Somatic hybrids of Sinapis alba + Brassica juncea: study of backcross progenies for morphological variations, chromosome constitution and reaction to Alternaria brassicae

The wild relatives of crops represent a rich reservoir of genes for introducing resilience to climate change into cultivated crops. To introgress genes from Sinapis alba, a wild relative of Brassicaceae, into Brassica juncea, a cultivated member of this family, we initially produced somatic hybrids between the two species and then produced a large number of backcross populations involving the two somatic hybrids (H1 and H2) with Brassica juncea. BC1 progeny were morphologically very similar. However, when they were challenge inoculated with a highly virulent Alternaria brassicae (ITCC No. 2542) culture under in vivo and in vitro conditions in two growing seasons, they showed wide variations in their disease reaction. Of the 40 BC1 lines tested in one season, 36 showed a resistant reaction. BC1F2 progenies derived from these resistant BC1 plants also showed resistance to Alternaria brassicae, indicating stable inheritance of the resistant phenotype. However, BC1F2 progenies showed a wide variation in morphological traits, including plant height, basal branching, leaf thickness, trichome density on leaves and stem. BC1 plants were examined by genomic in-situ hybridization (GISH) to determine their chromosome constitution. All five plants were found to possess 12 strong hybridization signals upon hybridization with a FITC-labeled S. alba-specific probe. GISH studies on BC1F2 plants indicated localized signals in addition to 12 full chromosome hybridization signals, suggesting alien introgressions into B. juncea that requires further validation. The BC2 generation was found to possess half of the haploid set of alien chromosomes. The BC1F2 and BC2 generations were further screened against A. Brassiceae and found to be resistant/tolerant.