Transpositions of mobile elements mdg4 (gypsy) and hobo in somatic and germ cells of a genetically unstable mutator strain of Drosophila melanogaster]

Analysis of distribution of the several families of mobile genetic elements has been performed. The analysis dealt with the X chromosomes of male progeny from the crosses of individual males of Mutator strain (MS) with attached-X females. The experimental results demonstrated different localization of the elements gypsy and hobo in the salivary gland squashes of different males-brothers. Location of other elements under study--mdg1, 412, mdg3, copia, 297, 17.6, Beagle, BS, Doc, FB, Springer--was invariant in all larvae. The analysis is equal to the study of transposition events at the level of gametes. Thus, doubtless, the capability of gypsy and hobo to transpose in germ cells of the MS individuals has been detected. Mobilization of the elements occurs at premiotic stages of gametes' development, as indicated by appearance of the clusters of transpositions. In the process of studies on coincidence of gypsy and hobo transposition acts, independent character of the elements' movement has been revealed. It has been detected in the same experiment that the distribution of the gypsy copies in different cells of the same salivary gland varies strongly. All hybridization sites were divided into two groups: "constant" sites common for all cells and "additional" ones, whose locations did not coincide in neighbouring cells of salivary gland. The existence of additional sites is major evidence of gypsy transpositions in somatic cells of MS. Transposition events have been as well discovered for hobo in somatic cells.     

Transposition of mobile elements gypsy (mdg4) and hobo in germ-line and somatic cells of a genetically unstable mutator strain of Drosophila melanogaster.

Summary Using the in situ hybridization technique, we have analysed the distribution of mobile elements in the X chromosomes of male offspring of individual mutator strain (MS) males crossed to attached-X females. The experiments demonstrate varying cytological localization of the mobile elements gypsy (mdg4) and hobo among different individuals. The other mobile elements investigated (mdgl, mdg3, 412, 297, copia, 17.6, Doc, H.M.S. Beagle, Springer, FB) display no changes in insertion sites. Such an experiment is equivalent to analysis of separate gametes of an MS individual. Thus, the ability of gypsy and hobo to transpose in germ-line cells is demonstrated directly. Transpositions occur at premeiotic stages of germ cell development, since they appear in clusters. Analysis of gypsy and hobo transposition events shows that they occur independently. The same experiment demonstrates that gypsy localization varies significantly between different salivary gland cells of an MS individual. Two types of gypsy hybridization sites can be distinguished: permanent sites, common to all cells, and additional ones varying between neighbouring salivary gland cells. These additional sites indicate gypsy transposition in somatic cells of the MS. Transposition of the hobo element in somatic cells has also been observed.     

The genetics and cytology of a mutator factor in Drosophila melanogaster

A Drosophila melanogaster mutator factor is described whose effects include the induction of unique chromosomal aberrations and male crossing over. Results of experiments to map the factor suggest that genetic transmission is somehow chromosomally associated but not localizable to the X, Y, second or third chromosome. There appears to be a good correlation between the distributions of male crossover exchange points and unique aberration breakpoints for the second chromosome but not for the third chromosome. The male crossovers, which occur more frequently in the centromeric region, occur in euchromatin rather than in the centric heterochromatin. The male crossovers tend to be rather precise reciprocal exchanges, since cytologically detectable deletions and duplications are only infrequently produced. It is suggested that the present mutator may be identical to earlier reported mutators of D. melanogaster.     

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.     

Retrotransposon MDG4 and its role in genetic instability of a mutator strain of Drosophila melanogaster

This article summarizes the results of a ten-year study of genetic instability of a mutator strain of Drosophila melanogaster caused by transposition of the gypsy retrotransposon. The results of other authors working with an analogous system are analyzed. Possible mechanisms are suggested for the interaction of gypsy with the cell gene flamenco that participates in transposition control of this mobile element.     

The cytogenetics of mutator gene-induced X-linked lethals in Drosophila melanogaster

A third chromosome mutator gene effectively increases the spontaneous frequency of sex-linked recessive lethals in females but not in females of Drosophila melanogaster. Approximately half the mutator-induced mutants occur as clusters of the same mutant implying a premeiotic origin. An appreciable number of the mutator-induced lethals are associated with comparatively long deficiencies of several salivary gland chromosome bands. The possible modes of mutator gene action are conjectured.     

Molecular nature of 11 spontaneous de novo mutations in Drosophila melanogaster

To investigate the molecular nature and rate of spontaneous mutation in Drosophila melanogaster, we screened 887,000 individuals for de novo recessive loss-of-function mutations at eight loci that affect eye color. In total, 28 mutants were found in 16 independent events (13 singletons and three clusters). The molecular nature of the 13 events was analyzed. Coding exons of the locus were affected by insertions or deletions >100 nucleotides long (6 events), short frameshift insertions or deletions (4 events), and replacement nucleotide substitutions (1 event). In the case of 2 mutant alleles, coding regions were not affected. Because approximately 70% of spontaneous de novo loss-of-function mutations in Homo sapiens are due to nucleotide substitutions within coding regions, insertions and deletions appear to play a much larger role in spontaneous mutation in D. melanogaster than in H. sapiens. If so, the per nucleotide mutation rate in D. melanogaster may be lower than in H. sapiens, even if their per locus mutation rates are similar.     

