Repeat turnover meets stable chromosomes: repetitive DNA sequences mark speciation and gene pool boundaries in sugar beet and wild beets.

Sugar beet and its wild relatives share a base chromosome number of nine and similar chromosome morphologies. Yet, interspecific breeding is impeded by chromosome and sequence divergence that is still not fully understood. Since repetitive DNAs are among the fastest evolving parts of the genome, we investigated, if repeatome innovations and losses are linked to chromosomal differentiation and speciation. We traced genome and chromosome-wide evolution across 13 beet species comprising all sections of the genera Beta and Patellifolia. For this, we combined short and long read sequencing, flow cytometry, and cytogenetics to build a comprehensive framework that spans the complete scale from DNA to chromosome to genome. Genome sizes and repeat profiles reflect the separation into three gene pools with contrasting evolutionary patterns. Among all repeats, satellite DNAs harbor most genomic variability, leading to fundamentally different centromere architectures, ranging from chromosomal uniformity in Beta and Patellifolia to the formation of patchwork chromosomes in Corollinae/Nanae. We show that repetitive DNAs are causal for the genome expansions and contractions across the beet genera, providing insights into the genomic underpinnings of beet speciation. Satellite DNAs in particular vary considerably between beet genomes, leading to the evolution of distinct chromosomal setups in the three gene pools, likely contributing to the barriers in beet breeding. Thus, with their isokaryotypic chromosome sets, beet genomes present an ideal system for studying the link between repeats, genomic variability, and chromosomal differentiation and provide a theoretical fundament for understanding barriers in any crop breeding effort.

Evolving Together: Cassandra Retrotransposons Gradually Mirror Promoter Mutations of the 5S rRNA Genes.

The 5S rRNA genes are among the most conserved nucleotide sequences across all species. Similar to the 5S preservation we observe the occurrence of 5S-related nonautonomous retrotransposons, so-called Cassandras. Cassandras harbor highly conserved 5S rDNA-related sequences within their long terminal repeats, advantageously providing them with the 5S internal promoter. However, the dynamics of Cassandra retrotransposon evolution in the context of 5S rRNA gene sequence information and structural arrangement are still unclear, especially: (1) do we observe repeated or gradual domestication of the highly conserved 5S promoter by Cassandras and (2) do changes in 5S organization such as in the linked 35S-5S rDNA arrangements impact Cassandra evolution? Here, we show evidence for gradual co-evolution of Cassandra sequences with their corresponding 5S rDNAs. To follow the impact of 5S rDNA variability on Cassandra TEs, we investigate the Asteraceae family where highly variable 5S rDNAs, including 5S promoter shifts and both linked and separated 35S-5S rDNA arrangements have been reported. Cassandras within the Asteraceae mirror 5S rDNA promoter mutations of their host genome, likely as an adaptation to the host's specific 5S transcription factors and hence compensating for evolutionary changes in the 5S rDNA sequence. Changes in the 5S rDNA sequence and in Cassandras seem uncorrelated with linked/separated rDNA arrangements. We place all these observations into the context of angiosperm 5S rDNA-Cassandra evolution, discuss Cassandra's origin hypotheses (single or multiple) and Cassandra's possible impact on rDNA and plant genome organization, giving new insights into the interplay of ribosomal genes and transposable elements.

The Dynamic Interplay Between Ribosomal DNA and Transposable Elements: A Perspective From Genomics and Cytogenetics.

Although both are salient features of genomes, at first glance ribosomal DNAs and transposable elements are genetic elements with not much in common: whereas ribosomal DNAs are mainly viewed as housekeeping genes that uphold all prime genome functions, transposable elements are generally portrayed as selfish and disruptive. These opposing characteristics are also mirrored in other attributes: organization in tandem (ribosomal DNAs) versus organization in a dispersed manner (transposable elements); evolution in a concerted manner (ribosomal DNAs) versus evolution by diversification (transposable elements); and activity that prolongs genomic stability (ribosomal DNAs) versus activity that shortens it (transposable elements). Re-visiting relevant instances in which ribosomal DNA-transposable element interactions have been reported, we note that both repeat types share at least four structural and functional hallmarks: (1) they are repetitive DNAs that shape genomes in evolutionary timescales, (2) they exchange structural motifs and can enter co-evolution processes, (3) they are tightly controlled genomic stress sensors playing key roles in senescence/aging, and (4) they share common epigenetic marks such as DNA methylation and histone modification. Here, we give an overview of the structural, functional, and evolutionary characteristics of both ribosomal DNAs and transposable elements, discuss their roles and interactions, and highlight trends and future directions as we move forward in understanding ribosomal DNA-transposable element associations.

