Terminal Repeat Retrotransposons Research Articles

Fine mapping and candidate gene analysis of CRA8.1.6, which confers clubroot resistance in turnip (Brassica rapa ssp. rapa)

Clubroot disease poses a significant threat to Brassica crops, necessitating ongoing updates on resistance gene sources. In F2 segregants of the clubroot-resistant inbred line BrT18-6-4-3 and susceptible DH line Y510, the genetic analysis identified a single dominant gene responsible for clubroot resistance. Through bulk segregant sequencing analysis and kompetitive allele-specific polymerase chain reaction assays, CRA8.1.6 was mapped within 110 kb (12,255–12,365 Mb) between markers L-CR11 and L-CR12 on chromosome A08. We identified B raA08g015220.3.5C as the candidate gene of CRA8.1.6. Upon comparison with the sequence of disease-resistant material BrT18-6-4-3, we found 249 single-nucleotide polymorphisms, seven insertions, six deletions, and a long terminal repeat (LTR) retrotransposon (5,310 bp) at 909 bp of the first intron. However, the LTR retrotransposon was absent in the coding sequence of the susceptible DH line Y510. Given the presence of a non-functional LTR insertion in other materials, it showed that the LTR insertion might not be associated with susceptibility. Sequence alignment analysis revealed that the fourth exon of the susceptible line harbored two deletions and an insertion, resulting in a frameshift mutation at 8,551 bp, leading to translation termination at the leucine-rich repeat domain’s C-terminal in susceptible material. Sequence alignment of the CDS revealed a 99.4% similarity to Crr1a, which indicate that CRA8.1.6 is likely an allele of the Crr1a gene. Two functional markers, CRA08-InDel and CRA08-KASP1, have been developed for marker-assisted selection in CR turnip cultivars. Our findings could facilitate the development of clubroot-resistance turnip cultivars through marker-assisted selection.

Characterization of the CsCENH3 protein and centromeric DNA profiles reveal the structures of centromeres in cucumber

Abstract Centromeres in eukaryotes mediate the accurate segregation of chromosomes during cell division. They serve as essential functional units of chromosomes and play a core role in the process of genome evolution. Centromeres are composed of satellite repeats and highly repetitive centromeric retrotransposons (CRs), which vary greatly even among closely related species. Cucumber (Cucumis sativus) is a globally cultivated and economically important vegetable and the only species in the Cucumis genus with seven pairs of chromosomes. Therefore, studying the centromeres of the Cucumis subgenus may yield valuable insights into its genome structure and evolution. Using chromatin immunoprecipitation (ChIP) techniques, we isolated centromeric DNA from cucumber reference line 9930. Our investigation into cucumber centromeres uncovered the centromeric satellite sequence, designated as CentCs, and the prevalent of Ty1/Copia long terminal repeat (LTR) retrotransposons. In addition, active genes were identified in the CsCENH3 nucleosome regions with low transcription levels. To the best of our knowledge, this is the first time that characterization of centromeres has been achieved on cucumber. Meanwhile, our results on the distribution of CentCs and CsCRs in the subgenus Cucumis indicate that the content of centromeric repeats in the wild variants was significantly reduced compared to the cultivated cucumber. The results provide evidence for centromeric DNA amplification that occurred during the domestication process from wild to cultivated cucumber. Furthermore, these findings may offer new information for enhancing our understanding of the phylogenetic relationship in the Cucumis genus.

Chromosome-level genome assembly of Pedicularis kansuensis illuminates genome evolution of facultative parasitic plant.

Parasitic plants have a heterotrophic lifestyle, in which they withdraw all or part of their nutrients from their host through the haustorium. Despite the release of many draft genomes of parasitic plants, the genome evolution related to the parasitism feature of facultative parasites remains largely unknown. In this study, we present a high-quality chromosomal-level genome assembly for the facultative parasite Pedicularis kansuensis (Orobanchaceae), which invades both legume and grass host species in degraded grasslands on the Qinghai-Tibet Plateau. This species has the largest genome size compared with other parasitic species, and expansions of long terminal repeat retrotransposons accounting for 62.37% of the assembly greatly contributed to the genome size expansion of this species. A total of 42,782 genes were annotated, and the patterns of gene loss in P. kansuensis differed from other parasitic species. We also found many mobile mRNAs between P. kansuensis and one of its host species, but these mobile mRNAs could not compensate for the functional losses of missing genes in P. kansuensis. In addition, we identified nine horizontal gene transfer (HGT) events from rosids and monocots, as well as one single-gene duplication events from HGT genes, which differ distinctly from that of other parasitic species. Furthermore, we found evidence for HGT through transferring genomic fragments from phylogenetically remote host species. Taken together, these findings provide genomic insights into the evolution of facultative parasites and broaden our understanding of the diversified genome evolution in parasitic plants and the molecular mechanisms of plant parasitism.

