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Root growth and thickening plays a key role in the final productivity and even the quality of storage roots in root crops. This study was conducted to identify and map quantitative trait loci (QTLs) affecting root morphological traits in Brassica rapa by using molecular markers. An F2 population was developed from a cross between Chinese cabbage (Brassica rapa ssp. chinensis) and turnip (B. rapa ssp. rapifera), which differed greatly in root characters. A genetic map covering 1837.1 cM, with 192 marker loci and 11 linkage groups, was constructed by using this F2 population. The F3 families derived from F2 plants were grown in the field and evaluated for taproot traits (thickness, length, and weight). QTL analysis via simple interval mapping detected 18 QTLs for the 3 root traits, including 7 QTLs for taproot thickness, 5 QTLs for taproot length, and 6 QTLs for taproot weight. Individually, the QTLs accounted for 8.4?27.4% of the phenotypic variation. The 2 major QTLs, qTRT4b for taproot thickness and qTRW4 for taproot weight, explained 27.4% and 24.8% of the total phenotypic variance, respectively. The QTLs for root traits, firstly detected in Brassica crops, may provide a basis for marker-assisted selection to improve productivity in root-crop breeding.
EN
With the improvement of seed quality, Brassica rapa oilseed germplasm went through 2 major breeding bottlenecks during the introgression of genes for zero erucic acid content and low glucosinolate content, respectively. This study investigates the impact of these bottlenecks on the genetic diversity in European winter B. rapa by comparing 3 open-pollinated cultivars, each representing a different breeding period. Diversity was estimated on 32 plants per cultivar, with 16 simple sequence repeat (SSR) markers covering each of the B. rapa linkage groups. There was no significant loss of genetic diversity over the 3 cultivars as indicated by allele number (ranging from 59 to 55), mean allele number (from 3.68 to 3.50), Shannon information index (from 0.94 to 0.87) and expected heterozygosity (from 0.53 to 0.48). About 83% of the total variation was attributed to within-cultivar variation, and the remaining 17% to between-cultivar variation by analysis of molecular variance (AMOVA). Individual plants were separated into the 3 cultivars by principal coordinate analysis (PCoA). In conclusion, genetic diversity within cultivars was high and quality breeding in B. rapa did not significantly reduce the genetic diversity of B. rapa winter cultivars, so there is no risk of decline in performance due to quality improvement.
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