How to start a LINE: 5' switching rejuvenates LINE retrotransposons in tobacco and related Nicotiana species.

By contrast to their conserved mammalian counterparts, plant long interspersed nuclear elements (LINEs) are highly variable, splitting into many low-copy families. Curiously, LINE families from the retrotransposable element (RTE) clade retain a stronger sequence conservation and hence reach higher copy numbers. The cause of this RTE-typical property is not yet understood, but would help clarify why some transposable elements are removed quickly, whereas others persist in plant genomes. Here, we bring forward a detailed study of RTE LINE structure, diversity and evolution in plants. For this, we argue that the nightshade family is the ideal taxon to follow the evolutionary trajectories of RTE LINEs, given their high abundance, recent activity and partnership to non-autonomous elements. Using bioinformatic, cytogenetic and molecular approaches, we detect 4029 full-length RTE LINEs across the Solanaceae. We finely characterize and manually curate a core group of 458 full-length LINEs in allotetraploid tobacco, show an integration event after polyploidization and trace hybridization by RTE LINE composition of parental genomes. Finally, we reveal the role of the untranslated regions (UTRs) as causes for the unique RTE LINE amplification and evolution pattern in plants. On the one hand, we detected a highly conserved motif at the 3' UTR, suggesting strong selective constraints acting on the RTE terminus. On the other hand, we observed successive rounds of 5' UTR cycling, constantly rejuvenating the promoter sequences. This interplay between exchangeable promoters and conserved LINE bodies and 3' UTR likely allows RTE LINEs to persist and thrive in plant genomes.

High-fidelity (repeat) consensus sequences from short reads using combined read clustering and assembly.

BACKGROUND: Despite the many cheap and fast ways to generate genomic data, good and exact genome assembly is still a problem, with especially the repeats being vastly underrepresented and often misassembled. As short reads in low coverage are already sufficient to represent the repeat landscape of any given genome, many read cluster algorithms were brought forward that provide repeat identification and classification. But how can trustworthy, reliable and representative repeat consensuses be derived from unassembled genomes? RESULTS: Here, we combine methods from repeat identification and genome assembly to derive these robust consensuses. We test several use cases, such as (1) consensus building from clustered short reads of non-model genomes, (2) from genome-wide amplification setups, and (3) specific repeat-centred questions, such as the linked vs. unlinked arrangement of ribosomal genes. In all our use cases, the derived consensuses are robust and representative. To evaluate overall performance, we compare our high-fidelity repeat consensuses to RepeatExplorer2-derived contigs and check, if they represent real transposable elements as found in long reads. Our results demonstrate that it is possible to generate useful, reliable and trustworthy consensuses from short reads by a combination from read cluster and genome assembly methods in an automatable way. CONCLUSION: We anticipate that our workflow opens the way towards more efficient and less manual repeat characterization and annotation, benefitting all genome studies, but especially those of non-model organisms.

ECCsplorer: a pipeline to detect extrachromosomal circular DNA (eccDNA) from next-generation sequencing data.

