Transpositional bursts and chromosome rearrangements in unstable lines of Drosophila

The phenomenon of transpositional bursts-massive simultaneous transpositions of mobile elements belonging to different structural classes and accompanied by multiple mutagenesis were earlier described. Although the mechanisms of this phenomenon are still unclear, it is obvious now that it embraces total genome and includes not only transpositions of different mobile elements but also recombination processes--homologous recombination for LTR's and gene conversion. It is shown in this work that transpositional bursts may be accompanied by appearance of grass chromosomal rearrangements. The chain of closely related mutations which is characterized, as well as pedigrees described earlier, by coordinated mutational transitions and multiple transpositions of mdg1, mdg2 and retrotransposon jokey was analyzed. Spontaneous appearance of mutations in the loci yellow, white (deficiency for 462 kb) and cut (insertion of mdg4, together with jokey) correlates with appearance of inversion In(I), 14A-20B, and the reversions of these mutations to the wild type (y+w+ct+) or to other alleles (ctMR2--insertion of mdg4 without jokey) are accompanied by reversions of inversion. The relationship of all lines analyzed in this work as well as the lines from other pedigrees was proved using analysis of polymorphic restriction sites at the scute and cut loci (5 probes were used). All "y w ctpN"--type mutants are shown to have insertion of about 7 kb at the scute locus which causes no alteration of phenotype. This once again proves multiple and coordinated character of changes taking place during hybrid dysgenesis.     

The nature of unstable insertion mutations and reversions in the locus cut of Drosophila melanogaster: molecular mechanism of transposition memory

The segment of the locus cut containing the mobile genetic element mdg4 (gypsy) insertions which induce unstable ct^MR2 and ct^MRpN10 mutations has been cloned. Both mutations depend on the insertion of mdg4 into the same sequence, which coincides with that in ct⁶ allele. The ct^MRpN10 mutation differs from ct^MR2 by additional insertion of a novel mobile element jockey into mdg4. Jockey is 2.8 kb long, represented by ˜2–100 copies per genome, very homogeneous and lacks long terminal repeats (LTRs). The excision of mdg4 takes place in stable ct⁺ reversions. On the other hand, a complete single LTR is retained in the case of unstable ct reversions characterized by a high level of reverse directed transpositions of mdg4 into the locus cut. The LTR serves as a guide for reinsertion of mdg4 itself or mdg4 with jockey into the same site of the genome. A possible mechanism of transposition memory (homologous recombination with extrachromosomal circular DNA) is discussed.     

Successive transposition explosions in Drosophila melanogaster and reverse transpositions of mobile dispersed genetic elements

Transposition outbursts occur in the destabilized Drosophila melanogaster strain ct^MR2 carrying a mutation in the locus cut induced by an insertion of mdg4. While the distribution of mobile genetic elements remained unchanged in the great majority of germ cells, in a few cells numerous transpositions had occurred involving mdg (copia-like), fold-back and P-elements. We used in situ hybridization to analyze the distribution of five families of mdg elements in the X-chromosome during several consequent mutational changes in D. melanogaster. Each of them was accompanied by many changes in mdg localization, all of which occurred in one and the same cell. Thus, we could observe the series consisting of up to five successive transposition explosions leading to an almost complete change in the distribution of the mdg elements tested. We also found that in the course of successive transposition explosions, mdg elements often inserted into those sub-sections of the X-chromosome where they had previously been located. This phenomenon, designated as reverse directed transposition, was studied in more detail on insertion into the locus yellow. The rate of reverse transposition of the same mdg element to the corresponding locus was 10–100 times as high as that of primary insertion. In some cases, `the transposon shuttle' into and out of the locus was observed. The existence of `transposition memory' partially explains the specificity of mdg localization in closely related strains as well as the co-ordinated behaviour of different mdg elements in independent transposition explosions. The evolutionary significance of transposition explosions and directed reverse transposition (transposon shuttle) is discussed.     

