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SummaryThe X-linked telomericPelements (TPs)TP5andTP6regulate the activity of the entirePelement family because they are inserted in a major locus for the production of Piwi-interacting RNAs (piRNAs). The potential for this cytotype regulation is significantly strengthened when eitherTP5orTP6is combined with a non-telomeric X-linked or autosomal transgene that contains aPelement. By themselves, none of the transgenicPelements have any regulatory ability. Synergism between the telomeric and transgenicPelements is much greater when the TP is derived from a female. Once an enhanced regulatory state is established in a female, it is transmitted to her offspring independently of either the telomeric or transgenicPelements – that is, it works through a strictly maternal effect. Synergistic regulation collapses when either the telomeric or the transgenicPelement is removed from the maternal genotype, and it is significantly impaired when the TPs come from stocks heterozygous for mutations in the genesaubergine,piwiorSu(var)205. The synergism between telomeric and transgenicPelements is consistent with a model in whichPpiRNAs are amplified by alternating, or ping-pong, targeting of primary piRNAs to sense and antisensePtranscripts, with the sense transcripts being derived from the transgenicPelement and the antisense transcripts being derived from the TP.

SummaryThe copia element is a retrotransposon that is hypothesized to have been horizontally transferred fromDrosophila melanogasterto some populations ofDrosophila willistoniin Florida. Here we have used PCR and Southern blots to screen for sequences similar tocopiaelement in South American populations ofD. willistoni, as well as in strains previously shown to be carriers of the element. We have not found the canonical copia element in any of these populations. Unlike thePelement, which invaded theD. melanogastergenome fromD. willistoniand quickly spread worldwide, the canonicalcopiaelement appears to have transferred in the opposite direction and has not spread. This may be explained by differences in the requirements for transposition and in the host control of transposition.

SummaryTP5, aPelement inserted in the telomere-associated sequences of the X chromosome, represses the excision of otherPelements in the germ line through a combination of maternal and zygotic effects. The maternal component of this repression is impaired by heterozygous mutations in theaubergineandSuppressor of variegation 205genes; one mutation in thepiwigene also appears to impair repression. In the female germ line, the level ofTP5mRNA is increased by these impairing mutations. The impairingaubergineandpiwimutations also increase the level of germ-line mRNA fromCP, a transgene that encodes theP-element transposase; however, theSuppressor of variegation 205mutation does not. These findings are discussed in terms of a model ofP-element regulation that involves post-transcriptional and chromatin re-organizing events mediated by maternally transmitted small RNAs derived from the telomericPelement.

SummaryThe X-linked telomericPelementsTP5andTP6interact synergistically with non-telomericPelements to repress hybrid dysgenesis. In this repression, the telomericPelements exert maternal effects, which, however, are not sufficient to establish synergism with the non-telomericPelements. Once synergism is established, the capacity to repress dysgenesis in the offspring of a cross persists for at least two generations after removing the telomericPelement from the genotype. At the molecular level, synergism between telomeric and non-telomericPelements is correlated with effective elimination ofP-element mRNA in the germ line. Maternally transmitted mutations in the genesaubergine,piwiandSuppressor of variegation 205[Su(var)205] block the establishment of synergism between telomeric and non-telomericPelements, and paternally transmitted mutations inpiwiandSu(var)205disrupt synergism that has already been established. These findings are discussed in terms of a model of cytotype regulation ofPelements based on Piwi-interacting RNAs (piRNAs) that are amplified by cycling between sense and antisense species.

Polymerase chain reaction (PCR) screening for P elements was carried out in 77 species with a primer set highly specific for the M-type subfamily. In the course of this search M-type elements were detected in 29 species: In the melanogaster (subgroups montium and rhopaloa) and obscura species groups of Drosophila (25 out of 71 species examined), and in the genus Scaptomyza (four out of six species). M-type elements are present in all species of the montium subgroup investigated so far (21), but occur only sporadically in other groups. Within the montium subgroup 20 species possess only incomplete copies, only one species (D. lacteicornis) harbours apparently full-sized elements. In contrast, outside the montium subgroup almost all species with M-type elements carry full-sized copies suggesting transpositional activity, at least in the recent past. The interior section of the full-sized M-type element of D. lacteicornis was partially sequenced (936 bp). In addition, the complete sequences of four internally deleted M-type elements of D. lacteicornis, D. rufa, D. quadraria, and D. triauraria were determined. Sequence comparisons (including sequence data from previous investigations) revealed striking discrepancies between P element phylogeny and the cladogenesis of their host species. Among several possible pathways for interspecific transfers of M-type elements, we favour the hypothesis assuming the invasion of Scaptomyza as well as the obscura group species of Drosophila via independent transmission routes originating from Asian species of the montium subgroup of Drosophila. The logical geographic scenario for these events would be East-Asia.

