P-M system of hybrid dysgenesis inDrosophila melanogaster: thermic modifications of the cytotype can be detected for several generations

Summary In the P-M system of hybrid dysgenesis inDrosophila melanogaster, the cytotype corresponds to an extrachromosomal state which controls the mobility of the P transposable elements. Previous experiments have shown that thermic treatments during development and ageing may dramatically influence the cytotype transformation process in certain types of hybrid females. We show here that these temperature-induced modifications can be detected in subsequent generations. The consequences for the cytotypic characterisation of strains are discussed.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Nature and Inheritance of P Element Regulation

The genetic determination of the control of resistance or susceptibility to germ line changes mediated by P elements was studied in two strains and in derivatives of crosses between them. One strain, characterized as true M, completely lacked P elements. The second strain, pseudo-M (M'), carried a number of P elements, but these did not have the potential to induce the gonadal sterility that is associated with P-M hybrid dysgenesis. Individuals from the true M strain were invariably unable to suppress P factor activity (i.e., all daughters of outcrosses of M females and P males were sterile). In contrast, individuals from the M' strain showed variable degrees of suppression that were manifested in a wide range of gonadal sterility frequencies in standard tests. This continuous distribution pattern was reproducible for more than 25 generations.--The results of the genetic analysis indicate that a strain with a variable degree of suppression of gonadal dysgenesis is not necessarily in a transient state between the extreme conditions of P and M cytotype. A large variance in the ability to suppress gonadal dysgenesis with a mean value intermediate between the extremes of P and M cytotype may be a relatively stable strain characteristic. No reciprocal cross effect was observed in the suppression of sterility of F1 females from M X M' matings. Thus, the existence of M' strains indicates a Mendelian component in P element regulation and suggests that cytotype, which has an extrachromosomal aspect, may be only one of perhaps several mechanisms involved in regulation. Analysis of the effects of individual chromosomes from the M' strain showed that each chromosome contributed to the reduction of gonadal dysgenesis in the progeny of test matings. The results are consistent with a one-component titration model for P element regulation.     

Gonadal Dysgenesis Determinants in a Natural Population of DROSOPHILA MELANOGASTER

Three populations of Drosophila melanogaster from northern California were surveyed for the ability to produce and resist gonadal dysgenesis in the P-M system of hybrid dysgenesis. Males from all three populations produced low to moderate levels of gonadal dysgenesis in crosses to Oregon-R M females. Most females had the P cytotype, but the M cytotype occurred occasionally. The three populations could not be statistically differentiated from one another, but were easily distinguished from populations from Australia and Wisconsin on the basis of gonadal dysgenesis potential. The California populations had higher levels of M cytotype than did the Wisconsin population. Thirteen X chromosomes and 11 pairs of autosomes were extracted from one of the California populations, using a modification of the standard balancer chromosome technique to suppress hybrid dysgenesis during extraction. All lines produced strongly skewed sterility distributions in crosses to M-strain females, and mean levels of sterility were less than 50%. There was evidence of nonadditive interactions between the autosomes. Most extraction lines had the P cytotype, but M and intermediate cytotypes were observed. Some of the intermediate cytotypes were stable over time. Lines were tested at two different times after extraction. Some lines evolved higher sterility potential as they were kept in the laboratory, even in the presence of P cytotype. The results point out a number of deficiencies in current genetic and population genetic models of hybrid dysgenesis and imply that gonadal dysgenesis is unlikely to be an important evolutionary force in this population.     

Quantitative models of hybrid dysgenesis: rapid evolution under transposition, extrachromosomal inheritance, and fertility selection

A model of the P-M system of hybrid dysgenesis is presented which incorporates single-site transposition of P factors in M cytotype, determination of offspring cytotype by both maternal cytotype and maternal or offspring nuclear genotype, and strong fertility selection in dysgenic individuals. The conditions required for the initial invasion of P factors into a pure M population, information concerning stable polymorphisms, and results of numerical iterations depicting the dynamic, nonequilibrium behavior of the system are summarized. While conditions for initial increase are independent of the rate of cytotype switching, the rate of evolution is accelerated by increased production of dysgenic individuals. If the transposition rate is sufficiently high to overcome the fertility barrier opposing P factors introduced into M populations, then convergence to high frequencies of the P factor occurs very rapidly. Under intense fertility depression, the phase of rapid increase may be preceded by an extended period of gradual increase at low frequencies.     

