Gene instability induced by mobile genetic elements in Drosophila melanogaster]

The genetic instability of Drosophila melanogaster genes induced by the mobile genetic elements is reviewed. The main attention is paid to genetic instability depended on types of crossing. Data on the possibility of genetic instability induction by the chemical and physical (X-rays, heat-shock) agents and their complex effect are cited. It was shown that a number of agents which cause mutagenic effect realize their action by involving of mobile genetic elements.     

Alteration of chromosome structure in ovarian nurse cells of Drosophila melanogaster by hybrid dysgenesis

The impact of hybrid dysgenesis on the chromosome structure of Drosophila melanogaster ovarian nurse cells was studied. In the examined lines and interlinear hybrids (including those yielded by dysgenic crosses in the P-M and I-R systems of hybrid dysgenesis), disturbed chromosome synapsis was revealed. The disturbance was somewhat similar to that observed in interspecific hybrids. Quantitative analysis showed that the mean frequency of nuclei with defective chromosome pairing ranged from 60.4 to 76%. FISH analysis of ovarian nurse chromosomes of Canton S x Berlin hybrids showed differences in the label localization in asynaptic homologs of arm 2L, which probably results in disrupted homolog pairing and reveal interlinear differences in localization of mobile genetic elements. Our results conform to Sved's model stating that hybrid dysgenesis is based on disorganization of the germline nuclear space.     

Induction of unstable mutations in Drosophila melanogaster by microinjection of oncogenic virus DNA into the embryo polar plasma. Insertional nature of mutations]

We have demonstrated that mutations induced in Drosophila melanogaster by the microinjections of adenovirus Sa7 DNA in early embryos are of insertional nature. The role of insertional elements is played by the Drosophila transposons, but not by the virus DNA. The ability of oncoviral DNA to induce transpositions of mobile elements in recipient genome is the molecular basis of this system of genetic instability.     

Molecular genetic analysis of hobo mobile genetic element polymorphism in the genome of Drosophila melanogaster line subjected to long-term selection

The distribution of mobile genetic element hobo was examined in Drosophila melanogaster lines HA (high male mating activity) and LA (low male mating activity) before and after their isogenization using Southern blot hybridization. The probe containing a full-size hobo copy was shown to produce polymorphic multilocus hybridization with chromosomal DNA. The polymorphism was line-specific. A comparison of hybridization patterns in isogenic and original lines showed that isogenization in dysgenic crosses resulted in the appearance of additional hobo localization sites in LA but not in HA. The hobo destabilization in the LA genome correlated with genetic instability and the ability to induce H-E hybrid dysgenesis. The results obtained are discussed in relation to the possible role of hobo in inducing genetic variability in lines with low male mating activity, which may counteract deleterious consequences of inbreeding and selection in the negative direction.     

The nature of spontaneous and induced mutagenesis in a genetically unstable mutator strain of Drosophila melanogaster]

The spontaneous and induced frequencies of visible mutations by N-nitroso-N-ethylurea in male cells of Drosophila melanogaster genetically unstable mutator strain have been investigated. The spontaneous and induced by N-nitroso-N-ethylurea genetic instability in mutator strain have similar manifestation, that evidently testifies the existence of general mechanisms of the appearance of unstable mutations, namely the transpositions of the mobile genetic elements.     

Variability in the longevity of Drosophila imagoes under chronic irradiation at low doses

The investigation of life span variability induced by a chronic influence of low doses gamma irradiation on the laboratory stocks of D. melanogaster, distinguishing by mobile genetic units, were carried out. Shown was the link of life span alterations in D. melanogaster with features of cytotype and genotype in tested stocks and with induced apoptotic cell death. The life span variation can be determined by a genomic destabilisation with an induction of mobile genetic elements in conditions of chronic gamma irradiation.     

