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Sequencing and Chromosome-Scale Assembly of Plant Genomes, Brassica rapa as a Use Case

Google Scholar Preprints 2021
Istace B. et al

Benjamin Istace 1*, Caroline Belser 1*, Cyril Falentin2
, Karine Labadie3
, Franz Boideau2
, Gwenaëlle Deniot2
, Loeiz
, Corinne Cruaud3
, Laurie Bertrand1
, Anne-Marie Chèvre2
, Patrick Wincker1
, Mathieu Rousseau-Gueutin2
Jean-Marc Aury1

1 Génomique Métabolique, Genoscope, Institut François Jacob, CEA, CNRS, Univ Evry, Université Paris-Saclay, 2
Rue Gaston Crémieux, Evry, 91057, France; bistace@genoscope.cns.fr; cbelser@genoscope.cns.fr; lbertrand@genoscope.cns.fr; pwincker@genoscope.cns.fr; jmaury@genoscope.cns.fr
IGEPP, INRAE, Institut Agro, Université de Rennes, Domaine de la Motte, 35653 Le Rheu, France; cyril.falentin@inrae.fr; franz.boideau@inrae.fr; gwenaelle.deniot@inrae.fr; loeiz.maillet@inrae.fr;
anne-marie.chevre@inrae.fr; mathieu.rousseau-gueutin@inrae.fr
3 Genoscope, Institut François Jacob, Commissariat à l’Energie Atomique (CEA), Université Paris-Saclay, 2 Rue
Gaston Crémieux, Evry, 91057, France; klabadie@genoscope.cns.fr; cruaud@genoscope.cns.fr
* These authors contributed equally.
Correspondence: jmaury@genoscope.cns.fr
Simple Summary: Reconstructing plant genomes is a difficult task due to their often large sizes,
unusual ploidy and large numbers of repeated elements. However, the field of sequencing is
changing very rapidly, with new and improved methods released every year. The ultimate goal of
this study is to provide readers with insights into techniques that currently exist for obtaining
high-quality and chromosome-scale assemblies of plant genomes. In this work, we presented the
advanced techniques already existing in the field and illustrated their application to reconstruct the
genome of the yellow sarson, Brassica rapa cv. Z1.
Abstract: With the rise of long-read sequencers and long-range technologies, delivering
high-quality plant genome assemblies is no longer reserved to large consortia. Indeed, sequencing
techniques but also computer algorithms have reached a point where the reconstruction of assemblies at the chromosome-scale is now feasible at the laboratory scale. Current technologies, and
especially long-range technologies, are numerous and selecting the most promising one for the
genome of interest is crucial to obtain optimal results. In this study, we resequenced the genome of
the yellow sarson, Brassica rapa cv. Z1, using the Oxford Nanopore PromethION sequencer and
assembled the sequenced data using current assemblers. To reconstruct complete chromosomes,
we used and compared three long-range techniques, optical mapping, Omni-C and Pore-C sequencing libraries commercialized by Bionano Genomics, Dovetail Genomics and Oxford Nanopore Technologies respectively, or a combination of the three, in order to evaluate the capability
of each technology.
Keywords: Genome; Assembly; Scaffolding; Chromosome-scale; Nanopore; Optical map; Bionano;
Omni-C; Pore-C; Plants
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© 2021 by the author(s). Distributed under a Creative Commons CC BY license.
1. Introduction
Assembling plant genomes has always been one of the most complex tasks in bioinformatics applied to genomics. Indeed, they often contain many repeated elements,
such as satellites or transposable elements. This leads to an increase in the size of the
genome which can then reach tens of gigabases, like the loblolly pine genome, which is
22Gb in size[1]. Moreover, the difficulty is further increased due to the high levels of
heterozygosity and highly variable ploidy[2]. All these characteristics made reconstruction of such genomes almost impossible without the help of a large consortium. However, the appearance of Illumina[3] technology, fifteen years ago, combined with significant
sequencing depth and the advent of assemblers using De Bruijn graphs paved the way
for low-cost genome assemblies. However, the resulting assemblies remained highly
fragmented and most repeats were not resolved.
Recently, the Pacific Biosciences (PacBio) and Oxford Nanopore Technologies (ONT)
single-molecule sequencing technologies have been commercialized, which offered the
opportunity to sequence fragments of several tens of kilobases, thus facilitating the assembly of complex genomes. However, this increase in the size of the sequenced fragments has a cost in terms of read’s quality. Indeed, Nanopore and PacBio raw reads show
an error rate of about 7% and 10% respectively[4,5]. Due to errors remaining in long reads
assemblies, a significant proportion of predicted genes may contain frameshifts[6], thus
reducing the size of the predicted proteins, which in turn could cause problems in
downstream analyses. To circumvent this issue, a number of polishing algorithms have
surfaced. First, polishing algorithms were using the same type of data as what was used
to perform the assembly, as is the case in Nanopolish, for Nanopore data, or Quiver, for
PacBio data. However, errors like indels were still present in genome assemblies and researchers quickly realized that pairing long reads data to generate the assembly and
high-quality reads, like the ones produced by Illumina sequencers, lead to the least
number of errors. This was, as an example, implemented in Pilon[7], Racon[8] or
Hapo-G[9]. These gradual improvements in base quality and read length allowed researchers to generate high-quality plant genomes, reaching, for the simplest (haploid
genomes), chromosome scale, without the need to add long-range technologies[10,11].