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.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Sterility Resulting from Gonadal Dysgenesis in the P-M System

Crosses between two types of strains, called P and M, characteristically give high frequencies of F(1) sterility and other aberrant traits. Previous studies indicated that, in addition to the direction of the parental cross, many factors influence the manifestation of this phenomenon known as "hybrid dysgenesis."-The present study is concerned with the characteristics of GD (gonadal dysgenesis) sterility associated with the P-M system and its temperature dependence. Female sterility is accompanied by a complete absence of egg-laying, and this is not attributable to an inability to mate. Thus, it seems likely that sterility results from a defect in gametogenesis of hybrid individuals. This conclusion is supported by the morphological and cytological observations presented in an accompanying paper (Schaefer, Kidwell and Fausto-Sterling 1979).-A narrow, critical, developmental temperature range was found in which both female and male sterility rose sharply from a low level to a high maximum. The critical range was 27 to 29 degrees for males, slightly higher than the range of 24 to 26 degrees for females. Two other dysgenic traits, male recombination and transmission ratio distortion, were affected by developmental temperature, but temperature response curves were quite different from those for sterility. The temperature-sensitive stage for female sterility occurs during embryonic and early larval development.-GD sterility is compared and contrasted with SF sterility, another specific type of non-Mendelian sterility resulting from a different interstrain dysgenic interaction.     

Radiation-induced and transposon-induced chromosome damage in Drosophila: translocations and transmission distortion

The possible interaction between X-ray- and transposon-induced chromosome damage was monitored in the P-M system of hybrid dysgenesis in Drosophila melanogaster. One- to two-day-old F1 dysgenic males originating from a cross between M strain females and P strain males were irradiated with 5.5 Gy (550 rad) or used as controls to monitor X-Y translocations and transmission ratio distortion. Two 3-day sperm broods were sampled for the former and two 4-day broods for the latter to detect damage induced in the most radiosensitive cells. F1 nondysgenic males derived from M female to M male crosses (controls) were treated identically. X-Y chromosome translocations induced by P element mobility alone declined sharply with a decrease in temperature (18 versus 21 degrees C) and they were significantly reduced with aging of hybrid males from brood 2, 4-8 days of age, to brood 3, 7-11 days of age. No significant increase in translocations was observed when X irradiation and P-M dysgenesis were combined, showing no interaction between damages induced by the two mutator systems. In contrast, interaction was observed in transmission ratio distortion which was significantly increased by X irradiation of hybrid males derived from both reciprocal M X P and P x M crosses. The preferential elimination of P element-bearing autosomes occurred when either spermatocytes or spermatids were irradiated. An aging effect was also observed, resulting in less distortion in 9- to 14-day-old dysgenic males compared to 5- to 10-day-old hybrids.     

Germline hypermutability in Drosophila and its relation to hybrid dysgenesis and cytotype

In its hypermutable state, an unstable singed allele, sn^w, mutates in the germline to two other alleleic forms at a total frequency usually between 40 and 60%. In its stable state, the mutation rate of sn^w is essentially zero. Its state depends on an extrachromosomal condition indistinguishable from a property called cytotype previously studied as a component of hybrid dysgenesis. Of the two known systems of hybrid dysgenesis, denoted P-M and I-R, sn^w hypermutability is determined by the P-M system and appears to be independent of the I-R system. Cytotype, as defined by the control of sn^w mutability, is self-reproducing in the cytoplasm or nucleoplasm of the germline through at least two generations. However, it is not entirely autonomous, being ultimately determined by the chromosomes after sufficiently many generations of backcrossing. This combination of chromosomal and extrachromosomal transmission agrees well with previous studies on cytotype. Temperature differences have little effect on the mean mutation rates, but they have a pronounced effect on the intrinsic variance among individuals. The latter effect suggests that high temperatures reduce germ-cell survival during the development of dysgenic flies. Chromosomal rearrangements produce no apparent effects on the behavior of sn^w. Hypermutability is thought to be caused by the excision or other alteration of an inserted genetic element in the sn^w gene. This element might be a copy of the "P factor," which is though to be a mobile sequence capable of causing female sterility and other dysgenic traits in the P-M system.     

