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.     

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.     

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.     

Hybrid dysgenesis in Drosophila melanogaster: influence of temperature on cytotype determination in the P-M system

Summary In Drosophila melanogaster, the P-M system of hybrid dysgenesis is a syndrome of germ line abnormalities, including temperature dependent gonadal dysgenesis (GD sterility), high rates of mutation and male recombination, which occurs in some interstrain hybrids but only from one of the two crosses. In the P-M system, hybrid dysgenesis results from interaction between chromosomally transposable elements of the P element family and a particular extrachromosomal state referred to as the M cytotype. Cytotype (M or P) is known to be determined by the absence or presence of chromosomal factors, but principally with limited cytoplasmic transmission.In a series of experiments in which F1 hybrid females from various P and M strains were submitted to different preadult and ageing temperature treatments, it was found that the cytotype switch is strongly temperature-dependent in the F1 females from M x P but not in the reciprocal cross. In the F1 females from the former cross, a strong M cytotype occurs at a low developmental temperature (18° C) and a weak M cytotype occurs at a high developmental temperature (26.5° C). On the other hand, a high ageing temperature applied after a low developmental temperature switches the cytotype from M to P and reciprocally, a low ageing temperature applied after a high developmental temperature switches the cytotype from P to M.This thermo-reversibility of the extrachromosomal state exists only in the F1 females from M mothers but not in the F1 females from P mothers; this dissymmetrical behavior is discussed in relation to the mechanism proposed by O'Hare and Rubin (1983) which explains cytotype determination by a positive feedback of the regulator of the P transposase on its own level of activity.     

The distribution of the P–M hybrid dysgenesis system in Drosophila melanogaster strains from Brazil

Wild-caught flies of Drosophila melanogaster from seven natural populations of extreme regions of Brazil (São Luís, MA; Teresina, PI; Rio Cipó, MG; Maringá, PR; São José do Rio Preto, SP; Joinville, SC; and Porto Alegre, RS) were studied with the purpose of evaluating hybrid dysgenesis due to mobilization of P elements and the regulatory capacity of the strains' cytotypes. Diagnostic crosses were made and the strains classified according to their P–M phenotypes. Four strains were classified as moderate P (MA, MG, PI, and SP), two as Q (PR and RS) and one as M′ (SC). Females of southern strains (PR, SC, and RS) presented in A crosses lower degrees of gonadal dysgenesis scores than those from northern strains (MA and PI).     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: A Syndrome of Aberrant Traits Including Mutation, Sterility and Male Recombination

A syndrome of associated aberrant traits is described in Drosophila melanogaster. Six of these traits, mutation, sterility, male recombination, transmission ratio distortion, chromosomal aberrations and local increases in female recombination, have previously been reported. A seventh trait, nondisjunction, is described for the first time. All of the traits we have examined are found nonreciprocally in F(1) hybrids. We present evidence that at least four of the traits are not found in nonhybrids. Therefore we have proposed the name hybrid dysgenesis to describe this syndrome.-A partition of tested strains into two types, designated P and M, was made according to the paternal or maternal contribution required to produce hybrid dysgenesis. This classification seems to hold for crosses of strains from within the United States and Australia, as well as for crosses between strains from the two countries. Strains collected recently from natural populations are typically of the P type and those having a long laboratory history are generally of the M type. However, a group of six strains collected from the wild in the 1960's are unambiguously divided equally between the P and M types. The dichotomy of this latter group raises interesting questions concerning possible implications for speciation.-Temperature often has a critical effect on the manifestation of hybrid dysgenesis. High F(1 ) developmental temperatures tend to increase the expression of sterility, sometimes to extreme levels. Conversely, low developmental temperatures tend to inhibit the expression of some dysgenic traits.-There are potentially important practical implications of hybrid dysgenesis for laboratory experimentation. The results suggest that care should be exercised in planning experiments involving strain crosses.     

