The number of major rDNA loci (the genes coding for 18S-5.8S-26S rRNA)
was investigated in the economically important Brassica species and t
heir wild relatives by in situ hybridization of an rDNA probe to metap
hase chromosomes and interphase nuclei. The diploid species B. nigra (
B genome) has two major pairs of rDNA loci, B. oleracea (C genome) has
two major pairs and one minor pair of loci, while B. campestris (A ge
nome) has five pairs of loci. Among the three tetraploid species arisi
ng from these three diploid ancestors, B. carinata (BBCC genomes) has
four loci, B. juncea (AABB genomes) has five major pairs and one minor
pair of loci, and B. napus (AACC genomes) has six pairs of loci, indi
cating that the number of loci has been reduced during evolution. The
complexity of the known rDNA restriction fragment length polymorphism
patterns gave little indication of number of rDNA loci. It is probable
that chromosome rearrangements have occurred during evolution of the
amphidiploid species. The data will be useful for physical mapping of
genes relative to rDNA loci, micro- and macro-evolutionary studies and
analysis of aneuploids including addition and substitution lines used
in Brassica breeding programs.