The Dark Matter of Large Cereal Genomes: Long Tandem Repeats

Reference genomes of important cereals, including barley, emmer wheat and bread wheat, were released recently. Their comparison with genome size estimates obtained by flow cytometry indicated that the assemblies represent not more than 88–98% of the complete genome. This work is aimed at identifying the missing parts in two cereal genomes and proposing techniques to make the assemblies more complete. We focused on tandemly organised repetitive sequences, known to be underrepresented in genome assemblies generated from short-read sequence data. Our study found arrays of three tandem repeats with unit sizes of 1242 to 2726 bp present in the bread wheat reference genome generated from short reads. However, this and another wheat genome assembly employing long PacBio reads failed in integrating correctly the 2726-bp repeat in the pseudomolecule context. This suggests that tandem repeats of this size, frequently incorporated in unassigned scaffolds, may contribute to shrinking of pseudomolecules without reducing size of the entire assembly. We demonstrate how this missing information may be added to the pseudomolecules with the aid of nanopore sequencing of individual BAC clones and optical mapping. Using the latter technique, we identified and localised a 470-kb long array of 45S ribosomal DNA absent from the reference genome of barley.

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dnAQET: a framework to compute a consolidated metric for benchmarking quality of de novo assemblies

BMC Genomics ◽

10.1186/s12864-019-6070-x ◽

2019 ◽

Vol 20 (1) ◽

Author(s):

Gokhan Yavas ◽

Huixiao Hong ◽

Wenming Xiao

Keyword(s):

Quality Assessment ◽

Genome Assembly ◽

Reference Genome ◽

De Novo ◽

Quality Score ◽

De Novo Genome Assembly ◽

Genome Assemblies ◽

Reference Genomes ◽

Better Than

Abstract Background Accurate de novo genome assembly has become reality with the advancements in sequencing technology. With the ever-increasing number of de novo genome assembly tools, assessing the quality of assemblies has become of great importance in genome research. Although many quality metrics have been proposed and software tools for calculating those metrics have been developed, the existing tools do not produce a unified measure to reflect the overall quality of an assembly. Results To address this issue, we developed the de novo Assembly Quality Evaluation Tool (dnAQET) that generates a unified metric for benchmarking the quality assessment of assemblies. Our framework first calculates individual quality scores for the scaffolds/contigs of an assembly by aligning them to a reference genome. Next, it computes a quality score for the assembly using its overall reference genome coverage, the quality score distribution of its scaffolds and the redundancy identified in it. Using synthetic assemblies randomly generated from the latest human genome build, various builds of the reference genomes for five organisms and six de novo assemblies for sample NA24385, we tested dnAQET to assess its capability for benchmarking quality evaluation of genome assemblies. For synthetic data, our quality score increased with decreasing number of misassemblies and redundancy and increasing average contig length and coverage, as expected. For genome builds, dnAQET quality score calculated for a more recent reference genome was better than the score for an older version. To compare with some of the most frequently used measures, 13 other quality measures were calculated. The quality score from dnAQET was found to be better than all other measures in terms of consistency with the known quality of the reference genomes, indicating that dnAQET is reliable for benchmarking quality assessment of de novo genome assemblies. Conclusions The dnAQET is a scalable framework designed to evaluate a de novo genome assembly based on the aggregated quality of its scaffolds (or contigs). Our results demonstrated that dnAQET quality score is reliable for benchmarking quality assessment of genome assemblies. The dnQAET can help researchers to identify the most suitable assembly tools and to select high quality assemblies generated.

