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1. Off-target piRNA gene silencing in Drosophila melanogaster rescued by a transposable element insertion

   https://doi.org/10.1371/journal.pgen.1010598  

   Miller, Danny E.; Dorador, Ana P.; Van Vaerenberghe, Kelley; Li, Angela; Grantham, Emily K.; Cerbin, Stefan; Cummings, Celeste; Barragan, Marilyn; Egidy, Rhonda R.; Scott, Allison R.; et al (February 2023, PLOS Genetics)

   Bosco, Giovanni (Ed.)

   Transposable elements (TE) are selfish genetic elements that can cause harmful mutations. In Drosophila , it has been estimated that half of all spontaneous visible marker phenotypes are mutations caused by TE insertions. Several factors likely limit the accumulation of exponentially amplifying TEs within genomes. First, synergistic interactions between TEs that amplify their harm with increasing copy number are proposed to limit TE copy number. However, the nature of this synergy is poorly understood. Second, because of the harm posed by TEs, eukaryotes have evolved systems of small RNA-based genome defense to limit transposition. However, as in all immune systems, there is a cost of autoimmunity and small RNA-based systems that silence TEs can inadvertently silence genes flanking TE insertions. In a screen for essential meiotic genes in Drosophila melanogaster , a truncated Doc retrotransposon within a neighboring gene was found to trigger the germline silencing of ald , the Drosophila Mps1 homolog, a gene essential for proper chromosome segregation in meiosis. A subsequent screen for suppressors of this silencing identified a new insertion of a Hobo DNA transposon in the same neighboring gene. Here we describe how the original Doc insertion triggers flanking piRNA biogenesis and local gene silencing. We show that this local gene silencing occurs in cis and is dependent on deadlock , a component of the Rhino-Deadlock-Cutoff (RDC) complex, to trigger dual-strand piRNA biogenesis at TE insertions. We further show how the additional Hobo insertion leads to de-silencing by reducing flanking piRNA biogenesis triggered by the original Doc insertion. These results support a model of TE-mediated gene silencing by piRNA biogenesis in cis that depends on local determinants of transcription. This may explain complex patterns of off-target gene silencing triggered by TEs within populations and in the laboratory. It also provides a mechanism of sign epistasis among TE insertions, illuminates the complex nature of their interactions and supports a model in which off-target gene silencing shapes the evolution of the RDC complex. 

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2. Controlling for Variable Transposition Rate with an Age-Adjusted Site Frequency Spectrum

   https://doi.org/10.1093/gbe/evac016  

   Horvath, Robert; Menon, Mitra; Stitzer, Michelle; Ross-Ibarra, Jeffrey (February 2022, Genome Biology and Evolution)

   Betran, Esther (Ed.)

   Abstract Recognition of the important role of transposable elements (TEs) in eukaryotic genomes quickly led to a burgeoning literature modeling and estimating the effects of selection on TEs. Much of the empirical work on selection has focused on analyzing the site frequency spectrum (SFS) of TEs. But TE evolution differs from standard models in a number of ways that can impact the power and interpretation of the SFS. For example, rather than mutating under a clock-like model, transposition often occurs in bursts which can inflate particular frequency categories compared with expectations under a standard neutral model. If a TE burst has been recent, the excess of low-frequency polymorphisms can mimic the effect of purifying selection. Here, we investigate how transposition bursts affect the frequency distribution of TEs and the correlation between age and allele frequency. Using information on the TE age distribution, we propose an age-adjusted SFS to compare TEs and neutral polymorphisms to more effectively evaluate whether TEs are under selective constraints. We show that our approach can minimize instances of false inference of selective constraint, remains robust to simple demographic changes, and allows for a correct identification of even weak selection affecting TEs which experienced a transposition burst. The results presented here will help researchers working on TEs to more reliably identify the effects of selection on TEs without having to rely on the assumption of a constant transposition rate. 

