Development and molecular-genetic characterization of a stable Brassica allohexaploid

We report first-time synthesis of a stable Brassica allohexaploid. It may evolve into a new species and also advance our understanding of pairing regulation and genome evolution in complex allopolyploids. Crop Brassicas include both monogenomic and digenomic species. A trigenomic Brassica (AABBCC) is not known to exist in nature. Past attempts to synthesize a stable allohexaploid were not successful due to aberrant meiosis and very high proportion of aneuploid plants in the selfed progenies. We report the development of a stable allohexaploid Brassica (2n = 54; AABBCC). Genomic in situ hybridization confirmed the complete assemblage of three genomes. Only allohexaploids involving B. rapa cv. R01 (2n = 20; AA) as pollinator with a set of B. carinata (2n = 34; BBCC) were stable. These exhibited a high proportion (0.78-0.94) of pollen mother cells with normal meiosis and an excellent hexaploid ratio (0.80-0.94) in the selfed progenies. Stability of two allohexaploid combinations was demonstrated from H₁ to H₄ generations at two very diverse locations in India. Graphical genotyping of allohexaploids allowed detection of chromosome fragment exchanges among three genomes. These were much smaller for meiotically stable allohexaploids as compared to unstable ones. The putative hexaploids were morphologically closer to the female donor, B. carinata, for leaf morphology, inflorescence structure and flower shape. The newly formed allohexaploid may also provide unique opportunities to investigate the immediate genetic and genomic consequences of a Brassica allohexaploid with three resident genomes.