Biology subjects

Heitkam, T.

Publications and source records attributed to Heitkam, T..

17 recordsLinked to original sources

bioRxiv2026

Chromosomal variability in a clonal crop: Somaclonal change follows the emergence of triploid saffron crocus

(1) BackgroundSaffron crocus (Crocus sativus) is the source of saffron, the most expensive spice in the world. It evolved about 3000 years ago as a sterile triploid clone in Greece. Since then, saffron has spread across the globe, where regionally distinct practices of saffron cultivation have developed. Despite differences in morpho-physiological traits, genetic variability is low, if present at all. Here, we aim to resolve chromosomal and sequence-associated variability across saffron crocus cultivars from the crops main cultivation areas in Africa, Asia and Europe. (2) MethodsWe used genome-wide DNA polymorphisms obtained through genotyping-by-sequencing (GBS) of 33 saffron and 14 closely related Crocus accessions, which we place into a phylogenetic context. For karyotyping, we compare nine saffron accessions by multi-color fluorescent in situ hybridisation (FISH) with repetitive DNA probes. (3) Key resultsPhylogenetic analyses confirmed the single origin and clonal nature of all saffron accessions. We detected slight DNA differences among saffron crocus genotypes, which were minor compared with those in wild C. cartwrightianus populations. Still, the Iranian saffron accessions form a genetically very narrow group that differs from the other proveniences in population genetic analyses. However, chromosomes of some saffron accessions display variable FISH signals, likely resulting from gains and losses of tandemly repeated DNA. (4) Main conclusionsBased on the high genetic identity and small karyotypic differences, we confirm the clonal origin of the saffron accessions. Nevertheless, as we detected small and regional chromosomal variability, we conclude that at least four somaclonal saffron lineages emerged after saffrons origin. Societal Impact StatementFor millennia, many cultures developed cultivation practices and regional crop varieties. A notable case is saffron, the worlds most expensive spice that is harvested from stigmas of saffron crocus. This flower crop arose 3000 years ago in a singular genome triplication event and since then spread clonally across the globe. By identifying genetic and chromosomal variability in clonal saffron accessions, we highlight regional diversity, support the preservation of traditional knowledge, and underscore the risk of relying on only one clonal lineage. This informs strategies for saffron cultivation, linking cultural heritage with modern genomics to address biodiversity, evolution, and food security.

plant biology↗

bioRxiv2026

Hide and seek: de novo identification in sugar beet reveals impact of non-autonomous LTR retrotransposons

Plant genomes are filled with retrotransposons and their derivatives, subject to constant sequence turnover. As short, non-autonomous retrotransposons do not encode a protein product, they experience reduced selective constraints on their DNA sequence, leading to diversification into multiple families, usually limited to only a few species. This absence of any coding capacity and their tendency to form subfamilies are the reasons for the incomplete description of non-autonomous LTR retrotransposons in most to all genomic repeat annotations. Here, we focus on non-autonomous LTR retrotransposon identification. Are all of these sequences derivatives of easier-to-identify full-length elements? Or is there more variability, which is currently overlooked? For this, we capitalize on our comprehensive understanding of the TE landscape in sugar beet to assess the extent of the blind spot on non-autonomous LTR retrotransposons Here, we present a workflow to identify non-autonomous LTR retrotransposons without prior sequence information, retrieving more than 100 families within the sugar beet genome. We only include TEs without the ability for complete self mobilization. Spanning up to 15,000 bp, these non-autonomous families are often longer than expected and characterized by reshuffling and modular evolution. Most strikingly, only a few of these families are directly derived from autonomous partners, showing that there is a large, undiscovered TE variety in the non-autonomous TE fraction. We highlight that a large fraction of non-autonomous TEs wont be retrieved with the current TE identification workflows, even if the output is well-curated and condensed into TE libraries and suggest procedures to remedy this gap. This study is the first insight into the non-autonomous LTR retrotransposon landscape within a single genome and serves as an example to estimate the error in non-autonomous TE detection.

genomics↗

bioRxiv2024

Diverging repeatomes in holoparasitic Hydnoraceae uncover a playground of genome evolution

