Effect of crop improvement on genetic diversity in oilseed Brassica rapa (turnip-rape) cultivars, detected by SSR markers

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.