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The widely accepted hypothesis postulates that the first spliceosomal introns originated from group II self-splicing introns. However, it is evident that not all spliceosomal introns in the nuclear genes of modern eukaryotes are inherited through vertical transfer of intronic sequences. Several phenomena contribute to the formation of new introns but their most common origin seems to be the insertion of transposable elements. Recent analyses have highlighted instances of mass gains of new introns from transposable elements. These events often coincide with an increase or change in the spliceosome's tolerance to splicing signals, including the acceptance of noncanonical borders. Widespread acquisitions of transposon-derived introns occur across diverse evolutionary lineages, indicating convergent processes. These events, though independent, likely require a similar set of conditions. These conditions include the presence of transposon elements with features enabling their removal at the RNA level as introns and/or the existence of a splicing mechanism capable of excising unusual sequences that would otherwise not be recognized as introns by standard splicing machinery. Herein we summarize those mechanisms across different eukaryotic lineages. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "Transposon-derived introns as an element shaping the structure of eukaryotic genomes" published in the journal Mobile DNA. The correction addresses an error in Figure 2 and its legend, clarifying that the intron insertion shown is a hypothetical mechanism that has not been experimentally proven. The corrected figure provides a hypothesized mechanism of intron gain assisted by target site duplication. The authors state that the actual mechanisms of introner insertion are not known. [Extracted from the article]

Repetitive DNA make up a considerable fraction of most eukaryotic genomes. In fish, transposable element (TE) activity has coincided with rapid species diversification. Here, we annotated the repetitive content in 100 genome assemblies, covering the major branches of the diverse lineage of teleost fish. We investigated if TE content correlates with family level net diversification rates and found support for a weak negative correlation. Further, we demonstrated that TE proportion correlates with genome size, but not to the proportion of short tandem repeats (STRs), which implies independent evolutionary paths. Marine and freshwater fish had large differences in STR content, with the most extreme propagation detected in the genomes of codfish species and Atlantic herring. Such a high density of STRs is likely to increase the mutational load, which we propose could be counterbalanced by high fecundity as seen in codfishes and herring. [ABSTRACT FROM AUTHOR]

Plant, animal and protist genomes often contain endogenous viral elements (EVEs), which correspond to partial and sometimes entire viral genomes that have been captured in the genome of their host organism through a variety of integration mechanisms. While the number of sequenced eukaryotic genomes is rapidly increasing, the annotation and characterization of EVEs remains largely overlooked. EVEs that derive from members of the family Caulimoviridae are widespread across tracheophyte plants, and sometimes they occur in very high copy numbers. However, existing programs for annotating repetitive DNA elements in plant genomes are poor at identifying and then classifying these EVEs. Other than accurately annotating plant genomes, there is intrinsic value in a tool that could identify caulimovirid EVEs as they testify to recent or ancient host-virus interactions and provide valuable insights into virus evolution. In response to this research need, we have developed CAULIFINDER, an automated and sensitive annotation software package. CAULIFINDER consists of two complementary workflows, one to reconstruct, annotate and group caulimovirid EVEs in a given plant genome and the second to classify these genetic elements into officially recognized or tentative genera in the Caulimoviridae. We have benchmarked the CAULIFINDER package using the Vitis vinifera reference genome, which contains a rich assortment of caulimovirid EVEs that have previously been characterized using manual methods. The CAULIFINDER package is distributed in the form of a Docker image. [ABSTRACT FROM AUTHOR]