BACKGROUND: Extrachromosomal circular DNAs (eccDNAs) are ring-like DNA structures physically separated from the chromosomes with 100 bp to several megabasepairs in size. Apart from carrying tandemly repeated DNA, eccDNAs may also harbor extra copies of genes or recently activated transposable elements. As eccDNAs occur in all eukaryotes investigated so far and likely play roles in stress, cancer, and aging, they have been prime targets in recent research-with their investigation limited by the scarcity of computational tools. RESULTS: Here, we present the ECCsplorer, a bioinformatics pipeline to detect eccDNAs in any kind of organism or tissue using next-generation sequencing techniques. Following Illumina-sequencing of amplified circular DNA (circSeq), the ECCsplorer enables an easy and automated discovery of eccDNA candidates. The data analysis encompasses two major procedures: first, read mapping to the reference genome allows the detection of informative read distributions including high coverage, discordant mapping, and split reads. Second, reference-free comparison of read clusters from amplified eccDNA against control sample data reveals specifically enriched DNA circles. Both software parts can be run separately or jointly, depending on the individual aim or data availability. To illustrate the wide applicability of our approach, we analyzed semi-artificial and published circSeq data from the model organisms Homo sapiens and Arabidopsis thaliana, and generated circSeq reads from the non-model crop plant Beta vulgaris. We clearly identified eccDNA candidates from all datasets, with and without reference genomes. The ECCsplorer pipeline specifically detected mitochondrial mini-circles and retrotransposon activation, showcasing the ECCsplorer's sensitivity and specificity. CONCLUSION: The ECCsplorer (available online at https://github.com/crimBubble/ECCsplorer ) is a bioinformatics pipeline to detect eccDNAs in any kind of organism or tissue using next-generation sequencing data. The derived eccDNA targets are valuable for a wide range of downstream investigations-from analysis of cancer-related eccDNAs over organelle genomics to identification of active transposable elements.

Complete pan-plastome sequences enable high resolution phylogenetic classification of sugar beet and closely related crop wild relatives.

BACKGROUND: As the major source of sugar in moderate climates, sugar-producing beets (Beta vulgaris subsp. vulgaris) have a high economic value. However, the low genetic diversity within cultivated beets requires introduction of new traits, for example to increase their tolerance and resistance attributes - traits that often reside in the crop wild relatives. For this, genetic information of wild beet relatives and their phylogenetic placements to each other are crucial. To answer this need, we sequenced and assembled the complete plastome sequences from a broad species spectrum across the beet genera Beta and Patellifolia, both embedded in the Betoideae (order Caryophyllales). This pan-plastome dataset was then used to determine the wild beet phylogeny in high-resolution. RESULTS: We sequenced the plastomes of 18 closely related accessions representing 11 species of the Betoideae subfamily and provided high-quality plastome assemblies which represent an important resource for further studies of beet wild relatives and the diverse plant order Caryophyllales. Their assembly sizes range from 149,723 bp (Beta vulgaris subsp. vulgaris) to 152,816 bp (Beta nana), with most variability in the intergenic sequences. Combining plastome-derived phylogenies with read-based treatments based on mitochondrial information, we were able to suggest a unified and highly confident phylogenetic placement of the investigated Betoideae species. Our results show that the genus Beta can be divided into the two clearly separated sections Beta and Corollinae. Our analysis confirms the affiliation of B. nana with the other Corollinae species, and we argue against a separate placement in the Nanae section. Within the Patellifolia genus, the two diploid species Patellifolia procumbens and Patellifolia webbiana are, regarding the plastome sequences, genetically more similar to each other than to the tetraploid Patellifolia patellaris. Nevertheless, all three Patellifolia species are clearly separated. CONCLUSION: In conclusion, our wild beet plastome assemblies represent a new resource to understand the molecular base of the beet germplasm. Despite large differences on the phenotypic level, our pan-plastome dataset is highly conserved. For the first time in beets, our whole plastome sequences overcome the low sequence variation in individual genes and provide the molecular backbone for highly resolved beet phylogenomics. Hence, our plastome sequencing strategy can also guide genomic approaches to unravel other closely related taxa.

The conserved 3' Angio-domain defines a superfamily of short interspersed nuclear elements (SINEs) in higher plants.

