The Absence of Somatic Effects of P-M Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

The wings and abdomens of dysgenic and nondysgenic control flies were scored for the presence of clones of cells mutant for first and third chromosome markers. These exceptional clones can arise from mitotic recombination, de novo mutation or deletion, and P-M hybrid dysgenesis has been shown to increase the frequency of parallel processes occurring in germ-line cells. Particular attention was given to careful genetic and molecular characterization of all stocks and to providing adequate and appropriate controls so that even very small increases in somatic clone frequency due to P-M hybrid dysgenesis would be detected. No difference was found in the frequency, size distribution or anatomical distribution of mutant somatic clones correlated to hybrid dysgenesis, confirming previous indications. The potential adaptive significance of a germ-line restriction of P-M hybrid dysgenesis is discussed.     

Characterization of MR (P) strains of Drosophila melanogaster: the number of intact P elements and their genetic effect.

To study the effect of mutagenic/carcinogenic agents on P-element transposition, the P strains used should be defined, especially with respect to the number of intact and functional P elements present. In this investigation, the relation between the number of complete P elements present in dysgenic males and P-insertion mutagenesis was studied in several MR (P) strains. The main conclusions from this investigation are: (1) Complete P elements can be present in the genome without genetic activity (even in a 'dysgenic' cross). As a consequence, the number of complete P elements present in particular dysgenic flies, is not necessarily an indication of their dysgenic genetic activity. (2) The MR-h12/Cy strain carries two complete P elements, one on the X chromosome without and one on the MR chromosome with genetic activity (making this strain most suitable for studies on P-transposition mechanisms).     

The molecular basis of P-M hybrid dysgenesis: The role of the P element,a P-strain-specific transposon family

We have shown previously that four of five white mutant alleles arising in P-M dysgenic hybrids result from the insertion of strongly homologous DNA sequence elements. We have named these P elements. We report that P elements are present in 30–50 copies per haploid genome in all P strains examined and apparently are missing entirely from all M strains examined, with one exception. Furthermore, members of the P family apparently transpose frequently in P-M dysgenic hybrids; chromosomes descendant from P-M dysgenic hybrids frequently show newly acquired P elements. Finally, the strain-specific breakpoint hotspots for the rearrangement of the π₂ P X chromsome occurring in P-M dysgenic hybrids are apparently sites of residence of P elements. These observations strongly support the P factor hypothesis for the mechanistic basis of P-M hybrid dysgenesis.     

Components of Hybrid Dysgenesis in a Wild Population of DROSOPHILA MELANOGASTER

Hybrid dysgenesis is a condition found in certain interstrain hybrids of Drosophila melanogaster caused by the interaction of chromosomal and cytoplasmic factors. Germ-line abnormalities, including sterility, high mutability and male recombination, appear in the affected individuals. There are at least two distinct systems of hybrid dysgenesis. We examined a Wisconsin wild population in two consecutive years to determine the distribution of the chromosomal P factor and the extrachromosomal M cytotype that together cause one kind of hybrid dysgenic sterility. The P factor was found to be very common in the population, with all three major chromosomes being polymorphic for it. This polymorphism was strongly correlated with variability for male recombination elements, suggesting that these two traits are part of the same system of hybrid dysgenesis. There was a slight tendency for the P factor to be lost in lines taken from this population and inbred in the laboratory for many generations. A large-scale search for the M cytotype, which causes susceptibility to the P factor, showed that it is present in the population at only very low frequencies. Further evidence that the population is mostly immune to the action of the P factor was our finding of a general lack of dysgenic sterility in the wild flies themselves. However, we were able to isolate several wild strains that consistently showed the M cytotype. In some cases, the frequency of the M cytotype could be maintained in these lines, but it could not usually be increased by artificial selection. Some possible consequences of hybrid dysgenesis for the evolutionary biology of Drosophila are suggested.     