Anaerobic fungi contain abundant, diverse, and transcriptionally active Long Terminal Repeat retrotransposons

Long Terminal Repeat (LTR) retrotransposons are a class of repetitive elements that are widespread in the genomes of plants and many fungi. LTR retrotransposons have been associated with rapidly evolving gene clusters in plants and virulence factor transfer in fungal-plant parasite-host interactions. We report here the abundance and transcriptional activity of LTR retrotransposons across several species of the early-branching Neocallimastigomycota, otherwise known as the anaerobic gut fungi. The ubiquity of LTR retrotransposons in these genomes suggests key evolutionary roles in these rumen-dwelling biomass degraders, whose genomes also contain many enzymes that are horizontally transferred from other rumen-dwelling prokaryotes. Up to 10% of anaerobic fungal genomes consist of LTR retrotransposons, and the mapping of sequences from LTR retrotransposons to transcriptomes shows that the majority of clusters are transcribed, with some exhibiting expression greater than 104 reads per kilobase million mapped reads (rpkm). Many LTR retrotransposons are strongly differentially expressed upon heat stress during fungal cultivation, with several exhibiting a nearly three-log10 fold increase in expression, whereas growth substrate variation modulated transcription to a lesser extent. We show that some LTR retrotransposons contain carbohydrate-active enzymes (CAZymes), and the expansion of CAZymes within genomes and among anaerobic fungal species may be linked to retrotransposon activity. We further discuss how these widespread sequences may be a source of promoters and other parts towards the bioengineering of anaerobic fungi.

Transposable elements in Rosaceae: insights into genome evolution, expression dynamics, and Syntenic gene regulation

Abstract Transposable elements (TEs) exert significant influence on plant genomic structure and gene expression. Here, we explored TE-related aspects across fourteen Rosaceae genomes, investigating genomic distribution, transposition activity, expression patterns, and nearby differentially expressed genes (DEGs). Analyses unveiled distinct long terminal repeat retrotransposon (LTR-RT) evolutionary patterns, reflecting varied genome size changes among nine species over the past million years. In the past 2.5 million years, Rubus idaeus showed a transposition rate twice as fast as Fragaria vesca, while Pyrus bretschneideri displayed significantly faster transposition compared to Crataegus pinnatifida. Genes adjacent to recent TE insertions were linked to adversity resistance, while those near previous insertions were functionally enriched in morphogenesis, enzyme activity, and metabolic processes. Expression analysis revealed diverse responses of LTR-RTs to internal or external conditions. Furthermore, we identified 3695 pairs of syntenic DEGs proximal to TEs in Malus domestica cv. Gala and Malus domestica (GDDH13), suggesting TE insertions may contribute to varietal trait differences in these apple varieties. Our study across representative Rosaceae species underscores the pivotal role of TEs in plant genome evolution within this diverse family. It elucidates how these elements regulate syntenic DEGs on a genome-wide scale, offering insights into Rosaceae-specific genomic evolution.

Pangenome analysis reveals transposon-driven genome evolution in cotton

BackgroundTransposable elements (TEs) have a profound influence on the trajectory of plant evolution, driving genome expansion and catalyzing phenotypic diversification. The pangenome, a comprehensive genetic pool encompassing all variations within a species, serves as an invaluable tool, unaffected by the confounding factors of intraspecific diversity. This allows for a more nuanced exploration of plant TE evolution.ResultsHere, we constructed a pangenome for diploid A-genome cotton using 344 accessions from representative geographical regions, including 223 from China as the main component. We found 511 Mb of non-reference sequences (NRSs) and revealed the presence of 5479 previously undiscovered protein-coding genes. Our comprehensive approach enabled us to decipher the genetic underpinnings of the distinct geographic distributions of cotton. Notably, we identified 3301 presence-absence variations (PAVs) that are closely tied to gene expression patterns within the pangenome, among which 2342 novel expression quantitative trait loci (eQTLs) were found residing in NRSs. Our investigation also unveiled contrasting patterns of transposon proliferation between diploid and tetraploid cotton, with long terminal repeat (LTR) retrotransposons exhibiting a synchronized surge in polyploids. Furthermore, the invasion of LTR retrotransposons from the A subgenome to the D subgenome triggered a substantial expansion of the latter following polyploidization. In addition, we found that TE insertions were responsible for the loss of 36.2% of species-specific genes, as well as the generation of entirely new species-specific genes.ConclusionsOur pangenome analyses provide new insights into cotton genomics and subgenome dynamics after polyploidization and demonstrate the power of pangenome approaches for elucidating transposon impacts and genome evolution.

