of hybrid females issued from crosses between females from reactive strains and males from inducer strains. It results in the production of high rates of mutations and chromosomal rearrangements as well as in a particular syndrome of sterility. The frequency of transposition of I factors is dependent on the amount of full-length RNA that is synthesized from an internal promoter. This full-length RNA serves both as an intermediate of transposition and presumably as a messenger for protein synthesis. Regulators of transposition apparently affect transcription initiation from the internal promoter. The data presented here lead to the proposal of a tentative model for transposition.

Properties of transgenic strains of Drosophila melanogaster containing I transposable elements from Drosophila teissieri

I factors are transposable elements of Drosophila melanogaster similar to mammalian LINEs, that transpose by reverse transcription of an RNA intermediate and are responsible for the I-R system of hybrid dysgenesis. There are two categories of strains in this species: inducer, that contain about 15 I elements at the various sites on chromosomal arms, and reactive, that lack active I factors. I elements occur in various Drosophila species. Potentially functional I factors from Drosophila teissieri can transpose when introduced by P-element-mediated transformation in a reactive strain of Drosophila melanogaster. We have studied the properties of Drosophila melanogaster strains into which such an I factor from Drosophila teissieri, named Itei, was introduced. Typical hybrid dysgenesis is produced when males carrying Itei are crossed with reactive females. However, more than one copy of the element seems necessary to produce dysgenic traits, whereas only one I factor of Drosophila melanogaster seems to be sufficient. The copy number of Itei in transformed lines maintained by endogamous crosses increases rapidly and stabilizes at values similar to those observed in inducer strains. As Drosophila teissieri contains much fewer copies than the Drosophila melanogaster strains, this suggests that the copy number of I elements is not simply regulated by sequences present in the element itself.

The 5' untranslated region of the I factor, a long interspersed nuclear element-like retrotransposon of Drosophila melanogaster, contains an internal promoter and sequences that regulate expression

The I-R system of hybrid dysgenesis in Drosophila melanogaster is controlled by a long interspersed nuclear element-like retroposon, the I factor. Transposition of the I factor occurs at a high frequency only in the ovaries of females produced by crossing males of inducer strains that contain functional I factors with females of reactive strains that lack them. In this study, the 5' untranslated region of the I factor was joined to the chloramphenicol acetyltransferase gene, and activity was assayed in transfected D. melanogaster tissue culture cells and transformed flies. The results have identified a promoter that lies within the first 186 pb of the I factor. Deletion analysis shows that nucleotides +1 to +40 are sufficient for high promoter activity and accurate transcription initiation. This region contains sequences that are found in a class of RNA polymerase II promoters that lack both a TATA box and CpG-rich motifs. In transformed flies, high levels of expression from nucleotides +1 to +186 are confined to the ovaries of reactive females, suggesting that the promoter is involved in the tissue and cytotype specificity of transposition.

I elements and the Drosophila genome

LINEs are a large class of transposable elements in eukaryotes. They transpose by reverse transcription of an RNA intermediate. I elements of Drosophila melanogaster belong to this class and are responsible for the I-R system of hybrid dysgenesis. Many results indicate that at the beginning of the century natural populations of this species were devoid of active I elements and that they were invaded by functional I elements in the last decades. Many Drosophila species contain both defective and active I elements. It seems that the latter were lost in Drosophila melanogaster before its spread throughout the world, and that the recent invasion results from the spread of functional elements originating either from another species by horizontal transfer or from an isolated population of the same species. These data are discussed, as well as their significance in evolutionary processes.

Evidence for retrotransposition of the I factor, a LINE element of Drosophila melanogaster

LINEs are transposable elements found in various eukaryotes such as plants, protists, insects, and mammals. Their transposition is usually difficult to study, particularly in humans, where some diseases have been shown to result from LINE insertion mutations. This is due to the fact that most copies of any particular family of elements are defective and that their transposition frequency is low. By contrast, the I factor of Drosophila melanogaster transposes at high frequency during I-R hybrid dysgenesis and is a good model for studying the LINE element superfamily. LINEs encode putative polypeptides showing similarities with viral reverse transcriptases but, unlike viral retrotransposons, they do not have terminal repeats and their ability to transpose by reverse transcription has previously only been inferred from structural analysis. Here we present direct evidence for LINE retrotransposition. Transposition of an I factor marked by an intron resulted in accurate removal of the intron.