Repetitive sequences are ubiquitous components of eukaryotic genomes affecting genome size and evolution as well as gene regulation. Among them, short interspersed nuclear elements (SINEs) are non-coding retrotransposons usually shorter than 1000 bp. They contain only few short conserved structural motifs, in particular an internal promoter derived from cellular RNAs and a mostly AT-rich 3' tail, whereas the remaining regions are highly variable. SINEs emerge and vanish during evolution, and often diversify into numerous families and subfamilies that are usually specific for only a limited number of species. In contrast, at the 3' end of multiple plant SINEs we detected the highly conserved 'Angio-domain'. This 37 bp segment defines the Angio-SINE superfamily, which encompasses 24 plant SINE families widely distributed across 13 orders within the plant kingdom. We retrieved 28 433 full-length Angio-SINE copies from genome assemblies of 46 plant species, frequently located in genes. Compensatory mutations in and adjacent to the Angio-domain imply selective restraints maintaining its RNA structure. Angio-SINE families share segmental sequence similarities, indicating a modular evolution with strong Angio-domain preservation. We suggest that the conserved domain contributes to the evolutionary success of Angio-SINEs through either structural interactions between SINE RNA and proteins increasing their transpositional efficiency, or by enhancing their accumulation in genes.

Satellite DNA landscapes after allotetraploidization of quinoa (Chenopodium quinoa) reveal unique A and B subgenomes.

If two related plant species hybridize, their genomes may be combined and duplicated within a single nucleus, thereby forming an allotetraploid. How the emerging plant balances two co-evolved genomes is still a matter of ongoing research. Here, we focus on satellite DNA (satDNA), the fastest turn-over sequence class in eukaryotes, aiming to trace its emergence, amplification, and loss during plant speciation and allopolyploidization. As a model, we used Chenopodium quinoa Willd. (quinoa), an allopolyploid crop with 2n = 4x = 36 chromosomes. Quinoa originated by hybridization of an unknown female American Chenopodium diploid (AA genome) with an unknown male Old World diploid species (BB genome), dating back 3.3-6.3 million years. Applying short read clustering to quinoa (AABB), C. pallidicaule (AA), and C. suecicum (BB) whole genome shotgun sequences, we classified their repetitive fractions, and identified and characterized seven satDNA families, together with the 5S rDNA model repeat. We show unequal satDNA amplification (two families) and exclusive occurrence (four families) in the AA and BB diploids by read mapping as well as Southern, genomic, and fluorescent in situ hybridization. Whereas the satDNA distributions support C. suecicum as possible parental species, we were able to exclude C. pallidicaule as progenitor due to unique repeat profiles. Using quinoa long reads and scaffolds, we detected only limited evidence of intergenomic homogenization of satDNA after allopolyploidization, but were able to exclude dispersal of 5S rRNA genes between subgenomes. Our results exemplify the complex route of tandem repeat evolution through Chenopodium speciation and allopolyploidization, and may provide sequence targets for the identification of quinoa's progenitors.

Divergence of 3' ends as a driver of short interspersed nuclear element (SINE) evolution in the Salicaceae.

Short interspersed nuclear elements (SINEs) are small, non-autonomous and heterogeneous retrotransposons that are widespread in plants. To explore the amplification dynamics and evolutionary history of SINE populations in representative deciduous tree species, we analyzed the genomes of the six following Salicaceae species: Populus deltoides, Populus euphratica, Populus tremula, Populus tremuloides, Populus trichocarpa, and Salix purpurea. We identified 11 Salicaceae SINE families (SaliS-I to SaliS-XI), comprising 27 077 full-length copies. Most of these families harbor segmental similarities, providing evidence for SINE emergence by reshuffling or heterodimerization. We observed two SINE groups, differing in phylogenetic distribution pattern, similarity and 3' end structure. These groups probably emerged during the 'salicoid duplication' (~65 million years ago) in the Salix-Populus progenitor and during the separation of the genus Salix (45-65 million years ago), respectively. In contrast to conserved 5' start motifs across species and SINE families, the 3' ends are highly variable in sequence and length. This extraordinary 3'-end variability results from mutations in the poly(A) tail, which were fixed by subsequent amplificational bursts. We show that the dissemination of newly evolved 3' ends is accomplished by a displacement of older motifs, leading to various 3'-end subpopulations within the SaliS families.

The Cassandra retrotransposon landscape in sugar beet (Beta vulgaris) and related Amaranthaceae: recombination and re-shuffling lead to a high structural variability.

