Summary: Homoeologous exchanges (HEs) have been shown to generate novel gene combinations and phenotypes in a range of polyploid species. Gene presence/absence variation (PAV) is also a major contributor to genetic diversity. In this study, we show that there is an association between these two events, particularly in recent Brassica napus synthetic accessions, and that these represent a novel source of genetic diversity, which can be captured for the improvement of this important crop species. By assembling the pangenome of B. napus, we show that 38% of the genes display PAV behaviour, with some of these variable genes predicted to be involved in important agronomic traits including flowering time, disease resistance, acyl lipid metabolism and glucosinolate metabolism. This study is a first and provides a detailed characterization of the association between HEs and PAVs in B. napus at the pangenome level.

Summary: Brassica oleracea is an important agricultural species encompassing many vegetable crops including cabbage, cauliflower, broccoli and kale; however, it can be susceptible to a variety of fungal diseases such as clubroot, blackleg, leaf spot and downy mildew. Resistance to these diseases is meditated by specific disease resistance genes analogs (RGAs) which are differently distributed across B. oleracea lines. The sequenced reference cultivar does not contain all B. oleracea genes due to gene presence/absence variation between individuals, which makes it necessary to search for RGA candidates in the B. oleracea pangenome. Here we present a comparative analysis of RGA candidates in the pangenome of B. oleracea. We show that the presence of RGA candidates differs between lines and suggests that in B. oleracea, SNPs and presence/absence variation drive RGA diversity using separate mechanisms. We identified 59 RGA candidates linked to Sclerotinia, clubroot, and Fusarium wilt resistance QTL, and these findings have implications for crop breeding in B. oleracea, which may also be applicable in other crops species.

Summary: Methods based on single nucleotide polymorphism (SNP), copy number variation (CNV) and presence/absence variation (PAV) discovery provide a valuable resource to study gene structure and evolution. However, as a result of these structural variations, a single reference genome is unable to cover the entire gene content of a species. Therefore, pangenomics analysis is needed to ensure that the genomic diversity within a species is fully represented. Brassica napus is one of the most important oilseed crops in the world and exhibits variability in its resistance genes across different cultivars. Here, we characterized resistance gene distribution across 50 B. napus lines. We identified a total of 1749 resistance gene analogs (RGAs), of which 996 are core and 753 are variable, 368 of which are not present in the reference genome (cv. Darmor‐bzh). In addition, a total of 15 318 SNPs were predicted within 1030 of the RGAs. The results showed that core R‐genes harbour more SNPs than variable genes. More nucleotide binding site‐leucine‐rich repeat (NBS‐LRR) genes were located in clusters than as singletons, with variable genes more likely to be found in clusters. We identified 106 RGA candidates linked to blackleg resistance quantitative trait locus (QTL). This study provides a better understanding of resistance genes to target for genomics‐based improvement and improved disease resistance.

Plant disease-resistance genes play a critical role in providing resistance against pathogens. The largest family of resistance genes are the nucleotide-binding site (NBS) and leucine-rich repeat (LRR) genes. They are classified into two major subfamilies, toll/interleukin-1 receptor (TIR)-NBS-LRR (TNL) and coiled-coil (CC)-NBS-LRR (CNL) proteins. We have identified and characterised 641 NBS-LRR genes in Brassica napus, 249 in B. rapa and 443 in B. oleracea. A ratio of 1 : 2 of CNL : TNL genes was found in the three species. Domain structure analysis revealed that 57% of the NBS-LRR genes are typical resistance genes and contain all three domains (TIR/CC, NBS, LRR), whereas the remaining genes are partially deleted or truncated. Of the NBS-LRR genes, 59% were found to be physically clustered, and individual genes involved in clusters were more polymorphic than those not clustered. Of the NBS-LRR genes in B. napus, 50% were identified as duplicates, reflecting a high level of genomic duplication and rearrangement. Comparative analysis between B. napus and its progenitor species indicated that >60% of NBS-LRR genes are conserved in B. napus. This study provides a valuable resource for the identification and characterisation of candidate NBS-LRR genes.

A resource of resistance gene analogs within Brassicaceae species identifies functional resistance genes to improve plant breeding programs.

Amborella trichopoda (Amborellaceae) is the single living sister species of all other extant flowering plants and only occurs in rain forest habitats on the remote island of New Caledonia. These features make Amborella an important species in which to study genetic variation, including gene presence/absence variants (PAVs). Here, we apply the reference genome based iterative mapping and assembly strategy (Bayer et al., 2020) to assess gene diversity across ten diverse individuals.