Genetic instability and transposition of mobile element mdg4 in a mutator strain of Drosophila melanogaster

Laboratory mutator strain of Drosophila melanogaster is characterized by increased (up to 10(-3)-10(-4) frequency of spontaneous mutability. Mutations appear in premeiotic stages of gametes development. The majority of mutations were unstable (high frequencies of reversions, appearance of new mutations at the same and other loci, replicating instability). Localization of mobile elements mdg1, mdg2, mdg3, mdg4, copia and P element in X chromosomes of mutator individuals and its mutations y, ct, sbt was studied by hybridization in situ. In all strains P element was absent. The distribution of mdg1, mdg2, mdg3 and copia was identical in mutator strains and its derivatives, but distribution of mdg4 was different. The essential heterogeneity in localization of mdg4 and increased (up to 30-40) copy number in the mutator strain individuals was observed. The ability of single element mdg4 to autonomous transpositions was thus shown.     

Mobilization of the transposable element mdg4 by hybrid dysgenesis generates a family of unstable cut mutations in Drosophila melanogaster

Summary A family of unstable mutations at the cut locus in Drosophila melanogaster was obtained under the conditions of hybrid dysgenesis (Gerasimova 1981, 1982). The in situ hybridization experiments have shown that, in the original unstable ct ^MR2 mutation, the 7B region of the X chromosome (where cut is located) contains a mobile dispersed genetic element, mdg4. All other unstable ct mutations derived from ct ^MR2 including visible and lethal alleles and unstable ct ⁺ reversions, also contain mdg4 in the 7B region. The X chromosomes of the parent strain (wild type) do not contain mdg4 at all. All stable revertants derived from ct ^MR2, from other unstable ct mutations, or from ct lethals lost mdg4 from the 7B region. The ct ^MR2 X chromosome does not contain P-elements, although a few copies are present in the autosomes. The instability of the ct ^MR2./ct ^MR2 strain remained at a high level for 50 generations (1.5 years) and then rapidly decreased. A new cross with an MRh12/Cy strain (originally used for dysgenesis induction and containing a number of P-elements) increased the instability to a level exceeding the original one. The data strongly suggest that unstable ct mutations in our system are induced by transpositions of mdg4, possibly activated by P-elements.     

Interaction of mobile elements P and mdg3 in Drosophila melanogaster: genetic aspects

It was found earlier that two unstable sn mutants isolated from natural populations are connected with insertion of mobile element mdg3 into the 7D1-2 region where singed gene (1-21.0) is localised. From two original sn mutants, a series of unstable sn alleles, both mutant and normal for phenotype, was extracted. Then we studied, how they change the mutation rate in germinal and somatic cells of different hybrids with pi 2 stock having P cytotype and active P elements in the chromosomes. Addition of P chromosomes, independently of the background of cytoplasm, proved to reduce the sn instability. The level of sn mutability was decreased with increasing the dose of P chromosomes. It is suggested that mutation events are caused by transposition of mdg3 and that both mdg3 and P elements compete for the same cellular factor, capable of activation of transposition process.     

Mobilization of retrotransposon copia in the Drosophila melanogaster genome]

Considerable heterogeneity of retrotransposon copia sites of location on polytene chromosomes was revealed in one of the substocks of the inbred Drosophila melanogaster stock. Heterogeneity of copia sites of location was found in no other substocks analyzed. The heterogeneity was shown to be caused by copia insertions in new sites. The frequency of insertions is about 12% per haploid genome per generation. The retrotransposon excisions and somatic transpositions were not observed. The location of retrotransposons mdg1, mdg2, mdg3, mdg4, 297 and H.M.S. Beagle appeared to be stable in all the stocks analyzed. Thus, a model system allowing to study mechanisms of retrotransposon copia transpositions in D. melanogaster tissues as well as phenotypic effects of copia mobilization is described.     

Effect of mutations in the genes su(Hw) and mod(mdg4) on transvection between alleles of the Yellow locus in Drosophila melanogaster]

A typical example of transvection is a complementation between alleles in the yellow locus: y2 (mdg4 insertion inactivating certain y-enhancers) and y1 (deletion of the y-promoter but not of the enhancer). Transvection was explained by trans-activation of promoter in y2-allele by enhancer of y1-allele. Here we found that the mutation mod(mdg4)1u1 in the modifier of mdg4 locus (a regulatory gene controlling, together with suppressor of Hairy wing) expression of (mdg4) completely suppress the complementation. Removal of an acidic domain from su(Hw) protein product in su(Hw)j mutation partially suppress the complementation. We also have found that mod(mdg4)1u1 mutation trans-inactivates the yellow allele with a wild type phenotype (y+2MC) in heterozygote with the y2 allele, i.e. the negative transvection takes place. In this case, deletion removing an acidic domain even in one copy of su(Hw) suppresses the effect of mod(mdg4)1u1 mutation.     

