Genetic variation of copia suppression in Drosophila melanogaster

Transposable element landscapes in aging Drosophila

Genetic mechanisms that repress transposable elements (TEs) in young animals decline during aging, as reflected by increased TE expression in aged animals. Does increased TE expression during aging lead to more genomic TE copies in older animals? To address this question, we quantified TE Landscapes (TLs) via whole genome sequencing of young and aged Drosophila strains of wild-type and mutant backgrounds. We quantified TLs in whole flies and dissected brains and validated the feasibility of our approach in detecting new TE insertions in aging Drosophila genomes when small RNA and RNA interference (RNAi) pathways are compromised. We also describe improved sequencing methods to quantify extra-chromosomal DNA circles (eccDNAs) in Drosophila as an additional source of TE copies that accumulate during aging. Lastly, to combat the natural progression of aging-associated TE expression, we show that knocking down PAF1, a conserved transcription elongation factor that antagonizes RNAi pathways, may bolster suppression of TEs during aging and extend lifespan. Our study suggests that in addition to a possible influence by different genetic backgrounds, small RNA and RNAi mechanisms may mitigate genomic TL expansion despite the increase in TE transcripts during aging.

Transposable elements, also called transposons, are genetic parasites found in all animal genomes. Normally, transposons are compacted away in silent chromatin in young animals. But, as animals age and transposon-silencing defense mechanisms break down, transposon RNAs accumulate to significant levels in old animals like fruit flies. An open question is whether the increased levels of transposon RNAs in older animals also correspond to increased genomic copies of transposons. This study approached this question by sequencing the whole genomes of young and old wild-type and mutant flies lacking a functional RNA interference (RNAi) pathway, which naturally silences transposon RNAs. Although the wild-type flies with intact RNAi activity had little new accumulation of transposon copies, the sequencing approach was able to detect several transposon accumulation occurrences in some RNAi mutants. In addition, we found that some fly transposon families can also accumulate as extra-chromosomal circular DNA copies. Lastly, we showed that genetically augmenting the expression of RNAi factors can counteract the rising transposon RNA levels in aging and promote longevity. This study improves our understanding of the animal host genome relationship with transposons during natural aging processes.

Identification of a novel retrotransposon with sex chromosome-specific distribution in Silene latifolia

Silene latifolia is a dioecious plant species with chromosomal sex determination. Although the evolution of sex chromosomes in S. latifolia has been the subject of numerous studies, a global view of X chromosome structure in this species is still missing. Here, we combine X chromosome microdissection and BAC library screening to isolate new X chromosome-linked sequences. Out of 8 identified BAC clones, only BAC 86M14 showed an X-preferential signal after FISH experiments. Further analysis revealed the existence of the Athila retroelement which is enriched in the X chromosome and nearly absent in the Y chromosome. Based on previous data, the Athila retroelement belongs to the CL3 group of most repetitive sequences in the S. latifolia genome. Structural, transcriptomics and phylogenetic analyses revealed that Athila CL3 represents an old clade in the Athila lineage. We propose a mechanism responsible for Athila CL3 distribution in the S. latifolia genome.

Possible mechanisms responsible for absence of a retrotransposon family on a plant Y chromosome

Some transposable elements (TEs) show extraordinary variance in abundance along sex chromosomes but the mechanisms responsible for this variance are unknown. Here, we studied Ogre long terminal repeat (LTR) retrotransposons in Silene latifolia, a dioecious plant with evolutionarily young heteromorphic sex chromosomes. Ogre elements are ubiquitous in the S. latifolia genome but surprisingly absent on the Y chromosome. Bacterial artificial chromosome (BAC) library analysis and fluorescence in situ hybridization (FISH) were used to determine Ogre structure and chromosomal localization. Next generation sequencing (NGS) data were analysed to assess the transcription level and abundance of small RNAs. Methylation of Ogres was determined by bisulphite sequencing. Phylogenetic analysis was used to determine mobilization time and selection forces acting on Ogre elements. We characterized three Ogre families ubiquitous in the S. latifolia genome. One family is nearly absent on the Y chromosome despite all the families having similar structures and spreading mechanisms. We showed that Ogre retrotransposons evolved before sex chromosomes appeared but were mobilized after formation of the Y chromosome. Our data suggest that the absence of one Ogre family on the Y chromosome may be caused by 24-nucleotide (24-nt) small RNA-mediated silencing leading to female-specific spreading. Our findings highlight epigenetic silencing mechanisms as potentially crucial factors in sex-specific spreading of some TEs, but other possible mechanisms are also discussed.

A genome-wide survey of genetic instability by transposition in Drosophila hybrids

Hybridization between species is a genomic instability factor involved in increasing mutation rate and new chromosomal rearrangements. Evidence of a relationship between interspecific hybridization and transposable element mobilization has been reported in different organisms, but most studies are usually performed with particular TEs and do not discuss the real effect of hybridization on the whole genome. We have therefore studied whole genome instability of Drosophila interspecific hybrids, looking for the presence of new AFLP markers in hybrids. A high percentage (27–90%) of the instability markers detected corresponds to TEs belonging to classes I and II. Moreover, three transposable elements (Osvaldo, Helena and Galileo) representative of different families, showed an overall increase of transposition rate in hybrids compared to parental species. This research confirms the hypothesis that hybridization induces genomic instability by transposition bursts and suggests that genomic stress by transposition could contribute to a relaxation of mechanisms controlling TEs in the Drosophila genome.

