Molecular-cytogenetic characterization of C-genome chromosome substitution lines in Brassica juncea (L.) Czern and Coss.

C genome chromosome substitution lines of B. juncea constitute a key genetic resource for increased genetic diversity and hybrid performance. C genome chromosome substitution lines were found in the progenies of derived B. juncea (2n = 36; AABB), synthesized through hybridization between B. napus and B. carinata. These were originally recognized based on the morphology and genomic in situ hybridization. Genotyping using the Brassica Illumina 60K Infinium SNP array confirmed the presence of C genome chromosomes in a large number of derived B. juncea genotypes. Three whole chromosome substitutions and 13 major C genome fragment substitutions were identified. Fragment substitutions were primarily terminal, but intercalary substitution(s) involving chromosome C1 and C2 were identified in three genotypes. The size of substituted C genome fragments varied from 0.04 Mbp (C1) to 64.85 Mbp (C3). In terms of proportions, these ranged from 0.10 % (C1) to 100 % (C1, C3 and C7) of the substituted chromosome. SSR genotyping with B genome specific primers suggested that substituting C genome chromosome(s) are likely to have replaced B genome chromosome(s). C1 was the most common substituting chromosome while substituted B chromosome seemed random. Study of the phenotypic traits underlined the importance of the substitution lines (especially of chromosome C1) for conferring superior trait performance (main shoot length and pods on the main shoot). High heterosis was also indicated in hybrid combinations of substitution lines with natural B. juncea. These substitution genotypes constituted a valuable resource for targeted gene transfer and QTL identification.