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Assembly of chromosome-scale contigs by efficiently resolving repetitive sequences with long reads

Nature Communications ◽

10.1038/s41467-019-13355-3 ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 10

Author(s):

Huilong Du ◽

Chengzhi Liang

Keyword(s):

Single Molecule ◽

Repetitive Sequences ◽

Tartary Buckwheat ◽

Gene Sequences ◽

Assembly Method ◽

Long Reads ◽

A Genome ◽

Genome Assemblies ◽

Reference Genomes

AbstractThe abundant repetitive sequences in complex eukaryotic genomes cause fragmented assemblies, which lose value as reference genomes, often due to incomplete gene sequences and unanchored or mispositioned contigs on chromosomes. Here we report a genome assembly method HERA, which resolves repeats efficiently by constructing a connection graph from an overlap graph. We test HERA on the genomes of rice, maize, human, and Tartary buckwheat with single-molecule sequencing and mapping data. HERA correctly assembles most of the previously unassembled regions, resulting in dramatically improved, highly contiguous genome assemblies with newly assembled gene sequences. For example, the maize contig N50 size reaches 61.2 Mb and the Tartary buckwheat genome comprises only 20 contigs. HERA can also be used to fill gaps and fix errors in reference genomes. The application of HERA will greatly improve the quality of new or existing assemblies of complex genomes.

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Construction of a new chromosome-scale, long-read reference genome assembly of the Syrian hamster, Mesocricetus auratus

10.1101/2021.07.05.451071 ◽

2021 ◽

Author(s):

R. Alan Harris ◽

Muthuswamy Raveendran ◽

Dustin T Lyfoung ◽

Fritz J Sedlazeck ◽

Medhat Mahmoud ◽

...

Keyword(s):

Genome Assembly ◽

Syrian Hamster ◽

Reference Genome ◽

Sequence Data ◽

Mesocricetus Auratus ◽

Protein Coding ◽

Protein Coding Genes ◽

Sequencing Technologies ◽

Long Read ◽

Short Read Sequence

Background The Syrian hamster (Mesocricetus auratus) has been suggested as a useful mammalian model for a variety of diseases and infections, including infection with respiratory viruses such as SARS-CoV-2. The MesAur1.0 genome assembly was published in 2013 using whole-genome shotgun sequencing with short-read sequence data. Current more advanced sequencing technologies and assembly methods now permit the generation of near-complete genome assemblies with higher quality and higher continuity. Findings Here, we report an improved assembly of the M. auratus genome (BCM_Maur_2.0) using Oxford Nanopore Technologies long-read sequencing to produce a chromosome-scale assembly. The total length of the new assembly is 2.46 Gbp, similar to the 2.50 Gbp length of a previous assembly of this genome, MesAur1.0. BCM_Maur_2.0 exhibits significantly improved continuity with a scaffold N50 that is 6.7 times greater than MesAur1.0. Furthermore, 21,616 protein coding genes and 10,459 noncoding genes were annotated in BCM_Maur_2.0 compared to 20,495 protein coding genes and 4,168 noncoding genes in MesAur1.0. This new assembly also improves the unresolved regions as measured by nucleotide ambiguities, where approximately 17.11% of bases in MesAur1.0 were unresolved compared to BCM_Maur_2.0 in which the number of unresolved bases is reduced to 3.00%. Conclusions Access to a more complete reference genome with improved accuracy and continuity will facilitate more detailed, comprehensive, and meaningful research results for a wide variety of future studies using Syrian hamsters as models.

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Refgenie: a reference genome resource manager

GigaScience ◽

10.1093/gigascience/giz149 ◽

2020 ◽

Vol 9 (2) ◽

Cited By ~ 3

Author(s):

Michał Stolarczyk ◽

Vincent P Reuter ◽

Jason P Smith ◽

Neal E Magee ◽

Nathan C Sheffield

Keyword(s):

Genome Analysis ◽

High Throughput Sequencing ◽

Reference Genome ◽

Sequencing Analysis ◽

Resource Manager ◽

Computing Environments ◽

Downstream Analysis ◽

Genome Assemblies ◽

Server Application ◽

Reference Genomes

Abstract Background Reference genome assemblies are essential for high-throughput sequencing analysis projects. Typically, genome assemblies are stored on disk alongside related resources; e.g., many sequence aligners require the assembly to be indexed. The resulting indexes are broadly applicable for downstream analysis, so it makes sense to share them. However, there is no simple tool to do this. Results Here, we introduce refgenie, a reference genome assembly asset manager. Refgenie makes it easier to organize, retrieve, and share genome analysis resources. In addition to genome indexes, refgenie can manage any files related to reference genomes, including sequences and annotation files. Refgenie includes a command line interface and a server application that provides a RESTful API, so it is useful for both tool development and analysis. Conclusions Refgenie streamlines sharing genome analysis resources among groups and across computing environments. Refgenie is available at https://refgenie.databio.org.