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3. The impact of insertion bias into piRNA clusters on the invasion of transposable elements

   https://doi.org/10.1186/s12915-025-02370-0  

   Pritam, Shashank; Scarpa, Almorò; Kofler, Robert; Signor, Sarah (December 2025, BMC Biology)

   Abstract BackgroundIn our current understanding of transposable element (TE) invasions, TEs move freely until they accidentally insert into a piRNA cluster, where they are silenced by the production of piRNA cognate to the TE. Under this model, one would expect that selection might favor TEs that avoid piRNA clusters. However, empirical observations show that some TEs, such as theP-element, insert into piRNA clusters preferentially. We were thus wondering if such a bias, by minimizing harm to the host, could facilitate the spread of TEs throughout a population. ResultsWe performed extensive forward simulations of TE invasions with different insertion biases into piRNA clusters to determine if there was ever a situation in which the insertion bias was beneficial to the TE. We found that insertion bias significantly altered the invasion dynamics of TEs, primarily by changing the number of TE copies in individuals before silencing. Insertion into a piRNA cluster reduced the deleterious effects of TEs to the host population, but we found that TEs avoiding piRNA clusters out-compete TEs with a bias toward cluster insertions. Insertion bias was only beneficial to the TE when there was negative selection against TEs and a lack of recombination. ConclusionsDifferent TEs show different insertion biases into piRNA clusters suggesting they are an attribute of the TE not the host, yet scenarios in which this is beneficial for TE propagation are quite limited. This opens up an interesting area for future research into the dynamics of insertion bias during TE invasions. 

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4. Differential Conservation and Loss of Chicken Repeat 1 (CR1) Retrotransposons in Squamates Reveal Lineage-Specific Genome Dynamics Across Reptiles

   https://doi.org/10.1093/gbe/evae157  

   Gable, Simone M; Bushroe, Nicholas A; Mendez, Jasmine M; Wilson, Adam; Pinto, Brendan J; Gamble, Tony; Tollis, Marc (July 2024, Genome Biology and Evolution)

   Mueller, Rachel (Ed.)

   Abstract Transposable elements (TEs) are repetitive DNA sequences which create mutations and generate genetic diversity across the tree of life. In amniote vertebrates, TEs have been mainly studied in mammals and birds, whose genomes generally display low TE diversity. Squamates (Order Squamata; including ∼11,000 extant species of lizards and snakes) show as much variation in TE abundance and activity as they do in species and phenotypes. Despite this high TE activity, squamate genomes are remarkably uniform in size. We hypothesize that novel, lineage-specific genome dynamics have evolved over the course of squamate evolution. To understand the interplay between TEs and host genomes, we analyzed the evolutionary history of the chicken repeat 1 (CR1) retrotransposon, a TE family found in most tetrapod genomes which is the dominant TE in most reptiles. We compared 113 squamate genomes to the genomes of turtles, crocodilians, and birds and used ancestral state reconstruction to identify shifts in the rate of CR1 copy number evolution across reptiles. We analyzed the repeat landscapes of CR1 in squamate genomes and determined that shifts in the rate of CR1 copy number evolution are associated with lineage-specific variation in CR1 activity. We then used phylogenetic reconstruction of CR1 subfamilies across amniotes to reveal both recent and ancient CR1 subclades across the squamate tree of life. The patterns of CR1 evolution in squamates contrast other amniotes, suggesting key differences in how TEs interact with different host genomes and at different points across evolutionary history. 

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5. Transposable elements employ distinct integration strategies with respect to transcriptional landscapes in eukaryotic genomes

   https://doi.org/10.1093/nar/gkaa370  

   Zhang, Xinyan; Zhao, Meixia; McCarty, Donald R; Lisch, Damon (May 2020, Nucleic Acids Research)

   null (Ed.)

   Abstract Transposable elements (TEs) are ubiquitous DNA segments capable of moving from one site to another within host genomes. The extant distributions of TEs in eukaryotic genomes have been shaped by both bona fide TE integration preferences in eukaryotic genomes and by selection following integration. Here, we compare TE target site distribution in host genomes using multiple de novo transposon insertion datasets in both plants and animals and compare them in the context of genome-wide transcriptional landscapes. We showcase two distinct types of transcription-associated TE targeting strategies that suggest a process of convergent evolution among eukaryotic TE families. The integration of two precision-targeting elements are specifically associated with initiation of RNA Polymerase II transcription of highly expressed genes, suggesting the existence of novel mechanisms of precision TE targeting in addition to passive targeting of open chromatin. We also highlight two features that can facilitate TE survival and rapid proliferation: tissue-specific transposition and minimization of negative impacts on nearby gene function due to precision targeting. 

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