Distinct subgenome stabilities in synthesized Brassica allohexaploids

Trigenomic Brassica allohexaploids synthesized from three crossing strategies showed diploidized and non-diploidized meiotic behaviors and produced both euploid and aneuploid progenies during successive generations, revealing the distinct subgenome stabilities (B > A> C). Three cultivated allotetraploid Brassica species (Brassica napus, B. juncea, B. carinata) represent the model system of speciation through interspecific hybridization and allopolyploidization, but no Brassica species at higher ploidy level exists in nature. In this study, Brassica allohexaploids (2n = 54, AABBCC) were artificially synthesized using three crossing strategies, and had combinations of the genomes from the extant allotetraploids and diploids (B. rapa, B. oleracea and B. nigra). The chromosome numbers and complements of these allohexaploids and the self-pollinated progenies of successive generations (S0-S7) were determined using multicolor fluorescent in situ hybridization that distinguished the chromosomes of three constituent genomes from each other. Both euploid and aneuploid progenies were identified. The most aneuploids maintained all B- and A-genome chromosomes and variable number of C-genome chromosomes, suggesting that genome stability was B > A > C. In the extreme case, loss of whole set of C-genome chromosomes led to the production of B. juncea-type progeny. Some aneuploid progenies had the same number of chromosomes (2n = 54) as the euploid, but the simultaneous loss and gain of A- and C-genome chromosomes. The diploidized and non-diploidized meiotic behaviors co-occurred in all allohexaploid individuals of consecutive generations. The aberrant chromosome pairing and segregation mainly involved the chromosomes of A and C genomes, which resulted in aneuploidy in self-pollinated progenies. The mechanisms for the differential stability of three genomes and the stabilization of the new allohexaploids are discussed.