P transposable element in Drosophila melanogaster: horizontal transfer]

The P transposable element family in Drosophila melanogaster is responsible for the syndrome of hybrid dysgenesis which includes chromosomal rearrangements, male recombination, high mutability and temperature sensitive agametic sterility (called gonadal dysgenesis sterility). P element activity is controlled by a complex regulation system, encoded by the elements themselves, which keeps their transposition rate low within the strain bearing P elements and limits copy number by genome. A second regulatory mechanism, which acts on the level of RNA processing, prevents P mobility to somatic cells. The oldest available strains, representing most major geographical regions of the world, exhibited no detectable hybridization to the P-element. In contrast, all recently collected natural populations that were tested carried P-element sequences. The available evidence is consistent with the hypothesis of a worldwide P-element invasion of D. melanogaster during the past 30 years. Timing and direction of the invasion are discussed. The lack of P-element in older strains of Drosophila melanogaster as well as in the species must closely related to Drosophila melanogaster, suggests that P entered the Drosophila melanogaster genome recently, probably by horizontal transfer from an other species. The analysis of P-element elsewhere in the genus Drosophila reveals that several more distantly related species carried transposable elements with sequences quite similar to P. The species with the best-matching P-element is D. willistoni. A P-element from this species was found to match all but one of the 2907 nucleotides of the Drosophila melanogaster P-element. The phylogenic distributions and the likely horizontal transfers of the two other Drosophila transposable elements are discussed.     

黑腹果蝇P转座因子研究 Ⅰ.我国黑腹果蝇的P-M测定及其地理分布

采用性腺败育(GD不育)作为标准检定方法。对我国20个地方的黑腹果蝇的P因子活性和细胞型进行了测定。结果表明我国北部沿海城市为Q型;南部沿海和内地皆为M型。各地的M品系所产生的GD不育能力各不相同,但表现出与地理位置相关的梯度变化。这一变化规律为研究我国黑腹果蝇的P因子起源及P和M品系的形成提供了重要的理论依据。     

Molecular Analysis of the P-M Gonadal Dysgenesis Cline in Eastern Australian Drosophila Melanogaster

The latitudinal cline in P-M gonadal dysgenesis potential in eastern Australia has been shown to comprise three regions which are, from north to south respectively, P, Q, and M, with the P-to-Q and Q-to-M transitions occurring over relatively short distances. The P element complements of 30 lines from different regions of the cline were determined by molecular techniques. The total amount of P element-hybridizing DNA was high in all lines, and it did not correlate in any obvious way with the P-M phenotypes of individual lines. The number of potentially full-sized P elements per genome was high in lines from the P regions, but variable or low among lines from the Q and M regions, and thus declined overall from north to south. A particular P element deletion-derivative, the KP element, occurred in all the tested lines. The number of KP elements was low in lines from the P region, much higher in lines from the Q region, and highest among lines from the M region, thus forming a cline reciprocal to that of the full-sized P elements. Another transposable element, hobo, which has been described as causing dysgenic traits similar to those of P-M hybrid dysgenesis, was shown to be present in all lines and to vary among them in number, but not in any latitudinal pattern. The P-M cline in gonadal dysgenesis potential can be inferred to be based on underlying clinal patterns of genomic P element complements. P activity of a line was positively correlated with the number of full-sized P elements in the line, and negatively correlated with the number of KP elements. Among Q and M lines, regulatory ability was not correlated with numbers of KP elements.     

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.     

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.     

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.     

Wake up of transposable elements following Drosophila simulans worldwide colonization.

Transposable elements (TEs) make up around 10%-15% of the Drosophila melanogaster genome, but its sibling species Drosophila simulans carries only one third as many such repeat sequences. We do not, however, have an overall view of copy numbers of the various classes of TEs (long terminal repeat [LTR] retrotransposons, non-LTR retrotransposons, and transposons) in genomes of natural populations of both species. We analyzed 34 elements in individuals from various natural populations of these species. We show that D. melanogaster has higher average chromosomal insertion site numbers per genome than D. simulans for all TEs except five. The LTR retrotransposons gypsy, ZAM, and 1731 and the transposon bari-1 present similar low copy numbers in both species. The transposon hobo has a large number of insertion sites, with significantly more sites in D. simulans. High variation between populations in number of insertion sites of some elements of D. simulans suggests that these elements can invade the genome of the entire species starting from a local population. We propose that TEs in the D. simulans genome are being awakened and amplified as they had been a long time ago in D. melanogaster.     

