Giemsa staining was used to characterize the satellited chromosomes, and silver staining, to detect nucleolus organizer regions and nucleoli in Brassica campestris (genome: AA, 2n = 20), B. nigra (BB, 2n = 16) and Sinapis arvensis (SS, 2n = 18). B. campestris had one pair of satellited chromosomes, while B. nigra had three pairs. S. arvensis had five distinct satellited chromosomes and a sixth chromosome with a faint satellite. Silver-stained nucleolus organizer regions (Ag-NORs) were localized at the secondary constrictions of the satellited chromosomes. Two Ag-NORs and a maximum number of two nucleoli were revealed in B. campestris. Heteromorphism for size of Ag-NORs and nucleoli was observed in one accession of B. campestris. In B. nigra, six Ag-NORs and six nucleoli were observed at early and late telophase, respectively. In S. arvensis, six Ag-NORs and six nucleoli were revealed. Size differences were noticed within and between pairs of Ag-NORs. The presence of three pairs of active NORs in B. nigra and S. arvensis is an additional support for the hypothesis of a basic number of x = 3 in the Brassiceae.