Background: DNA transposons are ubiquitous components of eukaryotic genomes. A major group of them encode a DDD/E transposase and contain terminal inverted repeats (TIRs) of varying lengths. The Kolobok superfamily of DNA transposons has been found in a wide spectrum of organisms. Results: Here we report a new Kolobok lineage, designated KolobokP. They were identified in 7 animal phyla (Mollusca, Phoronida, Annelida, Nemertea, Bryozoa, Chordata, and Echinodermata), and are especially rich in bivalves. Unlike other Kolobok families, KolobokP adopts a composite-like architecture: an internal region (INT) flanked by two long terminal direct repeats (LTDRs), which exhibit their own short terminal inverted repeats ranging up to 18 bps. The excision of LTDRs was strongly suggested. The LTDR lengths seem to be constrained to be either around 450-bp or around 660-bp. The internal region encodes a DDD/E transposase and a small His-Me finger nuclease, which likely originated from the homing endonuclease encoded by a group I intron from a eukaryotic species. The architecture of KolobokP resembles composite DNA transposons, usually observed in bacterial genomes, and long terminal repeat (LTR) retrotransposons. In addition to this monomeric LTDR-INT-LTDR structure, plenty of solo LTDRs and multimers represented as (LTDR-INT)n-LTDR are also observed. Our structural and phylogenetic analysis supported the birth of KolobokP in the late stage of the Kolobok evolution. We propose KolobokP families propagate themselves in two ways: the canonical transposition catalyzed by their transposase and the sequence-specific cleavage by their endonuclease followed by the multimerization through the unequal crossover. Conclusions: The presence of homing endonuclease and long terminal direct repeats of KolobokP families suggest their unique dual replication mechanisms: transposition and induced unequal crossover. [ABSTRACT FROM AUTHOR]

Transposable elements (TEs) are major genomic components in most eukaryotic genomes and play an important role in genome evolution. However, despite their relevance the identification of TEs is not an easy task and a number of tools were developed to tackle this problem. To better understand how they perform, we tested several widely used tools for de novo TE detection and compared their performance on both simulated data and well curated genomic sequences. As expected, tools that build TE-models performed better than k-mer counting ones, with RepeatModeler beating competitors in most datasets. However, there is a tendency for most tools to identify TE-regions in a fragmented manner and it is also frequent that small TEs or fragmented TEs are not detected. Consequently, the identification of TEs is still a challenging endeavor and it requires a significant manual curation by an experienced expert. The results will be helpful for identifying common issues associated with TE-annotation and for evaluating how comparable are the results obtained with different tools. [ABSTRACT FROM AUTHOR]

Transposable elements (TEs) play powerful and varied evolutionary and functional roles, and are widespread in most eukaryotic genomes. Research into their unique biology has driven the creation of a large collection of databases, software, classification systems, and annotation guidelines. The diversity of available TE-related methods and resources raises compatibility concerns and can be overwhelming to researchers and communicators seeking straightforward guidance or materials. To address these challenges, we have initiated a new resource, TE Hub, that provides a space where members of the TE community can collaborate to document and create resources and methods. The space consists of (1) a website organized with an open wiki framework, https://tehub.org, (2) a conversation framework via a Twitter account and a Slack channel, and (3) bi-monthly Hub Update video chats on the platform's development. In addition to serving as a centralized repository and communication platform, TE Hub lays the foundation for improved integration, standardization, and effectiveness of diverse tools and protocols. We invite the TE community, both novices and experts in TE identification and analysis, to join us in expanding our community-oriented resource. [ABSTRACT FROM AUTHOR]

Background: Retroelements (REs) occupy a significant part of all eukaryotic genomes including humans. The majority of retroelements in the human genome are inactive and unable to retrotranspose. Dozens of active copies are repressed in most normal tissues by various cellular mechanisms. These copies can become active in normal germline and brain tissues or in cancer, leading to new retroposition events. The consequences of such events and their role in normal cell functioning and carcinogenesis are not yet fully understood. If new insertions occur in a small portion of cells they can be found only with the use of specific methods based on RE enrichment and high-throughput sequencing. The downside of the high sensitivity of such methods is the presence of various artifacts imitating real insertions, which in many cases cannot be validated due to lack of the initial template DNA. For this reason, adequate assessment of rare (< 1%) subclonal cancer specific RE insertions is complicated. Results: Here we describe a new copy-capture technique which we implemented in a method called SeqURE for Sequencing Unknown of Retroposition Events that allows for efficient and reliable identification of new genomic RE insertions. The method is based on the capture of copies of target molecules (copy-capture), selective amplification and sequencing of genomic regions adjacent to active RE insertions from both sides. Importantly, the template genomic DNA remains intact and can be used for validation experiments. In addition, we applied a novel system for testing method sensitivity and precisely showed the ability of the developed method to reliably detect insertions present in 1 out of 100 cells and a substantial portion of insertions present in 1 out of 1000 cells. Using advantages of the method we showed the absence of somatic Alu insertions in colorectal cancer samples bearing tumor-specific L1HS insertions. Conclusions: This study presents the first description and implementation of the copy-capture technique and provides the first methodological basis for the quantitative assessment of RE insertions present in a small portion of cells. [ABSTRACT FROM AUTHOR]