Genetic effects in Drosophila melanogaster induced by chronic low-dose irradiation

It was investigated the influence of the chronic gamma-irradiation in the dose rate of 0.17 sGy/h on the rate of genetic variability in the laboratory strains of Drosophila melanogaster with genotypic distinguishes in mobile genetic elements and defects in the DNA repair processes. It was shown that the rates of induction of recessive lethal mutations depended on genotype of investigated strains. In the different strains we have observed an increase as well as a decrease of the mutation rate. Also in was established that irradiation leads to the frequencies of the GD-sterility and mutability of the snw and h(w+) in the P-M and H-E dysgenic crosses. The obtained results suggest that mobile genetic elements play an important role in the forming of genetic effects in response to low dose irradiation.     

Hybrid Dysgenesis in Drosophila Simulans Lines Transformed with Autonomous P Elements

The molecular and phenotypic analysis of several previously described P element-transformed lines of Drosophila simulans was extended in order to determine whether they had the potential to produce a syndrome of P-M hybrid dysgenesis analogous to the one in Drosophila melanogaster. The transformed line with the highest number of P elements at the beginning of the analysis, DsP pi-5C, developed strong P activity potential and P element regulation, properties characteristic of D. melanogaster P strains. The subsequent analysis of sublines derived from 34 single pair matings of DsP pi-5C revealed that they were heterogeneous with respect to both their P element complements and P activity potentials, but similar with respect to their regulatory capabilities. The subline with the highest P activity, DsP pi-5C-27, was subsequently used as a reference P strain in the genetic analysis of the D. simulans transformants. In these experiments, the reciprocal cross effect was observed with respect to both gonadal sterility and male recombination. As in D. melanogaster, the induction of gonadal sterility in D. simulans was shown to be temperature-dependent. Molecular analysis of DsP pi-5C-27 revealed that it has approximately 30 P elements per genome, at least some of which are defective. The number of potentially complete P elements in its genome is similar to the number in the D. melanogaster P strain, Harwich-77. Overall our analysis indicates that P-transformed lines of D. simulans are capable of expressing the major features of P-M hybrid dysgenesis previously demonstrated in D. melanogaster and that P elements appear to behave in a similar way in the two sibling species.     

Mobilization of hobo elements residing within the decapentaplegic gene complex: suggestion of a new hybrid dysgenesis system in Drosophila melanogaster

We present results demonstrating that the hobo family of transposable elements can promote high rates of chromosomal instability. Using strains with a hobo element inserted within the decapentaplegic gene complex (DPP-C), we have recovered numerous DPP-C mutations involving chromosomal rearrangements and deletions with one endpoint in the vicinity of the pre-existing hobo element. This hypermutability occurred in the germ lines of hybrid progeny from crosses involving strains containing hobo elements to strains lacking them. In some crosses, the offspring had rudimentary gonads, reminiscent of GD sterility. The germline hypermutability and infertility are similar to those produced by P-element-mediated hybrid dysgenesis. Given the many genetic and molecular similarities of the P and hobo systems, we propose that a system analogous to P-M hybrid dysgenesis has been activated in the hobo+ X hobo- crosses.     

Transposable elements as artisans of the heterochromatic genome in Drosophila melanogaster

Over 50 years ago Barbara McClintock discovered that maize contains mobile genetic elements, but her findings were at first considered nothing more than anomalies. Today it is widely recognized that transposable elements have colonized all eukaryotic genomes and represent a major force driving evolution of organisms. Our contribution to this special issue deals with the theme of transposable element-host genome interactions. We bring together published and unpublished work to provide a picture of the contribution of transposable elements to the evolution of the heterochromatic genome in Drosophila melanogaster. In particular, we discuss data on 1) colonization of constitutive heterochromatin by transposable elements, 2) instability of constitutive heterochromatin induced by the I factor, and 3) evolution of constitutive heterochromatin and heterochromatic genes driven by transposable elements. Drawing attention to these topics may have direct implications on important aspects of genome organization and gene expression.     