Celine, a long interspersed nuclear element retrotransposon, colonizes in the centromeres of poplar chromosomes.

Centromeres in most multicellular eukaryotes are composed of long arrays of repetitive DNA sequences. Interestingly, several transposable elements, including the well-known long terminal repeat (LTR) retrotransposon CRM (centromeric retrotransposon of maize), were found to be enriched in functional centromeres marked by the centromeric histone H3 (CENH3). Here we report a centromeric long interspersed nuclear element (LINE), Celine, in Populus species. Celine has colonized preferentially in the CENH3-associated chromatin of every poplar chromosome, with 84% of the Celine elements localized in the CENH3-binding domains. By contrast, only 51% of the CRM elements were bound to CENH3 domains in Populus trichocarpa. These results suggest different centromere targeting mechanisms employed by Celine and CRM elements. Nevertheless, the high target specificity seems to be detrimental to further amplification of the Celine elements, leading to a shorter life span and patchy distribution among plant species compared to the CRM elements. Using a phylogenetically guided approach we were able to identify Celine-like LINE elements in tea plant (Camellia sinensis) and green ash tree (Fraxinus pennsylvanica). The centromeric localization of these Celine-like LINEs was confirmed in both species. We demonstrate that the centromere targeting property of Celine-like LINEs is of primitive origin and has been conserved among distantly related plant species.

Fine Mapping and Candidate Gene Validation of Tomato Gene Carpelloid Stamen and Parthenocarpy (CSP)

Parthenocarpy and male sterility are highly desirable traits in tomato breeding and molecular study. The stamen carpelloid mutant generally displays male sterility. A natural mutant displaying carpelloid stamen and parthenocarpy (csp) was identified in our research group. In this study, the csp locus was finely mapped to a 65 kb interval, which contained six putative genes. One of them, Solyc04g081000, encodes the tomato class B MADS box gene TAP3 (syn. SlDEF). Sequencing data revealed that a copia long terminal repeat retrotransposon was inserted in the first intron of the TAP3 gene of the csp mutant. qRT-PCR showed that the expression of TAP3 was significantly down-regulated in the petals and stamens of the csp mutant. A phenotypic analysis of the TAP3 gene-edited mutants and allelism tests indicated that TAP3 was the gene underlying csp, and csp was a novel allelic mutation of TAP3. The results of this study will lay the foundation for a further analysis of the function of TAP3 and provide materials and a basis for a further study of the functional differentiation of tomato B-class genes.

Look4LTRs: a Long terminal repeat retrotransposon detection tool capable of cross species studies and discovering recently nested repeats

Plant genomes include large numbers of transposable elements. One particular type of these elements is flanked by two Long Terminal Repeats (LTRs) and can translocate using RNA. Such elements are known as LTR-retrotransposons; they are the most abundant type of transposons in plant genomes. They have many important functions involving gene regulation and the rise of new genes and pseudo genes in response to severe stress. Additionally, LTR-retrotransposons have several applications in biotechnology. Due to the abundance and the importance of LTR-retrotransposons, multiple computational tools have been developed for their detection. However, none of these tools take advantages of the availability of related genomes; they process one chromosome at a time. Further, recently nested LTR-retrotransposons (multiple elements of the same family are inserted into each other) cannot be annotated accurately — or cannot be annotated at all — by the currently available tools. Motivated to overcome these two limitations, we built Look4LTRs, which can annotate LTR-retrotransposons in multiple related genomes simultaneously and discover recently nested elements. The methodology of Look4LTRs depends on techniques imported from the signal-processing field, graph algorithms, and machine learning with a minimal use of alignment algorithms. Four plant genomes were used in developing Look4LTRs and eight plant genomes for evaluating it in contrast to three related tools. Look4LTRs is the fastest while maintaining better or comparable F1 scores (the harmonic average of recall and precision) to those obtained by the other tools. Our results demonstrate the added benefit of annotating LTR-retrotransposons in multiple related genomes simultaneously and the ability to discover recently nested elements. Expert human manual examination of six elements — not included in the ground truth — revealed that three elements belong to known families and two elements are likely from new families. With respect to examining recently nested LTR-retrotransposons, three out of five were confirmed to be valid elements. Look4LTRs — with its speed, accuracy, and novel features — represents a true advancement in the annotation of LTR-retrotransposons, opening the door to many studies focused on understanding their functions in plants.

