Mdg-1 mobile element polymorphism in selected Drosophila melanogaster populations

The changes in mdg-1 mobile element polymorphism that followed artificial selection for either high or low egg-to-adult viability in a Drosophila melanogaster population were investigated. The two selected subpopulations were thus characterized for fecundity, wing length, and number and location of the mdg-1 mobile element by in situ hybridization of the biotinylated-DNA on salivary gland chromosomes. The selected populations that differed greatly in egg-to-adult viability showed the same mean fecundity and identical values for intra and inter components of variances, intraclass correlation coefficient, and fluctuating asymmetry estimated on the wing length measurement. This indicates a non-correlated effect between deleterious mutations affecting viability and other fitness components. However, the two selected populations differed in their pattern of mdg-1 location, although the mean number of insertions per genome was not different from that of the initial population hence, the number of insertions of the mdg-1 mobile element was independent of the effective population size. These results suggest that the mdg-1 copy number was regulated, and that during the selection process, drift and inbreeding made up new insertion patterns of the mdg-1 element in the selected populations. The results are discussed in the light of some recent theoretical models of the population dynamics of transposable elements.     

Copy number and distribution of P and I mobile elements in Drosophila melanogaster populations

The distribution of the number of copies of P and I transposable elements per genome was investigated by in situ hybridization for a large set of Drosophila melanogaster strains. These included the P, Q and M types of the P-M system of hybrid dysgenesis. P element copy number varied widely (range 5–59). P and Q strains had around 40 copies whereas M strains generally had lower numbers (between 5 and 35) with one extreme value (52). The copy number of I elements appeared to be precisely regulated, as no strains were found outside the 15±5 range. The number of copies of the two families were independent. An excess of P copies on the X chromosome compared with the autosomes was found for the P and Q strains, but not for M strains. Among X-inserted P sites, a very high frequency of occupation was found at the tip of the X chromosome (cytological site 1A), especially for P and Q strains. The possible regulatory role in the P-M system of X-inserted P sites is discussed.     

Cloning and characterization of variable-sized gypsy mobile elements in Drosophila melanogaster

A cosmid genomic library from a known gypsy-induced forked mutation, f1, was screened by 32P-labeled gypsy transposable element. Of more than 250 positive clones we randomly selected 21 for in situ hybridization to wild-type polytene chromosomes. Two clones hybridized to region 15F on the X-chromosome, the cytological position of forked. A third clone hybridized to at least 17 sites on the chromosomes indicating the presence of repetitive sequences in the gypsy flanking DNA. All clones labeled the centromeric regions heavily. Ten clones, including the two hybridizing at 15F, were chosen for further analysis, and restriction mapping allowed us to place them into three groups: (1) full-length, (2) slightly diverging, and (3) highly diverging gypsy elements. Group (2) is missing the XbaI site in both their long terminal repeats (LTRs) as well as the middle HindIII site; four of these gypsy elements also have a approximately 100-bp deletion at the 5' LTR. The group (3) gypsy transposons are missing one LTR and also have highly diverging DNA sequences. The restriction analyses further imply that most of these different gypsy elements are present in more than one copy in the genome of the f1 stock used in this study. The results raise intriguing questions regarding the significance of transposable elements in evolution and biological functions.     

Autonomous transposition of gypsy mobile elements and genetic instability in Drosophila melanogaster

Summary The laboratory imitator strain (MS) of Drosophila melanogaster is characterized by an elevated frequency of spontaneous mutation (10^–3–10^–4). Mutations occur in both sexes at premeiotic stages of germ cell development. The increased mutability is a characteristic feature of MS itself, since it appears in the absence of outcrossing. Most of the mutations arising in this strain are unstable: reversions to wild type, high frequency mutation to new mutant states and replicating instability were observed. We have investigated the localization of the transposable genetic elements mdg1, 412, mdg3, gypsy (mdg4), copia and P in the X chromosomes of the MS and in the mutant lines y, ct, sbt derived from it by in situ hybridization. The P element was not found in any of these strains. The distributions of mdg1, 412, mdg3 and copia were identical in the X chromosomes of the MS and its derivatives. However, the sites of hybridization with gypsy differ in the various lines tested. In the polytene chromosomes of MS animals significant variation in location and number of copies of the gypsy element was demonstrated between different larvae; copy numbers as high as 30–40 were observed. These results suggest autonomous transposition of gypsy in the MS genome while several other mobile elements remain stable.     

Distribution of Drosophila melanogaster mobile genetic elements along X-chromosome and autosomes

The distribution along Drosophila melanogaster polytene X-chromosome and autosomes of 10911 in situ hybridization sites of a broad spectrum of copialike mobile elements is investigated. It is shown that against DNA content X-chromsomal cytological sections 14 + 15 and 16 + 17 contain much less mobile elements than other chromosomal regions. These X-chromosomal regions are also characterized both by significant decrease in the meiotic recombination frequencies and the amount of poly(dC-dA).poly(dG-dT) sequences which are capable to generate the Z form of DNA.     

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.     

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.     

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.     

Concerted transpositions of mobile genetic elements coupled with fitness changes in Drosophila melanogaster

In an inbred low-activity (LA) strain of Drosophila melanogaster with a low level of fitness and a complex of inadaptive characters, in situ hybridization reveals an invariant pattern of distribution of three copia-like elements (mdg-1, mdg-3, and copia). Rare, spontaneous, multiple transpositions of mobile elements in the LA strain were shown to be coupled with a drastic increase of fitness. A changed pattern of various types of mobile elements was also observed on selecting the LA strain for higher fitness. High-fitness strains show transpositions of mobile elements to definite chromosomal sites ("hot spots"). Concerted changes in the location of three different mobile elements were found to be coupled with an increase of fitness. The mdg-1 distribution patterns were also examined in two low-fitness strains independently selected from the high-fitness ones. Fitness decrease was accompanied by mdg-1 excision from the hot spots of their location usually detected in the high-fitness strains. The results suggest the existence of a system of adaptive transpositions of mobile elements that takes part in fitness control.      

