Meiotic Karyotypes of the B Genomes of Brassica Nigra and B. Carinata

Abstract

The diakinesis bivalents of the B genomes of Brassica nigra (BB) and B. carinata(BBCC) have been characterized, classified into groups and arranged as karyotypes. Chromosome size, differential condensation and staining of the chromatin, and the location of a darkly stained region comprising the centromere and in some cases most of the short arm as well, were basic criteria for bivalent classification. Although of different origin, the diakinesis chromosomes of the B genomes in B. nigra and B. carinata exhibited the same pattern of chromatin condensation. The karyotype comprised three groups contaaining pairs 1-2, 3-5 and 6-8. Ring bivalents were more frequent than rod bivalents. The morphology of pairs 6-8 and their frequent association with nucleoli were suggestive that they were nucleolar chromosome pairs. Nucleolar and possible homoeologous bivalent associations were more frequent in B. nigra than in B. carinata. The B and C genomes of B. carinata were easy to differentiate, because in contrast to the differentially stained bivalents of the B genomes, the nine chromosome pairs of the C genomes were largely homogeneously stained.