The Influence of Nonautonomous P Elements on Hybrid Dysgenesis in Drosophila melanogaster

An inbred line of the M' strain Muller-5 Birmingham was studied for its abilities to affect P-M hybrid dysgenesis. This strain possesses 57 P elements, all of which are apparently defective in the production of the P transposase. In combination with transposase-producing elements, these nonautonomous elements can enhance or diminish the incidence of hybrid dysgenesis, depending on the trait that is studied. Dysgenic flies that have one or more paternally-derived chromosomes with these elements partially repress the instability of the P element insertion mutation, snw; however, such flies have elevated frequencies of another dysgenic trait, GD sterility, and also show distorted segregation ratios. An explanation is presented in which all of these phenomena are unified as manifestations of the kinetics of P element activation in the germ line. The progeny of Muller-5 Birmingham females exhibit partial repression of both snw instability and GD sterility. This repression appears to involve a factor that can be transmitted maternally through at least two generations. This mode of repression therefore conforms to the pattern of inheritance of the P cytotype, the condition that brings about nearly total repression of P element activity in some strains. Models in which this repression could arise from the nonautonomous P elements of Muller-5 Birmingham are discussed.     

Movement of Drosophila melanogaster transposable elements other than P elements in a P-M hybrid dysgenic cross

Summary The Drosophila melanogaster mobile DNA sequences P factors and P elements transpose at elevated rates when P strain males are mated to M strain females in a hybrid dysgenic cross (Engels 1983). Isofemale lines derived from such a cross were analysed by in situ hybridisation using cloned copies of the transposable elements copia, 412 and F. It was found that lines derived from dysgenic crosses showed a statistically significant number of new sites for these elements when compared to a non-dysgenic control cross. This result suggests a functional coupling of copia, 412 and F transposition and some component present in the P-M dysgenic system.     

Chromosomal Effects on Mutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Two manifestations of hybrid dysgenesis were studied in flies with chromosomes derived from two different P strains. In one set of experiments, the occurrence of recessive X-linked lethal mutations in the germ cells of dysgenic males was monitored. In the other, the behavior of an X-linked P-element insertion mutation, snw, was studied in dysgenic males and also in dysgenic females. The chromosomes of one P strain were more proficient at causing dysgenesis in both sets of experiments. However, there was variation among the chromosomes of each strain in regard to the ability to induce lethals or to destabilize snw. The X chromosome, especially when it came from the stronger P strain, had a pronounced effect on both measures of dysgenesis, but in combination with the major autosomes, these effects were reduced. For the stronger P strain, the autosomes by themselves contributed significantly to the production of X-linked lethals and also had large effects on the behavior of snw, but they did not act additively on these two characters. For this strain, the effects of the autosomes on the X-linked lethal mutation rate suggest that only 1/100 P element transpositions causes a recessive lethal mutation. For the weaker P strain, the autosomes had only slight effects on the behavior of snw and appeared to have negligible effects on the X-linked lethal mutation rate. Combinations of chromosomes from either the strong or the weak P strain affected both aspects of dysgenesis in a nonadditive fashion, suggesting that the P elements on these chromosomes competed with each other for transposase, the P-encoded function that triggers P element activity. Age and sex also influenced the ability of chromosomes and combinations of chromosomes to cause dysgenesis.     

Transposable Element-Induced Response to Artificial Selection in Drosophila Melanogaster: Molecular Analysis of Selection Lines

Artificial selection lines for abdominal bristle score of Drosophila melanogaster established from P-M hybrid dysgenic crosses showed increases in selection response, heritability and phenotypic variance compared to similar lines started from nondysgenic crosses. To determine whether this increased genetic variance could be due to enhanced transposition of P elements following the dysgenic cross, the cytological locations (sites) of P elements were determined by in situ hybridization for the whole genome of samples of 20 individuals from the parental P strain, 20 individuals from each of the eight dysgenic selection lines, and ten individuals from each of the eight nondysgenic selection lines. Variation among and within the selection lines and the parental P strain in P element insertion sites was exceptionally high. A total of 601 sites were identified, but there was no difference in total number of sites per line, mean number of sites per individual, mean copy number per individual, or site frequency between dysgenic and nondysgenic selection lines, or between lines selected for high and low bristle score. Transposition following nondysgenic crosses may explain additional observations of accelerated selection responses in nondysgenic selection lines. It was not possible to deduce which, if any, of the several hundred insertions in the dysgenic selection lines were responsible for their extreme bristle phenotypes.     

