Ribosomal RNA genes in diploid and amphidiploid Brassica and related species: organization, polymorphism, and evolution

Restriction maps for rRNA genes of the three cultivated diploid species Brassica nigra, B. oleracea, and B. campestris are described using the rDNA map of radish as a standard. rDNA subunit heterogeneity similar to that found in radish was observed in the Brassica species, thus resulting in complex hybridization patterns. Brassica campestris could be distinguished from the other two Brassica diploids by the absence of the EcoRI site E3 in the rDNA subunits in almost all the accessions tested, and by its smaller external intergenic spacer. Two radish probes covering specific regions of the rDNA subunit as well as two other heterologous probes permitted the assignment of specific fragments to coding and intergenic spacer regions. Amphidiploid species B. napus and B. juncea had rDNA profiles reflecting the combination of EcoRI fragments observed in their parental species. A series of alien addition lines disclosed rDNA regions on two chromosomes in the B. oleracea genome, distinguished by syntenic associations to other markers and by specific intergenic spacer fragment.