A chromosome-level genome reveals genome evolution and molecular basis of anthraquinone biosynthesis in Rheum palmatum

BackgroundRhubarb is one of common traditional Chinese medicine with a diverse array of therapeutic efficacies. Despite its widespread use, molecular research into rhubarb remains limited, constraining our comprehension of the geoherbalism.ResultsWe assembled the genome of Rheum palmatum L., one of the source plants of rhubarb, to elucidate its genome evolution and unpack the biosynthetic pathways of its bioactive compounds using a combination of PacBio HiFi, Oxford Nanopore, Illumina, and Hi-C scaffolding approaches. Around 2.8 Gb genome was obtained after assembly with more than 99.9% sequences anchored to 11 pseudochromosomes (scaffold N50 = 259.19 Mb). Transposable elements (TE) with a continuous expansion of long terminal repeat retrotransposons (LTRs) is predominant in genome size, contributing to the genome expansion of R. palmatum. Totally 30,480 genes were predicted to be protein-coding genes with 473 significantly expanded gene families enriched in diverse pathways associated with high-altitude adaptation for this species. Two successive rounds of whole genome duplication event (WGD) shared by Fagopyrum tataricum and R. palmatum were confirmed. We also identified 54 genes involved in anthraquinone biosynthesis and other 97 genes entangled in flavonoid biosynthesis. Notably, RpALS emerged as a compelling candidate gene for the octaketide biosynthesis after the key residual screening.ConclusionOverall, our findings offer not only an enhanced understanding of this remarkable medicinal plant but also pave the way for future innovations in its genetic breeding, molecular design, and functional genomic studies.

Chromosome-level genome assembly and characterization of the Calophaca sinica genome.

Calophaca sinica is a rare plant endemic to northern China which belongs to the Fabaceae family and possesses rich nutritional value. To support the preservation of the genetic resources of this plant, we have successfully generated a high-quality genome of C. sinica (1.06 Gb). Notably, transposable elements (TEs) constituted ~73% of the genome, with long terminal repeat retrotransposons (LTR-RTs) dominating this group of elements (~54% of the genome). The average intron length of the C. sinica genome was noticeably longer than what has been observed for closely related species. The expansion of LTR-RTs and elongated introns emerged had the largest influence on the enlarged genome size of C. sinica in comparison to other Fabaceae species. The proliferation of TEs could be explained by certain modes of gene duplication, namely, whole genome duplication (WGD) and dispersed duplication (DSD). Gene family expansion, which was found to enhance genes associated with metabolism, genetic maintenance, and environmental stress resistance, was a result of transposed duplicated genes (TRD) and WGD. The presented genomic analysis sheds light on the genetic architecture of C. sinica, as well as provides a starting point for future evolutionary biology, ecology, and functional genomics studies centred around C. sinica and closely related species.