Molecular characteristics of the heterochromatic I elements from a reactive strain of Drosophila melanogaster

There are two categories of strains in Drosophila melanogaster with respect to the I-R system of hybrid dysgenesis. The inducer strains contain particular transposable elements named I factors. They are not present in the strains of the other category called reactive (R) strains. Defective I elements are present in the pericentromeric regions of both categories of strains. This last subfamily of I sequences has not yet been described in detail and little is known about its origin. In this paper, we report that the defective I elements display an average of 94% of sequence identity with each other and with the transposable I factor. The results suggest that they cannot be the progenitors of the present day I factors, but that each of these two subfamilies started to evolve independently several million years ago. Furthermore, the sequence comparison of these I elements with an active I factor from Drosophila teissieri provides useful information about when the deleted I elements became immobilized.

Identification of a potential RNA intermediate for transposition of the LINE-like element I factor in Drosophila melanogaster

The I factor, a transposable element related to mammalian LINEs, controls the I-R system of hybrid dysgenesis in Drosophila melanogaster. It transposes at high frequency in the germ-line of the female progeny of crosses between females of the reactive class of strains and males of the inducer class. The structure and DNA sequence of the I factor suggest that it transposes by reverse transcription of an RNA intermediate. Northern blot and S1 mapping experiments show that a full-length RNA of the I factor is synthesized specifically in the conditions of which I factors transpose. This RNA has all characteristics of a transposition intermediate. It is only found in the ovaries of dysgenic females suggesting that I factor activity is restricted to this tissue because of regulation at the level of the initiation of transcription or RNA stability.

I transposable elements and I-R hybrid dysgenesis in Drosophila

I factors, transposable elements related to mammalian LINEs, are responsible for I-R hybrid dysgenesis in Drosophila melanogaster. Although they are not structurally related to retrovirus-like transposable elements, they appear to move around the genome via reverse transcription of a full-length RNA intermediate. The mechanism and control of this process are now being dissected at the molecular level.

A long interspersed repetitive element--the I factor of Drosophila teissieri--is able to transpose in different Drosophila species

Long interspersed repetitive elements (LINEs) are transposable elements present in many species. In mammals they are difficult to study because most of them are defective and their transposition frequency is low. The I factor of Drosophila melanogaster is a LINE element that is particularly interesting because its transposition occurs at high frequency during I-R hybrid dysgenesis. This phenomenon occurs when males from the class of inducer strains are crossed with females from the class of reactive strains. Inducer strains contain several complete 5.4-kilobase I factors at various sites on the chromosomal arms. Reactive strains are devoid of complete I factors. Many results indicate that active I factors have invaded the D. melanogaster genome recently. To study the evolutionary history of I elements, we have cloned and sequenced a potentially active I factor from Drosophila teissieri. It is flanked by a target-site duplication and terminates at the 3' end by tandem repeats of the sequence TAA. When introduced into the germ line of a reactive strain of D. melanogaster by P element-mediated transformation, it is able to transpose and induces hybrid dysgenesis. This strengthens the hypothesis of a recent reinvasion of the D. melanogaster genome by active I factors giving rise to the inducer strains. They could have originated by horizontal transfer from another species. Such events also could occur for other LINE elements and might explain the spread of new variants in mammalian genomes. Moreover, the results give a further insight into I factor functional organization.

Characterization of 5' truncated transposed copies of the I factor in Drosophila melanogaster

I factors in Drosophila melanogaster are transposable elements structurally related to Mammalian LINEs. Their transposition is activated at high frequencies during I-R hybrid dysgenesis and is associated with the production of mutations of various sorts. Very few of these mutations have been studied at the molecular level; those reported so far result either from chromosomal rearrangements or from insertions of complete I factors. We have analysed three I-R induced yellow mutations and have found that one of them is due to the insertion of an I element very similar to the complete I factor, whereas the other two are due to insertions of I elements that are truncated at their 5' ends; one of them exhibits an unusual 3' end. We discuss possible mechanisms of production of such modified I elements.

