Cohabitation of KP and full-length P elements in the genome of MR strains inducing P-M-like hybrid dysgenesis in Drosophila melanogaster

Summary P strains of Drosophila melanogaster are characterized by the presence of both full-length and deletion derivatives of the transposable element P in their genome, and by their ability to induce the syndrome of hybrid dysgenesis among the progeny of certain intra-strain crosses, when introduced through the male parents. In contrast, strains belonging to the M' class, and which were also found to bear P element-homologous sequences, lack this ability and this has been attributed to the presence in the genome of most of these strains of a distinct class of deletion derivatives termed KP, which can suppress the action of functional P factors. Here we demonstrate that KP elements are present, next to full-length ones, in the genome of at least three strains which induce P-M-like dysgenic symptoms, including GD sterility. KP elements form the majority of the P-homologous sequences in the strains MR-h12, 23.5/CyL ⁴ and the latter's derivative 23.5 ^*/Cy. While the first one is a genuine P strain and the second one depicts a strong P cytotype, the third is a genuine M' strain. The hybrid dysgenesis induced by the two 23.5 MRF strains seems to be due, not primarily to the P elements, but to the action of hobo elements.     

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.     

The Alteration of the Chromosome Structure in Ovarian Nurse Cells of Drosophila melanogaster upon 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 × 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.     

The sources of genetic variability in highly inbred long-term selected strains of Drosophila melanogaster

A highly inbred, long-term selected for low fitness strain LA of Drosophila melanogaster posesses a significant mutational load and unusually high rates of spontaneous mutability as revealed by CyL/Pm method. Our results indicate that during cross of CyL/Pm to LA strains, destabilization of copia-like and mobile elements and induction of H-E hybrid dysgenesis take place. The role of these processes in causing considerable genetic variability of LA strain is discussed.     

Polymorphism of the Mdg-1 and I mobile elements in Drosophila melanogaster

The polymorphism of the mobile elements Mdg-1 (a copia-like element) and I (an element involved in I-R hybrid dysgenesis) was analysed in a mass-mated population of Drosophila melanogaster by in situ hybridization, using biotinylated DNA probes, on polytene chromosomes. The Mdg-1 and I elements were inserted independently but were within the same bulk of DNA insertion points of the Drosophila genome, which contained on average about 30 insertion sites for each element. The X chromosome contained the lowest copy number of elements while 2R and 3R had the highest number: 3R had the highest variability. There was no correlation between the copy numbers of elements among the chromosome arms. The average expected per locus heterozygosity was equal to 0.17 for both the Mdg-1 and the I elements. Although these two elements differ in sequence, they appeared to behave similarly in the Drosophila melanogaster genome. This suggests that they may compete for target insertion sites and may be under the same control mechanisms.     

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.     

Evolution of the LINE-like I element in the Drosophila melanogaster species subgroup

LINE-like retrotransposons, the so-called I elements, control the system of I-R (inducer-reactive) hybrid dysgenesis in Drosophila melanogaster. I elements are present in many Drosophila species. It has been suggested that active, complete I elements, located at different sites on the chromosomes, invaded natural populations of D. melanogaster recently (1920–1970). But old strains lacking active I elements have only defective I elements located in the chromocenter. We have cloned I elements from D. melanogaster and the melanogaster subgroup. In D. melanogaster, the nucleotide sequences of chromocentral I elements differed from those on chromosome arms by as much as 7%. All the I elements of D. mauritiana and D. sechellia are more closely related to the chromosomal I elements of D. melanogaster than to the chromocentral I elements in any species. No sequence difference was observed in the surveyed region between two chromosomal I elements isolated from D. melanogaster and one from D. simulans. These findings strongly support the idea that the defective chromocentral I elements of D. melanogaster originated before the species diverged and the chromosomal I elements were eliminated. The chromosomal I elements reinvaded natural populations of D. melanogaster recently, and were possibly introduced from D. simulans by horizontal transmission.     

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.     

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.     

Mariner-like sequences are present in the genome of the fruitfly,Drosophila melanogaster

No mariner-like elements (MLEs) have been described until now in the genome of Drosophila melanogaster despite many experiments using molecular methods. However, analyses of sequence data from the Berkeley Drosophila Genome Project show that there are DNA sequences corresponding to pieces of MLE in the genome of D. melanogaster. The sequences of these elements have diverged considerably (about 40%) from any other sequences observed elsewhere. Moreover, the putative amino acid sequences encoded by the best conserved regions reveal that these sequences are clearly homologous to MLEs transposase.     

The evolutionary genetics of thehobo transposable element in theDrosophila melanogaster complex

Hobo elements are a family of transposable elements found inDrosophila melanogaster and its three sibling species:D. simulans, D. mauritiana andD. sechellia. Studies inD. melanogaster have shown thathobo may be mobilized, and that the genetic effects of such mobilizations included the general features of hybrid dysgenesis: mutations, chromosomal rearrangements and gonadal dysgenis in F1 individuals. At the evolutionary level somehobo-hybridizing sequences have also been found in the other members of themelanogaster subgroup and in many members of the relatedmontium subgroup. Surveys of older collected strains ofD. melanogaster suggest that completehobo elements were absent prior to 50 years ago and that they have recently been introduced into this species by horizontal transfer. In this paper we review our findings and those of others, in order to precisely describe the geographical distribution and the evolutionary history ofhobo in theD. melanogaster complex. Studies of the DNA sequences reveal a different level of divergence between the groupD. melanogaster, D. simulans andD. mauritiana and the fourth speciesD. sechellia. The hypothesis of multiple transfers in the recent past into theD. melanogaster complex from a common outside source is discussed.     