Modification of the repair processes in chemical mutagenesis in Drosophila melanogaster]

The effect of 1,4-dihydroisonicotinic acid derivative (1,4-DHINA), glutapyrone, on the frequency of sex-linked recessive lethals (SLRL), which were induced by ethyl methanesulfonate (EMS) in spermatozoa, was studied in Drosophila males and females under different exposure conditions. This test was used to analyze repair processes in EMS mutagenesis. Glutapyrone manifested the protective effect after precultivation of males at the stage of larvae and females with glutapyrone. This preparation was shown to have no effect on the frequency of EMS-induced mutations under other conditions (precultivation and subsequent cultivation of mature males with glutapyrone). These data confirmed a key role of indirect mechanisms responsible for the effect of the examined antimutagen. Glutapyrone may be assumed to improve the synthesis or functioning of enzymes involved in the repair of O6-ethylguanine. After adaptive treatment of Drosophila with low doses of the mutagen, glutapyrone increased the fertility of parents but did not suppress chemical mutagenesis.     

The nature of reverse mutations in Drosophila melanogaster

A selection system for the screening of reversions has been constructed and used to test reversions of lethals located in the proximal region of the X chromosome of Drosophila and of Kpn mutations.Spontaneous and induced reversions have been screened, X-rays and ethyl methanesulphonate (EMS) being the mutagens used in the induction experiments.No genuine back-mutation was found in 6·10⁵ gametes scored. Sterile reversions of all four lethals tested were obtained. Their frequency suggested that at least in three of the lethals the sterile reversions represented “escapers” of the lethal effect rather than true revertants.Three fertile reversions of l^x4 were found and analyzed. All three were autosomal suppressors, located on the second chromosome, allelic to each other, dominant in males and recessive in females.One fertile reversion of l^3DES was found to be an X-linked suppressor. It is suggested that this suppressor is a Y-suppressed lethal, showing a V-type position effect, resulting from an aberration included in the proximal heterochromatin of the X chromosome.Reversions of Kpn were obtained at a similar rate to that found in previous reports²².The absence of true back-mutants in our experiments, in contrast to findings in previous reports, is discussed. From the existing literature on spontaneous and induced back-mutations in Drosophila melanogaster it appears that for several mutations the rates of forward and back-mutation are of the same order of magnitude. It is suggested that reported cases of back-mutations represent mainly inter- and intrachromosomal recombination in duplicated regions rather than mutational events and that the frequency of true back-mutation in Drosophila is usually of an order of magnitude, similar to that known for microorganisms and fungi.     

Characteristic features of induced mutagenesis in hybrid dysgenesis systems of Drosophila melanogaster

The mutagenic effect of low-dose gamma-irradiation was studied in Drosophila melanogaster systems of hybrid dysgenesis by estimating polytene chromosome rearrangements, recombination frequency, and viability at the embryonic and postembryonic developmental stages. A dose of gamma-irradiation which had no effect detectable by routine line crossing proved to significantly reduce the number of recombinants in the H-E and P-M systems and mortality at postembryonic stages. However, this combined effect was obtained if irradiation followed transposition, i.e., it depended on the application sequence of the mutagenic factors. The reverse order of the mutagenic treatment led to summation of the effects: as compared to either control, the frequencies of the dominant allele mutations as well as the larval and pupal mortality in F2 increased significantly (at the level of 99.9%). This allowed us to estimate the contribution of extremely low-dose gamma-irradiation into the mutagenic effect, which was impossible under routine conditions.     

Properties of super unstable mutations in the Drosophila melanogaster yellow locus]

The properties of super-unstable systems of the white, singed and ocetilless loci obtained as a result of P-M dysgenesis induction in the strains with a mobilized Stalker were described earlier. In the studies of super-instability in ocetilless locus, six super-unstable mutations in the yellow locus were obtained. Detailed genetic analysis was performed resulting in isolation of 80 alleles with different phenotype expression. In general, super-instability in the yellow locus reminds that in the white and ocetilless loci. Most of alleles are highly unstable possessing a characteristic pattern of mutation changes. Also, sub-systems were found in the yellow super-unstable system. Each consists of several mutually inter-converting alleles which possess a characteristic phenotype, mode and rate of mutation changes.     

Vinyl chloride mutagenesis in Drosophila melanogaster.

In inhalation experiments, Drosophila males were exposed to vinyl chloride at concentrations of 200, 850, 10,000 30,000 or 50,000 ppm for 2 days, and to 30 or 850 ppm for 17 days. VCM was mutagenic in the recessive-lethal test both after short-term and long-term exposures. The lowest effective concentration (LEC) was 850 ppm after 2 day exposure, and this value could be lowered to 30 ppm by prolonging the exposure time to 17 days. With the concentration levels tested, the mutation frequency increased with concentrations and reached a plateau at 10,000 ppm. This indicates a substrate saturation effect. In contrast with the recessive lethal assay, negative results were obtained when tests on dominant lethals, translocations, entire and partial sex-chromosome loss were carried out with VCM at 30,000 ppm for 2 days. This finding of a false negative seems a logical consequence of the observed saturation effect, and strengthens the concept that there exist two effective concentrations for point mutations vs the induction of chromosome breakage events. Vinyl chloride monomer provides another example to support our view that chromosome breakage is not a reliable measure of mutagenic activity.