Even if the base quality of long reads has improved over the years, assembling and
phasing heterozygous genomes still remains a difficult task. To try to solve this problem,
PacBio drastically improved the quality of its generated sequencing reads by using a
technique called Circular Consensus Sequencing (CCS) to generate high fidelity (HiFi)
reads, thus breaking the 1% error rate barrier[12]. While standard PacBio reads are generated by a single-pass of an enzyme around a circularized template, HiFi reads are
produced thanks to multiple passes of the same enzyme. This led to improvements in the
base quality of the assemblies and made it possible to assemble and phase large genomes[13]. In a similar manner, ONT recently revealed their new Q20+ sequencing kit,
which uses a new enzyme and optimized run conditions to lower the raw read error rate
to 1%. It should be noted however that, at the moment of writing, we could not test the
Q20+ kit and, therefore, could not validate the results shown by Oxford Nanopore.
However, this increase in read quality is not sufficient for all plant genomes. Indeed,
highly heterozygous genomes or duplicated genomes suffer from regions that are often
collapsed in genome assemblies. As an example, the recently sequenced tetraploid genome of potato[14] featured large regions (of the Mb order) that are identical between
haplotypes, making the assembly impossible even with reads of several kilobases. To
overcome this problem, a new technique called “gamete binning” has been developed
recently. This technique, used to assemble the diploid apricot genome[15] and the autotetraploid potato genome[14], relies on single cell sequencing of many gametes. This, in
turn, makes it possible to reliably assign sequencing reads to a particular haplotype and
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
assemble separately long reads from the same haplotype. This simplifies the genome assembly process by reducing the complexity of the dataset and enables a haplotype-by-haplotype assembly approach.
Although these technologies significantly simplify genome assembly, the resulting
contigs often do not reflect the chromosomal organization of the original genome. Optical
mapping is one of the long-range technologies now commonly used, in particular since
the development of the Direct Label and Stain (DLS) protocol by Bionano Genomics
(BNG). This protocol is designed to fluorescently label and repair high molecular weight
(HMW) DNA at a specific location composed of 6 nucleotides (5’CTTAAG3’) using
nicking endonucleases. In contrast to the previous version (Nick Label Repair and Stain,
NLRS), this labelling preserves the double stranded DNA and avoids fragmentation of
long DNA molecules[16]. These labelled molecules are charged into a flow cell and migrate into nanochannels in order to stay linear. The fluorescence signal is scanned and the
images are converted into molecule files (containing the position of the markers on each
molecule) with a provided software. The optical map is generated by assembling the individual molecules into larger ones which can represent chromosomes. An optical card
does not contain sequences but only labeling positions and distances between these positions, which can be seen as a barcode. The older NLRS protocol is less and less used, as
optical maps generated thanks to this protocol are generally less contiguous due to the
labelling process that tends to induce breaks into DNA molecules. Indeed, the enzyme
recognizes a double stranded sequence of 7 nucleotides (5’GCTCTTC3’) and operates as a
restriction enzyme. If several restriction sites are very close to each other, it could be a
possible break point during the labelling step[17].
These two optical maps can be combined in the scaffolding process allowing users to
add the benefit of the two labelling methods. Sequences from a given assembly are in
silico digested and the estimated position of the labels on the contigs are compared to
those of the optical maps. The scaffolding process detects assembly errors in the contigs,
breaks them in order to be consistent with the optical maps, and finally orients and orders the contigs to produce chromosome or chromosome arm scale sequences.
The Chromosomal conformation capture technique can also be used to obtain
high-continuity assemblies and give information on chromosomal regions adjacent in the
nucleus[18]. Indeed, chromosomes fold into topologically associating domains
(TADs)[19] and then sequences form loops and other folds. The number of contacts decreases as a function of genomic distance. The chromatin is fixed in order to preserve its
Three-Dimensional (3D) organization. An enzymatic digestion and a proximity ligation
step produce a chimeric fragment containing two portions of DNA distant on the chromosomal sequence but close in the nucleus space. The relative abundance of each ligation
product is related to the probability that those DNA sequences interact in the 3D space.
After library preparation and short read sequencing, the paired reads represent long
distance linking information. The mapping of the Illumina paired reads on the assembly
highlights the long-distance links between two contigs or scaffolds. Library preparation
kits are commercialized by several companies, for example Dovetail Genomics who
propose a kit named Omni-C. It has the advantage of using an endonuclease for the digestion step instead of restriction enzymes, as in other kits, which avoids biases in digestion.