Reduction of spontaneous somatic mutation frequency by a low-dose X irradiation of Drosophila larvae and possible involvement of DNA single-strand damage repair

The third instar larvae of Drosophila were irradiated with X rays, and the somatic mutation frequency in their wings was measured after their eclosion. In the flies with normal DNA repair and apoptosis functions, 0.2 Gy irradiation at 0.05 Gy/min reduced the frequency of the so-called small spot (mutant cell clone with reduced reproductive activity) compared with that in the sham-irradiated flies. When apoptosis was suppressed using the baculovirus p35 gene, the small spot frequency increased four times in the sham-irradiated control group, but the reduction by the 0.2-Gy irradiation was still evident. In a non-homologous end joining-deficient mutant, the small spot frequency was also reduced by 0.2 Gy radiation. In a mutant deficient in single-strand break repair, no reduction in the small spot frequency by 0.2 Gy radiation was observed, and the small spot frequency increased with the radiation dose. Large spot (mutant cell clone with normal reproductive activity) frequency was not affected by suppression of apoptosis and increased monotonically with radiation dose in wild-type larvae and in mutants for single- or double-strand break repair. It is hypothesized that some of the small spots resulted from single-strand damage and, in wild-type larvae, 0.2 Gy radiation activated the normal single-strand break repair gene, which reduced the background somatic mutation frequency.     

Novel events associated with phenotypic reversion of a P element mutant in Drosophila melanogaster.

Transposable P elements have been used extensively for Drosophila mutagenesis. While their mutagenic activity has long been recognized, the mechanisms by which P elements cause mutations are varied and not completely understood. We describe here an experiment to replace a P element at vestigial (vg) that caused a strong mutant phenotype (P[21-3]) with a P element (P[21]) known to produce a very weak phenotype when inserted at vg. In addition to testing the feasibility of P element replacements at vg, our investigation led to the production of 7 new vg alleles and 1 apparent second site suppressor. All the vg21-3 revertants that we recovered had a P element inserted into the first exon of vg at the same location and in the same orientation as the original element in vg21-3, providing a unique opportunity to study the mechanism of transposon mutagenesis. A majority of the revertants arose from a previously described event: internal deletion of P sequences, including the P promoter. In addition, 3 novel reversions of the vg21-3 wing phenotype were recovered. The wings of homozygous vg21r36 flies were normal. However, vg21r36 in combination with a deletion of the vg locus exhibited a strong mutant wing phenotype. This was surprising, because the P element insertion in vg21r36 was very similar to that found in the vg21 allele, which showed only slight nicking of the wings in combination with a deletion. In vg21r4, reversion was caused by a tandem insertion of P[21] and the original P[21-3] element present in vg21-3. Finally, the vg21r7 revertant had a P[21-3] insert at vg and 3 additional P elements elsewhere in the genome. We hypothesize that reversion in the 3 novel cases might be caused by P repressor produced by an element at vg or, in the case of vg21r7, elsewhere in the genome. This raises an interesting aspect of P element evolution. While P transposons produce mutations that might prove deleterious to their host, their success in invading the genome of D. melanogaster may be explained by their ability to silence those same mutations by a range of repressor-producing elements.     

Hybrid dysgenesis-induced response to selection in Drosophila melanogaster

In Drosophila melanogaster, the P-M and I-R systems of hybrid dysgenesis are associated with high rates of transposition of P and I elements, 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-mediated P transposition contributes to mutational variance for quantitative characters by comparing the responses to selection of P-M dysgenic and nondysgenic hybrids have given variable results. This experimental design has been extended to include an additional quantitative trait and the I-R hybrid 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.     

Aggressive behaviours of female Drosophila melanogaster are influenced by their social experience and food resources

Abstract Aggressive behaviours of female Drosophila melanogaster (Meigen) (Diptera: Drosophilidae) were studied in the laboratory. These behaviours included 'approach', a seemingly intentional movement of a fly to another fly, 'lunge', a dash at another fly, and 'wings erect', a brief erection of wings toward another fly. These behaviours were influenced by social experience and food resources. First, event frequencies of aggressive behaviours were higher in flies that had been held in isolation than in those that had been held in crowded conditions. This difference was the result of the social experience of attacking flies and not that of attacked flies. Second, flies showed aggressive behaviours more frequently when they were on food containing yeast colonies than when they were on food without yeast colonies. Third, flies conditioned to a food solution containing live yeast prior to observation showed lower frequency of aggressive behaviours than flies conditioned to a boiled food solution. Development of aggressive behaviours became apparent one day after eclosion.     

hobo is responsible for the induction of hybrid dysgenesis by strains of Drosophila melanogaster bearing the male recombination factor 23.5MRF