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Benchmark study comparing liftover tools for genome conversion of epigenome sequencing data

NAR Genomics and Bioinformatics ◽

10.1093/nargab/lqaa054 ◽

2020 ◽

Vol 2 (3) ◽

Cited By ~ 1

Author(s):

Phuc-Loi Luu ◽

Phuc-Thinh Ong ◽

Thanh-Phuoc Dinh ◽

Susan J Clark

Keyword(s):

Reference Genome ◽

Sequence Data ◽

Cost Effective ◽

Sequencing Data ◽

Original Sequence ◽

Benchmark Study ◽

Paired Samples ◽

Bisulphite Sequencing ◽

Genome Assemblies ◽

Mapping File

Abstract As reference genome assemblies are updated there is a need to convert epigenome sequence data from older genome assemblies to newer versions, to facilitate data integration and visualization on the same coordinate system. Conversion can be done by re-alignment of the original sequence data to the new assembly or by converting the coordinates of the data between assemblies using a mapping file, an approach referred to as ‘liftover’. Compared to re-alignment approaches, liftover is a more rapid and cost-effective solution. Here, we benchmark six liftover tools commonly used for conversion between genome assemblies by coordinates, including UCSC liftOver, rtracklayer::liftOver, CrossMap, NCBI Remap, flo and segment_liftover to determine how they performed for whole genome bisulphite sequencing (WGBS) and ChIP-seq data. Our results show high correlation between the six tools for conversion of 43 WGBS paired samples. For the chromatin sequencing data we found from interval conversion of 366 ChIP-Seq datasets, segment_liftover generates more reliable results than USCS liftOver. However, we found some regions do not always remain the same after liftover. To further increase the accuracy of liftover and avoid misleading results, we developed a three-step guideline that removes aberrant regions to ensure more robust genome conversion between reference assemblies.

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Accurate sequence variant genotyping in cattle using variation-aware genome graphs

10.1101/460345 ◽

2018 ◽

Cited By ~ 1

Author(s):

Danang Crysnanto ◽

Christine Wurmser ◽

Hubert Pausch

Keyword(s):

Dna Sequences ◽

Reference Genome ◽

Sequence Data ◽

Sequence Variant ◽

Sequence Variants ◽

Sequencing Data ◽

Reference Allele ◽

Reference Genomes ◽

Genome Graph ◽

Genotype Concordance

Background: The genotyping of sequence variants typically involves as a first step the alignment of sequencing reads to a linear reference genome. Because a linear reference genome represents only a small fraction of sequence variation within a species, reference allele bias may occur at highly polymorphic or diverged regions of the genome. Graph-based methods facilitate to compare sequencing reads to a variation-aware genome graph that incorporates non-redundant DNA sequences that segregate within a species. We compared accuracy and sensitivity of graph-based sequence variant genotyping using the Graphtyper software to two widely used methods, i.e., GATK and SAMtools, that rely on linear reference genomes using whole-genomes sequencing data of 49 Original Braunvieh cattle. Results: We discovered 21,140,196, 20,262,913 and 20,668,459 polymorphic sites using GATK, Graphtyper, and SAMtools, respectively. Comparisons between sequence variant and microarray-derived genotypes showed that Graphtyper outperformed both GATK and SAMtools in terms of genotype concordance, non-reference sensitivity, and non-reference discrepancy. The sequence variant genotypes that were obtained using Graphtyper had the lowest number of mendelian inconsistencies for both SNPs and indels in nine sire-son pairs with sequence data. Genotype phasing and imputation using the Beagle software improved the quality of the sequence variant genotypes for all tools evaluated particularly for animals that have been sequenced at low coverage. Following imputation, the concordance between sequence- and microarray-derived genotypes was almost identical for the three methods evaluated, i.e., 99.32, 99.46, and 99.24 % for GATK, Graphtyper, and SAMtools, respectively. Variant filtration based on commonly used criteria improved the genotype concordance slightly but it also decreased sensitivity. Graphtyper required considerably more computing resources than SAMtools but it required less than GATK. Conclusions: Sequence variant genotyping using Graphtyper is accurate, sensitive and computationally feasible in cattle. Graph-based methods enable sequence variant genotyping from variation-aware reference genomes that may incorporate cohort-specific sequence variants which is not possible with the current implementations of state-of-the-art methods that rely on linear reference genomes.