Background: Short interspersed elements (SINEs) are ubiquitous components of eukaryotic genomes. SINEs are composite transposable elements that are mobilized by non-long terminal repeat (non-LTR) retrotransposons, also called long interspersed elements (LINEs). The 3′ part of SINEs usually originated from that of counterpart non-LTR retrotransposons. The 5′ part of SINEs mostly originated from small RNA genes. SINE1 is a group of SINEs whose 5′ part originated from 7SL RNA, and is represented by primate Alu and murine B1. Well-defined SINE1 has been found only from Euarchontoglires, a group of mammals, in contrast to the wide distribution of SINE2, which has a tRNA-derived sequence, from animals to plants to protists. Both Alu and B1 are mobilized by L1-type non-LTR retrotransposons, which are the only lineage of autonomous non-LTR retrotransposons active in these mammalian lineages. Results: Here a new lineage of SINE1 is characterized from the seashore hagfish Eptatretus burgeri genome. This SINE1 family, designated SINE1-1_EBu, is young, and is transposed by RTE-type non-LTR retrotransposon, not L1-type. Comparison with other SINE families from hagfish indicated the birth of SINE1-1_EBu through chimera formation of a 7SL RNA-derived sequence and an older tRNA-derived SINE family. It reveals parallel evolution of SINE1 in two vertebrate lineages with different autonomous non-LTR retrotransposon partners. The comparison between two SINE1 lineages supports that the RNA secondary structure of the Alu domain of 7SL RNA is required for the efficient retrotransposition. Conclusions: The hagfish SINE1 is the first evident SINE1 family found outside of Euarchontoglires. Independent evolution of SINE1 with similar RNA secondary structure originated in 7SL RNA indicates the functional importance of 7SL RNA-derived sequence in the proliferation of SINEs. [ABSTRACT FROM AUTHOR]

Background: DNA transposons are ubiquitous components of eukaryotic genomes. Academ superfamily of DNA transposons is one of the least characterized DNA transposon superfamilies in eukaryotes. DNA transposons belonging to the Academ superfamily have been reported from various animals, one red algal species Chondrus crispus, and one fungal species Puccinia graminis. Six Academ families from P. graminis encode a helicase in addition to putative transposase, while some other families encode a single protein which contains a putative transposase and an XPG nuclease. Results: Systematic searches on Repbase and BLAST searches against publicly available genome sequences revealed that several species of fungi and animals contain multiple Academ transposon families encoding a helicase. These AcademH families generate 9 or 10-bp target site duplications (TSDs) while Academ families lacking helicase generate 3 or 4-bp TSDs. Phylogenetic analysis clearly shows two lineages inside of Academ, designated here as AcademH and AcademX for encoding helicase or XPG nuclease, respectively. One sublineage of AcademH in animals encodes plant homeodomain (PHD) finger in its transposase, and its remnants are found in several fish genomes. Conclusions: The AcademH lineage of TEs is widely distributed in animals and fungi, and originated early in the evolution of Academ DNA transposons. This analysis highlights the structural diversity in one less studied superfamily of eukaryotic DNA transposons. [ABSTRACT FROM AUTHOR]

Background: Transposable elements (TEs) are an important source of genomic variability in eukaryotic genomes. Their activity impacts genome architecture and gene expression and can lead to drastic phenotypic changes. Therefore, identifying TE polymorphisms is key to better understand the link between genotype and phenotype. However, most genotype-to-phenotype analyses have concentrated on single nucleotide polymorphisms as they are easier to reliable detect using short-read data. Many bioinformatic tools have been developed to identify transposon insertions from resequencing data using short reads. Nevertheless, the performance of most of these tools has been tested using simulated insertions, which do not accurately reproduce the complexity of natural insertions. Results: We have overcome this limitation by building a dataset of insertions from the comparison of two high-quality rice genomes, followed by extensive manual curation. This dataset contains validated insertions of two very different types of TEs, LTR-retrotransposons and MITEs. Using this dataset, we have benchmarked the sensitivity and precision of 12 commonly used tools, and our results suggest that in general their sensitivity was previously overestimated when using simulated data. Our results also show that, increasing coverage leads to a better sensitivity but with a cost in precision. Moreover, we found important differences in tool performance, with some tools performing better on a specific type of TEs. We have also used two sets of experimentally validated insertions in Drosophila and humans and show that this trend is maintained in genomes of different size and complexity. Conclusions: We discuss the possible choice of tools depending on the goals of the study and show that the appropriate combination of tools could be an option for most approaches, increasing the sensitivity while maintaining a good precision. [ABSTRACT FROM AUTHOR]