The Pore-C library preparation is quite similar to the Hi-C library preparation using
restriction enzymes, but the DNA fragments can be a concatenation of several chimeric
junctions. After sequencing, it is possible and expected that Nanopore reads contain the
multiple interacting sites. Indeed, as Nanopore reads are long, they span entire amplicons. To determine from which part of the genome the Nanopore reads originates, an
in-silico digestion of the reference genome is performed and each segment of the read is
assigned to a subsection of the reference. Although relatively young[20], the Pore-C
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
technology seems to be interesting and competitive with traditional Hi-C for the scaffolding of complex genomes[21].
Traditionally the anchoring of sequences along the chromosomes was performed
using genetic maps[22]. These are obtained by genotyping a segregating population using genetic markers, such as SNPs (Single Nucleotide Polymorphism). The polymorph
SNPS are then arranged on linkage groups according to the recombination frequency
between markers. Thereafter, the localization of genetically mapped markers on scaffolds
allows the validation, orientation and anchoring of the scaffolds onto pseudomolecules[23]. However, some regions are still difficult to anchor due to a low density of genetic markers or to the absence of recombination between markers, especially in (peri)centromeric regions[24] that are particularly rich in repetitive sequences[25] and thus
require specific markers for anchoring[26].
Although sequenced with long reads in a previous study[27], the Brassica rapa (cv. Z1,
AA, 2n=20) genome still contains a large number of unknown bases (33Mb, representing
8.2% of the assembly). Brassica genus includes many important crops that are cultivated
worldwide notably for their oil production or as vegetables. In addition, these species are
one of the best models to study the importance of polyploidy in plant evolution, diversification and adaptation, due to the occurrence of both ancient and recent polyploidization events[28]. Moreover, the B.rapa chromosomes [29] contain large centromeric regions
that are notoriously difficult to assemble (at least more complicated than the C genome
which contains, however, more transposable elements)[30], motivating us to resequence
B. rapa cv.Z1 with the Oxford Nanopore technology and obtain the best assembly possible using current assemblers and long-range techniques.
2. Materials and Methods
2.1. DNA extraction for nanopore sequencing
High-quality and high-molecular-weight (HMW) DNA was extracted in order to generate long reads using ONT. For this purpose, DNA was isolated from one gram of plant
leaves previously placed in the dark following the protocol provided by Oxford Nanopore Technologies, “High molecular weight gDNA extraction from plant leaves”
downloaded from the ONT Community in March, 2019. This protocol involves a conventional CTAB extraction followed by a purification using the commercial Qiagen Genomic tip (QIAGEN, MD, USA), and is described in detail in Belser et al.[11]. HMW
gDNA quality was checked on a 2200 TapeStation automated electrophoresis system
(Agilent, CA, USA) and the length of the DNA molecules was estimated to be over 60 Kb.
2.2. Nanopore sequencing
Two libraries were prepared simultaneously according to the following protocol and
using the Oxford Nanopore SQK-LSK109 kit. Genomic DNA fragments (2µg) were repaired and 3’-adenylated with the NEBNext FFPE DNA Repair Mix and the NEBNext®
Ultra™ II End Repair/dA-Tailing Module (New England Biolabs, Ipswich, MA, USA).
Sequencing adapters provided by ONT (Oxford Nanopore Technologies Ltd, Oxford,
UK) were then ligated using the NEBNext Quick Ligation Module (NEB). After purification with AMPure XP beads (Beckmann Coulter, Brea, CA, USA), the two libraries have
been pooled into one. One third of the library was mixed with the Sequencing Buffer
(ONT) and the Loading Bead (ONT) and loaded on a PromethION R9.4.1 flow cell. A
second third of the library was then loaded onto the flow cell after a Nuclease Flush using
the Flow Cell Wash Kit EXP-WSH003 (ONT) according to the Oxford Nanopore protocol.
ONT reads were basecalled using Guppy version 4.0.1.
2.3. Nanopore genome assembly and polishing
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Three sets of reads were generated (Supplementary Table 1), in order to test assemblers
with different read coverages and datasets. The first one was composed of all reads and
for the second one, we selected a 30X coverage of the longest reads. The last one was
composed of 30X of the highest-scoring Filtlong[31] reads. Then, we launched
Smartdenovo[32] (git commit 8488de9), Wtdbg2[33] (git commit b77c565) and Flye[34]
(version 2.8.3) on all sets. In addition, Necat[35] (git commit d377878) was launched with
the complete readset, as it corrects reads given as input and applies its own downsampling algorithm. We launched Smartdenovo with “-k 17”, as advised by the developers in
case of larger genomes and “-c 1” to generate a consensus sequence. Redbean was
launched with “-xont -X5000 -g500m” and Flye with “-g 500m”. NECAT was launched
with a genome size of 500Mb and other parameters were left as default.
The assembly produced by Necat was retained as it was the closest to the expected size of
the genome and the most contiguous (Supplementary Tables 2, 3, 4 and 5). The Necat
assembly was polished one time by using Racon (version 1.4.13) and Medaka[36] (version
1.2.0) with Nanopore reads and twice with Illumina reads by using Hapo-G (version 1.0).
Racon was launched with the following parameters: “-m 8 -x -6 -g -8 -w 500 -u”, as advised by the Racon developers and Medaka was launched with the “-m
r941_prom_high_g360” parameter, in order to comply with the version of the basecaller
that we used. Finally, Hapo-G was launched with the “-u” parameter.