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Refgenie: a reference genome resource manager

10.1101/698704 ◽

2019 ◽

Cited By ~ 2

Author(s):

Michal Stolarczyk ◽

Vincent P. Reuter ◽

Neal E. Magee ◽

Nathan C. Sheffield

Keyword(s):

High Throughput Sequencing ◽

Reference Genome ◽

Sequencing Analysis ◽

Tool Development ◽

Resource Manager ◽

Reference Genome Assembly ◽

Downstream Analysis ◽

Genome Assemblies ◽

Server Application ◽

Reference Genomes

Reference genome assemblies are essential for high-throughput sequencing analysis projects. Typically, genome assemblies are stored on disk alongside related resources; for example, many sequence aligners require the assembly to be indexed. The resulting indexes are broadly applicable for downstream analysis, so it makes sense to share them. However, there is no simple tool to do this. To this end, we introduce refgenie, a reference genome assembly asset manager. Refgenie makes it easier to organize, retrieve, and share genome analysis resources. In addition to genome indexes, refgenie can manage any files related to reference genomes, including sequences and annotation files. Refgenie includes a command-line interface and a server application that provides a RESTful API, so it is useful for both tool development and analysis.Availabilityhttps://refgenie.databio.org

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Mapping whole genome shotgun sequence and variant calling in mammalian species without their reference genomes

F1000Research ◽

10.12688/f1000research.2-244.v1 ◽

2013 ◽

Vol 2 ◽

pp. 244 ◽

Cited By ~ 4

Author(s):

Ted Kalbfleisch ◽

Michael P. Heaton

Keyword(s):

Gene Function ◽

Reference Genome ◽

Sequence Data ◽

Association Studies ◽

Mammalian Species ◽

Variant Calling ◽

Ovis Aries ◽

Genome Sequences ◽

Mammalian Gene ◽

Reference Genomes

Genomics research in mammals has produced reference genome sequences that are essential for identifying variation associated with disease. High quality reference genome sequences are now available for humans, model species, and economically important agricultural animals. Comparisons between these species have provided unique insights into mammalian gene function. However, the number of species with reference genomes is small compared to those needed for studying molecular evolutionary relationships in the tree of life. For example, among the even-toed ungulates there are approximately 300 species whose phylogenetic relationships have been calculated in the 10k trees project. Only six of these have reference genomes: cattle, swine, sheep, goat, water buffalo, and bison. Although reference sequences will eventually be developed for additional hoof stock, the resources in terms of time, money, infrastructure and expertise required to develop a quality reference genome may be unattainable for most species for at least another decade. In this work we mapped 35 Gb of next generation sequence data of a Katahdin sheep to its own species’ reference genome (Ovis aries Oar3.1) and to that of a species that diverged 15 to 30 million years ago (Bos taurus UMD3.1). In total, 56% of reads covered 76% of UMD3.1 to an average depth of 6.8 reads per site, 83 million variants were identified, of which 78 million were homozygous and likely represent interspecies nucleotide differences. Excluding genome repeat regions and sex chromosomes, approximately 3.7 million heterozygous sites were identified in this animal vs. bovine UMD3.1, representing polymorphisms occurring in sheep. Of these, 41% could be readily mapped to orthologous positions in ovine Oar3.1 with 80% corroborated as heterozygous. These variant sites, identified via interspecies mapping could be used for comparative genomics, disease association studies, and ultimately to understand mammalian gene function.

