Interspecific F1 hybrids were obtained from a cross between a male sterile Brassica napus (2n = 4x = 38, AA (20) and CC (18) genomes) and an inbreeding line B. rapa (Purple Cai-Tai inbred line 9418, 2n = 2x = 20, AA (20) genome) to introgress male sterility from a tetraploid into a diploid through backcrossing. The morphological characteristics of F1 plants were more like the female parent B. napus and segregated considerably in BC1 when backcrossed to the recurrent parent Purple Cai-Tai. The progeny became stable and more similar to Purple Cai-Tai by BC4. Most C genome chromosomes were found to be eliminated, based on cytogenetic analysis. The majority of chromosomes were eliminated at very early backcross stages, with only 20–26 chromosomes in BC1 plants, and some chromosomes were eliminated gradually with increased backcross generations. The BC4 plants were generally stable with exactly 20 chromosomes. Analysis by AFLP indicated that 49.5–68.7% of the total bands eliminated from F1 to BC4 were female parent specific, and ~12% of B. napus bands were retained with increased backcrossing. The genetic materials controlling sterility from the female parent B. napus were introgressed successfully into the BC4 plants even though most B. napus chromosomes/genetic materials were eliminated during the backcross process.