O_LIThe nuclear genomes of parasitic plants have undergone unique evolutionary trajectories to adapt to the heterotrophic lifestyle. These adaptations often involve large genomic alterations, potentially driven by repetitive DNAs. Despite the recognized role of repetitive DNAs for shaping plant genomes, their contribution to parasitic plant genome evolution remains largely unexplored. This study presents the first analysis of repetitive DNAs in eleven genomes of Hydnoraceae, a holoparasitic plant family. C_LIO_LIThe repetitive DNAs were identified and annotated de novo. Patterns of abundance and presence-absence were aligned with the phylogenetic relationships, geographical distribution, and host shifts, suggesting repetitive DNAs key role in shaping Hydnoraceae genomes. C_LIO_LIThe two Hydnoraceae genera, Hydnora and Prosopanche, exhibit distinct profiles. Eight Hydnora genomes are largely populated by long terminal repeat retrotransposons, while three Prosopanche genomes vary greatly in repetitive DNA composition, including P. bonacinae with massive amplifications of a single DNA transposon and P. panguanensis with over 15% 5S rDNA, compared to <0.1% in some Hydnoraceae. Both extremely low and very high abundance of 5S rDNA challenges our current understanding on chromosomal stabilization and rDNA transcription. C_LIO_LIThese genome dynamics, while inviting for further investigations, suggest rapidly evolving repeat profiles, potentially accelerated by the adaptation to the parasitic lifestyle. C_LI

evolutionary biology↗

bioRxiv2024

Preparing high-quality chromosome spreads from Crocus species for karyotyping and FISH

BackgroundThe saffron-producing Crocus sativus and its wild relative C. cartwrightianus are key species for understanding genetic evolution in this genus. Molecular-cytogenetic methods, especially fluorescent in situ hybridization (FISH), are essential for exploring the genetic relationships in this genus. Yet, preparing high-quality chromosomes for FISH analysis across Crocus species remains difficult. A standardized protocol for achieving clear and well-separated mitotic chromosomes is still lacking. This study assesses the effectiveness of four chromosome fixation methods for optimal chromosome spread preparation in Crocus. Root tips of different Crocus species were treated with four chromosome preparation methods namely hydroxyurea-colchicine (HC), nitrous oxide (NO), hydroxyquinoline (HQ), and ice water (IW) pretreatments to investigate their effectiveness in producing high-quality mitotic chromosome spreads. Metaphases obtained by the four methods were analyzed to assess their quality and metaphase index. ResultsEvaluation of 22,507 nuclei allowed us to confidently recommend a protocol for Crocus chromosome preparation. Among the methods, ice water pretreatment yielded the highest metaphase index (2.05%), more than doubling the results of HC (1.08%), NO (1.15%), and HQ (1.16%). Ice water-treated chromosomes exhibited better chromosome morphology, with relatively proper size, and non-overlapping chromosomes that were optimal for FISH analysis. Ice water pretreatment was also applied to C. cartwrightianus, the diploid progenitor of C. sativus, where it demonstrated similar efficacy. DAPI staining of chromosomes in both species allowed for clear visualization of intercalary and terminal heterochromatin. FISH analysis using 18S-5.8S-25S and 5S rDNA probes confirmed the utility of IW-prepared chromosome spreads for cytogenetic studies. ConclusionsWe strongly recommend ice water pretreatment as a suitable and effective method for obtaining many metaphase spreads of high-quality in C. sativus and related species, particularly for applications involving a detailed cytogenetic analysis.

plant biology↗

bioRxiv2024

Repeatome landscapes and cytogenetics of hortensias provide a framework to trace Hydrangea evolution and domestication

Background and AimsOrnamental hortensias are bred from a reservoir of over 200 species in the genus Hydrangea s.l. and are valued in gardens, households and landscapes across the globe. The phenotypic diversity of hortensia cultivars, hybrids and wild relatives is mirrored by their genomic variation, with differences in genome size, base chromosome numbers and ploidy level. We aim to understand the genomic and chromosomal basis of hortensia genome variation. Therefore, we analyze six hortensias with different origins and chromosomal setups for repeatome divergence, the genome fraction with the highest sequence turnover. This holds information from the hortensias evolutionary paths and can inform breeding initiatives. MethodsWe compiled a hortensia genotype panel representing members of the sections Macrophyllae, Hydrangea, Asperae, and Heteromallae and reconstructed a plastome-based phylogenetic hypothesis as evolutionary basis for all our analyses. We comprehensively characterized the repeatomes by whole genome sequencing and comparative repeat clustering. Major tandem repeats were localized by multi-color FISH. Key ResultsThe Hydrangea species show differing repeat profiles reflecting their separation into the two major Hydrangea clades: Diploid Hydrangea species from Japan show a conserved repeat profile, distinguishing them from Japanese polyploids as well as Chinese and American hortensias. These results are in line with plastome-based phylogenies. The presence of specific repeats indicates that H. paniculata was not polyploidized directly from the common ancestor of Japanese Hydrangea species, but evolved from a distinct progenitor. Major satellite DNAs were detected over all H. macrophylla chromosomes. ConclusionsRepeat composition among the Hydrangea species varies in congruence with their origins and phylogeny. Identified species-specific satDNAs may be used as cytogenetic markers to identify Hydrangea species and cultivars, and to infer parental species of old Hydrangea varieties. This repeatome and cytogenetics information helps to expand the genetic toolbox for tracing hortensia evolution and informing future hortensia breeding.