Clines of P-induced hybrid dysgenesis provide a means for monitoring the evolution of transposition repression over space and time. We have studied the molecular and phenotypic profiles of flies taken from a 2900 km cline along the eastern coast of Australia, which had previously been characterized over 10 years ago as having P populations in the north, Q populations at central sites and M′ populations in the south. We have found that Q and M′ populations of flies have increased their range within the cline at the expense of P lines. Q populations were found to be in the north of the cline and M′ populations in the south. Some of the northern Q lines transmit repression through both sexes and type I deletion elements have been isolated from them. We suggest that these elements are responsible for Q type repression. The results support our model that populations made up of Q individuals with strong biparentally transmitted repression form an evolutionarily stable strategy for the repression of hybrid dysgenesis in Drosophila melanogaster.

The influence of gamma-irradiation on P element excision and excision-site repair mechanisms was directly tested by embryonic somatic excision assays. Preblastoderm P[ry+, Δ2–3](99B) embryos, having a stable source of somatically active P transposase, were irradiated previous to injection with P excision indicator plasmids. Frequencies of precise or nearly precise P excision increased with gamma-ray doses ranging from 0·5 to 3·5 Gy. Higher doses resulted in frequencies close to that in unirradiated embryos, though considerable embryonic lethality was also evident at these doses. A direct positive interaction between gamma-irradiation and P element activity is concluded.

SummaryAnalysis of the transposition behaviour of thePandhoboelements borne by the31·1/CyL4MRF (P), 23·5Δ/CyL4MRF (hobo)and23·5*/Cy MRF (hobo)strains in the progeny of dysgenic crosses with two ME laboratory strains (Berlin-k and dp b en bw) at 25 °C revealed that: (a) the two ME laboratory strains affect differently the transposition rates ofPandhoboelements. More precisely,Pelement transposition is higher in heterozygotes withdp b en bwthan in those withBerlin-k. In contrast, the transposition rate ofhoboelements is higher inBerlin-kthan indp b cn bwheterozygotes. (b) LikeP, hobohas the potential to transpose at high frequencies and to nonhomologous chromosomes, (c) The dysgenically inactivehoboelements of the3·11 MRFstrain transpose more frequently than the dysgenically activehoboelements of the23·5 MRFstrains in certain crosses, (d) There are insertion hot spots forPandhoboelements. For thePelements there are enough data to suggest that the insertion hot spots are different in the two EM strains. The data are discussed on the basis of the involvement of putative host factors in transposition regulation of thePandhoboelements.

SummaryThree Harwich P sublines with differentP-element activity potential were used to investigate the influence of P-derived chromosomes on snωmutability andvgsuppression and to relate the induction of these dysgenic traits to the number and structure ofPelements. Destabilization of thesnωallele, a measure ofPtransposase activity, was differentially influenced by the major autosomes. Chromosome 2 of the standard Harwich subline, Hw, induced only 60% of the level of mutability relative to chromosome 3, whereas chromosome 3 of the weakest Harwich subline, Hf, induced only 50% of the mutability relative to chromosome 2. In somatic suppression of thevg21–3allele, chromosome 3 of the Hfsubline produced a lower level of complete suppression as compared to chromosome 3 of the Hwor the Hssubline (the high hybrid-dysgenesis-inducing subline). The level of these dysgenic traits and GD sterility, was not correlated with the number ofPelements per individual (67–68) or per chromosome arm which was very similar among the sublines. The number of completePelements per genome, based on Southern blot analysis of the X and major autosomes, ranged from 15 to 19. Destabilization of thesnωallele andvgsuppression by chromosome 3 was correlated with a greater number of completePelements. Two novel unexpected observations emerged from these studies: bothsnωmutability andvgsuppression data demonstrated highP-element activity in hybrids derived from non-dysgenic crosses irrespective of Harwich subline, indicating a lack of P-cytotype regulation. Mutability in non-dysgenic males ranged from 40 to 60% of the level found in dysgenic males. The highsnωmutability and low GD sterility in non-dysgenic hybrids suggests that these traits may arise by a different mechanism.

