Epigenomic and structural events preclude recombination in Brassica napus.

Meiotic recombination is a major evolutionary process generating genetic diversity at each generation in sexual organisms. However, this process is highly regulated, with the majority of crossovers lying in the distal chromosomal regions that harbor low DNA methylation levels. Even in these regions, some islands without recombination remain, for which we investigated the underlying causes. Genetic maps were established in two Brassica napus hybrids to detect the presence of such large nonrecombinant islands. The role played by DNA methylation and structural variations in this local absence of recombination was determined by performing bisulfite sequencing and whole genome comparisons. Inferred structural variations were validated using either optical mapping or oligo fluorescence in situ hybridization. Hypermethylated or inverted regions between Brassica genomes were associated with the absence of recombination. Pairwise comparisons of nine B. napus genome assemblies revealed that such inversions occur frequently and may contain key agronomic genes such as resistance to biotic stresses. We conclude that such islands without recombination can have different origins, such as DNA methylation or structural variations in B. napus. It is thus essential to take into account these features in breeding programs as they may hamper the efficient combination of favorable alleles in elite varieties.

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

With the rise of long-read sequencers and long-range technologies, delivering high-quality plant genome assemblies is no longer reserved to large consortia. Not only 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, in particular 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 scaffolding 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.

A Modified Meiotic Recombination in Brassica napus Largely Improves Its Breeding Efficiency.

Meiotic recombination is the main tool used by breeders to generate biodiversity, allowing genetic reshuffling at each generation. It enables the accumulation of favorable alleles while purging deleterious mutations. However, this mechanism is highly regulated with the formation of one to rarely more than three crossovers, which are not randomly distributed. In this study, we showed that it is possible to modify these controls in oilseed rape (Brassica napus, AACC, 2n = 4x = 38) and that it is linked to AAC allotriploidy and not to polyploidy per se. To that purpose, we compared the frequency and the distribution of crossovers along A chromosomes from hybrids carrying exactly the same A nucleotide sequence, but presenting three different ploidy levels: AA, AAC and AACC. Genetic maps established with 202 SNPs anchored on reference genomes revealed that the crossover rate is 3.6-fold higher in the AAC allotriploid hybrids compared to AA and AACC hybrids. Using a higher SNP density, we demonstrated that smaller and numerous introgressions of B. rapa were present in AAC hybrids compared to AACC allotetraploid hybrids, with 7.6 Mb vs. 16.9 Mb on average and 21 B. rapa regions per plant vs. nine regions, respectively. Therefore, this boost of recombination is highly efficient to reduce the size of QTL carried in cold regions of the oilseed rape genome, as exemplified here for a QTL conferring blackleg resistance.

Long-read assembly of the Brassica napus reference genome Darmor-bzh.

BACKGROUND: The combination of long reads and long-range information to produce genome assemblies is now accepted as a common standard. This strategy not only allows access to the gene catalogue of a given species but also reveals the architecture and organization of chromosomes, including complex regions such as telomeres and centromeres. The Brassica genus is not exempt, and many assemblies based on long reads are now available. The reference genome for Brassica napus, Darmor-bzh, which was published in 2014, was produced using short reads and its contiguity was extremely low compared with current assemblies of the Brassica genus. FINDINGS: Herein, we report the new long-read assembly of Darmor-bzh genome (Brassica napus) generated by combining long-read sequencing data and optical and genetic maps. Using the PromethION device and 6 flowcells, we generated ∼16 million long reads representing 93× coverage and, more importantly, 6× with reads longer than 100 kb. This ultralong-read dataset allows us to generate one of the most contiguous and complete assemblies of a Brassica genome to date (contig N50 > 10 Mb). In addition, we exploited all the advantages of the nanopore technology to detect modified bases and sequence transcriptomic data using direct RNA to annotate the genome and focus on resistance genes. CONCLUSION: Using these cutting-edge technologies, and in particular by relying on all the advantages of the nanopore technology, we provide the most contiguous Brassica napus assembly, a resource that will be valuable to the Brassica community for crop improvement and will facilitate the rapid selection of agronomically important traits.

Genome Size Variation and Comparative Genomics Reveal Intraspecific Diversity in Brassica rapa.

Traditionally, reference genomes in crop species rely on the assembly of one accession, thus occulting most of intraspecific diversity. However, rearrangements, gene duplications, and transposable element content may have a large impact on the genomic structure, which could generate new phenotypic traits. Comparing two Brassica rapa genomes recently sequenced and assembled using long-read technology and optical mapping, we investigated structural variants and repetitive content between the two accessions and genome size variation among a core collection. We explored the structural consequences of the presence of large repeated sequences in B. rapa 'Z1' genome vs. the B. rapa 'Chiifu' genome, using comparative genomics and cytogenetic approaches. First, we showed that large genomic variants on chromosomes A05, A06, A09, and A10 are due to large insertions and inversions when comparing B. rapa 'Z1' and B. rapa 'Chiifu' at the origin of important length differences in some chromosomes. For instance, lengths of 'Z1' and 'Chiifu' A06 chromosomes were estimated in silico to be 55 and 29 Mb, respectively. To validate these observations, we compared using fluorescent in situ hybridization (FISH) the two A06 chromosomes present in an F1 hybrid produced by crossing these two varieties. We confirmed a length difference of 17.6% between the A06 chromosomes of 'Z1' compared to 'Chiifu.' Alternatively, using a copy number variation approach, we were able to quantify the presence of a higher number of rDNA and gypsy elements in 'Z1' genome compared to 'Chiifu' on different chromosomes including A06. Using flow cytometry, the total genome size of 12 Brassica accessions corresponding to a B. rapa available core collection was estimated and revealed a genome size variation of up to 16% between these accessions as well as some shared inversions. This study revealed the contribution of long-read sequencing of new accessions belonging to different cultigroups of B. rapa and highlighted the potential impact of differential insertion of repeat elements and inversions of large genomic regions in genome size intraspecific variability.