genomics↗

bioRxiv2023

Comparative cytogenetics of kenaf (Hibiscus cannabinus L.) breeding lines reveal chromosomal variability and instability

Kenaf (Hibiscus cannabinus), a native warm-seasonal crop in Africa, is being considered for genetic improvement for local bast fiber production. To expedite its genetic improvement through breeding, kenaf genotypes from Ghana were assessed for genomic diversity regarding their chromosomal composition and ploidy levels. To gain insight into the repetitive DNA fractions in kenaf, the organization of 5S and 35S rRNA genes, as well as telomeric signal patterns were studied by a molecular cytogenetic approach. Using multi-color fluorescent in situ hybridization, distinct rDNA loci and Arabidopsis-type telomeres were revealed. The 5S rRNA genes were conserved in kenaf and localized in interstitial regions of two chromosomes across all accessions. The 35S rRNA genes were variable across the kenaf accessions and localized at sub-terminal ends and rarely interstitially in eight or six chromosome arms. Telomeric signals were observed at terminal ends of all chromosomes, with smaller signals also interstitially. The chromosome configuration of Ghana kenaf accessions was confirmed to be 2n=2x=36, each. We discuss the chromosomal variability and the likely genomic instability in the kenaf breeding lines from Ghana. To our knowledge, this is the first report on molecular cytogenetics on kenaf and thus, provides valuable insights into the genome of kenaf that will be useful for breeding. Additionally, this study provides a basis for further studies to analyze the repetitive DNA sequences and develop reference karyotypes to reveal genetic and evolutionary relationships between cultivated and wild Hibiscus species.

plant biology↗

bioRxiv2023

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

BackgroundDespite 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 full-length repeat consensuses be derived from unassembled genomes? ResultsHere, 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. ConclusionWe 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.

genomics↗

bioRxiv2023

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

BackgroundSugar beet (Beta vulgaris subsp. vulgaris) and its crop 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 DNA sequences represent the fastest evolving parts of the genome, they likely impact genomic variability and contribute to the separation of beet gene pools. Hence, we investigated if innovations and losses in the repeatome can be linked to chromosomal differentiation and speciation. ResultsWe traced genome- and chromosome-wide evolution across sugar beet and twelve wild beets comprising all sections of the beet genera Beta and Patellifolia. For this, we combined data from short and long read sequencing, flow cytometry, and cytogenetics to build a comprehensive data framework for our beet panel that spans the complete scale from DNA sequence to chromosome up to the genome. Genome sizes and repeat profiles reflect the separation of the beet species into three gene pools. These gene pools harbor repeats with contrasting evolutionary patterns: We identified section- and species-specific repeat emergences and losses, e.g. of the retrotransposons causal for genome expansions in the section Corollinae/Nanae. Since most genomic variability was found in the satellite DNAs, we focused on tracing the 19 beetSat families across the three beet sections/genera. These taxa harbor evidence for contrasting strategies in repeat evolution, leading to contrasting satellite DNA profiles and fundamentally different centromere architectures, ranging from chromosomal uniformity in Beta and Patellifolia species to the formation of patchwork chromosomes in Corollinae/Nanae species. ConclusionsWe show that repetitive DNA sequences are causal for genome size expansion and contraction across the beet genera, providing insights into the genomic underpinnings of beet speciation. Satellite DNAs in particular vary considerably among beet taxa, leading to the evolution of distinct chromosomal setups. These differences likely contribute to the barriers in beet breeding between the three gene pools. Thus, with their isokaryotypic chromosome sets, beet genomes present an ideal system for studying the link between repeats, genome variability, and chromosomal differentiation/evolution and provide a theoretical basis for understanding barriers in crop breeding.