Behavior of the hobo transposable element with regard to TPE repeats in transgenic lines of Drosophila melanogaster

The hobo transposable element of Drosophila melanogaster is known to induce a hybrid dysgenesis syndrome. Moreover it displays a polymorphism of a microsatellite in its coding region: TPE repeats. In European populations, surveys of the distribution of hobo elements with regard to TPE repeats revealed that the 5TPE element is distributed along a frequency gradient, and it is even more frequent than the 3TPE element in Western populations. This suggests that the invasive ability of the hobo elements could be related to the number of TPE repeats they contain. To test this hypothesis we monitored the evolution of 16 lines derived from five initial independent transgenic lines bearing the 3TPE element and/or the 5TPE element. Four lines bearing 5TPE elements and four bearing 3TPE elements were used as a noncompetitive genetic background to compare the evolution of the 5TPE element to that of the 3TPE element. Eight lines bearing both elements provided a competitive genetic context to study potential interactions between these two elements. We studied genetic and molecular aspects of the first 20 generations. At the molecular level, we showed that the 5TPE element is able to spread within the genome at least as efficiently as the 3TPE element. Surprisingly, at the genetic level we found that the 5TPE element is less active than the 3TPE element, and moreover may be able to regulate the activity of the 3TPE element. Our findings suggest that the invasive potential of the 5TPE element could be due not only to its intrinsic transposition capacity but also to a regulatory potential.     

Horizontal transfer of hobo transposable elements within the Drosophila melanogaster species complex: evidence from DNA sequencing.

The hobo family of transposable elements, one of three transposable-element families that cause hybrid dysgenesis in Drosophila melanogaster, appears to be present in all members of the D. melanogaster species complex: D. melanogaster, D. simulans, D. mauritiana, and D. sechellia. Some hobo-hybridizing sequences are also found in the other members of the melanogaster subgroup and in many members of the related montium subgroup. Surveys of older isofemale lines of D. melanogaster suggest that complete hobo elements were absent prior to 50 years ago and that hobo has recently been introduced into the species by horizontal transfer. To test the horizontal transfer hypothesis, the 2.6-kb XhoI fragments of hobo elements from D. melanogaster, D. simulans, and D. mauritiana were cloned and sequenced. The DNA sequences reveal an extremely low level of divergence and support the conclusion that the active hobo element has been horizontally transferred into or among these species in the recent past.     

Amplification of Kp Elements Associated with the Repression of Hybrid Dysgenesis in Drosophila Melanogaster

Mobile P elements in Drosophila melanogaster cause hybrid dysgenesis if their mobility is not repressed. One type of repression, termed P cytotype, is a complex interaction between chromosomes carrying P elements and cytoplasm and is transmitted through the cytoplasm only of females. Another type of repression is found in worldwide M' strains that contain approximately 30 copies per individual of one particular P element deletion-derivative termed the KP element. This repression is transmitted equally through both sexes. In the present study we show that biparentally transmitted repression increases in magnitude together with a rapid increase in KP copy-number in genotypes starting with one or a few KP elements and no other deletion-derivatives. Such correlated increases in repression and KP number per genome occur only in the presence of complete P elements, supporting the interpretation that they are probably a consequence of the selective advantage enjoyed by flies carrying the highest numbers of KP elements. Analysis of Q strains also reveals the presence of qualitative differences in the way the repression of dysgenesis is transmitted. In general, Q strains not containing KP elements have the P cytotype mode of repression, whereas Q strains with KP elements transmit repression through both sexes. This difference among Q strains further supports the existence of at least two types of repression of P-induced hybrid dysgenesis in natural populations of D. melanogaster.     

Induction of unstable mutations in Drosophila melanogaster by microinjections of oncogenic virus DNA into the embryo polar plasma. Prolonged genetic instability of mutations at the lobe locus]

Mutations Lobe induced by the microinjections of RSV cDNA into Drosophila melanogaster early embryos are characterized by permanent genetic instability; the level of this instability is being changed in time. Based on the results of genetic analyses of Lobe mutations and molecular analysis of white and ADH mutations induced at high frequency in this system of gene instability, we supposed that unstable mutations which arose under the influence of retroviral cDNA are of the insertional nature.     

