Meiotic karyotypes of the B genomes of Brassica nigra and B-carinata

The diakinesis bivalents of the B genomes of Brassica nigra (BB) and B. car inata (BBCC) have been characterized, classified into groups and arranged a s karyotypes. Chromosome size. differential condensation and staining of th e chromatin. and the location of a darkly stained region comprising the cen tromere and in some cases most of the short arm as well. were basic criteri a for bivalent classification. Although of different origin, the diakinesis chromosomes of the B genomes in B. nigra and B. carcinata exhibited the sa me pattern of chromatin condensation. The karyotype comprised three groups containing pairs 1-2, 3-5 and 6-8. Ring bivalents were more frequent than r od bivalents. The morphology of pairs 6-8 and their frequent association wi th nucleoli were suggestive that they were nucleolar chromosome pairs. Nucl eolar and possible homoeologous bivalent associations were more frequent in B. nigra than in B. carinata. The B and C genomes of B. carcinata were eas y to differentiate, because in contrast to the differentially stained bival ents of the B genomes. the nine chromosome pairs of the C genomes were larg ely homogeneously stained.