SummaryThe frequency, distribution and structure of P elements on the second and third chromosomes of Texas 1, a wild-type inbred strain ofDrosophila melanogaster, were investigated byin situhybridization. These autosomes were isolated individually and used as P-element donors to study the frequency and distribution of male recombination events generated on recipient chromosomes which were originally devoid of P sequences. The P-element array of chromosome 2 was shown to generate higher male recombination frequencies on chromosome 3 than vice versa, despite having fewer P factors and fewer P elements in general. This is likely to be due to the presence and distribution of specific P-deletion derivatives, which vary in their ability to repress P mobility. The male recombination generated on recipient chromosomes is associated with the insertion of donated P sequences, but only in a small minority of cases could a novel P-element site be detected at, or near, the recombination breakpoint. The majority of such breakpoints appear to be associated either with unsuccessful P insertion, or with the action of P transposase attracted by P elements newly inserted elsewhere on the recipient chromosome. Recent evidence also suggests that a small proportion of the breakpoints may be associated with the action of P transposase alone. Male recombination breakpoints appear to be distributed effectively at random along the recipient autosomes, and their frequency of occurrence was shown to correlate with the physical length of DNA available between markers, as revealed by the polytene map distance.

SummaryThe accumulation of a transposable element inside chromosomal inversions is examined theoretically by a mathematical model, and empirically by counts of P elements associated with inversion polymorphisms in natural populations ofDrosophila melanogaster. The model demonstrates that, if heterozygosity for an inversion effectively reduces element associated production of detrimental chromosome rearrangements, a differential accumulation of elements is expected, with increased copy number inside the minority inversion. Several-fold differential accumulations are possible with certain parameter values. We present data on P element counts for inversion polymorphisms on all five chromosome arms of 157 haploid genomes from two African populations. Our observations show significantly increased numbers of elements within the regions associated with the least common, or minority arrangements, in natural inversion polymorphisms.

SummaryX-rays and deficiencies in DNA repair had a synergistic effect on genetic damage associated with P-element mobility inDrosophila melanogaster. These interactions, using sterility and fecundity as endpoints, were tested in dysgenic males deficient in either excision or post-replication DNA repair. Three sublines of the Harwich P strain were used for the construction of hybrid males. These sublines differ in P-induction ability based on gonadal dysgenesis sterility (GD) andsnwmutability tests, in P-element insertion site pattern, and in the types of defective P-elements, such as KP elements, they possess. A lower degree of gonadal dysgenesis was correlated with the presence of KP elements. GD sterility andsnwmutability were not always correlated. Dysgenic hybrids originating from the standard reference subline, Harwichwhite, were much more sensitive to the post-replication repair than the excision repair defect. In contrast, sterility of hybrids derived from the weak subline was least affected by, and that of hybrids of the strongest subline was most affected by either DNA repair deficiency. The exacerbation by X-rays of the effects of DNA repair deficiencies on genetic damage indicates that both repair mechanisms are required for processing DNA lesions induced by the combined effect of P activity and ionizing radiation.

SummaryThe ability to repress P-element-induced gonadal dysgenesis was studied in 14 wild-type strains ofD. melanogasterderived from populations in the central and eastern United States. Females from each of these strains had a high ability to repress gonadal dysgenesis in their daughters. Reciprocal hybrids produced by crossing each of the wild-type strains with an M strain demonstrated that repression ability was determined by a complex mixture of chromosomal and cytoplasmic factors. Cytoplasmic transmission of repression ability was observed in all 14 strains and chromosomal transmission was observed in 12 of them. Genomic Southern blots indicated that four of the strains possessed a particular type of P element, called KP, which has been proposed to account for the chromosomal transmission of repression ability. However, in this study several of the strains that lacked KP elements exhibited as much chromosomal transmission of repression ability as the strains that had KP elements, suggesting that other kinds of P elements may be involved.