A high level of hybrid dysgenesis in Drosophila: High thermosensitivity, dependence on DNA repair, and incomplete cytotype regulation

Summary An unusually high level of P-M hybrid dysgenesis in Drosophila melanogaster is characteristic of hybrid offspring originating from both, A (M × ) and B (P × M) crosses of a subline of the Harwich P strain, termed H ^s . The novel properties induced by mobility of P elements carried by H ^s paternal chromosomes include: very high (over 95%) gonadal dysgenesis (GD) in both sexes at the low restrictive temperature of 21°C, and highly premature sterility when males are reared at 18°C and aged at 21°C. Although all three major chromosomes of the H ^s subline contributed to this atypical pattern of gonadal dysgenesis, chromosome 3 had the largest effect. Gonadal dysgenesis showed a temperature- and sex-dependent repression pattern by the defective P elements of Muller-5 Birmingham chromosomes; at 21°C there was virtually no repression of male sterility, but most effective repression of GD in females. At 29°C repression was effective in males, but declined in females. The high thermosensitive sterility, low fecundity, and premature aging of the male germ line were greatly exacerbated when males derived from either A or B crosses were deficient either in excision repair (mei-9 mutation) or in post-replication repair (mei-41 mutation). These findings demonstrate that both DNA repair pathways are essential for the repair of lesions induced by P element transposition and support the hypothesis that P element-induced chromosome breaks are responsible for the virtual abolition of the germ line. The relatively high premature sterility of cross B DNA repair-deficient males, reared at 18°C and aged at 21°C, indicates that there is incomplete cytotype regulation in H ^s subline hybrids.     

Elements causing hybrid dysgenesis on the second chromosome ofDrosophila melanogaster

Summary Hybrid dysgenesis inDrosophila melanogaster is a syndrome of germline abnormalities including temperature-dependent gonadal dysgenesis (GD sterility), high rates of mutation and male recombination. In theP-M system, hybrid dysgenesis results from interaction between chromosomally-linked factors (P factors) and a particular extrachromosomal state refered to as theM cytotype. TheT007/Cy strain, shown by other authors to induce a high level of mutation and male recombination, is presently studied with respect to gonadal dysgenesis. TheP activity appears mainly linked with theT007 second chromosome and has been essentially mapped to a 0.6 centimorgan long interval, i.e. betweenhk andpr. On the other hand, 14 strains balanced for deficiencies on the left arm of the second chromosome are studied for their relative level ofM cytotype activity.In F1 females, inheriting the same maternal cytotype and the same paternalT007 chromosome, significant differences inGD sterility are found between flies receiving the maternal deficiency and those receiving the alternate non-deleted chromosome. This effect appears only when the chromosomes are deleted for a common region (37F5-38A7), suggesting the presence of elements intervening in the determinism ofGD sterility in this zone. As this region is included in the correspondinghk-pr interval (37C1-38B6), these results state the problem of the nature of the elements located in this interval and two hypotheses are discussed.     

Drosophila melanogaster P Transposable Elements: Mechanisms of Transposition and Regulation

The molecular mechanisms that control P element transposition and determine its tissue specificity remain incompletely understood, although much information has been compiled about this element in the last decade. This review summarizes the currently available information about P element transposition, P-M hybrid dysgenesis and P cytotype features, P element-encoded repressors, and regulation of transposition.     