BACKGROUND AND AIMS: Plant genomes contain many retrotransposons and their derivatives, which are subject to rapid sequence turnover. As non-autonomous retrotransposons do not encode any proteins, they experience reduced selective constraints leading to their diversification into multiple families, usually limited to a few closely related species. In contrast, the non-coding Cassandra terminal repeat retrotransposons in miniature (TRIMs) are widespread in many plants. Their hallmark is a conserved 5S rDNA-derived promoter in their long terminal repeats (LTRs). As sugar beet (Beta vulgaris) has a well-described LTR retrotransposon landscape, we aim to characterize TRIMs in beet and related genomes. METHODS: We identified Cassandra retrotransposons in the sugar beet reference genome and characterized their structural relationships. Genomic organization, chromosomal localization, and distribution of Cassandra-TRIMs across the Amaranthaceae were verified by Southern and fluorescent in situ hybridization. KEY RESULTS: All 638 Cassandra sequences in the sugar beet genome contain conserved LTRs and thus constitute a single family. Nevertheless, variable internal regions required a subdivision into two Cassandra subfamilies within B. vulgaris. The related Chenopodium quinoa harbours a third subfamily. These subfamilies vary in their distribution within Amaranthaceae genomes, their insertion times and the degree of silencing by small RNAs. Cassandra retrotransposons gave rise to many structural variants, such as solo LTRs or tandemly arranged Cassandra retrotransposons. These Cassandra derivatives point to an interplay of template switch and recombination processes - mechanisms that likely caused Cassandra's subfamily formation and diversification. CONCLUSIONS: We traced the evolution of Cassandra in the Amaranthaceae and detected a considerable variability within the short internal regions, whereas the LTRs are strongly conserved in sequence and length. Presumably these hallmarks make Cassandra a prime target for unequal recombination, resulting in the observed structural diversity, an example of the impact of LTR-mediated evolutionary mechanisms on the host genome.

Broken, silent, and in hiding: tamed endogenous pararetroviruses escape elimination from the genome of sugar beet (Beta vulgaris).

BACKGROUND AND AIMS: Endogenous pararetroviruses (EPRVs) are widespread components of plant genomes that originated from episomal DNA viruses of the Caulimoviridae family. Due to fragmentation and rearrangements, most EPRVs have lost their ability to replicate through reverse transcription and to initiate viral infection. Similar to the closely related retrotransposons, extant EPRVs were retained and often amplified in plant genomes for several million years. Here, we characterize the complete genomic EPRV fraction of the crop sugar beet (Beta vulgaris, Amaranthaceae) to understand how they shaped the beet genome and to suggest explanations for their absent virulence. METHODS: Using next- and third-generation sequencing data and genome assembly, we reconstructed full-length in silico representatives for the three host-specific EPRVs (beetEPRVs) in the B. vulgaris genome. Focusing on the endogenous caulimovirid beetEPRV3, we investigated its chromosomal localization, abundance and distribution by fluorescent in situ and Southern hybridization. KEY RESULTS: Full-length beetEPRVs range between 7.5 and 10.7 kb in size, are heterogeneous in structure and sequence, and occupy about 0.3 % of the beet genome. Although all three beetEPRVs were assigned to the florendoviruses, they showed variably arranged protein-coding domains, different fragmentation, and preferences for diverse sequence contexts. We observed small RNAs that specifically target the individual beetEPRVs, indicating stringent epigenetic suppression. BeetEPRV3 sequences occur along all sugar beet chromosomes, preferentially in the vicinity of each other and are associated with heterochromatic, centromeric and intercalary satellite DNAs. BeetEPRV3 members also exist in genomes of related wild species, indicating an initial beetEPRV3 integration 13.4-7.2 million years ago. CONCLUSIONS: Our study in beet illustrates the variability of EPRV structure and sequence in a single host genome. Evidence of sequence fragmentation and epigenetic silencing implies possible plant strategies to cope with long-term persistence of EPRVs, including amplification, fixation in the heterochromatin, and containment of EPRV virulence.

FlexiDot: highly customizable, ambiguity-aware dotplots for visual sequence analyses.

