A high-density SNP genotyping array for Brassica napus and its ancestral diploid species based on optimised selection of single-locus markers in the allotetraploid genome

The Brassica napus Illumina array provides genome-wide markers linked to the available genome sequence, a significant tool for genetic analyses of the allotetraploid B. napus and its progenitor diploid genomes. A high-density single nucleotide polymorphism (SNP) Illumina Infinium array, containing 52,157 markers, was developed for the allotetraploid Brassica napus. A stringent selection process employing the short probe sequence for each SNP assay was used to limit the majority of the selected markers to those represented a minimum number of times across the highly replicated genome. As a result approximately 60 % of the SNP assays display genome-specificity, resolving as three clearly separated clusters (AA, AB, and BB) when tested with a diverse range of B. napus material. This genome specificity was supported by the analysis of the diploid ancestors of B. napus, whereby 26,504 and 29,720 markers were scorable in B. oleracea and B. rapa, respectively. Forty-four percent of the assayed loci on the array were genetically mapped in a single doubled-haploid B. napus population allowing alignment of their physical and genetic coordinates. Although strong conservation of the two positions was shown, at least 3 % of the loci were genetically mapped to a homoeologous position compared to their presumed physical position in the respective genome, underlying the importance of genetic corroboration of locus identity. In addition, the alignments identified multiple rearrangements between the diploid and tetraploid Brassica genomes. Although mostly attributed to genome assembly errors, some are likely evidence of rearrangements that occurred since the hybridisation of the progenitor genomes in the B. napus nucleus. Based on estimates for linkage disequilibrium decay, the array is a valuable tool for genetic fine mapping and genome-wide association studies in B. napus and its progenitor genomes.

Sequencing-based variant detection in the polyploid crop oilseed rape

BACKGROUND

The detection and exploitation of genetic variation underpins crop improvement. However, the polyploid nature of the genomes of many of our most important crops represents a barrier, particularly for the analysis of variation within genes. To overcome this, we aimed to develop methodologies based on amplicon sequencing that involve the incorporation of barcoded amplification tags (BATs) into PCR products.

RESULTS

A protocol was developed to tag PCR products with 5' 6-base oligonucleotide barcode extensions before pooling for sequencing library production using standard Illumina adapters. A computational method was developed for the de-convolution of products and the robust detection and scoring of sequence variants. Using this methodology, amplicons targeted to gene sequences were screened across a B. napus mapping population and the resulting allele scoring strings for 24 markers linkage mapped to the expected regions of the genome. Furthermore, using one-dimensional 8-fold pooling, 4608 lines of a B. napus mutation population were screened for induced mutations in a locus-specific amplicon (an orthologue of GL2.b) and mixed product of three co-amplified loci (orthologues of FAD2), identifying 10 and 41 mutants respectively.

CONCLUSIONS

The utilisation of barcode tags to de-convolute pooled PCR products in multiplexed, variation screening via Illumina sequencing provides a cost effective method for SNP genotyping and mutation detection and, potentially, markers for causative changes, even in polyploid species. Combining this approach with existing Illumina multiplexing workflows allows the analysis of thousands of lines cheaply and efficiently in a single sequencing run with minimal library production costs.

High-density SNP-based genetic map development and linkage disequilibrium assessment in Brassica napus L

Background

High density genetic maps built with SNP markers that are polymorphic in various genetic backgrounds are very useful for studying the genetics of agronomical traits as well as genome organization and evolution. Simultaneous dense SNP genotyping of segregating populations and variety collections was applied to oilseed rape (Brassica napus L.) to obtain a high density genetic map for this species and to study the linkage disequilibrium pattern.