Repbase is a comprehensive database of eukaryotic transposable elements (TEs) and repeat sequences, containing over 1300 human repeat sequences. Recent analyses of these repeat sequences have accumulated evidences for their contribution to human evolution through becoming functional elements, such as protein-coding regions or binding sites of transcriptional regulators. However, resolving the origins of repeat sequences is a challenge, due to their age, divergence, and degradation. Ancient repeats have been continuously classified as TEs by finding similar TEs from other organisms. Here, the most comprehensive picture of human repeat sequences is presented. The human genome contains traces of 10 clades (L1, CR1, L2, Crack, RTE, RTEX, R4, Vingi, Tx1 and Penelope) of non-long terminal repeat (non-LTR) retrotransposons (long interspersed elements, LINEs), 3 types (SINE1/7SL, SINE2/tRNA, and SINE3/5S) of short interspersed elements (SINEs), 1 composite retrotransposon (SVA) family, 5 classes (ERV1, ERV2, ERV3, Gypsy and DIRS) of LTR retrotransposons, and 12 superfamilies (Crypton, Ginger1, Harbinger, hAT, Helitron, Kolobok, Mariner, Merlin, MuDR, P, piggyBac and Transib) of DNA transposons. These TE footprints demonstrate an evolutionary continuum of the human genome. [ABSTRACT FROM AUTHOR]

In recent years, much attention has been paid to comparative genomic studies of transposable elements (TEs) and the ensuing problems of their identification, classification, and annotation. Different approaches and diverse automated pipelines are being used to catalogue and categorize mobile genetic elements in the ever-increasing number of prokaryotic and eukaryotic genomes, with little or no connectivity between different domains of life. Here, an overview of the current picture of TE classification and evolutionary relationships is presented, updating the diversity of TE types uncovered in sequenced genomes. A tripartite TE classification scheme is proposed to account for their replicative, integrative, and structural components, and the need to expand in vitro and in vivo studies of their structural and biological properties is emphasized. Bioinformatic studies have now become front and center of novel TE discovery, and experimental pursuits of these discoveries hold great promise for both basic and applied science. [ABSTRACT FROM AUTHOR]

Background: Transposable elements (TEs) are highly abundant genomic parasites in eukaryote genomes. Although several genomes have been screened for TEs, so far very limited information is available regarding avian TEs and their evolutionary histories. Taking advantage of the rich genomic data available for birds, we characterized the evolutionary history of the galluhop element, originally described in Gallus gallus, through the use of several bioinformatic analyses. Results: galluhop homologous sequences were found in 6 of 72 genomes analyzed: 5 species of Galliformes (Gallus gallus, Meleagris gallopavo, Coturnix japonica, Colinus virginianus, Lyrurus tetrix) and one Buceritiformes (Buceros rhinoceros). The copy number ranged from 5 to 10,158, in the genomes of C. japonica and G. gallus respectively. All 6 species possessed short elements, suggesting the presence of Miniature Inverted repeats Transposable Elements (MITEs), which underwent an ancient massive amplification in the G. gallus and M. gallopavo genomes. Only 4 species showed potential MITE full-length partners, although no potential coding copies were detected. Phylogenetic analysis of reconstructed coding sequences showed that galluhop homolog sequences form a new mariner subfamily, which we termed Gallus. Inter-species and intragenomic galluhop distance analyses indicated a high identity between the consensus of B. rhinoceros and the other 5 related species, and different emergence ages of the element between the Galliformes species and B. rhinocerus, suggesting that horizontal transfer took place from Galliformes to a Buceritiformes ancestor, probably through an intermediate species. Conclusions: Overall, our results showed that mariner elements have amplified to high copy numbers in some avian species, and that this transposition burst probably occurred in the common ancestor of G. gallus and M. gallopavo. In addition, although no coding sequences could be found currently, they probably existed, allowing an ancient massive MITE amplification in these 2 species. The other 4 species also have MITEs, suggesting that this new mariner family is prone to give rise to such non-autonomous derivatives. Last, our results suggest that a horizontal transfer event of a galluhop element occurred between Galliformes and Buceritiformes. [ABSTRACT FROM AUTHOR]