The level of dysgenic sterility and recessive mutations induced in laboratory strains of Drosophila melanogaster exposed to chronic low-dose gamma irradiation

Recent investigations showed that genetic instability accounts for many radiobiological effects. However, mechanisms underlying this phenomenon are still poorly understood. Assuming that mobile genetic elements may be involved in the induction of genetic instability, we studied parameters that characterize the activity of these elements in Drosophila melanogaster: hybrid dysgenesis and the level of recessive lethal mutations. In our experiments, we used D. melanogaster strains that differed in the type of hybrid dysgenesis (P-M and H-E). It was demonstrated that chronic exposure to radiation leads to substantial changes in the genetic structure of a population and an enhanced level of dysgenic sterility. Our results indicate that genetic instability and adaptation to the effect of chronic gamma-radiation are associated with the radiation-induced mobilization of mobile genetic elements.     

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.     

Comparative analysis of transposable elements in the melanogaster subgroup sequenced genomes

Transposable elements (TEs) are indwelling components of genomes, and their dynamics have been a driving force in genome evolution. Although we now have more information concerning their amounts and characteristics in various organisms, we still have little data from overall comparisons of their sequences in very closely-related species. While the Drosophila melanogaster genome has been extensively studied, we have only limited knowledge regarding the precise TE sequences in the genomes of the related species Drosophila simulans, Drosophila sechellia and Drosophila yakuba. In this study we analyzed the number and structure of TE copies in the sequenced genomes of these four species. Our findings show that, unexpectedly, the number of TE insertions in D. simulans is greater than that in D. melanogaster, but that most of the copies in D. simulans are degraded and in small fragments, as in D. sechellia and D. yakuba. This suggests that all three species were invaded by numerous TEs a long time ago, but have since regulated their activity, as the present TE copies are degraded, with very few full-length elements. In contrast, in D. melanogaster, a recent activation of TEs has resulted in a large number of almost-identical TE copies. We have detected variants of some TEs in D. simulans and D. sechellia, that are almost identical to the reference TE sequences in D. melanogaster, suggesting that D. melanogaster has recently been invaded by active TE variants from the other species. Our results indicate that the three species D. simulans, D. sechellia, and D. yakuba seem to be at a different stage of their TE life cycle when compared to D. melanogaster. Moreover, we show that D. melanogaster has been invaded by active TE variants for several TE families likely to come from D. simulans or the ancestor of D. simulans and D. sechellia. The numerous horizontal transfer events implied to explain these results could indicate introgression events between these species.     

Distribution of hobo transposable elements in natural populations of Drosophila melanogaster.

Forty-six strains derived from American and French natural populations of Drosophila melanogaster were tested for the presence and activity of hobo elements by using Southern blotting and a gonadal dysgenesis assay. The oldest available strains exhibited weak detectable hybridization to the hobo-element probe and revealed neither hobo-activity potential nor hobo-repression potential. In contrast, all recently collected strains harbored hobo sequences and revealed a strong hobo-repression potential but no strong hobo-activity potential. On the basis of restriction-enzyme analysis, old strains appear to have numerous fragments hybridizable to hobo sequences, several probably conserved at the same locations in the genome of the tested strain and others dispersed. In recently isolated strains, and unlike the situation in the published sequence of the cloned hobo108 element, a PvuII site is present in the great majority of full-sized hobo elements and their deletion derivatives. When the genetic and molecular characteristics are considered together, the available evidence is consistent with the hypothesis of a worldwide hobo-element invasion of D. melanogaster during the past 50 years. Comparison of data from the I-R and P-M systems suggests that the putative invasion followed the introduction of the I element but preceded that of the P element. This hypothesis poses the problem of the plausibility of three virtually simultaneous element invasions in this species. Such a possibility might be due to a modification of the genetic structure of American populations of D. melanogaster during the first part of the 20th century.     

Alteration of chromosome structure in ovarian nurse cells of Drosophila melanogaster by hybrid dysgenesis

The impact of hybrid dysgenesis on the chromosome structure of Drosophila melanogaster ovarian nurse cells was studied. In the examined lines and interlinear hybrids (including those yielded by dysgenic crosses in the P-M and I-R systems of hybrid dysgenesis), disturbed chromosome synapsis was revealed. The disturbance was somewhat similar to that observed in interspecific hybrids. Quantitative analysis showed that the mean frequency of nuclei with defective chromosome pairing ranged from 60.4 to 76%. FISH analysis of ovarian nurse chromosomes of Canton S x Berlin hybrids showed differences in the label localization in asynaptic homologs of arm 2L, which probably results in disrupted homolog pairing and reveal interlinear differences in localization of mobile genetic elements. Our results conform to Sved's model stating that hybrid dysgenesis is based on disorganization of the germline nuclear space.     