Recovery and characterization of hybrid dysgenesis-induced mei-9 and mei-41 alleles of Drosophila melanogaster

X-Linked methyl methanesulfonate (MMS)-sensitive mutations were induced with hybrid dysgenesis using four P strains: pi 2, Harwich, T-007 and OK-1. Mutations were identified after two generations of backcrosses to M strain females to replace the autosomes. Among 51,471 X-chromosomes examined 10 carried stable MMS-sensitive mutations representing 8 independent events. Males of the mutant strains failed to induce gonadal dysgenesis in crosses to Oregon-R females at 28.5 degrees C. Complementation tests showed that 3 of the induced mutations were mei-9 alleles, 2 were mei-41 alleles, 1 was a mus102 allele, and 2 were alleles at a newly identified MMS-sensitive locus, mus112 (map position: 1-32.8). As assayed by in situ hybridization on polytene chromosomes, each X-chromosome had no more than four P element insertions. 4 of the 8 mutations recovered in this study proved to have P element insertions at or very close to sites to which MMS sensitivity has been mapped. Hybrid dysgenesis-induced reversion of 2 mutants, mei-9RT1 and mei-41RT2, is associated with the loss of the P element from regions 4B and 14C respectively.     

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.     

Flow cytometric assessment of cytotype distributions within local populations of Phragmites australis (Poaceae) around Lake Biwa,the largest lake in Japan

Phragmites australis (Poaceae) is a clonal perennial that is an important component of wetland ecosystems worldwide. Using flow cytometry, we examined the cytotype distributions within five populations of P. australis located in the vicinity of Lake Biwa. As in previous reports, two ploidy levels, octoploid (2n = 8x = 96) and decaploid (2n = 10x = 120), were identified, which are assumed to be the main cytotypes around the lake. The coexistence of two cytotypes was detected in four of the five populations, suggesting the relatively common occurrence of mixed ploidy levels in the populations around Lake Biwa. Although intermingled cytotype distributions were observed in some populations, the 9x cytotype, that is, the expected outcome of inter‐cytotype crosses, was not observed, indicating limited gene flow between the two cytotypes.     

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.     

Evolution of Hybrid Dysgenesis Potential following P Element Contamination in Drosophila Melanogaster

P elements were introduced into M strain genomes by chromosomal contamination (transposition) from P strain chromosomes under conditions of P-M hybrid dysgenesis. A number of independently maintained contaminated lines were subsequently monitored for their ability to induce gonadal (GD) sterility in the progeny of reference crosses, over a period of 60 generations, in two experiments. The efficiency of chromosomal contamination was high; all tested lines acquired P elements following the association of M and P chromosomes in the same genome for a single generation. All the contaminated lines also sustained an initial unstable phase, marked by high frequencies of transposition and sterility within lines, in the absence of P element regulation. Subsequently, each of the lines rapidly evolved to one of three relatively stable strain types whose phenotypic and molecular properties correspond rather closely to those of the P, Q and M' strains that have previously been characterized. The numbers and structures of P elements and the presence or absence of P element regulation during the early generations appeared to be critical factors determining the subsequent course of evolution. On the basis of GD sterility frequencies, both the mean level of P activity, and the average capacity for P element regulation, were reduced in lines raised at 25 degrees, relative to those raised at 20 degrees, during the early generations. This latter result is consistent with the expectation that natural selection will tend to modify the manifestation of dysgenic traits, such as high temperature sterility, which cause a reduction of fitness. However, overall, stochastic factors appeared to predominate in determining the course of evolution of individual lines.     

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.     

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-Induced Quantitative Variation on the X Chromosome of Drosophila Melanogaster

To determine the ability of the P-M hybrid dysgenesis system of Drosophila melanogaster to generate mutations affecting quantitative traits, X chromosome lines were constructed in which replicates of isogenic M and P strain X chromosomes were exposed to a dysgenic cross, a nondysgenic cross, or a control cross, and recovered in common autosomal backgrounds. Mutational heritabilities of abdominal and sternopleural bristle score were in general exceptionally high-of the same magnitude as heritabilities of these traits in natural populations. P strain chromosomes were eight times more mutable than M strain chromosomes, and dysgenic crosses three times more effective than nondysgenic crosses in inducing polygenic variation. However, mutational heritabilities of the bristle traits were appreciable for P strain chromosomes passed through one nondysgenic cross, and for M strain chromosomes backcrossed for seven generations to inbred P strain females, a result consistent with previous observations on mutations affecting quantitative traits arising from nondysgenic crosses. The new variation resulting from one generation of mutagenesis was caused by a few lines with large effects on bristle score, and all mutations reduced bristle number.     

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.     

KP elements repress P-induced hybrid dysgenesis in Drosophila melanogaster.