Sterility and Hypermutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Inbred wild strains of Drosophila melanogaster derived from the central and eastern United States were used to make dysgenic hybrids in the P-M system. These strains possessed P elements and the P cytotype, the condition that represses P element transposition. Their hybrids were studied for the mutability of the P element insertion mutation, snw, and for the incidence of gonadal dysgenesis (GD) sterility. All the strains tested were able to induce hybrid dysgenesis by one or both of these assays; however, high levels of dysgenesis were rare. Sets of X chromosomes and autosomes from the inbred wild strains were more effective at inducing GD sterility than were sets of Y chromosomes and autosomes. In two separate analyses, GD sterility was positively correlated with snw mutability, suggesting a linear relationship. However, one strain appeared to induce too much GD sterility for its level of snw destabilization, indicating an uncoupling of these two manifestations of hybrid dysgenesis.     

Dynamic equilibrium between insertion and excision of P elements in highly inbred lines from an M′ strain of Drosophila melanogaster

Six highly inbred lines of Drosophila melanogaster extracted from an M strain (in the P/M system of hybrid dysgenesis) were studied for the evolution of the number and chromosomal location of complete and defective P elements through generations 52–200. These lines possessed full-sized P elements but differed in their cytotype (M or P). Three lines with P cytotype and full-sized P elements at site 1A had a constant P copy number over generations with low rates of insertion and excision. Three lines with M cytotype and at least one full-sized P element accumulated P copies over the generations and reached a plateau near generation 196, at which rates of transposition and excision were equal to 1.2 × 10^–3 to 3 × 10^–3 events per element per generation. At that time these three lines still presented an M cytotype, produced transposase, and were able to regulate P copy number. The similarity at equilibrium between insertion and excision rates was exactly what was expected from theoretical models for a self-regulated element. The large number of generations necessary to attain the equilibrium in copy number indicates, however, that caution may be de rigueur when testing theoretical models of copy-number containment based on transposition and excision-rate comparison.     

The P-M and the 23.5 Mrf (Hobo) Systems of Hybrid Dysgenesis in Drosophila Melanogaster Are Independent of Each Other

Strains of Drosophila melanogaster bearing the male recombination factor 23.5 MRF induce hybrid dysgenesis in a way which is highly reminiscent of the P-M system, and, most probably, causally related to the activity of the transposable element hobo. We have investigated potential interactions between the two systems of hybrid dysgenesis by studying mixed lines derived from bidirectional crosses between 23.5 MRF and P strains, and analyzed their potentials to induce or suppress the occurrence of dysgenesis. All new lines possess the P induction abilities, as determined by two different procedures, and have also acquired a P cytotype. In contrast, some of them lost their ability to induce the non-P-M dysgenesis, as well as to suppress the action of 23.5 MRF. This loss of the 23.5 MRF induction abilities parallels the selective loss of full-length hobo elements from the genome of these lines, providing further substantiation to the notion that the 23.5 MRF activity is directly linked to this transposable element.     

Hybrid dysgenesis in Drosophila melanogaster: predominance of Q factor in Japanese populations and its change in the laboratory

The P-M system of hybrid dysgenesis was investigated in two Japanese regions, Katsunuma and Ohzu. Most strains were Q in that they did not produce appreciable gonadal dysgenesis with P males or with M females. A few strains produced ambiguous results. Some of these were unstable and on later testing were P, Q or M. Eleven laboratory strains which had been derived from various sites in Japan and had been maintained for 8 to 27 years were also examined. Eight (including all four that were 27 years old) were M, two were Q, and one which had been kept as a large cage population showed some P activity.     

Hybrid dysgenesis in natural populations ofDrosophila melanogaster in Japan

The P-M system of hybrid dysgenesis inDrosophila melanogaster was investigated on the basis of gonadal dysgenesis, using 1,590 strains from 28 natural populations in Japan, and 20 populations from Southeast Asia, the Pacific area and Africa. Strong P strains were found sporadically in several populations in Japan. Few strong M strains were observed. Q strains were present at a high frequency in most populations. Thus, most populations in these areas were regarded as Q populations. The distribution of the P element and the evolution of P, Q and M populations are also discussed.     

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.     