Background: Transposable elements (TEs) are major components of eukaryotic genomes and drivers of genome evolution, producing intraspecific polymorphism and interspecific differences through mobilization and nonhomologous recombination. TE insertion sites are often highly variable within species, creating a need for targeted genome re-sequencing (TGS) methods to identify TE insertion sites. Methods: We present a hemi-specific PCR approach for TGS of P-elements in Drosophila genomes on the Illumina platform. We also present a computational framework for identifying new insertions from TGS reads. Finally, we describe a new method for estimating the frequency of TE insertions from WGS data, which is based precise insertion sites provided by TGS annotations. Results: By comparing our results to TE annotations based on whole genome re-sequencing (WGS) data for the same Drosophila melanogaster strain, we demonstrate that TGS is powerful for identifying true insertions, even in repeat-rich heterochromatic regions. We also demonstrate that TGS offers enhanced annotation of precise insertion sites, which facilitates estimation of TE insertion frequency. Conclusions: TGS by hemi-specific PCR is a powerful approach for identifying TE insertions of particular TE families in species with a high-quality reference genome, at greatly reduced cost as compared to WGS. It may therefore be ideal for population genomic studies of particular TE families. Additionally, TGS and WGS can be used as complementary approaches, with TGS annotations identifying more annotated insertions with greater precision for a target TE family, and WGS data allowing for estimates of TE insertion frequencies, and a broader picture of the location of non-target TEs across the genome. [ABSTRACT FROM AUTHOR]

Background: The revolutionary concept of "jumping genes" was conceived by McClintock in the late 1940s while studying the Activator/Dissociation (Ac/Ds) system in maize. Transposable elements (TEs) represent the most abundant component of many eukaryotic genomes. Mobile elements are a driving force of eukaryotic genome evolution. McClintock's Ac, the autonomous element of the Ac/Ds system, together with hobo from Drosophila and Tam3 from snapdragon define an ancient and diverse DNA transposon superfamily named hAT. Other members of the hAT superfamily include the insect element Hermes and Tol2 from medaka. In recent years, genetic tools derived from the 'cut' and 'paste' Tol2 DNA transposon have been widely used for genomic manipulation in zebrafish, mammals and in cells in vitro. Results: We report the purification of a functional recombinant Tol2 protein from E.coli. We demonstrate here that following microinjection using a zebrafish embryo test system, purified Tol2 transposase protein readily catalyzes gene transfer in both somatic and germline tissues in vivo. We show that purified Tol2 transposase can promote both in vitro cutting and pasting in a defined system lacking other cellular factors. Notably, our analysis of Tol2 transposition in vitro reveals that the target site preference observed for Tol2 in complex host genomes is maintained using a simpler target plasmid test system, indicating that the primary sequence might encode intrinsic cues for transposon integration. Conclusions: This active Tol2 protein is an important new tool for diverse applications including gene discovery and molecular medicine, as well as for the biochemical analysis of transposition and regulation of haT transposon/genome interactions. The measurable but comparatively modest insertion site selection bias noted for Tol2 is largely determined by the primary sequence encoded in the target sequence as assessed through studying Tol2 protein-mediated transposition in a cell-free system. [ABSTRACT FROM AUTHOR]