Heterochromatic Recombination in Germ Cells of DROSOPHILA MELANOGASTER Females

Heterochromatic recombination in germ cells was found to occur in females of Drosophila melanogaster having a specific genotype. Results of the present study can be summarized as follows: (1) The frequency of heterochromatic recombination descreases consistently and markedly as the female ages. (2) The female that induces heterochromatic recombination is associated with reduced number of progeny when she is young, but as she gets older, the number of progeny increases, approaching that of the normal female. The reduction in the number of progeny is due to unhatchability of eggs produced, not to reduced egg laying. (3) Cytoplasmic factors affect the above two traits. These traits seem to be due to interaction between chromosomal and cytoplasmic elements. (4) These traits are not expressed in males. (5) The increase in recombination frequency seems to be limited to the centric heterochromatin.—It is suggested that heterochromatic recombination is one of the traits associated with the I-R system of hybrid dysgenesis in D. melanogaster.     

A cloned I-factor is fully functional in Drosophila melanogaster

Summary I-R hybrid dysgenesis in Drosophila melanogaster occurs in female progeny of crosses between reactive strain females and inducer strain males, and is controlled by transposable elements called I-factors. These are 5.3 kb elements that are structurally similar to mammalian LINE elements and other retroposons. We have tested the activity of an I-factor directly, by introducing it into the genome of a reactive strain, using P-element mediated transformation. It confers the complete inducer phenotype on the reactive strain, and can stimulate dysgenesis when transformed males are mated with reactive females. It has transposed in the transformed lines, and we have cloned one of the transposed copies. This is the first time that it has been possible to demonstrate that a particular retroposon is transposition proficient, and to compare donor and transposed elements. We propose a mechanism for I-factor transposition based on these results, and the coding capacity of these elements. We have been unable to detect either autonomous transposition of a complete I-factor from a plasmid injected into reactive strain embryos, or transposition of a marked I-factor when co-injected with a complete element.     

Molecular characteristics of the heterochromatic I elements from a reactive strain ofDrosophila melanogaster

Summary There are two categories of strains inDrosophila melanogaster with respect to the I-R system of hybrid dysgenesis. The inducer strains contain particular transposable elements named I factors. They are not present in the strains of the other category called reactive (R) strains. Defective I elements are present in the pericentromeric regions of both categories of strains. This last subfamily of I sequences has not yet been described in detail and little is known about its origin. In this paper, we report that the defective I elements display an average of 94% of sequence identity with each other and with the transposable I factor. The results suggest that they cannot be the progenitors of the present day I factors, but that each of these two subfamilies started to evolve independently several million years ago. Furthermore, the sequence comparison of these I elements with an active I factor fromDrosophila teissieri provides useful information about when the deleted I elements became immobilized.     

Domesticated P Elements in the Drosophila montium Species Subgroup Have a New Function Related to a DNA Binding Property

Molecular domestication of a transposable element is defined as its functional recruitment by the host genome. To date, two independent events of molecular domestication of the P transposable element have been described: in the Drosophila obscura species group and in the Drosophila montium species subgroup. These P neogenes consist of stationary, nonrepeated sequences, potentially encoding 66-kDa repressor-like (RL) proteins. Here we investigate the function of the montium P neogenes. We provide evidence for the presence of RL proteins in two montium species (D. tsacasi and D. bocqueti) specifically expressed in adult and larval brain and gonads. We tested the hypothesis that the montium P neogenes’ function is related to the repression of the transposition of distantly related mobile P elements which coexist in the genome. Our results strongly suggest that the montium P neogenes are not recruited to downregulate the P element transposition. Given that all the proteins encoded by mobile or stationary P homologous sequences show a strong conservation of the DNA binding domain, we tested the capacity of the RL proteins to bind DNA in vivo. Immunostaining of polytene chromosomes in D. melanogaster transgenic lines strongly suggests that montium P neogenes encode proteins that bind DNA in vivo. RL proteins show multiple binding to the chromosomes. We suggest that the property recruited in the case of the montium P neoproteins is their DNA binding property. The possible functions of these neogenes are discussed. [Reviewing Editor: Dr. Dmitri Petrov     

Stability of European natural populations of Drosophila melanogaster with regard to the P–M system: a buffer zone made up of Q populations

Current natural populations of Drosophila melanogaster from Eurasia, Africa and Oceania were investigated with regard to the P–M system of hybrid dysgenesis, for both genetic properties (gonadal dysgenesis sterility analyses) and molecular characteristics (number of full-size elements and particular P element deletion-derivatives, the KP elements). Full-size and KP elements are, respectively, at the origin of two distinct regulation systems, the maternally transmitted P cytotype and the KP-mediated repression whose transmission is biparental. The results show both qualitative and quantitative differences in the geographical distribution of P elements. Comparison with distributions observed in 1980–1983 reveals a great stability of natural populations with regard to this system. In particular, the eastward gradient of P susceptibility previously described in Europe is still observed. This stability could result from the existence of a ’buffer zone’ made up of the French and bordering Q populations (with no P activity and completely regulating the transposition of active P elements). Indeed, in such populations repression mechanisms are redundant, as revealed by the study of repression inheritance. These populations are thus potentially able to limit the progression of P elements that occurs by step by step migrations. This distribution also allows us to enrich the P element invasion model, which can be divided into three steps: (1) a decrease in the number of full-size elements which coincides with an increase in the number of KP elements due to a regulatory role or a high transposition capacity; (2) an equilibrium, when the number of KP elements reaches a maximum and in which populations still have some full-size elements; (3) KP elements reduce in number in the absence of full-size elements allowing transposition, the populations losing their repression potential.