Dominique Anxolabéhère

aubergine mutations in Drosophila melanogaster impair P cytotype determination by telomeric P elements inserted in heterochromatin.

Transposable P elements inserted in the heterochromatic Telomeric Associated Sequences on the X chromosome (1A site) of Drosophila melanogaster have a very strong capacity to elicit the P cytotype, a maternally transmitted condition which represses P element transposition and P-induced hybrid dysgenesis. This repressive capacity has previously been shown to be sensitive to mutant alleles of the gene Su(var)205, which encodes HP1 (Heterochromatin Protein 1), thus suggesting a role for chromatin structure in repression. Since an interaction between heterochromatin formation and RNA interference has been reported in various organisms, we tested the effect of mutant alleles of aubergine, a gene that has been shown to play a role in RNA interference in Drosophila, on the repressive properties of telomeric P elements. Seven out of the eight mutant alleles tested clearly impaired the repressive capacities of the two independent telomeric P insertions at 1A analyzed. P repression by P strains whose repressive capacities are not linked to the presence of P copies at 1A were previously found to be insensitive to Su(var)205; here, we show that they are also insensitive to aubergine mutations. These results strongly suggest that both RNA interference and heterochromatin structure are involved in the establishment of the P cytotype elicited by telomeric P elements, and reinforce the hypothesis that different mechanisms for repression of P elements exist which depend on the chromosomal location of the regulatory copies of P.

Telomeric transgenes and Trans-silencing in Drosophila

Autonomous P elements, inserted in heterochromatic telomeric associated sequences (TAS) at the X chromosome telomere (site 1A) have strong P element regulatory properties that include repression of P-induced hybrid-dysgenesis and of P-lacZ expression in the germline. P-lacZ insertions or defective P elements at 1A in TAS can also repress in trans a euchromatic P-lacZ in the germline. This property has been called a trans-silencing effect (TSE). It requires some sequence-homology between the telomeric insertion and the euchromatic transgene. When repression is partial, variegating lacZ expression is observed, suggesting a chromatin-based component. TSE is observed only when the silencer transgenes are maternally inherited and occurs only in the female germline. We have evidence that this silencing also works in the presence of homologous non-P element sequences suggesting that homology-dependent silencing could be a general phenomenon in the female germline; such a system might have been subsequently adopted by the P element family, allowing its own repression.

P elements and MITE relatives in the whole genome sequence of Anopheles gambiae

BackgroundMiniature Inverted-repeat Terminal Elements (MITEs), which are particular class-II transposable elements (TEs), play an important role in genome evolution, because they have very high copy numbers and display recurrent bursts of transposition. The 5 and 3 subterminal regions of a given MITE family often show a high sequence similarity with the corresponding regions of an autonomous Class-II TE family. However, the sustained presence over a prolonged evolutionary time of MITEs and TE master copies able to promote their mobility has been rarely reported within the same genome, and this raises fascinating evolutionary questions.ResultsWe report here the presence of P transposable elements with related MITE families in the Anopheles gambiae genome. Using a TE annotation pipeline we have identified and analyzed all the P sequences in the sequenced A. gambiae PEST strain genome. More than 0.49% of the genome consists of P elements and derivates. P elements can be divided into 9 different subfamilies, separated by more than 30% of nucleotide divergence. Seven of them present full length copies. Ten MITE families are associated with 6 out of the 9 P subfamilies. Comparing their intra-element nucleotide diversities and their structures allows us to propose the putative dynamics of their emergence. In particular, one MITE family which has a hybrid structure, with ends each of which is related to a different P-subfamily, suggests a new mechanism for their emergence and their mobility.ConclusionThis work contributes to a greater understanding of the relationship between full-length class-II TEs and MITEs, in this case P elements and their derivatives in the genome of A. gambiae. Moreover, it provides the most comprehensive catalogue to date of P- like transposons in this genome and provides convincing yet indirect evidence that some of the subfamilies have been recently active.