Background: Transposable elements are a major player contributing to genetic variation and shaping genome evolution. Multiple independent transposon domestication events have occurred in ciliates, recruiting transposases to key roles in cellular processes. In the ciliate Oxytricha trifallax, the telomere-bearing elements (TBE), a Tc1/mariner transposon, occupy a significant portion of the germline genome and are involved in programmed genome rearrangements that produce a transcriptionally active somatic nucleus from a copy of the germline nucleus during development. Results: Here we provide a thorough characterization of the distribution and sequences of TBE transposons in the Oxytricha germline genome. We annotate more than 10,000 complete and 24,000 partial TBE sequences. TBEs cluster into four major families and display a preference for either insertion into DNA segments that are retained in the somatic genome or their maintenance at such sites. The three TBE-encoded genes in all four families display dN/dS ratios much lower than 1, suggesting genome-wide purifying selection. We also identify TBE homologs in other ciliate species for phylogenomic analysis. Conclusions: This paper provides genome-wide characterization of a major class of ciliate transposons. Phylogenomic analysis reveals selective constraints on transposon-encoded genes, shedding light on the evolution and domesticated functions of these transposons. [ABSTRACT FROM AUTHOR]

Repbase Update (RU) is a database of representative repeat sequences in eukaryotic genomes. Since its first development as a database of human repetitive sequences in 1992, RU has been serving as a well-curated reference database fundamental for almost all eukaryotic genome sequence analyses. Here, we introduce recent updates of RU, focusing on technical issues concerning the submission and updating of Repbase entries and will give short examples of using RU data. RU sincerely invites a broader submission of repeat sequences from the research community. [ABSTRACT FROM AUTHOR]

Background: PHIS transposon superfamily belongs to DNA transposons and includes PIF/Harbinger, ISL2EU, and Spy transposon groups. These three groups have similar DDE domain-containing transposases; however, their coding capacity, species distribution, and target site duplications (TSDs) are significantly different. Results: In this study, we systematically identified and analyzed PHIS transposons in 836 sequenced eukaryotic genomes using transposase homology search and structure approach. In total, 380 PHIS families were identified in 112 genomes and 168 of 380 families were firstly reported in this study. Besides previous identified PIF/Harbinger, ISL2EU, and Spy groups, three new types (called Pangu, NuwaI, and NuwaII) of PHIS superfamily were identified; each has its own distinctive characteristics, especially in TSDs. Pangu and NuwaII transposons are characterized by 5'-ANT-3' and 5'-C|TNA|G-3' TSDs, respectively. Both transposons are widely distributed in plants, fungi, and animals; the NuwaI transposons are characterized by 5'-CWG-3' TSDs and mainly distributed in animals. Conclusions: Here, in total, 380 PHIS families were identified in eukaryotes. Among these 380 families, 168 were firstly reported in this study. Furthermore, three new types of PHIS superfamily were identified. Our results not only enrich the transposon diversity but also have extensive significance for improving genome sequence assembly and annotation of higher organisms. [ABSTRACT FROM AUTHOR]

Background: Penelope-like elements (PLEs) are an enigmatic group of retroelements sharing a common ancestor with telomerase reverse transcriptases. In our previous studies, we identified endonuclease-deficient PLEs that are associated with telomeres in bdelloid rotifers, small freshwater invertebrates best known for their long-term asexuality and high foreign DNA content. Completion of the high-quality draft genome sequence of the bdelloid rotifer Adineta vaga provides us with the opportunity to examine its genomic transposable element (TE) content, as well as TE impact on genome function and evolution. Results: We performed an exhaustive search of the A. vaga genome assembly, aimed at identification of canonical PLEs combining both the reverse transcriptase (RT) and the GIY-YIG endonuclease (EN) domains. We find that the RT/EN-containing Penelope families co-exist in the A. vaga genome with the EN-deficient RT-containing Athena retroelements. Canonical PLEs are present at very low copy numbers, often as a single-copy, and there is no evidence that they might preferentially co-mobilize EN-deficient PLEs. We also find that Penelope elements can participate in expansion of A. vaga multigene families via trans-action of their enzymatic machinery, as evidenced by identification of intron-containing host genes framed by the Penelope terminal repeats and characteristic targetsite duplications generated upon insertion. In addition, we find that Penelope open reading frames (ORFs) in several families have incorporated long stretches of coding sequence several hundred amino acids (aa) in length that are highly enriched in asparagine residues, a phenomenon not observed in other retrotransposons. Conclusions: Our results show that, despite their low abundance and low transcriptional activity in the A. vaga genome, endonuclease-containing Penelope elements can participate in expansion of host multigene families. We conclude that the terminal repeats represent the cis-acting sequences required for mobilization of the intervening region in trans by the Penelope-encoded enzymatic activities. We also hypothesize that the unusual capture of long N-rich segments by the Penelope ORF occurs as a consequence of peculiarities of its replication mechanism. These findings emphasize the unconventional nature of Penelope retrotransposons, which, in contrast to all other retrotransposon types, are capable of dispersing intron-containing genes, thereby questioning the validity of traditional estimates of gene retrocopies in PLE-containing eukaryotic genomes. [ABSTRACT FROM AUTHOR]