DNA methylation of mobile genetic elements in human cancers

Mobile genetic elements are responsible for half of the human genome, creating the host genomic instability or variability through several mechanisms. Two types of abnormal DNA methylation in the genome, hypomethylation and hypermethylation, are associated with cancer progression. Genomic hypermethylation has been most often observed on the CpG islands around gene promoter regions in cancer cells. In contrast, hypomethylation has been observed on mobile genetic elements in the cancer cells. It is recently considered that the hypomethylation of mobile genetic elements may play a biological role in cancer cells along with the DNA hypermethylation on CpG islands. Growing evidence has indicated that mobile genetic elements could be associated with the cancer initiation and progression through the hypomethylation. Here we review the recent progress on the relationship between DNA methylation and mobile genetic elements, focusing on the hypomethylation of LINE-1 and HERV elements in various human cancers and suggest that DNA hypomethylation of mobile genetic elements could have potential to be a new cancer therapy target in the future.     

P transposable element in Drosophila melanogaster: horizontal transfer]

The P transposable element family in Drosophila melanogaster is responsible for the syndrome of hybrid dysgenesis which includes chromosomal rearrangements, male recombination, high mutability and temperature sensitive agametic sterility (called gonadal dysgenesis sterility). P element activity is controlled by a complex regulation system, encoded by the elements themselves, which keeps their transposition rate low within the strain bearing P elements and limits copy number by genome. A second regulatory mechanism, which acts on the level of RNA processing, prevents P mobility to somatic cells. The oldest available strains, representing most major geographical regions of the world, exhibited no detectable hybridization to the P-element. In contrast, all recently collected natural populations that were tested carried P-element sequences. The available evidence is consistent with the hypothesis of a worldwide P-element invasion of D. melanogaster during the past 30 years. Timing and direction of the invasion are discussed. The lack of P-element in older strains of Drosophila melanogaster as well as in the species must closely related to Drosophila melanogaster, suggests that P entered the Drosophila melanogaster genome recently, probably by horizontal transfer from an other species. The analysis of P-element elsewhere in the genus Drosophila reveals that several more distantly related species carried transposable elements with sequences quite similar to P. The species with the best-matching P-element is D. willistoni. A P-element from this species was found to match all but one of the 2907 nucleotides of the Drosophila melanogaster P-element. The phylogenic distributions and the likely horizontal transfers of the two other Drosophila transposable elements are discussed.     

A novel transposition system in Drosophila melanogaster depending on the Stalker mobile genetic element.

Crosses between the Drosophila melanogaster y2sc1waG strain or some of its derivatives and the FM4 strain yielded insertional mutagenesis with a frequency of 10(-3)-10(-4). The system differs in several respects from the known cases of hybrid dysgenesis: (i) it does not depend on the direction of a cross; (ii) destabilization continues for a long time after initial crosses; (iii) mutations may occur at different stages of development. The mutation in the yellow locus has been cloned and found to depend on insertion into the coding region of the gene of a novel mobile genetic element designated as Stalker. The sequencing of Stalker termini reveals 405 bp direct repeats (LTRs) and a target 3 bp duplication, as well as some other sequences typical of retrovirus-like retrotransposons. The number of Stalker copies per genome and chromosomal localization vary among D. melanogaster strains. Before crosses, the location of Stalker on chromosomes is fairly stable in a particular strain but thereafter numerous changes in Stalker distribution take place. Most novel substrains are internally heterogenous which is indicative of the continuing Stalker transposition. Other mobile elements tested do not move. Possibly, only Stalker is mobilized in the system. Many known and novel mutations have been obtained. Comparison of their genetic localization with Stalker distribution suggests that the majority of them have been induced by the Stalker insertion.