2.4. Optical mapping and hybrid scaffolding
Two optical maps were generated using previously generated molecules[27] and the
assembly pipeline developed by Bionano Genomics (BNG) with the following two options “add pre-assembly” and “non haplotype without extend and split”. The
two-enzyme hybrid scaffolding pipeline (bionano solve and tools Version: 1.6.1) was
used to generate the hybrid scaffolds (Table 1). BisCoT[37] was used to correct artifactual
duplications (negative gaps) introduced during the scaffolding process. A final step of
polishing was performed using Hapo-G and Illumina[27] sequencing data.
2.5. Omni-C library preparation and Illumina sequencing
The Dovetail Omni-C library was prepared using the Dovetail Hi-C preparation kit
(Dovetail Genomics, Scotts Valley, CA, USA), according to the manufacturer’s protocol
(manual version 1.0 for non-mammalian samples), using young frozen leaves previously
placed in the dark. Briefly, after sample crosslinking, chromatin was digested using a
sequence-independent endonuclease. Proximity ligation (which creates chimeric molecules) was performed using a biotin-labeled bridge between the ends of the digested
DNA. After reversal crosslinking, the DNA was purified and followed by library generation (omitting the fragmentation step). Finally, the biotinylated chimeric molecules were
captured and amplified before sequencing on the Novaseq 6000 instrument (Illumina,
San Diego, CA, USA) using 150 base-length read chemistry in paired-end mode.
2.6. Scaffolding using the Omni-C library
Scaffolding was realized thanks to the 3D de novo assembly (3D-DNA[38]) pipeline
(version 180419). Hi-C raw reads were aligned against the assembly (-s none option) using Juicer. The resulting merged_nodups.txt file and the assembly were given to the
run-asm-pipeline.sh script with the options “–editor-repeat-coverage 5
–splitter-coarse-stringency 30 –editor-coarse-resolution 100000”. Contact maps were
visualized through the Juicebox tool [39](version 1.11.08) and edited to adjust the construction of scaffolds or break misjoins. After edition, the new .assembly file was downloaded from the Juicebox interface. The file is filtered and converted into a fasta file
thanks to the juicebox_assembly_converter.py script[40].
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2.7. Pore-C library preparation and nanopore sequencing
The RE-Pore-C library was carried out as described in the Oxford Nanopore Technologies RE-Pore-C protocol for plant samples (30th July, 2020 version), with the exception of
tissue fixation, which was performed following the protocol of Chang Liu[41]. Crosslinked plant nuclei were isolated prior to in situ restriction digestion with NlaIII (New
England Biolabs, Ipswich, MA, USA). After overnight incubation, the restriction enzyme
was heat-denatured; crosslinked DNA clusters were ligated in proximity, followed by
protein degradation and de-crosslinking, releasing the chimeric Pore-C dsDNA polymers. Libraries were constructed using the ONT Ligation Sequencing Kit (SQK-LSK109),
following the library preparation recommendations. Sequencing was carried out on a
R.9.4.1 PromethION flowcell. Nuclease washes were used to maximize output.
2.8. Scaffolding using the Pore-C library
Prior to scaffolding, we removed all nanopore Pore-C reads that were larger than 100kb
in size, as it drastically increases running times and can be problematic for the pipeline to
handle as stated in a github issue[42]. Then, we used the Pore-C Snakemake[43] pipeline
(git commit 6b2f762) developed by ONT to generate the necessary files for scaffolding
with the Salsa2[44] scaffolder. Both Salsa2 and the Pore-C Snakemake pipeline were
launched with default parameters.
2.9. Super-scaffolding
In addition to testing the scaffolding of the nanopore assembly with one long-range
technology, we combined several techniques to test if combining scaffolding methods
would lead to better results. In particular, we super-scaffolded the BNG scaffolds with
the Pore-C and Omni-C libraries and vice versa. Tools and parameters were the same as
already described in previous sections. The 3D-DNA additional option -r 0 was used for
scaffolding the BNG scaffolds with the Omni-C library.
2.10. Validation of the assemblies
2.10.1. Comparison to reference genomes
We used the B.rapa cv. Chiifu v3[45] and the B.napus cv. Darmor-BZH v10[30]
A-subgenome to generate dotplots and compare our different assemblies to established
reference genomes. To do so, we used minimap2 with the “-x asm20” parameter to generate alignments of the assemblies against each reference. Then, we used D-Genies[46] in
order to visualize the alignments and enabled the “Sort contigs” and “Hide noise” options.
2.10.2 Quality assessment with merqury
We used merqury (version 1.3, git commit 6b5405e) to obtain a quality score for each of
our assemblies. First, we used the bundled best_k.sh script with a tolerable collision rate
of 0.0001 and a genome size of 500Mb to find the best size of kmer for our genome, which
gave us a kmer size of 21. Then, we used meryl (version 1.3, git commit 3400615) to
compute the 21-mer counts with Illumina reads via the meryl count command with default parameters. Finally, we used merqury to compare the kmers of each assembly to
kmers extracted from the Illumina reads.
