Contributions to the analysis of the Brassica-genome

Abstract

1. Pachytene chromosome structure of the 3 Brassica species B. campestris (genome a; n=10), B. oleracea (genome c; n=9), and B. nigra (genome b; n=8) is described. In each of these species the same 6 chromosome types can be recognized by such structural characteristics as total length, symmetry of arms, and especially shape of the heterochromatic centromere region. 2. B. campestris is doubly tetrasomic for 2 chromosomes of different type and hexasomic for another chromosome type. B. oleracea is a triple tetrasomic, B. nigra a double tretasomic. The chromosome types present in increased numbers are different ones in the 3 species. The formulas for the genomes are: a = AA B C DD E FFF, c = A BB CC D EE F, and b = A B C DD E FF. 3. Secondary paring has been observed in pachytene of a triploid aac-hybrid, proving partial homology in the different genomes of some chromosomes belonging to the same structural type. 4. In each of the 3 genomes the duplicated chromosomes can be recognized by the occurrence of secondary pairing, the frequency of which depends on the degree of structural similarity. Secondary pairing is found in pachytene as well as metaphase I and II. 5. These results agree with earlier observations (see Discussion) and confirm the presence of balanced secondary polyploidy in Brassica derivable from a basic chromosome number x=6.