Telomeric trans-silencing in Drosophila melanogaster: tissue specificity, development and functional interactions between non-homologous telomeres.

Background The study of P element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, “TAS”) has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. TSE can show variegation in ovaries, displays a maternal effect as well as an epigenetic transmission through meiosis and involves heterochromatin and RNA silencing pathways. Principal Findings Here, we analyze phenotypic and genetic properties of TSE. We report that TSE does not occur in the soma at the adult stage, but appears restricted to the female germline. It is detectable during development at the third instar larvae where it presents the same tissue specificity and maternal effect as in adults. Transgenes located in TAS at the telomeres of the main chromosomes can be silencers which in each case show the maternal effect. Silencers located at non-homologous telomeres functionally interact since they stimulate each other via the maternally-transmitted component. All germinally-expressed euchromatic transgenes tested, located on all major chromosomes, were found to be repressed by a telomeric silencer: thus we detected no TSE escaper. The presence of the euchromatic target transgene is not necessary to establish the maternal inheritance of TSE, responsible for its epigenetic behavior. A single telomeric silencer locus can simultaneously repress two P-lacZ targets located on different chromosomal arms. Conclusions and Significance Therefore TSE appears to be a widespread phenomenon which can involve different telomeres and work across the genome. It can explain the P cytotype establishment by telomeric P elements in natural Drosophila populations.

P element regulation and X-chromosome subtelomeric heterochromatin in Drosophila melanogaster

In Drosophila melanogaster, crossing males carrying autonomous P elements with females devoid of P copies results in hybrid dysgenesis in the germline of progeny. The reciprocal cross produces non-dysgenic progeny due to a maternally inherited state non-permissive for P transposition. The capacity of a P copy to repress transposition depends on both its structure and its chromosomal location. Naturally occuring regulatory P elements inserted at the telomere of the X chromosome have been genetically isolated in a genomic context devoid of other P elements. One or two copies of autonomous P elements at this site (1A) are sufficient to elicit a strong P repression in the germline. These elements are flanked by Telomeric Associated Sequences, previously identified and described by Karpen and Spradling (1992) as having heterochromatic properties. The regulatory properties of P elements at 1A are strongly impaired by mutations affecting Su(var)205, which encodes Heterochromatin Protein 1, a non-histone heterochromatin protein. The regulatory properties of classical P strains are not sensitive to Su(var)205. Models based on chromatin structure or on nuclear localisation of the telomeres are discussed in order to explain both the strong regulatory properties of P elements at the X chromosome telomere and their sensitivity to Su(var)205.

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.

P element-encoded regulatory products enhance Repeat-Induced Gene Silencing (RIGS) of P-lacZ-white clusters in Drosophila melanogaster

Abstract. In Drosophila melanogaster, some clusters of P transgenes (P-lacZ-white) display a variegating phenotype for the white marker in the eye, a phenomenon termed Repeat-Induced Gene Silencing (RIGS). We have tested the influence of the P element repression state (P cytotype) on the eye phenotype of several P-lac-w clusters that differ in transgene copy number or genomic insertion site. P element-encoded regulatory products strongly enhance RIGS. The effect occurs in both sexes, is detectable with clusters having at least three copies and is observed at both genomic locations tested (cytogenetic regions 50C and 92E). Single variegating P-lac-w transgenes located in pericentromeric heterochromatin are not affected by P regulatory products. All P strain backgrounds tested enhance RIGS, including chromosomes bearing a single P element encoding a truncated P transposase or carrying a single internally deleted KP element. Therefore, clusters are highly sensitive to different types of P repressors. Finally, a chimeric gene in which the 5′ portion of the P element is fused to the polyhomeotic coding sequence (php1) also strongly enhances silencing of P-lac-w clusters. These results have implications for the mechanism of action of the P repressors and show that P transgene clusters represent a new class of P-sensitive alleles, providing a simple assay for somatic P repression that can be completed in one generation.