The beta heterochromatic sequences flanking the I elements are themselves defective transposable elements

Phylogenetic studies suggest that mobile element families are unstable components of the Drosophila genome. Two examples of immobilization of a transposable element family are presented here: as judged by their constant genomic organization among unrelated strains, the F and I element families have been respectively immobilized for a long time in D. simulans and in the reactive D. melanogaster strains (these are the laboratory strains which escaped the recent I invasion of D. melanogaster natural populations). All the elements of these defective families are located in the beta heterochromatic portion of the genome. Moreover, most if not all of the beta heterochromatic sequences into which the defective I elements are embedded are themselves non-mobile members of various nomadic families such as mdg 4, 297, 1731, F and Doc. These results are discussed with special emphasis on the possible nomadic origin of beta heterochromatin components and on the mechanisms of evolutionary turnover of the transposable element families.

I elements of Drosophila melanogaster generate specific chromosomal rearrangements during transposition

We report a detailed molecular analysis of three chromosomal rearrangements, which have been produced during I-R hybrid dysgenesis in Drosophila melanogaster. They all disrupt the yellow gene. One of them is a deletion; the other two are inversions, which may be interpreted as the results of recombination events between I elements inserted at their break points. These events appear to occur at the time of transposition and involve integrating rather than resident I elements. They are produced by a mechanism very similar to homologous ectopic recombination.

A cloned I-factor is fully functional in Drosophila melanogaster

I-R hybrid dysgenesis in Drosophila melanogaster occurs in female progeny of crosses between reactive strain females and inducer strain males, and is controlled by transposable elements called I-factors. These are 5.4 kb elements that are structurally similar to mammalian LINE elements and other retroposons. We have tested the activity of an I-factor directly, by introducing it into the genome of a reactive strain, using P-element mediated transformation. It confers the complete inducer phenotype on the reactive strain, and can stimulate dysgenesis when transformed males are mated with reactive females. It has transposed in the transformed lines, and we have cloned one of the transposed copies. This is the first time that it has been possible to demonstrate that a particular retroposon is transposition proficient, and to compare donor and transposed elements. We propose a mechanism for I-factor transposition based on these results, and the coding capacity of these elements. We have been unable to detect either autonomous transposition of a complete I-factor from a plasmid injected into reactive strain embryos, or transposition of a marked I-factor when co-injected with a complete element.

Structure and genomic organization of I elements involved in I-R hybrid dysgenesis in Drosophila melanogaster

I-R hybrid dysgenesis in D. melanogaster is controlled by transposable elements known as I factors which terminate at their 3' ends by an A-rich sequence. Inducer strains contain active I factors. Both reactive and inducer stocks possess defective I elements. We have cloned various I elements from both categories of strains. The I elements having recently transposed in inducer strains have a structure closely related to that of active I factors. However we have isolated one such I element that is truncated at its 5' end. The I elements common to reactive and inducer strains are affected by various rearrangements and many point mutations. They do not appear to be simple derivatives of complete I factors.

Transposable and nontransposable elements similar to the I factor involved in inducer-reactive (IR) hybrid dysgenesis in Drosophila melanogaster coexist in various Drosophila species

The I factor is a transposable element controlling inducer-reactive (IR) hybrid dysgenesis in Drosophila melanogaster, which occurs when males from the class of inducer strains are crossed with females from the class of reactive strains. Inducer strains contain several copies of the complete 5.4-kilobase (kb) I factor at various sites on the chromosomal arms; reactive strains contain no complete I factor. Incomplete and defective I elements occur at constant locations in pericentromeric heterochromatin of both types of strains. The 5.4-kb I factors transpose, whereas incomplete I elements do not transpose. The constant location of defective I elements in all strains indicates that they were in the genome before the spread of D. melanogaster throughout the world. Sequences homologous to I occur in other Drosophila species, and their distribution correlates with the phylogenetic relationships between species. We have studied the organization of I homologues in Drosophila simulans and Drosophila teissieri. These species seem to contain both transposable I elements, even though their structure may differ from that of the 5.4-kb I factors of the inducer strains of D. melanogaster, and nontransposable I elements, which are always at the same place in the genome when different stocks of the same species are compared. These results suggest that both mobile and nonmobile I elements are very old components of the Drosophilidae genome.