Background: Transposable elements (TEs) are a very dynamic component of eukaryotic genomes with important implications (e.g., in evolution) and applications (e.g., as transgenic tools). They also represent a major challenge for the assembly and annotation of genomic sequences. However, they are still largely unknown in non-model species. Results: Here, we have annotated the repeats and transposable elements present in a 600 kb genomic region of the blowfly Calliphora vicina (Diptera: Calliphoridae) which contains most of the achaete-scute gene complex of this species. This is the largest genomic region to be sequenced and analyzed in higher flies outside the Drosophila genus. We find that the repeat content spans at least 24% of the sequence. It includes 318 insertions classified as 3 LTR retrotransposons, 21 LINEs, 14 cut-and-paste DNA transposons, 4 helitrons and 33 unclassified repeats. Conclusions: This is the most detailed description of TEs and repeats in the Calliphoridae to date. This contribution not only adds to our knowledge about TE evolution but will also help in the annotation of repeats on Dipteran whole genome sequences. [ABSTRACT FROM AUTHOR]

Sireviruses are an ancient and plant-specific LTR retrotransposon genus. They possess a unique genome structure that is characterized by a plethora of highly conserved sequence motifs in key domains of the non-coding genome, and often, by the presence of an envelope-like gene. Recently, their crucial role in the organization of the maize genome, where Sireviruses occupy approximately 21% of its nuclear content, was revealed, followed by an analysis of their distribution across the plant kingdom. It is now suggested that Sireviruses have been a major mediator of the evolution of many plant genomes. However, the name 'Sirevirus' has caused confusion in the scientific community in regards to their classification within the LTR retrotransposon order and their relationship with viruses - a situation that is not unique to Sireviruses, but also affects other LTR retrotransposon genera. Here, we clarify the phylogenetic position of Sireviruses as typical LTR retrotransposons of the Copia superfamily and explain that the confusion stems from the discrepancy in the categorization of LTR retrotransposons by the two main classification systems: the International Committee on the Taxonomy of Viruses (ICTV) system and the unified classification system for eukaryotic transposable elements. While the name 'Sirevirus' has been given by ICTV, we show that the transposable element system, which is more suitable for eukaryotic genome studies, lacks an appropriate taxonomic level for describing them. We urge for this inconsistency to be addressed. Finally, we provide data suggesting that of the three ICTV-proposed genera of the Pseudoviridae (that is, Copia) family, only Sireviruses form a monophyletic group, while the phylogenetic distinction between Pseudoviruses and Hemiviruses is unclear. We conclude that because of their ongoing important contribution to the classification of transposable elements, these schemes need to be frequently revisited and revised - as shown by the example of the Sirevirus LTR retrotransposon genus. [ABSTRACT FROM AUTHOR]

Background: Miniature inverted-repeat transposable elements (MITEs) are common in eukaryotic genomes, and are important for genomic evolution. Results: In the present study, the identification of MITEs in the mulberry genome revealed 286,122 MITE-related sequences, including 90,789 full-length elements. The amplification of mulberry MITEs and the influence of MITEs on the evolution of the mulberry genome were analyzed. The timing of MITE amplifications varied considerably among the various MITE families. Fifty-one MITE families have undergone a single round of amplification, while the other families developed from multiple amplifications. Most mulberry MITEs were inserted near genes and some could regulate gene expression through small RNAs. An analysis of transgenic plants indicated that MITE insertions can upregulate the expression of a target gene. Moreover, MITEs are frequently associated with alternative splicing events (exonizations). Conclusion: The data presented herein provide insights into the generation of MITEs as well as their impact on gene regulation and evolution in mulberry. [ABSTRACT FROM AUTHOR]