genomics↗

bioRxiv2023

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 non-autonomous retrotransposons, so-called Cassandra. Cassandras harbor highly conserved 5S rDNA-related sequences within their long terminal repeats (LTRs), 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 hosts 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 Cassandras origin hypotheses (single or multiple) and Cassandras possible impact on rDNA and plant genome organization, giving new insights into the interplay of ribosomal genes and transposable elements.

evolutionary biology↗

bioRxiv2023

Pangenome of cultivated beet and crop wild relatives reveals parental relationships of a tetraploid wild beet

Most crop plants, including sugar beet (Beta vulgaris subsp. vulgaris), suffer from domestication bottlenecks and low genetic diversity caused by extensive selection for few traits. However, crop wild relatives (CWRs) harbour useful traits relevant for crop improvement, including enhanced adaptation to biotic and abiotic stresses. Especially polyploids are interesting from an evolutionary perspective as genes undergo reorganisation after the polyploidisation event. Through neo-and subfunctionalisation, novel functions emerge, which enable plants to cope with changing environments and extreme/harsh conditions. Particularly in the face of climate change, specific stress and pathogen resistances or tolerances gain importance. To introduce such traits into breeding material, CWRs have already been identified as an important source for sustainable breeding. The identification of genes underlying traits of interest is crucial for crop improvement. For beets, the section Corollinae contains the tetraploid species Beta corolliflora (2n=4x=36) that harbours salt and frost tolerances as well as a wealth of pathogen resistances. The number of beneficial traits of B. corolliflora is increased compared to those of the known diploids in this section (all 2n=2x=18). Nevertheless, neither the parental relationships of B. corolliflora have been resolved, nor are genomic resources available to steer sustainable, genomics-informed breeding. To benefit from the resources offered by polyploid beet wild relatives, we generated a comprehensive pangenome dataset including B. corolliflora, Beta lomatogona, and Beta macrorhiza, as well as a more distant wild beet Patellifolia procumbens (2n=2x=18). Joined analyses with publicly available genome sequences of two additional wild beets allowed the identification of genomic regions absent from cultivated beet, providing a sequence database harbouring traits relevant for future breeding endeavours. In addition, we present strong evidence for the parental relationship of the B. corolliflora wild beet as an autotetraploid emerging from B. macrorhiza.

genomics↗

bioRxiv2022

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

In 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 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 clarifying why some transposable elements are removed quickly whereas others persist in plant genomes. Here, we bring forward the first 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 amplification after polyploidization, and trace hybridization events 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 one hand, we detect 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.

plant biology↗

bioRxiv2021

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

BackgroundAs 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. ResultsWe 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. ConclusionIn 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.

genomics↗

bioRxiv2021

Genome-wide analysis of long terminal repeat retrotransposons from the cranberry Vaccinium macrocarpon

BACKGROUNDLong terminal repeat (LTR) retrotransposons are widespread in plant genomes and play a large role in the generation of genomic variation. Despite this, their identification and characterization remains challenging, especially for non-model genomes. Hence, LTR retrotransposons remain undercharacterized in Vaccinium genomes, although they may be beneficial for current berry breeding efforts. OBJECTIVEExemplarily focusing on the genome of American cranberry (Vaccinium macrocarpon Aiton), we aim to generate an overview of the LTR retrotransposon landscape, highlighting the abundance, transcriptional activity, sequence, and structure of the major retrotransposon lineages. METHODSGraph-based clustering of whole genome shotgun Illumina reads was performed to identify the most abundant LTR retrotransposons and to reconstruct representative in silico full-length elements. To generate insights into the LTR retrotransposon diversity in V. macrocarpon, we also queried the genome assembly for presence of reverse transcriptases (RTs), the key domain of LTR retrotransposons. Using transcriptomic data, transcriptional activity of retrotransposons corresponding to the consensuses was analyzed. RESULTSWe provide an in-depth characterization of the LTR retrotransposon landscape in the V. macrocarpon genome. Based on 475 RTs harvested from the genome assembly, we detect a high retrotransposon variety, with all major lineages present. To better understand their structural hallmarks, we reconstructed 26 Ty1-copia and 28 Ty3-gypsy in silico consensuses that capture the detected diversity. Accordingly, we frequently identify association with tandemly repeated motifs, extra open reading frames, and specialized, lineage-typical domains. Based on the overall high genomic abundance and transcriptional activity, we suggest that retrotransposons of the Ale and Athila lineages are most promising to monitor retrotransposon-derived polymorphisms across accessions. CONCLUSIONSWe conclude that LTR retrotransposons are major components of the V. macrocarpon genome. The representative consensuses provide an entry point for further Vaccinium genome analyses and may be applied to derive molecular markers for enhancing cranberry selection and breeding.