Molecular and genetic analysis has revealed a specific P factor deletion derivative (the KP element) which is able to repress P-induced hybrid dysgenesis. All naturally occurring strains lacking the P cytotype (M') that were examined, throughout the world contain up to 30 copies of KP per haploid genome together with complete P factors. The KP element is derived from the P factor by an internal deletion of 1753 bp removing nucleotides 808-2560 and is transcribed to yield an abundant 0.8-kb poly(A)+ RNA with the coding capacity for an in-frame 207 amino acid polypeptide. Genetic crosses show that KP elements preferentially accumulate in the presence of P factors and suppress hybrid dysgenesis. Suppression is transmitted through both sexes and is thus distinct from the maternally transmitted P cytotype mode of suppression. The spread of KP elements is probably due to the continual selection of individuals with the highest numbers of KP elements in which P-induced hybrid dysgenesis is suppressed.     

The relationship between radiation-induced and transposon-induced genetic damage during Drosophila oogenesis

The combined effect of X-irradiation and transposon mobility on the frequencies of X-linked recessive lethals and dominant lethals was investigated in female hybrids in the P-M system of hybrid dysgenesis. X-linked lethals were measured in G2 hybrid dysgenic females whose X chromosome was derived from the M X P cross. To test for additivity or synergism, the mutation rate in irradiated dysgenic females was compared to that of unirradiated females as well as to irradiated nondysgenic hybrid females derived from M X M crosses. Eggs collected for 2 days after irradiation, were represented by the more radiation-sensitive A and B oocytes (about 75%) and the least sensitive C oocytes (about 25%). The production of X-linked lethal events in X-irradiated dysgenic females was 8.1%, as compared to 4.5% in dysgenic controls and 3.4% in irradiated, nondysgenic controls, demonstrating an additive effect of radiation and dysgenesis-induced genetic damage. The effect of irradiation on sterility of dysgenic hybrid females was a negative one, resulting in 20% less sterility than expected from an additive effect. The combined effect of radiation and dysgenesis on dominant lethality tested in A, B and C oocytes of the same hybrid females was synergistic. Egg broods collected for 3.5 days after irradiation showed that significantly more damage was induced in the presence of ionizing radiation in dysgenic females than in their nondysgenic counterparts. This effect was most obvious in B and C oocytes. The synergism observed may be related to the inability of cells to repair the increased number of chromosome breaks induced both by radiation and transposon mobility.     

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.     

DYNAMICS OF P-M HYBRID DYSGENESIS IN P-TRANSFORMED LINES OF DROSOPHILA SIMULANS

An autonomous P element from Drosophila melanogaster was introduced by microinjection into the germ line of its sibling species, Drosophila simulans. The invasion kinetics of P elements was studied in seven independent lines over 60 generations, using gel blotting, in situ hybridization, and dysgenic crosses. Some of the main phenotypic and molecular characteristics of P-M hybrid dysgenesis were observed, i.e., gonadal dysgenesis (GD sterility), chromosome rearrangements, and the occurrence of degenerate P elements. At least four lines reached a steady state with complete or nearly complete P-element regulation but with a moderate number of P elements (10–24 per haploid genome) and P activity (10–35% GD sterility). This failure to obtain strong P strains in D. simulans could be due to experimental conditions, a host-dependent component of P transposition, or more severe selection against the deleterious effects of this transposon.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Factors Affecting Chromosomal Contamination in the P-M System

The two interacting components of the P-M system of hybrid dysgenesis are chromosomally associated elements called P factors and a susceptible cytoplasmic state referred to as M cytotype. Previous experiments have indicated that P factors are a family of multiple-copy transposable genetic elements dispersed throughout the genome of P strains but absent in long-established M strains.—Evidence is presented that the sterility and male recombination-inducing potential of P elements may be acquired by X chromosomes, derived from M strains, through nonhomologous association with P strain autosomes, a process referred to as "chromosomal contamination." The frequencies of chromosomal contamination of X chromosomes by P strain autosomes were highly variable and depended on a number of factors. M cytotype (as opposed to P cytotype) was essential for high frequencies of P factor contamination. There were large differences in contamination potential among individual female families, and a weak negative correlation existed between family size and contamination frequency. Chromosomal contamination in the P-M system was shown to be independent of that in the I-R system.—Frequency distributions suggested that the relationship between sterility production and P factor insertion is complex. The majority of P element transpositions, identified by in situ hybridization in one X chromosome, were not associated with gonadal sterility. However, high sterility potential was found to be associated with the presence of at least one P element inserted into the X chromosome. This potential was lost at a rate of about one-sixth per generation in M cytotype but was stabilized in P cytotype. Various hypotheses concerning the relationship between transposition and chromosomal contamination are discussed.