Adaptation to P element transposon invasion in Drosophila melanogaster

Transposons evolve rapidly and can mobilize and trigger genetic instability. Piwi-interacting RNAs (piRNAs) silence these genome pathogens, but it is unclear how the piRNA pathway adapts to invasion of new transposons. In Drosophila, piRNAs are encoded by heterochromatic clusters and maternally deposited in the embryo. Paternally inherited P element transposons thus escape silencing and trigger a hybrid sterility syndrome termed P-M hybrid dysgenesis. We show that P-M hybrid dysgenesis activates both P elements and resident transposons and disrupts the piRNA biogenesis machinery. As dysgenic hybrids age, however, fertility is restored, P elements are silenced, and P element piRNAs are produced de novo. In addition, the piRNA biogenesis machinery assembles, and resident elements are silenced. Significantly, resident transposons insert into piRNA clusters, and these new insertions are transmitted to progeny, produce novel piRNAs, and are associated with reduced transposition. P element invasion thus triggers heritable changes in genome structure that appear to enhance transposon silencing.     

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.     

Temporal distribution of P elements in Drosophila melanogaster strains from natural populations in Japan

Genetic and molecular investigations were carried out with 10 Japanese Drosophila melanogaster strains on P-M system of hybrid dysgenesis. The strains used here were collected in the years from 1952 to 1984 from various natural populations, and have been maintained in our laboratory. The whole genomic Southern hybridization was performed by using the 2.9-kb P element and the internal fragments as probes. Five strains possessed no P element copy and the other 5 strains possessed mainly incomplete P elements which had internal deletions. The former 5 strains were M, 2 of the latter were Q, and the remaining 3 were M' strains. Hikone-R, collected in 1952, had no P element copy, while Hikone-H, collected in 1957, was the earliest observed to possess multicopies of an incomplete P element. This revealed that P elements in Drosophila melanogaster were present more than 30 years ago in Japan, as already shown to have been the case on the American continent.     

Comparison of the regulation of P elements in M and M' strains of Drosophila melanogaster

M and M' strains of Drosophila melanogaster in the P-M system of hybrid dysgenesis were compared in two series of tests, with the following results. (1) The singed-weak hypermutability regulation test showed that M' strains had lower P excision rates than M strains, suggesting that P-elements repression must occur in M' strains although it is not detectable by gonadal dysgenesis assays. (2) The evolution of mixed P+M and mixed P+M' populations was compared, using a strong P strain. The P+M cultures invariably evolved in a few generations into strong P cultures, while the P+M' cultures evolved into P-type cultures with reduced P-factor potentials. However, after 30 generations of culture, both these types of mixed cultures had similar P copy numbers, suggesting that regulation of copy number had occurred in them.     

Amplification of Kp Elements Associated with the Repression of Hybrid Dysgenesis in Drosophila Melanogaster

Mobile P elements in Drosophila melanogaster cause hybrid dysgenesis if their mobility is not repressed. One type of repression, termed P cytotype, is a complex interaction between chromosomes carrying P elements and cytoplasm and is transmitted through the cytoplasm only of females. Another type of repression is found in worldwide M' strains that contain approximately 30 copies per individual of one particular P element deletion-derivative termed the KP element. This repression is transmitted equally through both sexes. In the present study we show that biparentally transmitted repression increases in magnitude together with a rapid increase in KP copy-number in genotypes starting with one or a few KP elements and no other deletion-derivatives. Such correlated increases in repression and KP number per genome occur only in the presence of complete P elements, supporting the interpretation that they are probably a consequence of the selective advantage enjoyed by flies carrying the highest numbers of KP elements. Analysis of Q strains also reveals the presence of qualitative differences in the way the repression of dysgenesis is transmitted. In general, Q strains not containing KP elements have the P cytotype mode of repression, whereas Q strains with KP elements transmit repression through both sexes. This difference among Q strains further supports the existence of at least two types of repression of P-induced hybrid dysgenesis in natural populations of D. melanogaster.     

Genomic P elements content of a wild M' strain of Drosophila melanogaster: KP elements do not always function as type II repressor elements

The P element is one of the best-studied DNA transposons as a model system to study evolution of mobile DNAs. The P element is a causative factor for P-M hybrid dysgenesis in Drosophila melanogaster and the P-M phenotype (P, Q, or M) has been thought to reflect genomic P elements content. Recent survey of natural populations showed that full-size P (FP) and KP elements are predominant in almost all current populations, irrespective of their phenotype variation. It was also suggested that some P elements are functionally inactive and their inactivation plays an important role in determining P-M phenotype. In order to know how the genomic P elements are inactivated, we characterized molecular features and insertion sites of them in an M' strain. We isolated 20 P elements, one FP, 15 KP, and four other internally deleted defective elements, all of which appeared thoroughly inactive. These FP and KP elements had canonical sequences in each case, but no mutations abolishing their function. In addition, they were mostly located in or within the vicinity of presumably active genes. Our results suggest that inactivation of P elements is associated with neither mutations nor constitutional suppression by heterochromatinization in M' strains and that only a few elements inserted in some special chromosomal regions are likely to be involved in determination of the phenotype of individual flies. Existence of many copies of canonical, but inactive, KP elements in the M' strain is inconsistent with the assumption that type II repression of the KP element is the main reason for its increase in the wild populations of D. melanogaster.