Karyotype and LTR-RTs analysis provide insights into oak genomic evolution

BackgroundWhole-genome duplication and long terminal repeat retrotransposons (LTR-RTs) amplification in organisms are essential factors that affect speciation, local adaptation, and diversification of organisms. Understanding the karyotype projection and LTR-RTs amplification could contribute to untangling evolutionary history. This study compared the karyotype and LTR-RTs evolution in the genomes of eight oaks, a dominant lineage in Northern Hemisphere forests.ResultsKaryotype projections showed that chromosomal evolution was relatively conservative in oaks, especially on chromosomes 1 and 7. Modern oak chromosomes formed through multiple fusions, fissions, and rearrangements after an ancestral triplication event. Species-specific chromosomal rearrangements revealed fragments preserved through natural selection and adaptive evolution. A total of 441,449 full-length LTR-RTs were identified from eight oak genomes, and the number of LTR-RTs for oaks from section Cyclobalanopsis was larger than in other sections. Recent amplification of the species-specific LTR-RTs lineages resulted in significant variation in the abundance and composition of LTR-RTs among oaks. The LTR-RTs insertion suppresses gene expression, and the suppressed intensity in gene regions was larger than in promoter regions. Some centromere and rearrangement regions indicated high-density peaks of LTR/Copia and LTR/Gypsy. Different centromeric regional repeat units (32, 78, 79 bp) were detected on different Q. glauca chromosomes.ConclusionChromosome fusions and arm exchanges contribute to the formation of oak karyotypes. The composition and abundance of LTR-RTs are affected by its recent amplification. LTR-RTs random retrotransposition suppresses gene expression and is enriched in centromere and chromosomal rearrangement regions. This study provides novel insights into the evolutionary history of oak karyotypes and the organization, amplification, and function of LTR-RTs.

Distinct and overlapping RNA determinants for binding and target-primed reverse transcription by Bombyx mori R2 retrotransposon protein.

Eukaryotic retrotransposons encode a reverse transcriptase that binds RNA to template DNA synthesis. The ancestral non-long terminal repeat (non-LTR) retrotransposons encode a protein that performs target-primed reverse transcription (TPRT), in which the nicked genomic target site initiates complementary DNA (cDNA)synthesis directly into the genome. The best understood model system for biochemical studies of TPRT is the R2 protein from the silk moth Bombyx mori. The R2 protein selectively binds the 3' untranslated region of its encoding RNA as template for DNA insertion to its target site in 28S ribosomal DNA. Here, binding and TPRT assays define RNA contributions to RNA-protein interaction, template use for TPRT and the fidelity of template positioning for TPRT cDNA synthesis. We quantify both sequence and structure contributions to protein-RNA interaction. RNA determinants of binding affinity overlap but are not equivalent to RNA features required for TPRT and its fidelity of template positioning for full-length TPRT cDNA synthesis. Additionally, we show that a previously implicated RNA-binding protein surface of R2 protein makes RNA binding affinity dependent on the presence of two stem-loops. Our findings inform evolutionary relationships across R2 retrotransposon RNAs and are a step toward understanding the mechanism and template specificity of non-LTR retrotransposon mobility.

Transposition of HOPPLA in siRNA-deficient plants suggests a limited effect of the environment on retrotransposon mobility in Brachypodium distachyon.

Long terminal repeat retrotransposons (LTR-RTs) are powerful mutagens regarded as a major source of genetic novelty and important drivers of evolution. Yet, the uncontrolled and potentially selfish proliferation of LTR-RTs can lead to deleterious mutations and genome instability, with large fitness costs for their host. While population genomics data suggest that an ongoing LTR-RT mobility is common in many species, the understanding of their dual role in evolution is limited. Here, we harness the genetic diversity of 320 sequenced natural accessions of the Mediterranean grass Brachypodium distachyon to characterize how genetic and environmental factors influence plant LTR-RT dynamics in the wild. When combining a coverage-based approach to estimate global LTR-RT copy number variations with mobilome-sequencing of nine accessions exposed to eight different stresses, we find little evidence for a major role of environmental factors in LTR-RT accumulations in B. distachyon natural accessions. Instead, we show that loss of RNA polymerase IV (Pol IV), which mediates RNA-directed DNA methylation in plants, results in high transcriptional and transpositional activities of RLC_BdisC024 (HOPPLA) LTR-RT family elements, and that these effects are not stress-specific. This work supports findings indicating an ongoing mobility in B. distachyon and reveals that host RNA-directed DNA methylation rather than environmental factors controls their mobility in this wild grass model.

