Analysing the genetic architecture of clubroot resistance variation in Brassica napus by associative transcriptomics

Clubroot is a destructive soil-borne pathogen of Brassicaceae that causes significant recurrent reductions in yield of cruciferous crops. Although there is some resistance in oilseed rape (a crop type of the species Brassica napus), the genetic basis of that resistance is poorly understood. In this study, we used an associative transcriptomics approach to elucidate the genetic basis of resistance to clubroot pathotype ECD 17/31/31 across a genetic diversity panel of 245 accessions of B. napus. A single nucleotide polymorphism (SNP) association analysis was performed with 256,397 SNPs distributed across the genome of B. napus and combined with transcript abundance data of 53,889 coding DNA sequence (CDS) gene models. The SNP association analysis identified two major loci (on chromosomes A2 and A3) controlling resistance and seven minor loci. Within these were a total of 86 SNP markers. Altogether, 392 genes were found in these regions. Another 21 genes were implicated as potentially involved in resistance using gene expression marker (GEM) analysis. After GO enrichment analysis and InterPro functional analysis of the identified genes, 82 candidate genes were identified as having roles in clubroot resistance. These results provide useful information for marker-assisted breeding which could lead to acceleration of pyramiding of multiple clubroot resistance genes in new varieties

Extensive homoeologous genome exchanges in allopolyploid crops revealed by mRNAseq-based visualization

Polyploidy, the possession of multiple sets of chromosomes, has been a predominant factor in the evolution and success of the angiosperms. Although artificially formed allopolyploids show a high rate of genome rearrangement, the genomes of cultivars and germplasm used for crop breeding were assumed stable and genome structural variation under the artificial selection process of commercial breeding has remained little studied. Here, we show, using a repurposed visualization method based on transcriptome sequence data, that genome structural rearrangement occurs frequently in varieties of three polyploid crops (oilseed rape, mustard rape and bread wheat), meaning that the extent of genome structural variation present in commercial crops is much higher than expected. Exchanges were found to occur most frequently where homoeologous chromosome segments are collinear to telomeres and in material produced as doubled haploids. The new insights into genome structural evolution enable us to reinterpret the results of recent studies and implicate homoeologous exchanges, not deletions, as being responsible for variation controlling important seed quality traits in rapeseed. Having begun to identify the extent of genome structural variation in polyploid crops, we can envisage new strategies for the global challenge of broadening crop genetic diversity and accelerating adaptation, such as the molecular identification and selection of genome deletions or duplications encompassing genes with trait-controlling dosage effects

Co-expression Network Analysis of Diverse Wheat Landraces Reveals Marker of Early Thermotolerance and Candidate Master-regulator of Thermotolerance Genes

Bread wheat (Triticum aestivum L.) is a crop, relied on by billions of people around the world as a major source of both income and calories. Rising global temperatures, however, pose a genuine threat to the livelihood of these people, as wheat growth and yields are extremely vulnerable to damage by heat stress. Here we present the YoGI wheat landrace panel, comprised of 342 accessions which show remarkable phenotypic and genetic diversity thanks to their adaptation to different climates. We quantified the abundance of 110,790 transcripts from the panel and used these data to conduct weighted co-expression network analysis and identify hub genes in modules associated with abiotic stress tolerance. We found that the expression of three hub genes, all heat shock proteins (HSPs), were significantly correlated with early thermotolerance in a validation panel of landraces. These hub genes belonged to the same module, with one (TraesCS4D01G207500.1) likely regulating the expression of the other two hub genes, as well as a suite of other HSPs and heat stress transcription factors (Hsfs). In this work, therefore, we identify three validated hub genes, whose expression can serve as markers of thermotolerance during early development, and suggest that TraesCS4D01G207500.1 is a potential master regulator of HSP and Hsf expression – presenting the YoGI landrace panel as an invaluable tool for breeders wishing to determine and introduce novel alleles into modern varieties, for the production of climate-resilient crops