Summary: FlexiDot is a cross-platform dotplot suite generating high quality self, pairwise and all-against-all visualizations. To improve dotplot suitability for comparison of consensus and error-prone sequences, FlexiDot harbors routines for strict and relaxed handling of ambiguities and substitutions. Our shading modules facilitate dotplot interpretation and motif identification by adding information on sequence annotations and sequence similarities. Combined with collage-like outputs, FlexiDot supports simultaneous visual screening of large sequence sets, enabling dotplot use for routine analyses. Availability and implementation: FlexiDot is implemented in Python 2.7. Software and documentation are freely available at http://github.com/molbio-dresden/flexidot. Supplementary information: Supplementary data are available at Bioinformatics online.

Adding color to a century-old enigma: multi-color chromosome identification unravels the autotriploid nature of saffron (Crocus sativus) as a hybrid of wild Crocus cartwrightianus cytotypes.

Saffron crocus (Crocus sativus) is the source of the most expensive spice of the world, produced from manually harvested stigmas, thus serving as a cash crop for rural communities. However, despite its economic importance, its genome and chromosomes are poorly studied. C. sativus is a sterile triploid species harboring eight chromosome triplets, and propagated only as a clonal lineage by corms. Saffron's evolutionary origin, parental species and allo- or autotriploidy has been a matter of discussion for almost a century. We performed a survey sequencing of the saffron genome and selected cytogenetic landmark sequences consisting of major tandem repeats, which we used as probes in comparative multicolor fluorescent in situ hybridization (FISH). We tagged 92 chromosomal positions and resolved the chromosomal composition of saffron triplets. By comparative FISH of six Crocus species from 11 accessions, we demonstrate that C. sativus is an autotriploid hybrid derived from heterogeneous Crocus cartwrightianus cytotypes. The FISH reference karyotype of saffron is crucial for integrating genome sequencing data with chromosomes and for investigating the relationship among Crocus species. We provide an evolutionary model of the saffron emergence; the knowledge of the parental origin offers a route towards the resynthesis of C. sativus from C. cartwrightianus to broaden saffron's gene pool.

Next-generation sequencing reveals differentially amplified tandem repeats as a major genome component of Northern Europe's oldest Camellia japonica.

Northern Europe's oldest and largest Camellia japonica growing at the Pillnitz Castle (Germany) for over 200 years is of botanical and cultural importance and is a reference for C. japonica molecular scale analysis. In order to provide a fundament for genome analysis of the genus Camellia, we characterize the C. japonica tandem repeat fraction, constituting 12.5 % of the Pillnitz camellia's genome. A genomic library of the Pillnitz C. japonica was produced and Illumina sequenced to generate 36 Gb of paired-end reads. We performed graph-based read clustering implemented in the RepeatExplorer pipeline to estimate the C. japonica repeat fraction of 73 %. This enabled us to identify and characterize the most prominent satellite DNAs, Camellia japonica satellite 1-4 (CajaSat1-CajaSat4), and the 5S ribosomal DNA (rDNA) by bioinformatics, fluorescent in situ and Southern hybridization. Within the Camellia genus, satellite spreading, array expansion and formation of higher-order structures highlight different modes of repeat evolution. The CajaSat satellites localize at prominent chromosomal sites, including (peri)centromeres and subtelomeres of all chromosomes, thus serving as chromosomal landmarks for their identification. This work provides an insight into the C. japonica chromosome organization and significantly expands the Camellia genomic knowledge, also with respect to the tea plant Camellia sinensis.

Profiling of extensively diversified plant LINEs reveals distinct plant-specific subclades.

A large fraction of eukaryotic genomes is made up of long interspersed nuclear elements (LINEs). Due to their capability to create novel copies via error-prone reverse transcription, they generate multiple families and reach high copy numbers. Although mammalian LINEs have been well described, plant LINEs have been only poorly investigated. Here, we present a systematic cross-species survey of LINEs in higher plant genomes shedding light on plant LINE evolution as well as diversity, and facilitating their annotation in genome projects. Applying a Hidden Markov Model (HMM)-based analysis, 59 390 intact LINE reverse transcriptases (RTs) were extracted from 23 plant genomes. These fall in only two out of 28 LINE clades (L1 and RTE) known in eukaryotes. While plant RTE LINEs are highly homogenous and mostly constitute only a single family per genome, plant L1 LINEs are extremely diverse and form numerous families. Despite their heterogeneity, all members across the 23 species fall into only seven L1 subclades, some of them defined here. Exemplarily focusing on the L1 LINEs of a basal reference plant genome (Beta vulgaris), we show that the subclade classification level does not only reflect RT sequence similarity, but also mirrors structural aspects of complete LINE retrotransposons, like element size, position and type of encoded enzymatic domains. Our comprehensive catalogue of plant LINE RTs serves the classification of highly diverse plant LINEs, while the provided subclade-specific HMMs facilitate their annotation.