2.10.3 Gene-completeness estimation with Busco
In order to estimate the gene completeness of the genome assemblies, we launched Busco
version 5.1.2 with the embryophyta datasets (odb10). All other options were left as default.
2.11. Construction of a B. rapa genetic map and anchoring
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To construct a B. rapa genetic map, we created a F2 population (149 plants) deriving
from an initial cross between the doubled haploid B. rapa ssp. trilocularis cv. Z1 and the
inbred line B. rapa ssp pekinensis cv. Chiifu-401-42. DNA from the parental lines, the F1
hybrid and the 149 plants were extracted using the sbeadex plant kit (LGC Genomics,
Teddington Middlesex, UK) on the oKtopure robot at the GENTYANE platform (INRAE,
Clermont-Ferrand, France), and thereafter genotyped using the Brassica 19K Illumina
infinium SNP array (TraitGenetics, Germany). From these data, a total of 4030 markers
were found polymorph between the parental lines and were used to create a genetic map
(985.9 cM in total) using CarthaGene[47] software version 1.2.3, with a LOD score of 4
and a maximal genetic distance of 0.21 cM. For all these genetically mapped markers, we
then blasted their sequence contexts against the B. rapa cv. Z1 scaffolds obtained using
either additional Bionano data only or both Bionano and Omni-C data, totalling 3867 and
3865 physically anchored markers respectively. This step allowed us to help with the
ordering and orientation of scaffolds, as well as to compare the quality of both assemblies.
2.12. Putative position of peri-centromere and sub-telomere regions
The putative position of (peri)centromeres that were inferred by blasting against this
novel assembly the centromere-specific repeat sequences CentBr1 and CentBr2
(CW978699, CW978837[48]), as well as sequences that are found in the pericentromeric
heterochromatin blocks of Brassica chromosomes: the centromere-specific Ty1/copia-like
retrotransposon of Brassica (CRB, AC166739), the pericentromeric Ty3/Gypsy-like retrotransposon of B.rapa (PCRBr, ACC166740), a 238-bp degenerate tandem repeat (TR238,
AC166740) and a 805-bp tandem repeat (TR805, AC166739[49]) (Lim et al. 2007). The
putative position of subtelomeres were inferred by blasting against this novel assembly
the B. rapa subtelomeric satellite repeats (pBrSTRa/b, EU294384 and EU294385[50]).
2.12. Gene prediction
Gene prediction was performed using several proteomes: 8 from other genotypes of B.
napus[51] (Westar, Zs11, QuintaA, Zheyou73, N02127, GanganF73, Tapidor3, and Shengli3), Arabidopsis thaliana (UP000006548) and the 2021 annotation of Darmor-bzh[30].
Regions of low complexity in genomic sequences were masked with the DustMasker algorithms[52] (version 1.0.0 from the blast 2.10.0 package). Proteomes were then aligned
on the genome in a 2-step strategy. First, BLAT[53] (version 36 with default parameter)
was used to quickly localize corresponding putative regions of these proteins on the genome. The best match and the matches with a score ≥90% of the best match score have
been retained. Second, alignments were refined using Genewise[54] (version 2.2.0 default
parameters), which is more accurate for detecting intron/exon boundaries. Alignments
were kept if >75% of the length of the protein was aligned on the genome.
All the protein alignments were combined using Gmove[55], which is an easy-to-use
predictor with no need for a pre-calibration step. Briefly, putative exons and introns, extracted from alignments, were used to build a graph, where nodes and edges represent,
respectively, exons and introns. Gmove extracts all paths from the graph and searches
open reading frames that are consistent with the protein evidence. Finally, we decided to
exclude single-exon genes composed of >80% of untranslated regions. Following this
pipeline, we predicted 56,073 genes with 4.39 exons per gene on average.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
3. Results
3.1. Nanopore sequencing and long reads assembly
A single PromethION R9.4.1 flowcell produced 93Gb of data with an N50 of 26.9kb
(Table S1), representing a genome coverage of approximately 186X, with 38X being
composed of reads longer than 50Kb.
We subsampled data as previously described (30X longest, 30X highest scoring
Filtlong reads, and all reads). Flye and Wtdbg2 produced their most contiguous assembly
with a subset of reads, showing that experimenting with different coverages may be
beneficial for the assembly. Necat with the entire readset led to the most contiguous assembly with a cumulative size of 442Mb, a contig N50 of 10.4Mb, a merqury quality score
of 27.5 and 1,567 (97.1%) complete and single-copy embryophyta BUSCO genes (Table
S5). After polishing with Nanopore and Illumina reads (Table S6), the merqury quality
score rose to 36.4 and the number of complete embryophyta BUSCO genes increased to
1,604 (99.4%). By aligning the Necat assembly to the B. rapa cv. Chiifu v3 and the B. napus
Darmor-bzh v10 reference genomes (Figure S1), we could detect the presence of a chimeric contig, showing a translocation between the A07 and A03 chromosomes. This
chimeric junction was left as is and used to see if scaffolding algorithms would be able to
correct it.