Validation of a novel associative transcriptomics pipeline in Brassica oleracea: identifying candidates for vernalisation response

Background: Associative transcriptomics has been used extensively in Brassica napus to enable the rapid identification of markers correlated with traits of interest. However, within the important vegetable crop species, Brassica oleracea, the use of associative transcriptomics has been limited due to a lack of fixed genetic resources and the difficulties in generating material due to self-incompatibility. Within Brassica vegetables, the harvestable product can be vegetative or floral tissues and therefore synchronisation of the floral transition is an important goal for growers and breeders. Vernalisation is known to be a key determinant of the floral transition, yet how different vernalisation treatments influence flowering in B. oleracea is not well understood. Results: Here, we present results from phenotyping a diverse set of 69 B. oleracea accessions for heading and flowering traits under different environmental conditions. We developed a new associative transcriptomics pipeline, and inferred and validated a population structure, for the phenotyped accessions. A genome-wide association study identified miR172D as a candidate for the vernalisation response. Gene expression marker association identified variation in expression of BoFLC.C2 as a further candidate for vernalisation response. Conclusions: This study describes a new pipeline for performing associative transcriptomics studies in B. oleracea. Using flowering time as an example trait, it provides insights into the genetic basis of vernalisation response in B. oleracea through associative transcriptomics and confirms its characterisation as a complex G x E trait. Candidate leads were identified in miR172D and BoFLC.C2. These results could facilitate marker-based breeding efforts to produce B. oleracea lines with more synchronous heading dates, potentially leading to improved yields

The opium poppy genome and morphinan production.

Morphinan-based painkillers are derived from opium poppy (Papaver somniferum L.). We report a draft of the opium poppy genome, with 2.72 gigabases assembled into 11 chromosomes with contig N50 and scaffold N50 of 1.77 and 204 megabases, respectively. Synteny analysis suggests a whole-genome duplication at ∼7.8 million years ago and ancient segmental or whole-genome duplication(s) that occurred before the Papaveraceae-Ranunculaceae divergence 110 million years ago. Syntenic blocks representative of phthalideisoquinoline and morphinan components of a benzylisoquinoline alkaloid cluster of 15 genes provide insight into how this cluster evolved. Paralog analysis identified P450 and oxidoreductase genes that combined to form the STORR gene fusion essential for morphinan biosynthesis in opium poppy. Thus, gene duplication, rearrangement, and fusion events have led to evolution of specialized metabolic products in opium poppy

Cytonuclear interactions remain stable during allopolyploid evolution despite repeated whole-genome duplications in Brassica

Several plastid macromolecular protein complexes are encoded by both nuclear and plastid genes. Therefore, cytonuclear interactions are held in place to prevent genomic conflicts that may lead to incompatibilities. Allopolyploidy resulting from hybridization and genome doubling of two divergent species can disrupt these fine-tuned interactions, as newly formed allopolyploid species confront biparental nuclear chromosomes with a uniparentally inherited plastid genome. To avoid any deleterious effects of unequal genome inheritance, preferential transcription of the plastid donor over the other donor has been hypothesized to occur in allopolyploids. We used Brassica as a model to study the effects of paleopolyploidy in diploid parental species, as well as the effects of recent and ancient allopolyploidy in Brassica napus, on genes implicated in plastid protein complexes. We first identified redundant nuclear copies involved in those complexes. Compared with cytosolic protein complexes and with genome-wide retention rates, genes involved in plastid protein complexes show a higher retention of genes in duplicated and triplicated copies. Those redundant copies are functional and are undergoing strong purifying selection. We then compared transcription patterns and sequences of those redundant gene copies between resynthesized allopolyploids and their diploid parents. The neopolyploids showed no biased subgenome expression or maternal homogenization via gene conversion, despite the presence of some non-synonymous substitutions between plastid genomes of parental progenitors. Instead, subgenome dominance was observed regardless of the maternal progenitor. Our results provide new insights on the evolution of plastid protein complexes that could be tested and generalized in other allopolyploid species