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Mapping whole genome shotgun sequence and variant calling in mammalian species without their reference genomes

F1000Research ◽

10.12688/f1000research.2-244.v2 ◽

2014 ◽

Vol 2 ◽

pp. 244 ◽

Cited By ~ 5

Author(s):

Ted Kalbfleisch ◽

Michael P. Heaton

Keyword(s):

Gene Function ◽

Reference Genome ◽

Sequence Data ◽

Association Studies ◽

Mammalian Species ◽

Variant Calling ◽

Ovis Aries ◽

Genome Sequences ◽

Mammalian Gene ◽

Reference Genomes

Genomics research in mammals has produced reference genome sequences that are essential for identifying variation associated with disease. High quality reference genome sequences are now available for humans, model species, and economically important agricultural animals. Comparisons between these species have provided unique insights into mammalian gene function. However, the number of species with reference genomes is small compared to those needed for studying molecular evolutionary relationships in the tree of life. For example, among the even-toed ungulates there are approximately 300 species whose phylogenetic relationships have been calculated in the 10k trees project. Only six of these have reference genomes: cattle, swine, sheep, goat, water buffalo, and bison. Although reference sequences will eventually be developed for additional hoof stock, the resources in terms of time, money, infrastructure and expertise required to develop a quality reference genome may be unattainable for most species for at least another decade. In this work we mapped 35 Gb of next generation sequence data of a Katahdin sheep to its own species’ reference genome (Ovis aries Oar3.1) and to that of a species that diverged 15 to 30 million years ago (Bos taurus UMD3.1). In total, 56% of reads covered 76% of UMD3.1 to an average depth of 6.8 reads per site, 83 million variants were identified, of which 78 million were homozygous and likely represent interspecies nucleotide differences. Excluding repeat regions and sex chromosomes, nearly 3.7 million heterozygous sites were identified in this animal vs. bovine UMD3.1, representing polymorphisms occurring in sheep. Of these, 41% could be readily mapped to orthologous positions in ovine Oar3.1 with 80% corroborated as heterozygous. These variant sites, identified via interspecies mapping could be used for comparative genomics, disease association studies, and ultimately to understand mammalian gene function.

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One Major Challenge of Sequencing Large Plant Genomes Is to Know How Big They Really Are

International Journal of Molecular Sciences ◽

10.3390/ijms19113554 ◽

2018 ◽

Vol 19 (11) ◽

pp. 3554 ◽

Cited By ~ 5

Author(s):

Jaroslav Doležel ◽

Jana Čížková ◽

Hana Šimková ◽

Jan Bartoš

Keyword(s):

Genome Size ◽

Reference Genome ◽

Nuclear Dna ◽

Sequence Data ◽

Cereal Crops ◽

Dna Amount ◽

A Genome ◽

Human Genome Assembly ◽

Genome Assemblies ◽

Measured Mass

Any project seeking to deliver a plant or animal reference genome sequence must address the question as to the completeness of the assembly. Given the complexity introduced particularly by the presence of sequence redundancy, a problem which is especially acute in polyploid genomes, this question is not an easy one to answer. One approach is to use the sequence data, along with the appropriate computational tools, the other is to compare the estimate of genome size with an experimentally measured mass of nuclear DNA. The latter requires a reference standard in order to provide a robust relationship between the two independent measurements of genome size. Here, the proposal is to choose the human male leucocyte genome for this standard: its 1C DNA amount (the amount of DNA contained within unreplicated haploid chromosome set) of 3.50 pg is equivalent to a genome length of 3.423 Gbp, a size which is just 5% longer than predicted by the most current human genome assembly. Adopting this standard, this paper assesses the completeness of the reference genome assemblies of the leading cereal crops species wheat, barley and rye.

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