Results

We developed an integrated genetic map for oilseed rape by high throughput SNP genotyping of four segregating doubled haploid populations. A very high level of collinearity was observed between the four individual maps and a large number of markers (>59%) was common to more than two maps. The precise integrated map comprises 5764 SNP and 1603 PCR markers. With a total genetic length of 2250 cM, the integrated map contains a density of 3.27 markers (2.56 SNP) per cM. Genotyping of these mapped SNP markers in oilseed rape collections allowed polymorphism level and linkage disequilibrium (LD) to be studied across the different collections (winter vs spring, different seed quality types) and along the linkage groups. Overall, polymorphism level was higher and LD decayed faster in spring than in “00” winter oilseed rape types but this was shown to vary greatly along the linkage groups.

Conclusions

Our study provides a valuable resource for further genetic studies using linkage or association mapping, for marker assisted breeding and for Brassica napus sequence assembly and genome organization analyses.

Haplotype structure around Bru1 reveals a narrow genetic basis for brown rust resistance in modern sugarcane cultivars

Modern sugarcane cultivars (Saccharum spp., 2n = 100-130) are high polyploid, aneuploid and of interspecific origin. A major gene (Bru1) conferring resistance to brown rust, caused by the fungus Puccinia melanocephala, has been identified in cultivar R570. We analyzed 380 modern cultivars and breeding materials covering the worldwide diversity with 22 molecular markers genetically linked to Bru1 in R570 within a 8.2 cM segment. Our results revealed a strong LD in the Bru1 region and strong associations between most of the markers and rust resistance. Two PCR markers, that flank the Bru1-bearing segment, were found completely associated with one another and only in resistant clones representing efficient molecular diagnostic for Bru1. On this basis, Bru1 was inferred in 86 % of the 194 resistant sugarcane accessions, revealing that it constitutes the main source of brown rust resistance in modern cultivars. Bru1 PCR diagnostic markers should be particularly useful to identify cultivars with potentially alternative sources of resistance to diversify the basis of brown rust resistance in breeding programs.

Analysis of genome-wide linkage disequilibrium in the highly polyploid sugarcane

Linkage disequilibrium (LD) in crops, established by domestication and early breeding, can be a valuable basis for mapping the genome. We undertook an assessment of LD in sugarcane (Saccharum spp), characterized by one of the most complex crop genomes, with its high ploidy level (>or=8) and chromosome number (>100) as well as its interspecific origin. Using AFLP markers, we surveyed 1,537 polymorphisms among 72 modern sugarcane cultivars. We exploited information from available genetic maps to determine a relevant statistical threshold that discriminates marker associations due to linkage from other associations. LD is very common among closely linked markers and steadily decreases within a 0-30 cM window. Many instances of linked markers cannot be recognized due to the confounding effect of polyploidy. However, LD within a sample of cultivars appears as efficient as linkage analysis within a controlled progeny in terms of assigning markers to cosegregation groups. Saturating the genome coverage remains a challenge, but applying LD-based mapping within breeding programs will considerably speed up the localization of genes controlling important traits by making use of phenotypic information produced in the course of selection.

Genetic and physical mapping of the grapevine powdery mildew resistance gene, Run1, using a bacterial artificial chromosome library

Resistance to grapevine powdery mildew is controlled by Run1, a single dominant gene present in the wild grapevine species, Muscadinia rotundifolia, but absent from the cultivated species, Vitis vinifera. Run1 has been introgressed into V. vinifera using a pseudo-backcross strategy, and genetic markers have previously been identified that are linked to the resistance locus. Here we describe the construction of comprehensive genetic and physical maps spanning the resistance locus that will enable future positional cloning of the resistance gene. Physical mapping was performed using a bacterial artificial chromosome (BAC) library constructed using genomic DNA extracted from a resistant V. vinifera individual carrying Run1 within an introgression. BAC contig assembly has enabled 20 new genetic markers to be identified that are closely linked to Run1, and the position of the resistance locus has been refined, locating the gene between the simple sequence repeat (SSR) marker, VMC4f3.1, and the BAC end sequence-derived marker, CB292.294. This region contains two multigene families of resistance gene analogues (RGA). A comparison of physical and genetic mapping data indicates that recombination is severely repressed in the vicinity of Run1, possibly due to divergent sequence contained within the introgressed fragment from M. rotundifolia that carries the Run1 gene.