3.2. Long range genome assembly
3.2.1 Hybrid scaffolding
The produced DLE and BspQI maps achieved a N50 of 13.5Mb and 1.9Mb and a
cumulative size of 466Mb and 434Mb respectively. The combination of the two optical
maps with the nanopore contigs lead to an assembly that reached a N50 of 16.8Mb with a
cumulative size of 446.4Mb (Table S7). Very few gaps were introduced underlining the
great concordance between the length of the optical maps and the nanopore assembly.
Finally, as expected, BisCoT software increased the N50 and decreased the cumulative
size. Indeed, this was expected because BisCoT is designed to remove artifactual tandem
duplications (negative gaps) and the possible redundancies in the assembly. A final
round of polishing gave an assembly with a N50 of 17Mb and a cumulative size of
443,9Mb (Table S7). The comparisons with the Chiifu genome and Darmor-bzh
A-subgenome did neither show a chimeric scaffold nor a misorganization (Figure S2).
3.2.2. Hi-C scaffolding
The sequencing of the Omni-C library produced 108M paired reads (32Gb) that
were aligned on the nanopore polished contigs using the Juicer pipeline. The mapping
information was used by 3D-DNA[38] to orient and order sequences of the input assembly. A contact map was generated and visualized through the Juicebox[39] interface. We
edited the contact map and merged some scaffolds (two larger scaffolds were constructed
by merging two scaffolds for each) and broke one misjoin (Figure S9). We obtained an
assembly of 439Mb with a N50 of 25.5Mb (Table 1). Comparisons with the Chiifu genome
and B. napus A-subgenome did neither show a misjoin nor a misorganization (Figure S3).
3.2.3. Pore-C scaffolding
The sequencing of the Pore-C library (Table S8) produced 15.55Gb of data with an
N50 of 3.99Kb. As advised by the developers of the Pore-C Snakemake pipeline, we removed reads that were longer than 100Kb and obtained a resulting dataset composed of
5.8 million reads with an N50 of 3.97Kb, for a total cumulative size of 15.50Gb. This
readset was used to perform all scaffoldings with the Pore-C technology presented in this
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
We applied the Pore-C Snakemake pipeline released by Oxford Nanopore, as well as
Salsa2 to scaffold the Necat assembly (Table 1) and obtained 253 scaffolds, for a total
cumulative size of 443Mb with a scaffold N50 of 20.1Mb. After aligning scaffolds onto the
reference genomes and inspecting alignments and the resulting dotplots (Figure S4), we
could not find any evidence indicating the presence of chimeric sequences.
3.3. Combination of several long-range techniques
Additionally, we investigated whether the combination of long-range technologies
could add the benefits of each (Figure 1). For this purpose, we scaffolded the Omni-C and
Pore-C assemblies with the optical maps. Hi-C scaffolding sometimes introduces misjoins (chimera or chromosome arm inversion) that could be corrected by the comparison
with optical maps. Surprisingly, the scaffolding of the Omni-C assembly (Omni-C+BNG
assembly) decreased the contiguity and added more than 2% of undetermined bases. It
produced an assembly of 445Mb with a N50 of 20.3Mb. On the other hand, the
Pore-C+BNG assembly produced an assembly of 450Mb and a N50 of 25.9Mb. The N50
slightly increased but scaffolds did not achieve the chromosome scale. When comparing
final assemblies to reference genomes, no misjoins were detected with the exception of a
part of the A01 chromosome being duplicated in the Pore-C+BNG assembly.
Conversely, we scaffolded the BNG scaffolds with the Hi-C or the Pore-C libraries (Table
2). As the BNG scaffolds were close to reach chromosome scale, we expected that with the
long distance contacts of the Hi-C we could organize and orient the BNG scaffolds into
complete chromosomes. We obtained a BNG+Omni-C assembly of 440Mb with a N50 of
33.3Mb after editing the contact map (Figure S10). The size of the 10 largest scaffolds
were compatible with chromosome length and dotplots showed good consistency with
the B. napus A-subgenome and the Chiifu genome (Figure S5). Likewise, we obtained a
BNG+Pore-C assembly of nearly 444Mb with a N50 of 17Mb (Figure S6). Again the
alignments with reference genomes did not show any inconsistencies but no scaffold
achieved the chromosome scale. As metrics were similar to those obtained with the BNG
scaffolds, we put aside the BNG+Pore-C assembly.