SummaryP-element mutagenesis was used to contaminate M-strain second chromosomes with P elements. The effect of P-element transposition on abdominal and sternopleural bristle scores and on female productivity was deduced by comparing the distributions of these quantitative traits among the contaminated second-chromosome lines with a control population of M-strain second-chromosome lines free of P elements. Estimates of P-element-induced mutational variance,Vm, for these characters are very high, and mutational ‘heritabilities’ (Vm/Ve, the ratio of mutational variance to environmental variance) are of the same order as heritabilities of these traits from natural populations. P-element-induced mutational variance of abdominal bristle score is roughly two orders of magnitude greater than spontaneous and X-ray-inducedVm/Vefor this trait.

SummaryThe frequency and distribution of P elements were investigated in the third chromosomes of two wild-type strains ofDrosophila melanogasterusingin situhybridization of biotinylated probes to the polytene chromosomes. The relationship between these data and the extent of hybrid dysgenesis was determined through assays of egg production, egg hatchability (F2embryo lethality),snwdestabilization and male recombination along the third chromosome. The results suggest that P-element distribution, frequency and structure are all contributory factors in the regulation of hybrid dysgenesis. Texas 6 was shown consistently to be a stronger P strain than Texas 1, eliciting greater reductions in fertility, more extensivesnwdestabilization and higher frequencies of male recombination. Clustering of male recombination events, arising from pre-meiotic crossing over, was evident among the dysgenic progeny of each strain. Male recombination andsnwdestabilization were independently distributed among the dysgenic males studied, suggesting that these traits represent separate P-mediated functions. The third chromosome male recombination maps produced by the two strains differed significantly from each other and from the published female meiotic and polytene chromosome maps. Male recombination breakpoints were associated with the original distribution of P sequences in the two strains and the results suggest that this relationship may be closer for potentially complete P factors than for P sequences in general. An analysis of sub-lines derived from individual recombinant males revealed that chromosomal breakpoints could also be associated with novel insertions following P-element transposition.

SummaryInDrosophila melanogaster, theP−MandI−Rsystems of hybrid dysgenesis are associated with high rates of transposition ofPandIelements, respectively, in the germlines of dysgenic hybrids formed by crossing females of strains without active elements to males of strains containing them. Transposition rates are not markedly accelerated in the reciprocal, nondysgenic hybrids. Previous attempts to evaluate the extent to which hybrid dysgenesis-mediatedPtransposition contributes to mutational variance for quantitative characters by comparing the responses to selection ofP−Mdysgenic and nondysgenic hybrids have given variable results. This experimental design has been extended to include an additional quantitative trait and theI−Rhybrid dysgenesis system. The selection responses of lines founded from both dysgenic and nondysgenic crosses showed features that would be expected from the increase in frequency of initially rare genes with major effects on the selected traits. These results differ from those of previous experiments which showed additional selection response only in lines started from dysgenic crosses, and can be explained by the occasional occurrence of large effect transposable element-induced polygenic mutations in both dysgenic and nondysgenic selection lines. High rates of transposition in populations founded from nondysgenic crosses may account for the apparently contradictory results of the earlier selection experiments, and an explanation is proposed for its occurrence.

SummaryIn this study we estimate the frequency at whichP-element insertion events, as identified byin situhybridization, generate lethal and mild viability mutations. The frequency of lethal mutations generated per insertion event was 0·004. Viability dropped an average of 1% per insertion event. Our results indicate that it is deletions and rearrangements resulting from the mobilization ofPelements already in place and not the insertionsper sethat cause the drastic effects on viability and fitness observed in most studies of P–M dysgenesis-derived mutations. Elements of five other families (I, copia,412,B104, andgypsy) were not mobilized in these crosses. Finally, we contrast the density ofPelements on theXchromosome with the density on the four autosomal arms in a collection of thirty genomes from an African population. The relative number ofPelements on theXchromosome is too high to be explained by either a hemizygous selection or a neutrality model. The possible reasons for the failure to detect selection are discussed.