The diversification and activity of hAT transposons in Musa genomes.

Sequencing of plant genomes often identified the hAT superfamily as the largest group of DNA transposons. Nevertheless, detailed information on the diversity, abundance and chromosomal localization of plant hAT families are rare. By in silico analyses of the reference genome assembly and bacterial artificial chromosome (BAC) sequences, respectively, we performed the classification and molecular characterization of hAT transposon families in Musa acuminata. Musa hAT transposons are organized in three families designated MuhAT I, MuhAT II and MuhAT III. In total, 70 complete autonomous elements of the MuhAT I and MuhAT II families were detected, while no autonomous MuhAT III transposons were found. Based on the terminal inverted repeat (TIR)-specific sequence information of the autonomous transposons, 1722 MuhAT I- and MuhAT II-specific miniature inverted-repeat transposable elements (MuhMITEs) were identified. Autonomous MuhAT I and MuhAT II elements are only moderately abundant in the sections of the genus Musa, while the corresponding MITEs exhibit an amplification in Musa genomes. By fluorescent in situ hybridization (FISH), autonomous MuhAT transposons as well as MuhMITEs were localized in subtelomeric, most likely gene-rich regions of M. acuminata chromosomes. A comparison of homoeologous regions of M. acuminata and Musa balbisiana BACs revealed the species-specific mobility of MuhMITEs. In particular, the activity of MuhMITEs II showing transduplications of genomic sequences might indicate the presence of active MuhAT transposons, thus suggesting a potential role of MuhMITEs as modulators of genome evolution of Musa.

TE-Tracker: systematic identification of transposition events through whole-genome resequencing.

BACKGROUND: Transposable elements (TEs) are DNA sequences that are able to move from their location in the genome by cutting or copying themselves to another locus. As such, they are increasingly recognized as impacting all aspects of genome function. With the dramatic reduction in cost of DNA sequencing, it is now possible to resequence whole genomes in order to systematically characterize novel TE mobilization in a particular individual. However, this task is made difficult by the inherently repetitive nature of TE sequences, which in some eukaryotes compose over half of the genome sequence. Currently, only a few software tools dedicated to the detection of TE mobilization using next-generation-sequencing are described in the literature. They often target specific TEs for which annotation is available, and are only able to identify families of closely related TEs, rather than individual elements. RESULTS: We present TE-Tracker, a general and accurate computational method for the de-novo detection of germ line TE mobilization from re-sequenced genomes, as well as the identification of both their source and destination sequences. We compare our method with the two classes of existing software: specialized TE-detection tools and generic structural variant (SV) detection tools. We show that TE-Tracker, while working independently of any prior annotation, bridges the gap between these two approaches in terms of detection power. Indeed, its positive predictive value (PPV) is comparable to that of dedicated TE software while its sensitivity is typical of a generic SV detection tool. TE-Tracker demonstrates the benefit of adopting an annotation-independent, de novo approach for the detection of TE mobilization events. We use TE-Tracker to provide a comprehensive view of transposition events induced by loss of DNA methylation in Arabidopsis. TE-Tracker is freely available at http://www.genoscope.cns.fr/TE-Tracker . CONCLUSIONS: We show that TE-Tracker accurately detects both the source and destination of novel transposition events in re-sequenced genomes. Moreover, TE-Tracker is able to detect all potential donor sequences for a given insertion, and can identify the correct one among them. Furthermore, TE-Tracker produces significantly fewer false positives than common SV detection programs, thus greatly facilitating the detection and analysis of TE mobilization events.