Induction of single transpositions of mobile genetic elements in Drosophila melanogaster using mitomycin C

Intraperitoneal injections of mitomycin C into the males from laboratory strains of Drosophila melanogaster induce several mutation events in different loci of the X-chromosome in the offspring. These mutations are caused by transposition of mobile genetic elements. The transpositions are single and are not associated with transposition explosions.     

Transposable elements and fitness in Drosophila melanogaster

Transposable elements constitute a significant fraction of the Drosophila melanogaster genome. The five families of moderately repeated transposable elements identified to date occupy dispersed and variable genomic locations, but have relatively constant copy numbers per individual. What effect to these elements have on the fitness of the individuals harboring them? Experimental evidence relating to this question is reviewed. The relevant data fall into two broad categories. The first involves the determination of the distribution of transposable elements in natural populations, by restriction mapping or in situ hybridization, and the comparison of the observed distribution with different theoretical expectations. The second approach is to study directly the effects of new transposable element-induced mutations on fitness. The P family of transposable elements is a particularly efficient mutagen, and the results of experiments in which initially P-free chromosomes are contaminated with P elements are discussed with regard to P-induced fitness mutations.     

I elements of Drosophila melanogaster generate specific chromosomal rearrangements during transposition

Summary 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.     

Studies on the transposition rates of mobile genetic elements in a natural population of Drosophila melanogaster

In an isolated population of Drosophila melanogaster on Ishigaki Island the chromosomal distribution of several retrotransposons, including copia, 412, 297, 17.6, I, and jockey elements, was examined by in situ hybridization. In this population the cosmopolitan inversion, In(2L)t, is known to exist in high frequency. One major haplotype concerning the occupied sites of the transposable elements was identified in the In(2L)t-carrying chromosomes. This haplotype is suggested to be the ancestral one. The age of the inversion in this local population was estimated to be 1,400 generations. The transposition rates of these elements were estimated based on the age of the inversion and the number of the elements lost and gained. The excision rates were in the range from 9.13 x 10(-5) to 2.25 x 10(-4) per site per generation. They were similar each other in the copia-like elements as well as in the LINE-like elements. The rate was higher in the copia-like elements than in the LINE-like elements. Insertions occurred in the range from 6.79 x 10(-4) to 9.05 x 10(-4) per element per generation. It is herein shown that both insertions and excisions occurred at a significantly higher rate in this population than in the laboratory.      

Polymorphism and stability in the histone gene cluster of Drosophila melanogaster

Histone genes in Drosophila melanogaster are organized into repeats of 4.8 and 5.0 kb (Lifton et al., 1978). We find these repeat sizes in every one of the more than 20 Drosophila strains we have examined. Strains differ in the relative amounts of the two repeat types, with ratios varying from 11 to 14, the 5.0 kb repeat always present in equal or greater amounts than the 4.8 kb repeat. Restriction enzyme digestion and blotting analysis reveals that the strains also differ in a number of far less abundant fragments containing histone DNA sequences. In the Amherst and Samarkand strains, there are, in addition, many copies of 4.0 and 5.5 kb repeat-like fragments respectively. A series of stocks were made isogenic for single second chromosomes from the Amherst strain. The hybridization patterns of the histone DNA from these stocks containing different Amherst chromosomes are very similar but a number of differences in the minor fragments were seen. The stability of the histone locus restriction pattern was tested by following the DNA derived from a single second chromosome of the b Adhn² pr cn strain over a two year period. The restriction pattern of major and minor bands remained identical. Finally, histone loci distinguishable by their restriction pattern on blots were recombined with visible markers. These chromosomes will be useful in tracing the fate of specific histone loci during genetic manipulations.     

Constitutive heterochromatin and transposable elements in Drosophila melanogaster

Several families of transposable elements (TEs), most of them belonging to the retrotransposon catagory, are particularly enriched in Drosophila melanogaster constitutive heterochromatin. The enrichment of TE-homologous sequences into heterochromatin is not a peculiar feature of the Drosophila genome, but appears to be widespread among higher eukaryotes. The constitutive heterochromatin of D. melanogaster contains several genetically active domains; this raises the possibility that TE-homologous sequences inserted into functional heterochromatin compartments may be expressed. In this review, I present available data on the genetic and molecular organization of D. melanogaster constitutive heterochromatin and its relationship with transposable elements. The implications of these findings on the possible impact of heterochromatic TEs on the function and evolution of the host genome are also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fixation of transposable elements in the Drosophila melanogaster genome

We have investigated at the molecular level four cases in which D. melanogaster middle repetitive DNA probes consistently hybridized to a particular band on chromosomes sampled from a D. melanogaster natural population. Two corresponded to true fixations of a roo and a Stalker element, and the others were artefacts of the in situ hybridization technique caused by the presence of genomic DNA flanking the transposable elements (TEs) in the probes. The two fixed elements are located in the beta-heterochromatin (20A and 80B, respectively) and are embedded in large clusters of other elements, many of which may also be fixed. We also found evidence that this accumulation is an ongoing process. These results support the hypothesis that TEs accumulate in the non-recombining part of the genome. Their implications for the effects of TEs on determining the chromatin structure of the host genomes are discussed in the light of recent evidence for the role of TE-derived small interfering-RNAs as cis -acting determinants of heterochromatin formation.