SummaryThe DNA structures around the G6PD coding region in three high-G6PD activity mutants and their low-activity revertants ofDrosophila melanogasterwere analysed by Southern blot using a cloned G6PD gene as a probe. As a result, two kinds of insertion sequences were found; one was present just 5′ to exon I (Ins1), and the other within the intron (Ins2). The Ins1 sequence was 3·5 Kb in two mutants and 2·9 Kb in one mutant. In both cases, it consisted of a core sequence either 1·2 or 0·6 Kb long flanked by terminal repeats. On the other hand, low-activity revertants possessed either a defective Ins1 or no Ins1. The Ins2 sequence was found in all mutants and revertants, but not in Canton S. Although a recombinant phage carrying the DNA fragment spanning the entire Ins1 has not been obtained, sequencing data of the clone containing only the terminal repeats demonstrated that the repeats are defective P elements. Comparison of the genomic DNA structures of mutants and revertants suggested that the element responsible for the positive regulation of the G6PD gene in the mutants would probably be the core sequence, but not the flanking defective P elements. It was also conjectured that the 1·2 Kb core sequence might be composed of two identical elements, which might transpose independently.

SummaryDysgenic and non-dysgenic base populations were made by reciprocal crossing of Harwich (P) and Canton-S (M) strains. From each cross, two up and two down selection lines were established, with selection on abdominal bristle number for ten generations. The intensity of selection was 10 out of 50 individuals from each sex. Mean bristle number, phenotypic variation and heritabilities were compared between dysgenic and non-dysgenic populations under selection. Except for an anomalous non-dysgenic downline in which a mutation of large effect occurred, all lines showed similar responses to selection. These results contrast with the results reported by Mackay (1984, 1985) in which substantial increases were obtained for response to selection, phenotypic variation and heritability in the dysgenic compared to non-dysgenic lines. There are some indications that the higher response in our aberrant non-dysgenic downline is the result of transposition. Possible explanations for the occurrence of transposition and dysgenesis in the lines derived from nondysgenic crosses are discussed.

SummaryP element mutagenesis was used to contaminate M strain second chromosomes with P elements. The contaminated lines were compared to uncontaminated control lines for homozygous and heterozygous fitness and its components. Mean homozygous fitness, viability and fertility of chromosome lines contaminated with P elements is decreased relative to the uncontaminated control lines by, respectively, 55, 28 and 40%. Variance among contaminated homozygous lines of total fitness increases by a factor of 1·5, variance of viability by a factor of 5·9, and variance of fertility by a factor of 1·9, compared to variance of these traits among the population of uncontaminated homozygous chromosomes. Estimates of P-element-induced mutational variance among second chromosome lines for homozygous fitness, viability and fertility are, respectively, 2 × 10−2, 5 × 10−2and 2 × 10−2. This magnitude of mutational effect is equivalent, in terms of incidence of induced recessive lethal chromosomes and D:L ratio, to a dose of approximately 1·0–2·5 × 10−3m EMS. The distributions of fitness traits among M-derived second chromosome homozygous lines contaminated with P elements are remarkably similar in many regards to distributions of fitness and viability of chromosomal homozygotes derived from naturalDrosophilapopulations. It is possible that a proportion of the fitness variation previously observed (reviewed by Simmons & Crow, 1977) following homozygosis of wild chromosomes was not present in the natural populations, but was generated by P-element transposition during the chromosome extraction procedure. P-element-induced fitness mutations appear to be completely recessive. Implications for models of evolution of transposable elements are discussed.

SummaryThe P–M system of hybrid dysgenesis inDrosophila melanogasteris a syndrome of genetic abnormalities which appears in the progeny of crosses between strains different with regard to their possession of ‘P’ transposable elements. Cytotype is an extrachromosomal property which regulates the mobility of the P element. We report here data showing that a cytotype polymorphism previously observed in a natural population from North-Africa is stable over a period of 5 years. A potentially high rate of mutation is associated with this cytotype polymorphism. Explanations of the appearance of a cytotype polymorphism are proposed and the consequences for the genetic load induced by transposable elements are discussed.

SummaryLittle information exists about the mechanisms that determine the fate of mobile elements in natural populations. In this study we catalogue the distribution of 638 P-elements across 114Xchromosomes in samples drawn from three natural populations ofDrosophila melanogaster. There is an extremely high occurrence of elements at the tip relative to the rest of the euchromatic chromosome. We demonstrate that the distribution ofde novoinsertions of the P-element on a specific laboratory chromosome is markedly different; no P-elements were recovered at the tip in the 243 insertion events recorded. In contrast, insertion data for the π2chromosome suggests an elevated rate associated with the tip site although it does not appear sufficient to explain the large differential accumulation on wild chromosomes. This raises the issue of inter chromosome (or tip) variation in relative rates, as well as the possibility that rates of elimination are lower at the tip.