Impairment of cytotype regulation of P-element activity in Drosophila melanogaster by mutations in the Su(var)205 gene

Contribution of transposable elements to transgenerational effects of chronic radioactive exposure of natural populations of Drosophila melanogaster living for a long time in the zone of the Chernobyl nuclear disaster

The accident at the Chernobyl Nuclear Power Plant (ChNPP) led to the negative impact of chronic radioactive contamination on populations of organisms associated with the transgenerational transmission of genome instability. When the destabilization of genome, different genetic damages occur, the accumulation of which leads to the formation of mutations, morphological anomalies, and mortality in the offspring. The mechanisms underlying the manifestation of transgenerational events in the offspring of irradiated parents are not well understood. In this study, for the first time, the features of the influence of transposable elements (TEs) on the long-term biological consequences of the ChNPP are considered. In this work, specimens of D. melanogaster obtained from natural populations in 2007 in the areas of the ChNPP with heterogeneous radioactive contamination were studied. The descendants from these populations were maintained in laboratory (inbred) conditions for 160 generations. A stable transgenerational transmission of dominant lethal mutations (DLMs) to the offspring of all studied populations was shown. The DLM frequencies strongly were correlated with the level of survival of offspring. The mean frequencies of recessive sex-linked lethal mutations varied at the level of spontaneous point mutations. The simultaneous presence of P, hobo and I elements indicates that the studied populations do not have a definite cytotype, their phenotypic status is unstable. The behavior of TEs in the genomes of offspring depends not only on parental exposure, but also on origin of population, distance to the ChNPP, and inbred conditions. The obtained results confirm the hypothesis that TEs are involved in transgenerational transmission and accumulation of mutations by the offspring of irradiated parents. The TEs pattern present in the Chernobyl genomes of D. melanogaster is a peculiar of epigenetic mechanism for the regulation of plasticity and adaptation of populations living for many generations under conditions of a technogenically caused radiation background.

Radiobiological features in offspring of natural populations of Drosophila melanogaster after Chernobyl accident

In their natural habitats, populations of organisms are faced with different levels of chronic low-intensity radiation, causing a wide range of radiobiological effects (from radiosensitivity to radioadaptive response and hormesis). In this study, specimens of Drosophila melanogaster were selected from territories of the Chernobyl nuclear power plant with different levels of radioactive contamination. The isogenic stocks derived from these specimens represent the genetic systems of current populations and make it possible to study radioresistance and its mechanisms in future generations under controlled laboratory conditions. Previous studies have shown that transgenerational radiation effects at the level of lethal mutations and survival rate are unstable and depend not only on the level of chronic low-intensity irradiation, but also on other factors. A single acute irradiation exposure of offspring whose parents inhabited a site with a higher level of chronic irradiation made it possible to reveal pronounced radioresistant features in the offspring. And the offspring whose parents were exposed to radiation levels close to the natural radiation background, on the contrary, acquired radiosensitive features. Their response to acute exposure includes a high-frequency of lethal mutations and a short lifespan. The differential response to different levels of chronic parental exposure is caused by differences in the activities of certain transposons that destabilize the genome. Our data contribute to the understanding of genetic and epigenetic mechanisms (via transposon activity) of the effect of parental radiation exposure on the health and adaptive potential of populations affected by the technogenically increased radiation background.

Reexamining the P-Element Invasion of Drosophila melanogaster Through the Lens of piRNA Silencing

Transposable elements (TEs) are both important drivers of genome evolution and genetic parasites with potentially dramatic consequences for host fitness. The recent explosion of research on regulatory RNAs reveals that small RNA-mediated silencing is a conserved genetic mechanism through which hosts repress TE activity. The invasion of the Drosophila melanogaster genome by P elements, which happened on a historical timescale, represents an incomparable opportunity to understand how small RNA-mediated silencing of TEs evolves. Repression of P-element transposition emerged almost concurrently with its invasion. Recent studies suggest that this repression is implemented in part, and perhaps predominantly, by the Piwi-interacting RNA (piRNA) pathway, a small RNA-mediated silencing pathway that regulates TE activity in many metazoan germlines. In this review, I consider the P-element invasion from both a molecular and evolutionary genetic perspective, reconciling classic studies of P-element regulation with the new mechanistic framework provided by the piRNA pathway. I further explore the utility of the P-element invasion as an exemplar of the evolution of piRNA-mediated silencing. In light of the highly-conserved role for piRNAs in regulating TEs, discoveries from this system have taxonomically broad implications for the evolution of repression.