Background: Helitrons play an important role in shaping eukaryotic genomes due to their ability to transfer horizontally between distantly related species and capture gene fragments during the transposition. However, the mechanisms of horizontal transfer (HT) and the process of gene fragment capturing of Helitrons still remain to be further clarified. Results: Here, we characterized a novel Helitron family discontinuously distributed in 27 out of 256 insect genomes. The most prominent characteristic of Hel1 family is its high sequence similarity among species of different insect orders. Related elements were also identified in two spiders, representing the first report of spider Helitrons. All these elements were classified into 2 families, 9 subfamilies and 35 exemplars based on our new classification criteria. Autonomous partners of Helitron were reconstructed in the genomes of three insects and one spider. Integration pattern analysis showed that majority of Hel1A elements in Papilio xuthus and Pieris rapae inserted into introns. Consistent with filler DNA model, stepwise sequence acquisition was observed in Sfru_Hel1Aa, Sfru_Hel1Ab and Sfru_Hel1Ac in Spodoptera frugiperda. Remarkably, the evidence that Prap_Hel1Aa in a Lepdidoptera insect, Pieris rapae, was derived from Cves_Hel1Aa in a parasitoid wasp, Cotesia vestalis, suggested the role of nonregular host-parasite interactions in HT of Helitrons. Conclusions: We proposed a modified classification criteria of Helitrons based on the important role of the 5′-end of Helitrons in transposition, and provided evidence for stepwise sequence acquisition and recurrent HT of a novel Helitron family. Our findings of the nonregular host-parasite interactions may be more conducive to the HT of transposons. [ABSTRACT FROM AUTHOR]

Background: Transposable elements are biologically important components of eukaryote genomes. In particular, non-LTR retrotransposons (N-LTRrs) played a key role in shaping the human genome throughout evolution. In this study, we compared retrotransposon insertions differentially present in the genomes of Anatomically Modern Humans, Neanderthals, Denisovans and Chimpanzees, in order to assess the possible impact of retrotransposition in the differentiation of the human lineage. Results: We first identified species-specific N-LTRrs and established their distribution in present day human populations. These analyses shortlisted a group of N-LTRr insertions that were found exclusively in Anatomically Modern Humans. These insertions are associated with an increase in the number of transcriptional/splicing variants of those genes they inserted in. The analysis of the functionality of genes containing human-specific N-LTRr insertions reflects changes that occurred during human evolution. In particular, the expression of genes containing the most recent N-LTRr insertions is enriched in the brain, especially in undifferentiated neurons, and these genes associate in networks related to neuron maturation and migration. Additionally, we identified candidate N-LTRr insertions that have likely produced new functional variants exclusive to modern humans, whose genomic loci show traces of positive selection. Conclusions: Our results strongly suggest that N-LTRr impacted our differentiation as a species, most likely inducing an increase in neural complexity, and have been a constant source of genomic variability all throughout the evolution of the human lineage. [ABSTRACT FROM AUTHOR]

Background: The majority of Eukaryotic genomes are composed of a small portion of stable (non-mobile) genes and a large fraction of parasitic mobile elements such as transposable elements and endogenous viruses: the Mobilome. Such important component of many genomes are normally underscored in genomic analysis and detailed characterized mobilomes only exists for model species. In this study, we used a combination of de novo and homology approaches to characterize the Mobilome of two non-model parasitoid wasp species. Results: The different methodologies employed for TE characterization recovered TEs with different features as TE consensus number and size. Moreover, some TEs were detected only by one or few methodologies. RepeatExplorer and dnaPipeTE estimated a low TE content of 5.86 and 4.57% for Braconidae wasp and 5.22% and 7.42% for L. boulardi species, respectively. Both mobilomes are composed by a miscellaneous of ancient and recent elements. Braconidae wasps presented a large diversity of Maverick/Polintons Class II TEs while other TE superfamilies were more equally diverse in both species. Phylogenetic analysis of reconstructed elements showed that vertical transfer is the main mode of transmission. Conclusion: Different methodologies should be used complementarity in order to achieve better mobilome characterization. Both wasps genomes have one of the lower mobilome estimates among all Hymenoptera genomes studied so far and presented a higher proportion of Class II than Class I TEs. The large majority of superfamilies analyzed phylogenetically showed that the elements are being inherited by vertical transfer. Overall, we achieved a deep characterization of the mobilome in two non-model parasitoid wasps improving our understanding of their evolution. [ABSTRACT FROM AUTHOR]