Localization of P elements, copy number regulation, and cytotype determination in Drosophila melanogaster

Seventeen highly-inbred lines of Drosophila melanogaster extracted from an M' strain (in the P/M system of hybrid dysgenesis) were studied for their cytotype and the number and chromosomal location of complete and defective P elements. While most lines were of M cytotype, three presented a P cytotype (the condition that represses P-element activity) and one was intermediate between M and P. All lines were found to possess KP elements and only eight to bear full-sized P elements. Only the lines with full-sized P elements showed detectable changes in their P-insertion pattern over generations; their rates of gain and of loss of P-element sites were equal to 0.12 and 0.09 per genome, per generation, respectively. There was no correlation between these two rates within lines, suggesting independent transpositions and excisions in the inbred genomes. The results of both Southern blot analysis and in situ hybridization of probes made from left and right sides of the P element strongly suggested the presence of a putative complete P element in region 1A of the X chromosome in the three lines with a P cytotype; the absence of P copy in this 1A region in lines with an M cytotype, favours the hypothesis that the P element inserted in 1A could play a major role in the P-cytotype determination. Insertion of a defective 2 kb P element was also observed in region 93F in 9 of the 13 M lines. The regulation of the P-element copy number in our lines appeared not to be associated with the ratio of full-length and defective P elements.     

Evolution des potentialités dysgénésiques du système P-M dans des populations expérimentales mixtes P,Q, M et M′ de Drosophila melanogaster

Change of hybrid dysgenesis potentials in P-M system of Drosophila melanogaster — In the P-M system of hybrid dysgenesis, three types of Drosophila melanogaster strains have been described in relation to hybrid gonadal sterility: P, Q and M. When M strain females were mated with P strain males, the P factors resulted in variable level of sterility in their progeny. The Q strain had no significant potential for sterility in any hybrid strain combination. To observe the dynamics of chromosomal contamination, due to the P transposable elements in different genetic context, mixed populations of these three types of strains were set up and monitored for their gonadal sterility potential during at least 30 generations.A first set of 16 experimental populations was set up; each of these was initiated with a mixture of 50% of individuals from the Harwich strain (a strong P strain) and 50% of individuals from a M or Q strain collected in natural populations. The M activity levels of these strains corresponded to a range from 100% to 0%. For all of these populations, the M activity potential disappeared during the five first generations. However, the P activity potential reached an equilibrium level positively correlated with the M activity potential level introduced at the beginning. It is proposed that the force of invasion of the P type by chromosomal contamination through the transposition of the P elements is dependent on the copy number of P sequences present on the chromosome of the M strain in competition.A second set of 18 experimental populations was set up with a mixture of P, M or Q strains collected in France between 1965 and 1982 (this period probably corresponds to the invasion of the P elements in France). After 30 generations, all of these populations (except one) had lost all dysgenic sterility potentiality and seemed to be of the Q type. Taking into account the results obtained from the two sets of experimental populations, the temporal and geographical distribution of P elements in the world could be explained by a progressive diffusion of autonomous P elements, from America with an accompanying decrease of their ability to transpose.

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Ce travail a été réalisé dans le cadre de l'A.T.P. Biologie des populations et de l'UA 693 du C.N.R.S.     

P Element Insertions and Rearrangements at the Singed Locus of Drosophila Melanogaster

DNA from the singed gene of Drosophila melanogaster was isolated using an inversion between a previously cloned P element at cytological location 17C and the hypermutable allele singed-weak. Five out of nine singed mutants examined have alterations in their DNA maps in this region. The singed locus is a hotspot for mutation during P-M hybrid dysgenesis, and we have analyzed 22 mutations induced by P-M hybrid dysgenesis. All 22 have a P element inserted within a 700-bp region. The precise positions of 10 P element insertions were determined and they define 4 sites within a 100-bp interval. During P-M hybrid dysgenesis, the singed-weak allele is destabilized, producing two classes of phenotypically altered derivatives at high frequency. In singed-weak, two defective P elements are present in a "head-to-head" or inverse tandem arrangement. Excision of one element results in a more extreme singed bristle phenotype while excision of the other leads to a wild-type bristle phenotype.