Drosophila interspecific hybrids phenocopy piRNA-pathway mutants

Hybrids of two Drosophila species show transposable element derepression and piRNA pathway malfunction, revealing adaptive evolution of piRNA pathway components.

The Piwi-interacting RNA (piRNA) pathway defends the germline of animals from the deleterious activity of selfish transposable elements (TEs) through small-RNA mediated silencing. Adaptation to novel invasive TEs is proposed to occur by incorporating their sequences into the piRNA pool that females produce and deposit into their eggs, which then propagates immunity against specific TEs to future generations. In support of this model, the F1 offspring of crosses between strains of the same Drosophila species sometimes suffer from germline derepression of paternally inherited TE families, caused by a failure of the maternal strain to produce the piRNAs necessary for their regulation. However, many protein components of the Drosophila piRNA pathway exhibit signatures of positive selection, suggesting that they also contribute to the evolution of host genome defense. Here we investigate piRNA pathway function and TE regulation in the F1 hybrids of interspecific crosses between D. melanogaster and D. simulans and compare them with intraspecific control crosses of D. melanogaster. We confirm previous reports showing that intraspecific crosses are characterized by derepression of paternally inherited TE families that are rare or absent from the maternal genome and piRNA pool, consistent with the role of maternally deposited piRNAs in shaping TE silencing. In contrast to the intraspecific cross, we discover that interspecific hybrids are characterized by widespread derepression of both maternally and paternally inherited TE families. Furthermore, the pattern of derepression of TE families in interspecific hybrids cannot be attributed to their paucity or absence from the piRNA pool of the maternal species. Rather, we demonstrate that interspecific hybrids closely resemble piRNA effector-protein mutants in both TE misregulation and aberrant piRNA production. We suggest that TE derepression in interspecific hybrids largely reflects adaptive divergence of piRNA pathway genes rather than species-specific differences in TE-derived piRNAs.

Eukaryotic genomes contain large quantities of transposable elements (TEs), short self-replicating DNA sequences that can move within the genome. The selfish replication of TEs has potentially drastic consequences for the host, such as disruption of gene function, induction of sterility, and initiation or exacerbation of some cancers. Like the adaptive immune system that defends our bodies against pathogens, the Piwi-interacting RNA (piRNA) pathway defends animal genomes against the harmful effects of TEs. Fundamental to piRNA-mediated defense is the production of small noncoding RNAs that act like antibodies to target replicating TEs for destruction by piRNA-effector proteins. piRNAs are expected to diverge rapidly between species in response to genome infection by increasingly disparate TEs. Here, we tested this hypothesis by examining how differences in piRNAs between two species of fruit fly relate to TE “immunity” in their hybrid offspring. Because piRNAs are maternally deposited, we expected excessive replication of paternal TEs in hybrids. Surprisingly, we observe increased activity of both maternal and paternal TEs, together with defects in piRNA production that are reminiscent of piRNA effector-protein mutants. Our observations reveal that piRNA effector-proteins do not function properly in hybrids, and we propose that adaptive evolution among piRNA effector-proteins contributes to host genome defense and leads to the functional incompatibilities that we observe in hybrids.

What makes transposable elements move in the Drosophila genome?

Transposable elements (TEs), by their capacity of moving and inducing mutations in the genome, are considered important drivers of species evolution. The successful invasions of TEs in genomes, despite their mutational properties, are an apparent paradox. TEs' transposition is usually strongly regulated to low value, but in some cases these elements can also show high transposition rates, which has been associated sometimes to changes in environmental conditions. It is evident that factors susceptible to induce transpositions in natural populations contribute to TE perpetuation. Different factors were proposed as causative agents of TE mobilization in a wide range of organisms: biotic and abiotic stresses, inter- and intraspecific crosses and populational factors. However, there is no clear evidence of the factors capable of inducing TE mobilization in Drosophila, and data on laboratory stocks show contradictory results. The aim of this review is to have an update critical revision about mechanisms promoting transposition of TEs in Drosophila, and to provide to the readers a global vision of the dynamics of these genomic elements in the Drosophila genome.

Co-evolution between transposable elements and their hosts: a major factor in genome size evolution?

Most models of genome size evolution emphasize changes in relative rates of and/or the efficacy of selection on insertions and deletions. However, transposable elements (TEs) are a major contributor to genome size evolution, and since they experience their own selective pressures for expansion, genome size changes may in part be driven by the dynamics of co-evolution between TEs and their hosts. Under this perspective, predictions about the conditions that allow for genome expansion may be altered. In this review, we outline the evidence for TE-host co-evolution, discuss the conditions under which these dynamics can change, and explore the possible contribution to the evolution of genome size. Aided partly by advances in our understanding of the mechanisms of TE silencing via small RNAs, there is growing evidence that the evolution of transposition rates can be important in driving genome expansion and contraction. Shifts in genome size and transposon abundance associated with interspecific hybridization and changes in mating system are consistent with an important role for transposition rate evolution, although other possible explanations persist. More understanding of the potential for the breakdown of host silencing mechanisms and/or the potential for TEs to evade host immune responses will improve our understanding of the importance of changes in TE activity in driving genome size evolution.