The male recombination factor 23.5MRF, isolated ten years ago from a natural Greek population of Drosophila melanogaster, has been shown to induce hybrid dysgenesis when crossed to some M strains, in a fashion slightly different from that of most P strains. Furthermore, it was recently shown that 23.5MRF can also induce GD sterility when crossed to specific P strain females (e.g., Harwich, pi 2 and T-007). In these experiments, the P strains mentioned behaved like M strains in that they did not induce sterility in the reciprocal crosses involving 23.5MRF. We extended the analysis to show that 23.5MRF does not destabilize snW(M) and that a derivative with fewer full-length P elements behaves like an M strain toward the same P strains and still retains its dysgenic properties in the reciprocal crosses. We show that there is a strong correlation between the site of dysgenic chromosomal breakpoints induced by 23.5MRF and the localization of hobo elements on the second chromosome, and also that hobo elements are found associated with several 23.5MRF induced mutations. These results suggest that hobo elements are responsible for the aberrant dysgenic properties of this strain, and that they may express their dysgenic properties independent of the presence of P elements.     

Rearrangements Involving Repeated Sequences within a P Element Preferentially Occur between Units Close to the Transposon Extremities

In a previous report we described rearrangements occurring at a high rate (30% of the progeny of dysgenic flies) within a cluster of 5S genes internal to a P element. These events were characterized as precise amplifications and deletions of 5S units. Here we analyze recombination events within P elements containing two repeated arrays of 5S genes flanking a central white gene. Deletions (50%) and duplications (3%) of the white gene together with various amounts of flanking 5S genes were observed. These recombinations occur preferentially between the most external 5S units of P transposons. Such rearrangements could be favored by interactions between the proteins bound to the P terminal sequences.     

A Female Sterile Screen of the Drosophila Melanogaster X Chromosome Using Hybrid Dysgenesis: Identification and Characterization of Egg Morphology Mutants

We have conducted a hybrid dysgenic screen of the X chromosome for mutations affecting female fertility, with particular attention to those causing abnormal egg and eggshell morphology. In a screen of 4017 dysgenic strains, 398 mutants derived from 168 different germ lines were isolated and assigned to eight classes according to their diverse phenotypes. One interesting class consists of mutants that block oogenesis at specific stages. Our analysis has focused on mutations affecting eggshell formation, including mutants that lay morphologically abnormal sterile eggs as well as those that lay no eggs but exhibit blocks in the late stages of oogenesis. A subset of 48 mutants was assorted into 30 allelic groups by inter se complementation and genetically localized by interval mapping. Two multiallele complementation groups, de1 (7 alleles) and ne1 (8 alleles), were identified as well as five two-allele complementation groups. A search for alleles among mutants generated in other female sterile screens was unsuccessful, pointing to the distinctive nature of the dysgenic mutant collection. The single case of allelism determined in this study was one with a lethal allele of the Broad-Complex, l(1)npr, suggesting a possible involvement of ecdysone in choriogenesis. A subset of 18 dysgenic strains was analyzed for P element hybridization and 16 of these were found to have hybridization signals in the appropriate cytogenetic interval. By examining these signals in two or more alleles of the same complementation group, we have been able to tentatively localize two mutations. Light and electron microscopy of the eggshell in 43 different strains has revealed a variety of effects. The respiratory appendages were defective in 27 of these mutants. Effects on the ultrastructure of the main body of the endochorion were not strongly correlated with the appendage defects, and could be classified as minor (14 mutants) or major (16 mutants). Although 13 mutants showed no ultrastructural chorion defects, six of these had defective respiratory appendages.     

Evidence for rapid limitation of the I element copy number in a genome submitted to several generations of I-R hybrid dysgenesis in Drosophila melanogaster

Summary When Drosophila melanogaster males coming from a class of strains known as inducer are crossed with females from the complementary class (reactive), a quite specific kind of sterility is observed in the F₁ female progeny (denoted SF). The inducer chromosomes differ from the reactive chromosomes by the presence of a transposable element (called the I factor) that is responsible for the induction of this dysgenic symptom. In the germ line of dysgenic females, up to 100% of the reactive chromosomes may be contaminated, i.e. they acquire I factor(s) owing to very frequent replicative transpositions. A contaminated reactive stock was obtained by reconstructing the reactive genotype in the offspring of SF females and its kinetics of invasion by I elements was followed in the successive inbred dysgenic generations. The results show that the mean copy number of I elements increased very quickly up to the level of inducer strains and then stayed in equilibrium even though the dysgenic state was perpetuated by selection for SF sterility at every generation. The possible mechanisms of this copy number limitation are discussed.     