Near-gapless genome and transcriptome analyses provide insights into fruiting body development in Lentinula edodes

Fruiting body development in macrofungi is an intensive research subject. In this study, high-quality genomes were assembled for two sexually compatible monokaryons from a heterokaryotic Lentinula edodes strain WX1, and variations in L. edodes genomes were analyzed. Specifically, differential gene expression and allele-specific expression (ASE) were analyzed using the two monokaryotic genomes and transcriptome data from four different stages of fruiting body development in WX1. Results revealed that after aeration, mycelia sensed cell wall stress, pheromones, and a decrease in CO2 concentration, leading to up-regulated expression in genes related to cell adhesion, cell wall remodeling, proteolysis, and lipid metabolism, which may promote primordium differentiation. Aquaporin genes and those related to proteolysis, mitosis, lipid, and carbohydrate metabolism may play important roles in primordium development, while genes related to tissue differentiation and sexual reproduction were active in fruiting body. Several essential genes for fruiting body development were allele-specifically expressed and the two nuclear types could synergistically regulate fruiting body development by dominantly expressing genes with different functions. ASE was probably induced by long terminal repeat-retrotransposons. Findings here contribute to the further understanding of the mechanism of fruiting body development in macrofungi.

Chromosome-Scale Genome Assembly for Clubrush (Bolboschoenus planiculmis) Indicates a Karyotype with High Chromosome Number and Heterogeneous Centromere Distribution.

Bolboschoenus planiculmis (F.Schmidt) T.V.Egorova is a typical wetland plant in the species-rich Cyperaceae family. This species contributes prominently to carbon dynamics and trophic integration in wetland ecosystems. Previous studies have reported that the chromosomes of B. planiculmis are holocentric; i.e. they have kinetic activity along their entire length and carry multiple centromeres. This feature was suggested to lead to a rapid genome evolution through chromosomal fissions and fusions and participate to the diversification and ecological success of the Bolboschoenus genus. However, the specific mechanism remains uncertain, partly due to the scarcity of genetic information on Bolboschoenus. We present here the first chromosome-level genome assembly for B. planiculmis. Through the integration of high-quality long-read and short-read data, together with chromatin conformation using Hi-C technology, the ultimate genome assembly was 238.01 Mb with a contig N50 value of 3.61 Mb. Repetitive elements constituted 37.04% of the genome, and 18,760 protein-coding genes were predicted. The low proportion of long terminal repeat retrotransposons (∼9.62%) was similar to that reported for other Cyperaceae species. The Ks (synonymous substitutions per synonymous site) distribution suggested no recent large-scale genome duplication in this genome. The haploid assembly contained a large number of 54 pseudochromosomes with a small mean size of 4.10 Mb, covering most of the karyotype. The results of centromere detection support that not all the chromosomes in B. planiculmis have multiple centromeres, indicating more efforts are needed to fully reveal the specific style of holocentricity in cyperids and its evolutionary significance.

Evolution of a Restriction Factor by Domestication of a Yeast Retrotransposon.

Transposable elements drive genome evolution in all branches of life. Transposable element insertions are often deleterious to their hosts and necessitate evolution of control mechanisms to limit their spread. The long terminal repeat retrotransposon Ty1 prime (Ty1'), a subfamily of the Ty1 family, is present in many Saccharomyces cerevisiae strains, but little is known about what controls its copy number. Here, we provide evidence that a novel gene from an exapted Ty1' sequence, domesticated restriction of Ty1' relic 2 (DRT2), encodes a restriction factor that inhibits Ty1' movement. DRT2 arose through domestication of a Ty1' GAG gene and contains the C-terminal domain of capsid, which in the related Ty1 canonical subfamily functions as a self-encoded restriction factor. Bioinformatic analysis reveals the widespread nature of DRT2, its evolutionary history, and pronounced structural variation at the Ty1' relic 2 locus. Ty1' retromobility analyses demonstrate DRT2 restriction factor functionality, and northern blot and RNA-seq analysis indicate that DRT2 is transcribed in multiple strains. Velocity cosedimentation profiles indicate an association between Drt2 and Ty1' virus-like particles or assembly complexes. Chimeric Ty1' elements containing DRT2 retain retromobility, suggesting an ancestral role of productive Gag C-terminal domain of capsid functionality is present in the sequence. Unlike Ty1 canonical, Ty1' retromobility increases with copy number, suggesting that C-terminal domain of capsid-based restriction is not limited to the Ty1 canonical subfamily self-encoded restriction factor and drove the endogenization of DRT2. The discovery of an exapted Ty1' restriction factor provides insight into the evolution of the Ty1 family, evolutionary hot-spots, and host-transposable element interactions.

Genome assembly provides insights into the genome evolution of Baccaurea ramiflora Lour.