Conditional mutations in Drosophila melanogaster: On the occasion of the 150th anniversary of G. Mendel's report in Brünn

The basis for modern genetics was laid by Gregor Mendel. He proposed that traits belonging to the intraspecific variability class be studied. However, individuals of one species possess traits of another class. They are related to intraspecific similarity. Individuals never differ from each other in these traits. By analogy with traits varying within a species and determined by genes, it is conjectured that intraspecific similarity is determined by genes, too. If so, mutations in these genes can be obtained. This paper provides a review of works published in 2000-2014 that: (1) propose breeding methods for detection of mutations in Drosophila melanogaster genes that lead intraspecific similarity; these mutations were called conditional; (2) describe collections of conditional mutations in chromosomes X, 2, and 3 of Drosophila; (3) show unusual features of epigenetic nature in the mutants; and (4) analyze these features of the mutants. Based on the peculiarities of manifestation it is supposed that the recognized conditional mutations occur in genes responsible for intraspecific similarity. The genes presumably belong to the so-called regulatory network of the Drosophila genome. This approach expands the scope of breeding analysis introduced by G. Mendel for heredity studies 150 years ago.

P Transposable Elements in Drosophila and other Eukaryotic Organisms

P transposable elements were discovered in Drosophila as the causative agents of a syndrome of genetic traits called hybrid dysgenesis. Hybrid dysgenesis exhibits a unique pattern of maternal inheritance linked to the germline-specific small RNA piwi-interacting (piRNA) pathway. The use of P transposable elements as vectors for gene transfer and as genetic tools revolutionized the field of Drosophila molecular genetics. P element transposons have served as a useful model to investigate mechanisms of cut-and-paste transposition in eukaryotes. Biochemical studies have revealed new and unexpected insights into how eukaryotic DNA-based transposons are mobilized. For example, the P element transposase makes unusual 17nt-3' extended double-strand DNA breaks at the transposon termini and uses guanosine triphosphate (GTP) as a cofactor to promote synapsis of the two transposon ends early in the transposition pathway. The N-terminal DNA binding domain of the P element transposase, called a THAP domain, contains a C2CH zinc-coordinating motif and is the founding member of a large family of animal-specific site-specific DNA binding proteins. Over the past decade genome sequencing efforts have revealed the presence of P element-like transposable elements or P element transposase-like genes (called THAP9) in many eukaryotic genomes, including vertebrates, such as primates including humans, zebrafish and Xenopus, as well as the human parasite Trichomonas vaginalis, the sea squirt Ciona, sea urchin and hydra. Surprisingly, the human and zebrafish P element transposase-related THAP9 genes promote transposition of the Drosophila P element transposon DNA in human and Drosophila cells, indicating that the THAP9 genes encode active P element "transposase" proteins.

piRNA-mediated transposon regulation and the germ-line mutation rate in Drosophila melanogaster males

Transposons, especially retrotransposons, are abundant in the genome of Drosophila melanogaster. These mobile elements are regulated by small RNAs that interact with the Piwi family of proteins-the piwi-interacting or piRNAs. The Piwi proteins are encoded by the genes argonaute3 (ago3), aubergine (aub), and piwi. Heterochromatin Protein 1 (HP1), a chromatin-organizing protein encoded by the Suppressor of variegation 205 [Su(var)205] gene, also plays a role in this regulation. To assess the mutational impact of weakening the system for transposon regulation, we measured the frequency of recessive X-linked lethal mutations occurring in the germ lines of males from stocks that were heterozygous for mutant alleles of the ago3, aub, piwi, or Su(var)205 genes. These mutant alleles are expected to deplete the wild-type proteins encoded by these genes by as much as 50%. The mutant alleles of piwi and Su(var)205 significantly increased the X-linked lethal mutation frequency, whereas the mutant alleles of ago3 did not. An increased mutation frequency was also observed in males from one of two mutant aub stocks, but this increase may not have been due to the aub mutant. The increased mutation frequency caused by depleting Piwi or HP1suggests that chromatin-organizing proteins play important roles in minimizing the germ-line mutation rate, possibly by stabilizing the structure of the heterochromatin in which many transposons are situated.

