Neil Wratten

Genome-wide delineation of natural variation for pod shatter resistance in Brassica napus

Resistance to pod shattering (shatter resistance) is a target trait for global rapeseed (canola, Brassica napus L.), improvement programs to minimise grain loss in the mature standing crop, and during windrowing and mechanical harvest. We describe the genetic basis of natural variation for shatter resistance in B. napus and show that several quantitative trait loci (QTL) control this trait. To identify loci underlying shatter resistance, we used a novel genotyping-by-sequencing approach DArT-Seq. QTL analysis detected a total of 12 significant QTL on chromosomes A03, A07, A09, C03, C04, C06, and C08; which jointly account for approximately 57% of the genotypic variation in shatter resistance. Through Genome-Wide Association Studies, we show that a large number of loci, including those that are involved in shattering in Arabidopsis, account for variation in shatter resistance in diverse B. napus germplasm. Our results indicate that genetic diversity for shatter resistance genes in B. napus is limited; many of the genes that might control this trait were not included during the natural creation of this species, or were not retained during the domestication and selection process. We speculate that valuable diversity for this trait was lost during the natural creation of B. napus. To improve shatter resistance, breeders will need to target the introduction of useful alleles especially from genotypes of other related species of Brassica, such as those that we have identified.

Diversity array technology markers: Genetic diversity analyses and linkage map construction in rapeseed (Brassica napus L.)

We developed Diversity Array Technology (DArT) markers for application in genetic studies of Brassica napus and other Brassica species with A or C genomes. Genomic representation from 107 diverse genotypes of B. napus L. var. oleifera (rapeseed, AACC genomes) and B. rapa (AA genome) was used to develop a DArT array comprising 11 520 clones generated using PstI/BanII and PstI/BstN1 complexity reduction methods. In total, 1547 polymorphic DArT markers of high technical quality were identified and used to assess molecular diversity among 89 accessions of B. napus, B. rapa, B. juncea, and B. carinata collected from different parts of the world. Hierarchical cluster and principal component analyses based on genetic distance matrices identified distinct populations clustering mainly according to their origin/pedigrees. DArT markers were also mapped in a new doubled haploid population comprising 131 lines from a cross between spring rapeseed lines ‘Lynx-037DH’ and ‘Monty-028DH’. Linkage groups were assigned on the basis of previously mapped simple sequence repeat (SSRs), intron polymorphism (IP), and gene-based markers. The map consisted of 437 DArT, 135 SSR, 6 IP, and 6 gene-based markers and spanned 2288 cM. Our results demonstrate that DArT markers are suitable for genetic diversity analysis and linkage map construction in rapeseed.

Differential tolerance of high manganese among rapeseed genotypes

Cultivation of crop cultivars resistant to high soil manganese (Mn) may reduce the negative effects of Mn toxicity on crop yield. Three studies were carried out to select Brassica genotypes (B. napus and B. rapa) resistant to high Mn concentration and to characterise the nature of any Mn resistance found. In Experiment 1, 33 B. napus and nine B. rapa genotypes were screened in a sub-irrigated nutrient solution system. Based on visual symptoms and plant size, single plants were identified with resistance to high Mn from within cultivars of four B. napus and one B. rapa. Resistance was also identified in one B. napus doubled haploid genotype. In Experiment 2, a genotype resistant to high Mn and two genotypes (progenies from Experiment 1) sensitive to high Mn were exposed to eight Mn concentrations (9–500 μM) for 2 weeks in nutrient solution. The relative shoot weight (RSW) and the relative root weight (RRW) of the genotype resistant to Mn were significantly greater at ≥100 μM Mn than both genotypes sensitive to high Mn; the sensitive genotypes reacted similarly. The three genotypes had similar tissue Mn contents and the elevated Mn tissue contents did not induce deficiencies of Mg or Fe. In Experiment 3, 12 genotypes (progenies from Experiment 1) were screened in nutrient solution at 9 μM Mn and with an additional 125 μM Mn. The RRW and RSW of the genotypes ranged from 35 to 114 and 39 to 94%, respectively. All the selections sensitive to high Mn had a RSW <60% and thus were confirmed to be Mn sensitive, while all the selections resistant to Mn had a RSW >70% and thus were confirmed as Mn resistant. This evidence confirmed the availability of rapeseed germplasm resistant to Mn toxicity with an ability to withstand high content of Mn through internal tissue tolerance. Also, the observed Mn tolerance in this material is genetically controlled and not an artifact of our screening assays.