Analysis of somatic Brassica hybrids by genomic in situ hybridization (GISH)

The chromosomal composition of plants derived from asymmetric somatic hybridization between Brassica nigra and B. napus was investigated by genomic in situ hybridization (GISH). The aim of the interspecific fusion was the transfer of genes conferring resistance to the two fungal pathogens Plasmodiophora brassicae and Leptosphaeria maculans from a B. nigra line into the rapeseed genome. After inoculation of young hybrid plants with spores of the fungi, plants resistant to one or both pathogens were selected. The B. nigra/B. napus DNA ratio was determined by non-radioactive dot blots using species-specific repetitive sequences as probes. A remarkable elimination of the donor genome and resistance to both pathogens was the selection criterion for backcrossing a hybrid plant to the B. napus recipient line. An analysis of chromosome preparations of various hybrids by the GISH method enabled distinction of chromosomes from each species. The fate of B. nigra chromosomes in further backcross generations could be traced by these in situ experiments. In addition to the loss of B. nigra chromosomes we could also detect chromosome rearrangements in the hybrid nuclei. The latter is of relevance for the introgression of the resistance genes into B. napus cultivars.