3.4. Anchoring
Based on the previous results, the anchoring with the genetic map was performed on
two assemblies : the BNG scaffolds and the BNG+Omni-C scaffolds. The comparison of
the position of the markers allows us to verify the structure of the scaffolds, to assign and
orientate scaffolds onto chromosomes. We were able to anchor 384Mb of the BNG scaffolds and 369Mb of the BNG+Omni-C scaffolds on the chromosomes (compared to
357Mb of anchored sequences in the V1 version). The putative chromosomes are composed of two to five BNG scaffolds or one to two BNG+Omni-C scaffolds. No misjoins
were detected in the BNG scaffolds, but one inversion was detected in the BNG+Omni-C
scaffolds. To construct the final version, we decided to keep the 36 anchored BNG scaffolds that contained more sequences. However, we used the Omni-C data to add two
scaffolds in chromosome A03 and one scaffold in chromosome A09 which were difficult
to anchor with the genetic map. In the end, we succeeded to anchor 386.05Mb (86.96 % of
the whole genome assembly) . The final B. rapa cv. Z1 version 2 is composed of 210 scaffolds with a cumulative size of around 444Mb and a N50 of 39,2Mb (L50=5) (Table 3 and
Figure 2). Among the ten chromosomes, seven chromosomes contained telomeric repeats
(TTTAGGG motif) at both ends and the three others had telomeric repeats at one end. In
the same way, subtelomeric repeats have been found in each chromosome (except for
chromosome A03) (Figure 2). Compared to the B. rapa cv. Z1 version 1, the cumulative
size and the contig N50 have greatly increased (6.6Mb and 10.2Mb for versions 1 and 2
respectively) showing that the new assembly covered a higher proportion of the estimated genome length (Table 3). In the same way, the rate of undetermined bases
dropped from 8.22 to 0.66%. The final comparison of the genetic and physical positions
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
revealed that only 179 out of the 3867 markers (4.63%) were discordant, most often due to
an inaccurate position on the genetic map (of a few cM).
4. Discussion
In this study, we used the genome of B. rapa cv. Z1 to feature how plant genomes can
now be generated. We chose the Oxford Nanopore PromethION technology and also
compared current long-range scaffolding techniques, namely Bionano optical mapping
and the Hi-C and Pore-C chromatin conformation capture techniques, in order to obtain
chromosome-scale assembly.
We sequenced the genome of B. rapa cv. Z1 on a single R9.4.1 PromethION flowcell,
and showed that sequencing a medium-sized genome is now affordable to individual
laboratories as the total cost of sequencing, including consumables, was about
$1,110. The great amount of data we obtained, added to the size of the reads, made it
possible for us to assemble the genome using Necat into large contigs, with an N50 of
10.4Mb. However, after aligning this assembly to the B. rapa cv. Chiifu or the B. napus
cv. Darmor-bzh A-subgenome, we detected a chimeric contig, showing a translocation
from the A07 to the A03 chromosome. We polished the contigs using a combination of
Nanopore data and Illumina data and reached a quality score of 36.4, representing an
error rate of 0.02%. Remarkably, the quality score was already high, even when polishing
only with Nanopore reads (31.04), showing that the Nanopore sequencing technology
made sizable progress since its beginnings[57].
The polished contigs were then scaffolded using optical maps or two chromatin
conformation capture techniques, Omni-C and Pore-C. Regarding the continuity of the
resulting scaffolds, the ones obtained with Omni-C had the largest N50 (25Mb) but the
BNG and Pore-C scaffolds were very close to it, with an almost identical L50. However, a
scaffold N90 of 3.4Mb seemed to indicate that scaffolding with optical maps makes it
possible to anchor a higher proportion of small contigs. In comparison, the N90 of the
Pore-C and Omni-C assemblies were 1.47Mb and 222kb, respectively. Scaffolding with
optical maps also introduced 2.9Mb of unknown bases (Ns), since it is the only technique
tested here that can estimate gap sizes, other techniques only inserting an arbitrary
amount in each gap, giving the false impression that the assembly is complete. Finally,
when scaffolds were compared to reference genomes, we were pleased to see that every
technique was able to successfully correct the chimeric region that was initially present in
Nanopore contigs.
As we wanted to see if combining long-range technologies would lead to a better
assembly, we scaffolded polished Nanopore contigs by combining two long-range techniques. We were able to reach chromosome-scale by using optical maps first and then
Omni-C. Indeed, we obtained a single scaffold per chromosome assembly (N50 of 33.3Mb
and L50 of 5), with 2.9Mb (0.67%) of undetermined bases.
In contrast, we noticed that Omni-C or Pore-C scaffolds were fragmented when integrated with optical maps (Figures S11 and S12). Indeed, the sequences are broken when
conflicts are detected between the optical maps and the assembly. Since we know that in
our case the Hi-C scaffolding did not produce chimeras (Figures S3 and S4), we assume
that the conflicts may result from a poor estimate of the gap size during the Hi-C scaffolding process. In this case, the scaffolds are chained with fixed gap sizes (100 nucleotides) even if some small contigs can take place in these gaps, with the consequence of
generating mapping conflicts with the optical maps. This is because BNG software cannot handle underestimated gap sizes. Therefore, we do not recommend chaining, in this
order, Hi-C scaffolding and then optical maps.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
The use of the genetic map allowed us to obtain complete chromosomes from the
BNG assembly and to validate the BNG+Omni-C scaffolds. These assemblies were chosen for their global metrics: BNG scaffolds had good N50 and N90, BNG+Omni-C scaffolds already reached chromosome scale. The quantity of anchored bases is lower with
the BNG+Omni-C assembly showing that the Omni-C scaffolding had fragmented scaffolds during the scaffolding process. Highly repeated regions are more likely difficult to
organize with Hi-C data and the very large centromeres in Z1 were problematic. As an
advantage of the optical map, complex regions as centromere or rDNA genes clusters are
generally well covered thanks to the very long DNA molecules (>200Kb long), allowing
the organization of long-read assemblies.