Genetic interactions between P elements involved in piRNA-mediated repression of hybrid dysgenesis in Drosophila melanogaster

Previous studies have shown that telomeric P elements inserted at the left end of the X chromosome are anchors of the P cytotype, the maternally inherited state that regulates P-element activity in the germ line of Drosophila melanogaster. This regulation is mediated by small RNAs that associate with the Piwi family of proteins (piRNAs). We extend the analysis of cytotype regulation by studying new combinations of telomeric and nontelomeric P elements (TPs and non-TPs). TPs interact with each other to enhance cytotype regulation. This synergism involves a strictly maternal effect, called presetting, which is apparently mediated by piRNAs transmitted through the egg. Presetting by a maternal TP can elicit regulation by an inactive paternally inherited TP, possibly by stimulating its production of primary piRNAs. When one TP has come from a stock heterozygous for a mutation in the aubergine, piwi, or Suppressor of variegation 205 genes, the synergism between two TPs is impaired. TPs also interact with non-TPs to enhance cytotype regulation, even though the non-TPs lack regulatory ability on their own. Non-TPs are not susceptible to presetting by a TP, nor is a TP susceptible to presetting by a non-TP. The synergism between TPs and non-TPs is stronger when the TP was inherited maternally. This synergism may be due to the accumulation of secondary piRNAs created by ping-pong cycling between primary piRNAs from the TPs and mRNAs from the non-TPs. Maternal transmission of P-element piRNAs plays an important role in the maintenance of strong cytotype regulation over generations.

Maternal impairment of transposon regulation in Drosophila melanogaster by mutations in the genes aubergine, piwi and Suppressor of variegation 205

TP5, a P element inserted in the telomere-associated sequences of the X chromosome, represses the excision of other P elements in the germ line through a combination of maternal and zygotic effects. The maternal component of this repression is impaired by heterozygous mutations in the aubergine and Suppressor of variegation 205 genes; one mutation in the piwi gene also appears to impair repression. In the female germ line, the level of TP5 mRNA is increased by these impairing mutations. The impairing aubergine and piwi mutations also increase the level of germ-line mRNA from CP, a transgene that encodes the P-element transposase; however, the Suppressor of variegation 205 mutation does not. These findings are discussed in terms of a model of P-element regulation that involves post-transcriptional and chromatin re-organizing events mediated by maternally transmitted small RNAs derived from the telomeric P element.

The epigenetic trans-silencing effect in Drosophila involves maternally-transmitted small RNAs whose production depends on the piRNA pathway and HP1

Background

The study of P transposable element repression in Drosophila melanogaster led to the discovery of the 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. Phenotypic and genetic analysis have shown that TSE exhibits variegation in ovaries, displays a maternal effect as well as epigenetic transmission through meiosis and involves heterochromatin (including HP1) and RNA silencing.

Principal Findings

Here, we show that mutations in squash and zucchini, which are involved in the piwi-interacting RNA (piRNA) silencing pathway, strongly affect TSE. In addition, we carried out a molecular analysis of TSE and show that silencing is correlated to the accumulation of lacZ small RNAs in ovaries. Finally, we show that the production of these small RNAs is sensitive to mutations affecting squash and zucchini, as well as to the dose of HP1.

Conclusions and Significance

Thus, our results indicate that the TSE represents a bona fide piRNA-based repression. In addition, the sensitivity of TSE to HP1 dose suggests that in Drosophila, as previously shown in Schizosaccharomyces pombe, a RNA silencing pathway can depend on heterochromatin components.