plant biology↗

bioRxiv2021

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

MotivationExtrachromosomal 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. ResultsHere, 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 semiartificial and published circSeq data from the model organisms H. sapiens and A. thaliana, and generated circSeq reads from the non-model crop B. 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 ECCsplorers sensitivity and specificity. 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. Availability and implementationThe ECCsplorer pipeline is available on GitHub at https://github.com/crimBubble/ECCsplorer under the GNU license. ContactTony Heitkam (tony.heitkam@tu-dresden.de) Supplementary informationSupplementary data are available online.

bioinformatics↗

bioRxiv2021

Comparative repeat profiling of two closely related conifers (Larix decidua and Larix kaempferi) reveals high genome similarity with only one fast-evolving satellite DNA

In eukaryotic genomes, cycles of repeat expansion and removal lead to large-scale genomic changes and propel organisms forward in evolution. However, in conifers, active repeat removal is thought to be limited, leading to expansions of their genomes, mostly exceeding 10 gigabasepairs. As a result, conifer genomes are largely littered with fragmented and decayed repeats. Here, we aim to investigate how the repeat landscapes of two related conifers have diverged, given the conifers accumulative genome evolution mode. For this, we applied low coverage sequencing and read clustering to the genomes of European and Japanese larch, Larix decidua (Lamb.) Carriere and Larix kaempferi (Mill.), that arose from a common ancestor, but are now geographically isolated. We found that both Larix species harbored largely similar repeat landscapes, especially regarding the transposable element content. To pin down possible genomic changes, we focused on the repeat class with the fastest sequence turnover: satellite DNAs (satDNAs). Using comparative bioinformatics, Southern, and fluorescent in situ hybridization, we reveal the satDNAs organizational patterns, their abundances, and chromosomal locations. Four out of the five identified satDNAs are widespread in the Larix genus, with two even present in the more distantly related Pseudotsuga and Abies genera. Unexpectedly, the EulaSat3 family was restricted to L. decidua and absent from L. kaempferi, indicating its evolutionarily young age. Taken together, our results exemplify how the accumulative genome evolution of conifers may limit the overall divergence of repeats after speciation, producing only few repeat-induced genomic novelties.

plant biology↗

bioRxiv2020

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

Background and AimsEndogenous 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. MethodsUsing next- and third-generation sequencing data and the genome assembly, we reconstructed full-length in silico representatives for the three host-specific EPRV families (beetEPRVs) in the B. vulgaris genome. Focusing on the canonical family beetEPRV3, we investigated its chromosomal localization, abundance, and distribution by fluorescent in situ and Southern hybridization. Key ResultsBeetEPRVs range between 7.5 and 10.7 kb (0.3 % of the B. vulgaris genome) and are heterogeneous in structure and sequence. Although all three beetEPRV families were assigned to the florendoviruses, they showed variably arranged protein-coding domains, different degrees of fragmentation, and preferences for diverse sequence contexts. We observed small RNAs that target beetEPRVs in a family-specific manner, indicating stringent epigenetic suppression. We localized beetEPRV3 on all 18 sugar beet chromosomes, occurring preferentially in clusters and associated with heterochromatic, centromeric and intercalary satellite DNAs. BeetEPRV3 variants also exist in the genomes of related wild species, indicating an initial beetEPRV3 integration 13.4 to 7.2 million years ago. ConclusionsOur study in beet illustrates the variability of EPRV structure and sequence in a single host genome. Evidence of sequence fragmentation and epigenetic silencing imply possible plant strategies to cope with long-term persistence of EPRVs, including amplification, fixation in the heterochromatin, and containment of EPRV virulence.

plant biology↗

bioRxiv2019

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

If two related plant species hybridise, their genomes are combined 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 allopolyploidisation. As a model, we used Chenopodium quinoa Willd. (quinoa), an allopolyploid crop with 2n=4x=36 chromosomes. Quinoa originated by hybridisation of an unknown female American Chenopodium diploid (AA genome) with an unknown male Old World diploid species (BB genome), dating back 3.3 to 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 characterised 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 hybridisation. As C. pallidicaule harbours a unique satDNA profile, we are able to exclude it as quinoas parental species. Using quinoa long reads and scaffolds, we detected only limited evidence of interlocus homogenisation of satDNA after allopolyploidisation, 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 allopolyploidisation, and may provide sequence targets for the identification of quinoas progenitors.

plant biology↗