In our opinion, optical mapping is the recommended long-range technology that
leads to complete assemblies mostly in complex regions like centromeres but requires
additional tools which are not provided by BNG. Indeed, bioinformatic tools dedicated to
optical maps are rare which makes the use of this data more complicated than Hi-C
short-reads. Additionally, it could be difficult to extract high molecular weight DNA in
particular for plant genomes. Obtaining very long DNA molecules without any contaminant as polyphenol compounds that can inhibit the enzymatic reactions is critical. In our
own hands, the DLE-1 enzyme was more sensitive to contaminants than the previous
enzyme BspQI. The alternative, in the case of unsuccessful optical map preparation, is to
switch to Hi-C technology for which sequencing libraries and bioinformatic analysis are
easier. Indeed, Hi-C can produce chromosome-scale assemblies even if the anchoring of
complex regions, like centromeres, is less effective and results should be reviewed with
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Table S1:
Metrics of Nanopore datasets, Table S2: Metrics of raw Smartdenovo Nanopore assemblies, Table
S3: Metrics of raw Flye Nanopore assemblies, Table S4: Metrics of raw Wtdbg2 Nanopore assemblies, Table S5: Metrics of the raw Necat Nanopore assembly, Table S6: Metrics of the Necat assembly after each polishing step, Table S7: Hybrid scaffolding results, Table S8: Metrics of the
Pore-C raw PromethION run with (left column) or without (right column) reads of more than
100kb in size, Figure S1: Dotplots of polished Necat contigs aligned to Brassica rapa Chiifu (left) or
Brassica napus Darmor-BZH A genome (right), Figure S2: Dotplots of Bionano scaffolds aligned to
Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH A genome (right), Figure S3: Dotplots of
Omni-C scaffolds aligned to Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH A genome
(right), Figure S4: Dotplots of Pore-C scaffolds aligned to Brassica rapa Chiifu (left) or Brassica
napus Darmor-BZH A genome (right), Figure S5: Dotplots of Bionano + Omni-C scaffolds aligned
to Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH A genome (right), Figure S6. Dotplots
of Bionano + Pore-C scaffolds aligned to Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH
A genome (right), Figure S7: Dotplots of Omni-C + Bionano scaffolds aligned to Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH A genome (right), Figure S8: Dotplots of Pore-C + Bionano
scaffolds aligned to Brassica rapa Chiifu (left) or Brassica napus Darmor-BZH A genome (right),
Figure S9: Contact Map generated with the Omni-C library (mapping on the nanopore contigs),
Figure S10: Contact Map generated with the Omni-C library (mapping on the bionano scaffolds),
Figure S11: Conflict example detected by the optical maps on the Omni-C scaffolds, Figure S12:
Conflict example detected by the optical maps on the Pore-C scaffold.
Author Contributions: Benjamin Istace: Methodology, Software, Validation, Writing – Original
Draft Caroline Belser: Methodology, Software, Validation, Writing – Original Draft Cyril Falentin: Resources, Software, Validation, Formal analysis Karine Labadie: Resources, Franz Boideau:
Software, Formal analysis Gwenaëlle Deniot: Resources Loeiz Maillet: Software, Formal analysis Corinne Cruaud: Resources Laurie Bertrand: Resources Anne-Marie Chèvre: Supervision,
Writing – Review & Editing Patrick Wincker: Supervision Mathieu Rousseau-Gueutin: Supervision, Writing – Review & Editing Jean-Marc Aury: Conceptualization, Supervision, Writing – Review & Editing. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the Genoscope, the Commissariat à l’Energie Atomique et
aux Energies Alternatives (CEA), and France Génomique (ANR-10-INBS-09–08), the “Région Bretagne” as well as INRAE ‘Plant Breeding’ department that funded the Ph.D. scholarship of Franz
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The genome assembly and gene predictions are freely available at
http://www.genoscope.cns.fr/plants. The Illumina, and the bionano data of the B. rapa cv Z1 version 1 are available in the European Nucleotide Archive under the following projects: PRJEB26620.
The PromethION sequencing data, the Omni-C Illumina sequencing data, the Pore-C nanopore
sequencing data and the new optical maps are available in the European Nucleotide Archive under
the following projects: PRJEB46167.
Acknowledgments: We thank the UMR INRA 1095 ‘GENTYANE platform’ (Clermont-Ferrand,
France, http://gentyane.clermont.inra.fr/) for the DNA extraction of segregating population and the
platform TraitGenetics (Gatersleben, Germany, http://www.traitgenetics.com/) for genotyping using the Brassica 19K Illumina infinium SNP array.
Conflicts of Interest: JMA received travel and accommodation expenses to speak at Oxford Nanopore Technologies conferences. JMA and CB received accommodation expenses to speak during
Bionano Genomics user meetings. The authors declare that they have no other competing interests.
Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 12 July 2021 doi:10.20944/preprints202107.0275.v1
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