Cytotype regulation inDrosophila melanogaster: synergism between telomeric and non-telomericPelements

The X-linked telomericPelementsTP5andTP6interact synergistically with non-telomericPelements to repress hybrid dysgenesis. In this repression, the telomericPelements exert maternal effects, which, however, are not sufficient to establish synergism with the non-telomericPelements. Once synergism is established, the capacity to repress dysgenesis in the offspring of a cross persists for at least two generations after removing the telomericPelement from the genotype. At the molecular level, synergism between telomeric and non-telomericPelements is correlated with effective elimination ofP-element mRNA in the germ line. Maternally transmitted mutations in the genesaubergine,piwiandSuppressor of variegation 205[Su(var)205] block the establishment of synergism between telomeric and non-telomericPelements, and paternally transmitted mutations inpiwiandSu(var)205disrupt synergism that has already been established. These findings are discussed in terms of a model of cytotype regulation ofPelements based on Piwi-interacting RNAs (piRNAs) that are amplified by cycling between sense and antisense species.

Ecological selection maintains cytonuclear incompatibilities in hybridizing sunflowers

Despite the recent renaissance in studies of ecological speciation, the connection between ecological selection and the evolution of reproductive isolation remains tenuous. We tested whether habitat adaptation of cytoplasmic genomes contributes to the maintenance of reproductive barriers in hybridizing sunflower species, Helianthus annuus and Helianthus petiolaris. We transplanted genotypes of the parental species, reciprocal F1 hybrids and all eight possible backcross combinations of nuclear and cytoplasmic genomes into the contrasting xeric and mesic habitats of the parental species. Analysis of survivorship across two growing seasons revealed that the parental species' cytoplasms were strongly locally adapted and that cytonuclear interactions (CNIs) significantly affected the fitness and architecture of hybrid plants. A significant fraction of the CNIs have transgenerational effects, perhaps due to divergence in imprinting patterns. Our results suggest a common means by which ecological selection may contribute to speciation and have significant implications for the persistence of hybridizing species.

Genetics and biochemistry of RNAi in Drosophila

RNA interference (RNAi) is the technique employing double-stranded RNA to target the destruction of homologous messenger RNAs. It has gained wide usage in genetics. While having the potential for many practical applications, it is a reflection of a much broader spectrum of small RNA-mediated processes in the cell. The RNAi machinery was originally perceived as a defense mechanism against viruses and transposons. While this is certainly true, small RNAs have now been implicated in many other aspects of cell biology. Here we review the current knowledge of the biochemistry of RNAi in Drosophila and the involvement of small RNAs in RNAi, transposon silencing, virus defense, transgene silencing, pairing-sensitive silencing, telomere function, chromatin insulator activity, nucleolar stability, and heterochromatin formation. The discovery of the role of RNA molecules in the degradation of mRNA transcripts leading to decreased gene expression resulted in a paradigm shift in the field of molecular biology. Transgene silencing was first discovered in plant cells (Matzke et al. 1989; van der Krol et al. 1990; Napoli et al. 1990) and can occur on both the transcriptional and posttranscriptional levels, but both involve short RNA moieties in their mechanism. RNA interference (RNAi) is a type of gene silencing mechanism in which a double-stranded RNA (dsRNA) molecule directs the specific degradation of the corresponding mRNA (target RNA). The technique of RNAi was first discovered in Caenorhabditis elegans in 1994 (Guo and Kemphues 1994). Later the active component was found to be a dsRNA (Fire et al. 1998). In subsequent years, it has been found to occur in diverse eukaryotes

Cytotype regulation by telomeric P elements in Drosophila melanogaster: evidence for involvement of an RNA interference gene

P elements inserted at the left telomere of the X chromosome evoke the P cytotype, a maternally inherited condition that regulates the P-element family in the Drosophila germline. This regulation is completely disrupted in stocks heterozygous for mutations in aubergine, a gene whose protein product is involved in RNA interference. However, cytotype is not disrupted in stocks heterozygous for mutations in two other RNAi genes, piwi and homeless (spindle-E), or in a stock heterozygous for a mutation in the chromatin protein gene Enhancer of zeste. aubergine mutations exert their effects in the female germline, where the P cytotype is normally established and through which it is maintained. These effects are transmitted maternally to offspring of both sexes independently of the mutations themselves. Lines derived from mutant aubergine stocks reestablish the P cytotype quickly, unlike lines derived from stocks heterozygous for a mutation in Suppressor of variegation 205, the gene that encodes the telomere-capping protein HP1. Cytotype regulation by telomeric P elements may be tied to a system that uses RNAi to regulate the activities of telomeric retrotransposons in Drosophila.