The Fate of Chromosomes and Alleles in Brassica Allohexaploids

Cultivated Brassica crops B. napus (oilseed rape, 2n = AACC), B. carinata (Ethiopian mustard, 2n = BBCC) and B. juncea (Indian mustard, 2n =AABB) are all allopolyploid, with two genomes each resulting from ancestral hybridisation events between diploid species B. rapa (Chinese cabbage, turnip; 2n =AA), B. nigra (black mustard; 2n = BB) and B. oleracea (cabbage, cauliflower, broccoli; 2n = CC). Although no naturally occurring allohexaploid Brassica exists, combining the three genomes to create 2n = AABBCC plants offers significant potential for crop improvement as well as potential elucidation of the mechanisms for polyploid speciation in Brassica. We investigated genome stability and meiotic behaviour in different allohexaploid Brassica populations. Molecular genotyping and cytogenetics approaches were used to track A, B and C genome allele inheritance, DNA content was estimated by flow cytometry and self-pollinated seed set and pollen viability data were collected. Homologous chromosome pairs segregated with moderately high fidelity and with meiotic crossover frequencies similar to those in established Brassica species. However, no strong selective pressure for 2n = AABBCC karyotypes was observed, with frequent aneuploidy and no significant correlations between fertility and DNA ploidy or chromosome fragment loss. Most chromosomes present in only one copy (univalents) were equally likely to be lost as to be retained, and severe karyotype distortions were observed in some second-generation progeny. Production of stable allohexaploid lines is predicted to require rigorous selection for 2n = AABBCC karyotypes.