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.     

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.     

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.     

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.     

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.     

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.     

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.     

Frequent transpositions of Drosophila melanogaster HeT-A transposable elements to receding chromosome ends.

HeT-A elements are a new family of transposable elements in Drosophila that are found exclusively in telomeric regions and in the pericentric heterochromatin. Transposition of these elements onto broken chromosome ends has been implicated in chromosome healing. To monitor the fate of HeT-A elements that had attached to broken ends of the X chromosome, we examined individual X chromosomes from a defined population over a period of 17 generations. The ends of the X chromosomes with new HeT-A additions receded at the same rate as the broken ends before the HeT-A elements attached. In addition, some chromosomes, approximately 1% per generation, had acquired new HeT-A sequences of an average of 6 kb at their ends with oligo(A) tails at the junctions. Thus, the rate of addition of new material per generation matches the observed rate of terminal loss (70-75 bp) caused by incomplete replication at the end of the DNA molecule. One such recently transposed HeT-A element which is at least 12 kb in length has been examined in detail. It contains a single open reading frame of 2.8 kb which codes for a gag-like protein.     

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.      

Transpositions of mobile dispersed genes in Drosophila melanogaster and fitness of stocks

Summary In situ hybridization with polytene chromosomes was used to demonstrate the transposition of mobile dispersed genes (mdg)-1 and 3 following the selection of flies from low reproductive activity and vability (LA stock) for high reproductive activity, viability and fitness (LA⁺ and HA stocks).The inbred LA stock is continuously selected for low reproductive activity and viability and maintains at least for twentyfive generations a characteristic pattern of mdg-1 distribution in 14–15 sites. Inbred LA⁺ and HA stocks exhibit a changed pattern of mdg-1 locations and the number of sites reaches 21–25. Parallel and independent selection for higher viability may lead to similar characteristic changes in the localization of mdg-1.In several independent experiments we observed, within one generation, a spontaneous and saltatory growth of viability and fitness in the mass-bred LA stock. In these cases new mdg-1 and mdg-3 sites reproducibly appeared to within several bands, some of them characteristic of LA⁺ and HA stocks.We discuss the possible role of mdg in determining the quantitative characters of individuals and their fitness.     

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.     

The differential genetic and environmental canalization of fitness components in Drosophila melanogaster

Canalization describes the process by which phenotypic variation is reduced by developmental mechanisms. A trait can be canalized against environmental or genetic perturbations. Stabilizing selelction should favor improved canalization, and the degree of a trait's canalization should be positively correlated with its impact on fitness. Here we report, for Drosophila melanogaster, measurements of environmental canalization for five fitness components. We compare them with measurements of genetic canalization, and we discuss the impact of inbreeding on both. In three experiments we measured the variation of fitness components within lines nested within temperature, treatment, and experiment. Lines differed in the position of a P element insert or in genetic background. Within lines flies were genetically nearly identical. We designated trait variation within lines as environmental canalization. The canalization of the traits increased with their impact on fitness, and the pattern was similar to that found for the canalization of fitness components against genetic differences, measured as the variation among lines nested within temperature, treatment, and experiment. This suggests that developmental mechanisms buffer the phenotype against both genetic and environmental disturbance. The results also suggest, less strongly, that inbreeding weakens canalization.     

Rapid evolution of the intersexual genetic correlation for fitness in Drosophila melanogaster

Sexual antagonism (SA) arises when male and female phenotypes are under opposing selection, yet genetically correlated. Until resolved, antagonism limits evolution toward optimal sex‐specific phenotypes. Despite its importance for sex‐specific adaptation and existing theory, the dynamics of SA resolution are not well understood empirically. Here, we present data from Drosophila melanogaster, compatible with a resolution of SA. We compared two independent replicates of the “LH_M” population in which SA had previously been described. Both had been maintained under identical, controlled conditions, and separated for around 200 generations. Although heritabilities of male and female fitness were similar, the intersexual genetic correlation differed significantly, being negative in one replicate (indicating SA) but close to zero in the other. Using population sequencing, we show that phenotypic differences were associated with population divergence in allele frequencies at nonrandom loci across the genome. Large frequency changes were more prevalent in the population without SA and were enriched at loci mapping to genes previously shown to have sexually antagonistic relationships between expression and fitness. Our data suggest that rapid evolution toward SA resolution has occurred in one of the populations and open avenues toward studying the genetics of SA and its resolution.     

Transposition of mobile genetic elements in interspecific hybrids of Drosophila

In situ hybridization of labeled DNA of four mobile dispersed genetic elements (mdg), isolated from D. melanogaster and C. virilis genomes, with polytene chromosomes of the larvae of several Drosophila species has been carried out. The data show that the mdg elements exhibit a high degree of species specificity. The same conclusions are derived from filter hybridization using ³²P-labeled D. melanogaster and D. virilis DNA and cloned mdg sequences immobilized on nitrocellulose filters. We attempted to induce transpositions (jumping) of mdg elements specific for D. virilis chromosomes to the chromosomes of related species (e.g. D. littoralis Meigen) originally lacking the representatives of this family of repeats. For this purpose we produced hybrid stocks with synthetic karyotoypes characterized by different combinations of D. virilis homologous chromosomes and hybrid chromosomes. In one of such stocks we did find by in situ hybridization the insertion of a D. virilis mdg element into the fifth chromosome of D. littoralis Meigen. The transposition (jumping) took place in the only region where somatic pairing between the fifth chromosomes of D. virilis and D. littoralis occurs more or less regularly in the hybrids. Since crossing-over in hybrid chromosomes of males is excluded in such synthetic stocks, gene conversion may be responsible for this transposition. The possible bearing of the phenomenon observed on the problem of hybrid dysgenesis is discussed.     

Polymorphism of the Mdg-1 and I mobile elements in Drosophila melanogaster

The polymorphism of the mobile elements Mdg-1 (a copia-like element) and I (an element involved in I-R hybrid dysgenesis) was analysed in a mass-mated population of Drosophila melanogaster by in situ hybridization, using biotinylated DNA probes, on polytene chromosomes. The Mdg-1 and I elements were inserted independently but were within the same bulk of DNA insertion points of the Drosophila genome, which contained on average about 30 insertion sites for each element. The X chromosome contained the lowest copy number of elements while 2R and 3R had the highest number: 3R had the highest variability. There was no correlation between the copy numbers of elements among the chromosome arms. The average expected per locus heterozygosity was equal to 0.17 for both the Mdg-1 and the I elements. Although these two elements differ in sequence, they appeared to behave similarly in the Drosophila melanogaster genome. This suggests that they may compete for target insertion sites and may be under the same control mechanisms.     

The changes in genetic and environmental variance with inbreeding in Drosophila melanogaster

We performed a large-scale experiment on the effects of inbreeding and population bottlenecks on the additive genetic and environmental variance for morphological traits in Drosophila melanogaster. Fifty-two inbred lines were created from the progeny of single pairs, and 90 parent-offspring families on average were measured in each of these lines for six wing size and shape traits, as well as 1945 families from the outbred population from which the lines were derived. The amount of additive genetic variance has been observed to increase after such population bottlenecks in other studies; in contrast here the mean change in additive genetic variance was in very good agreement with classical additive theory, decreasing proportionally to the inbreeding coefficient of the lines. The residual, probably environmental, variance increased on average after inbreeding. Both components of variance were highly variable among inbred lines, with increases and decreases recorded for both. The variance among lines in the residual variance provides some evidence for a genetic basis of developmental stability. Changes in the phenotypic variance of these traits are largely due to changes in the genetic variance.