Molecular lesions associated with white gene mutations induced by I-R hybrid dysgenesis in Drosophila melanogaster

We have identified molecular lesions associated with six mutations, w and w, of the white gene of Drosophila melanogaster. These mutations arose in flies subject to I-R hybrid dysgenesis. Four of the mutations give rise to coloured eyes and are associated with insertions of 5.4-kb elements indistinguishable from the I factor controlling I-R dysgenesis. The insertion associated with w is at a site which, within the resolution of these experiments, is identical to that of two previously studied I factors. This appears to be a hot-spot for I factor insertion. We have compared the sites of these insertions with sequences complementary to white gene mRNA identified by Pirrotta and Bröckl. The hot-spot is in the fourth intron. The insertion carried by w is either within, or just beyond, the last exon. The insertion carried by w is near the junction of the first exon and first intron. The w mutation is a derivative of w. It contains an insertion of I factor DNA within, or immediately adjacent to, the F-like element associated with w, and results in restoration of some eye colour. This insertion is just upstream of the start of the white mRNA. Mutations w and w are deletions removing mRNA coding sequences. Both determine a bleached white phenotype.

The molecular basis of I-R hybrid dysgenesis in Drosophila melanogaster: identification, cloning, and properties of the I factor

We have analyzed two mutations of the white-eye gene, which arose in flies subject to I-R hybrid dysgenesis. These mutations are associated with insertions of apparently identical 5.4 kb sequences, which we have cloned. We believe that these insertions are copies of the I factor controlling I-R hybrid dysgenesis. The I factor is not a member of the copia-like or fold-back classes of transposable elements and has no sequence homology with the P factor that controls P-M dysgenesis. All strains of D. melanogaster contain I-factor sequences. Those present in reactive strains must represent inactive I elements. I elements have a remarkably similar sequence organization in all reactive strains and are located in peri-centromeric regions. Inducer strains appear to contain both I elements, located in peri-centromeric regions, and 10-15 copies of the complete I factor at sites on the chromosome arms.

Hybrid dysgenesis in Drosophila melanogaster

Several dysgenic traits may occur within the Drosophila melanogaster species as a result of crosses between different strains. Crossing two mutually interacting categories, named inducer and reactive, may lead, among other abnormalities, to a specific kind of female sterility that has proved useful for investigating the genetic factors involved in the interaction. The reactive state appears to result from a cytoplasmic state ultimately controlled by a chromosomal polygenic system. The inducer character is determined by a chromosomal factor that exhibits all characteristics of a transposable element. Overall, the data contribute to clarification of mutator activities in D. melanogaster and open new opportunities to investigate unusual genetic mechanisms.

Non-Mendelian female sterility in Drosophila melanogaster: influence of aging and thermic treatments. III. Cumulative effects induced by these factors

Crosses between various strains of Drosophila melanogaster may give rise to a female sterility of non-Mendelian determination. Reduced fertility is observed in females, known as SF females, bred from crosses between females of "reactive" strains and males of "inducer" strains. The reduced fertility of the SF females is the result of an interaction between an extrachromosomal property varies considerably in its ability to reduce fertility. The fertility reduction of the SF females corresponds to what is known as the reactivity level of their reactive mothers. Two nongenetic factors can modify the level of reactivity: aging and temperature. The action of aging is cumulative. When the flies of a reactive strain are submitted at each generation to the action of this factor, the level of reactivity of this strain is gradually modified. The modifications induced are reversible. Indeed, when such a modified strain is returned to standard breeding conditions, the reactivity returns progressively to its initial level. The effect of thermic treatments also seems to be cumulative and reversible.