Genetic variability of Canadian elite cultivars of summer turnip rape (Brassica rapa L.) revealed by simple sequence repeat markers

Abstract

Canadian public oilseed breeding programs have played a leading role in the development of canola-quality Brassica rapa oilseed cultivars, but the genetic variability of the cultivars developed over the past 60 yr is poorly understood. Simple sequence repeat (SSR) markers were applied to assess the genetic variability of 294 plants representing one landrace introduced from Poland ca. 1940 and nine Canadian elite B. rapa cultivars released by Agriculture and Agri-Food Canada since 1964. Application of 18 SSR primer pairs detected 27 likely loci on five or more linkage groups and 123 polymorphic alleles. The allelic frequencies ranged from 0.003 to 0.997 and averaged 0.262. The estimates of mean heterozygosity for these cultivars ranged from 0.126 to 0.197 and averaged 0.165. Significant decreases of SSR alleles and average dissimilarities were observed over the 60 yr of breeding effort. The proportion of total SSR variation residing among the cultivars was 10.4%; between high vs. low erucic acid cultivars 5% and between high- vs. low-glucosinate cultivars 5%. Pairwise genetic differentiation ranged from 0.025 to 0.184 and averaged 0.104. Genetic clustering of these cultivars revealed AC Sunbeam to be a genetically unique cultivar, and two distinct groups of the other nine cultivars were separated by high- vs. low-glucosinate content. Several distinct genotypes, largely derived from the landrace Polish, were identified. These findings are useful for broadening the genetic base of elite B. rapa germplasm, selecting genetically diverse genotypes for synthetic and hybrid combinations, and conserving this germplasm.Key words: Simple sequence repeat, turnip rape (summer), Brassica rapa, genetic diversity, genetic relationship, genetic structure