Distribution of Unlinked Transpositions of a Ds Element from a T-DNA Locus on Tomato Chromosome 4

In maize, receptor sites for unlinked transpositions of Activator (Ac) elements are not distributed randomly. To test whether the same is true in tomato, the receptor sites for a Dissociation (Ds) element derived from Ac, were mapped for 26 transpositions unlinked to a donor T-DNA locus on chromosome 4. Four independent transposed Dss mapped to sites on chromosome 4 genetically unlinked to the donor T-DNA, consistent with a preference for transposition to unlinked sites on the same chromosome as opposed to sites on other chromosomes. There was little preference among the nondonor chromosomes, except perhaps for chromosome 2, which carried seven transposed Dss, but these could not be proven to be independent. However, these data, when combined with those from other studies in tomato examining the distribution of transposed Acs or Dss among nondonor chromosomes, suggest there may be absolute preferences for transposition irrespective of the chromosomal location of the donor site. If true, transposition to nondonor chromosomes in tomato would differ from that in maize, where the preference seems to be determined by the spatial arrangement of chromosomes in the interphase nucleus. The tomato lines carrying Ds elements at known locations are available for targeted transposon tagging experiments.     

Doc and copia instability in an isogenic Drosophila melanogaster stock

A high degree of heterogeneity and an overall increase in number of insertion sites of the mobile elements Doc and copia were revealed in one substock of an isogenic Drosophila melanogaster stock, while in two other substocks the distribution of copia sites was highly homogenous, but that of Doc sites was again heterogenous. We therefore concluded that copia was unstable in one of the substocks and Doc was unstable in all. Doc instability presumably arose earlier than copia instability. Doc and copia transpositions were directly observed in experiments with one substock. An abundance of copia insertions was revealed in the X chromosome where insertions with deleterious effects are exposed to selection in hemizygous condition. The locations of many other mobile elements (mdg1, mdg2, mdg3, mdg4, 297, B104, H.M.S. Beagle, I, P, BS, FB) were found to be conserved in each substock and did not differ between them, indicating that these mobile elements were stable. This homogeneity is a strong argument against any possibility of inadvertent contamination.     

Mobile elements and transposition events in the cut locus of Drosophila melanogaster

We have cloned from the Oregon R strain of Drosophila melanogaster a 240 kb segment of DNA that contains the cut (ct) locus, and characterized the region for the presence of repetitive elements. Within this region at least five copies of the suffix element were detected, as well as several putatively novel mobile elements. A number of mutations obtained from the unstable ct ^MR2 strain and its derivatives were mapped within the cut locus. Comparison between parental and daughter strains indicates that frequently two or more independent transposition events involving the cut locus occur simultaneously within a single germ cell, thus providing a molecular basis for the transposition explosion phenomenon.     

Doc and copia instability in an isogenic Drosophila melanogaster stock

A high degree of heterogeneity and an overall increase in number of insertion sites of the mobile elements Doc and copia were revealed in one substock of an isogenic Drosophila melanogaster stock, while in two other substocks the distribution of copia sites was highly homogenous, but that of Doc sites was again heterogenous. We therefore concluded that copia was unstable in one of the substocks and Doc was unstable in all. Doc instability presumably arose earlier than copia instability. Doc and copia transpositions were directly observed in experiments with one substock. An abundance of copia insertions was revealed in the X chromosome where insertions with deleterious effects are exposed to selection in hemizygous condition. The locations of many other mobile elements (mdg1, mdg2, mdg3, mdg4, 297, B104, H.M.S. Beagle, I, P, BS, FB) were found to be conserved in each substock and did not differ between them, indicating that these mobile elements were stable. This homogeneity is a strong argument against any possibility of inadvertent contamination.