A genetic sexing system to improve the sterile insect technique against the Mediterranean fruit fly

The autosomal recessive allele v wing (v) in the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), produces flies that when reared at 30 degrees C have stubby wings. The mutant was used to construct a translocation-based genetic sexing system in an attempt to improve the efficiency and effectiveness of the sterile insect release method for field control.     

Impairment of cytotype regulation of P-element activity in Drosophila melanogaster by mutations in the Su(var)205 gene

Cytotype regulation of transposable P elements in the germ line of Drosophila melanogaster is associated with maternal transmission of P elements inserted at the left telomere of the X chromosome. This regulation is impaired in long-term stocks heterozygous for mutations in Suppressor of variegation 205 [Su(var)205], a gene implicated in the control of telomere length. Regulation by TP5, a structurally incomplete P element at the X telomere, is more profoundly impaired than regulation by TP6, a different incomplete P element inserted at the same site in a TAS repeat at the X telomere. Genetic analysis with the TP5 element indicates that its regulatory ability is not impaired in flies whose fathers came directly from a stock heterozygous for a Su(var)205 mutation, even when the flies themselves carry this mutation. However, it is impaired in flies whose grandfathers came from such a stock. Furthermore, this impairment occurs even when the Su(var)205 mutation is not present in the flies themselves or in their mothers. The impaired regulatory ability of TP5 persists for at least several generations after TP5 X chromosomes extracted from a long-term mutant Su(var)205 stock are made homozygous in the absence of the Su(var)205 mutation. Impairment of TP5-mediated regulation is therefore not directly dependent on the Su(var)205 mutation. However, it is characteristic of the six mutant Su(var)205 stocks that were tested and may be related to the elongated telomeres that develop in these stocks. Impairment of regulation by TP5 is also seen in a stock derived from Gaiano, a wild-type strain that has elongated telomeres due to a dominant mutation in the Telomere elongation (Tel) gene. Regulation by TP6 is not impaired in the Gaiano genetic background. The regulatory abilities of the TP5 and TP6 elements are therefore not equally susceptible to the effects of elongated telomeres in the mutant Su(var)205 and Gaiano stocks.     

SIZE,SYMMETRY, AND SEXUAL SELECTION IN THE HOUSEFLY,MUSCA DOMESTICA

Relationships between measures of body size, asymmetry, courtship effort, and mating success were investigated in the housefly, Musca domestica (Diptera: Muscidae). A previous study indicated that both male and female flies with low fluctuating asymmetry enjoyed enhanced mating success. The aim of our investigations was to determine whether the greater success of symmetrical males is due to variation in male mating effort or to female choice and whether males exhibited mate choice. However, our study found directional rather than fluctuating asymmetry with both male and female flies having, on average, longer left wings than right. Also, asymmetry was not related to mating success in either sex. Rather, both males and females appeared to exhibit choice on the basis of the size of potential mates, with males preferring females with long bodies and females preferring heavy males. Possible benefits from choice of large mates are discussed. The initial mating strikes (in which the male leaps onto the back of the female) did not appear to be targeted according to female morphology, and their frequency did not vary according to male morphology. This indicates that mate choice by both sexes according to size probably occurs during the later stages of courtship, when the flies are in intimate contact. Possible reasons for the absence of choice according to asymmetry are discussed.     

Exposure to static electric fields leads to changes in biogenic amine levels in the brains of Drosophila

Natural and anthropogenic static electric fields are commonly found in the environment and can have both beneficial and harmful effects on many animals. Here, we asked how the fruitfly responds to these fields and what the consequences of exposure are on the levels of biogenic amines in the brain. When given a choice in a Y-tube bioassay Drosophila avoided electric fields, and the greater the field strength the more likely Drosophila were to avoid it. By comparing wild-type flies, flies with wings surgically removed and vestigial winged flies we found that the presence of intact wings was necessary to produce avoidance behaviour. We also show that Coulomb forces produced by electric fields physically lift excised wings, with the smaller wings of males being raised by lower field strengths than larger female wings. An analysis of neurochemical changes in the brains showed that a suite of changes in biogenic amine levels occurs following chronic exposure. Taken together we conclude that physical movements of the wings are used by Drosophila in generating avoidance behaviour and are accompanied by changes in the levels of amines in the brain, which in turn impact on behaviour.