Intergeneric addition and substitution of Brassica napus with different chromosomes from Orychophragmus violaceus: Phenotype and cytology

• Chromosome addition and substitution of Brassica napus and Orychophragmus violaceus are obtained.
• These lines were distinguishable by their distinct phenotypes and chromosome markers.
• The 45S rDNA loci on three O. violaceus chromosomes show variable degrees of nucleolar dominance.

The crucifer Orychophragmus violaceus (2n = 24) with large purple flowers is cultivated as an ornamental plant in China. Previously intergeneric or even intertribal somatic hybrids between Brassica napus (2n = 38, AACC) and O. violaceus were produced and backcrossing progenies developed. In this study, eight out of potential twelve mono-/disomic alien addition lines (MAALs/DAALs) carrying one or two copies of individual O. violaceus chromosomes were further established. One disomic substitution line was selected by crossing the addition line with one B. napus nullisomics. These lines were distinguishable from each other by their phenotypes and chromosome markers, for some expressed the traits specific for O. violaceus (serrated leaves, basal clustering stems, purple flowers, respectively), and the others showed the new traits for two parents (female sterility, early flowering). The rDNA loci on three O. violaceus chromosomes were active and gave differential amounts of rRNA transcripts in the B. napus background, showing the variable degrees of nucleolar dominance. The O. violaceus chromosomes added to or substituted into the B. napus background were stably maintained in somatic and meiotic cells, and could be transmitted to the progeny through male and female gametes. These MAALs/DAALs were useful for elucidating the genome structure of O. violaceus, molecular mechanism behind the epigenetic phenomenon of nucleolar dominance.