Baccaurea ramiflora Lour., an evergreen tree of the Baccaurea genus of the Phyllanthaceae family, is primarily distributed in South Asia, Southeast Asia, and southern China, including southern Yunnan Province. It is a wild or semi-cultivated tree species with ornamental, edible, and medicinal value, exhibiting significant development potential. In this study, we present the whole-genome sequencing of B. ramiflora, employing a combination of PacBio SMRT and Illumina HiSeq 2500 sequencing techniques. The assembled genome size was 975.8 Mb, with a contig N50 of 509.33 kb and the longest contig measuring 7.74 Mb. The genome comprises approximately 73.47% highly repetitive sequences, of which 52.1% are long terminal repeat–retrotransposon sequences. A total of 29,172 protein-coding genes were predicted, of which 25,980 (89.06%) have been annotated, Additionally, 3452 non-coding RNAs were identified. Comparative genomic analysis revealed a close relationship between B. ramiflora and the Euphorbiaceae family, with both being sister groups that diverged approximately 59.9 million years ago. During the evolutionary process, B. ramiflora exhibited positive selection in 278 candidate genes. Synonymous substitution rate and collinearity analysis demonstrated that B. ramiflora underwent a single ancient genome-wide triploidization event, without recent genome-wide duplication events. This high-quality B. ramiflora genome provides a valuable resource for basic research and tree improvement programs focusing on the Phyllanthaceae family.

Retroviral Insertion Polymorphism (RIP) of Porcine Endogenous Retroviruses (PERVs) in Pig Genomes.

Endogenous retroviruses (ERVs) are one of the superfamilies of long terminal repeat retrotransposons (LTRs) in mice and humans. Approximately 8% of the pig genome is composed of sequences derived from LTRs. While the majority of ERVs in pigs have decayed, a small number of full-length copies can still mobilize within the genome. This study investigated the unexplored retroviral insertion polymorphisms (RIPs) generated by the mobilization of full-length ERVs (Fl-ERVs), and evaluated their impact on phenotypic variation to gain insights into the biological role of Fl-ERVs in pigs. Overall, 39 RIPs (insertions or deletions relative to the pig reference genome) generated by Fl-ERVs were predicted by comparative genomic analysis, and 18 of them were confirmed by PCR detection. Four RIP sites (D5, D14, D15, and D18) were further evaluated by population analysis, and all of them displayed polymorphisms in multiple breeds. The RIP site of ERV-D14, which is a Fl-ERV inserted in the STAB2-like gene, was further confirmed by sequencing. Population analysis of the polymorphic site of ERV-D14 reveals that it presents moderate polymorphism information in the Large White pig breed, and the association analysis reveals that the RIP of ERV-D14 is associated with age variations at 30 kg body weight (p < 0.05) and 100 kg body weight (p < 0.01) in the population of Large White pigs (N = 480). Furthermore, the ERV-D14 RIP is associated with changes in the expression of the target gene STAB2-like in the liver, backfat, and leaf fat in Sushan pigs. These data suggest that some Fl-ERVs are still mobilizing in the pig's genome, and contribute to genomic and phenotypic variations.

Helenus and Ajax, Two Groups of Non-Autonomous LTR Retrotransposons, Represent a New Type of Small RNA Gene-Derived Mobile Elements.

Terminal repeat retrotransposons in miniature (TRIMs) are short non-autonomous long terminal repeat (LTR) retrotransposons found from various eukaryotes. Cassandra is a unique TRIM lineage which contains a 5S rRNA-derived sequence in its LTRs. Here, two new groups of TRIMs, designated Helenus and Ajax, are reported based on bioinformatics analysis and the usage of Repbase. Helenus is found from fungi, animals, and plants, and its LTRs contain a tRNA-like sequence. It includes two LTRs and between them, a primer-binding site (PBS) and polypurine tract (PPT) exist. Fungal and plant Helenus generate 5 bp target site duplications (TSDs) upon integration, while animal Helenus generates 4 bp TSDs. Ajax includes a 5S rRNA-derived sequence in its LTR and is found from two nemertean genomes. Ajax generates 5 bp TSDs upon integration. These results suggest that despite their unique promoters, Helenus and Ajax are TRIMs whose transposition is dependent on autonomous LTR retrotransposon. These TRIMs can originate through an insertion of SINE in an LTR of TRIM. The discovery of Helenus and Ajax suggests the presence of TRIMs with a promoter for RNA polymerase III derived from a small RNA gene, which is here collectively termed TRIMp3.