Production and cytogenetics of intergeneric hybrids between the three cultivated Brassica diploids and Orychophragmus violaceus

It has been proposed that both complete and partial separation of the paren tal genomes during mitosis and meiosis occurs in the intergeneric hybrids b etween Orychophragmus violaceus (2n=24) and the three cultivated Brassica t etraploids (B. napus, B. carinata and B. juncea). The hypothesis has been t hat this and the variations in chromosome numbers of these hybrids and thei r progenies result from the different roles of the A, B and C genomes origi nating from Brassica. To test this hypothesis, we produced hybrids between O. violaceus and the cultivated Brassica diploids. The hybrids with B. oler acea (2n=18, CC) had an intermediate morphology, but their petals were purp le like those of O. violaceus. They were sterile and had the expected chrom osome number (2n=21) in their mitotic and meiotic cells. The hybrid with B. campestris (2n=20, AA) was morphologically intermediate, except for its pa rtial fertility and its yellow petals, which were similar to those of B. ca mpestris. It was mixoploid (2n=23-42), and cells with 2n=34 were most frequ ent. Partial separation of parental genomes during mitosis, leading to the addition of O. violaceus chromosomes to the B. campestris complement, was p roposed to explain the findings in the mitotic and meiotic cells of the hyb rid and its progeny. In crosses with B. nigra (2n=16, BE), the majority of the FI plants were of the maternal type (2n=16), a small fraction had B. ni gra morphology but were mixoploids (2n=16-18), predominantly with 2n=16 cel ls and three plants, each with a specific morphology, were mixoploids consi sting of cells with varying ranges of chromosome numbers (2n=17-26, 11-17 a nd 14-17). The origin of these different types of plants was inferred to be a result of the complete and partial separation of parental genomes and th e loss of O. violaceus chromosomes. Our findings in the three crosses sugge st that the A genome was more influential than the C genome with respect to complete genome separation during mitosis and meiosis of the hybrids with B. napus. Possible complete and partial genome separation during mitotic di visions of the hybrids with B. carinata was mainly attributed to the role o f the B genome. The combined roles of the A and B genomes would thus contri bute to the most variable chromosome numbers of mitotic and meiotic cells i n the hybrids with B. juncea and their progenies. The possible cytological mechanisms pertaining to these hybrids and the potential of genome separati on in the production of Brassica aneuploids and homozygous plants are discu ssed.