Performance of aneuploid backcross hybrids between the crop Brassica napus and its wild relative B. rapa.

Crossings between the diploid wild Brassica rapa (AA, 2n = 20) and the tetraploid cultivar B. napus (AACC, 2n = 38) can readily be made. Backcrosses to the wild B. rapa (BC1 ) produce aneuploids with variable chromosome numbers between 20 and 29. How does survival and performance relate to DNA content of plants? Growth of the BC1 plants was measured in the lab. One plant in the F1 self-pollinated spontaneously and produced abundant F2 seeds that were also examined. The number of C-chromosomes was estimated from DNA values obtained with flow cytometry. Average DNA value of the BC1 was similar to that of the parents, which shows that C-chromosomes do not reduce success of pollen or embryos. The average DNA value in the F2 was 13% higher than in the F1 , suggesting that extra C-chromosomes facilitated gamete success and/or embryo survival. Under both optimal and drought stress conditions growth and survival of BC1 hybrids was similar to that of B. rapa. No significant correlations existed between growth or survival and DNA value. Aneuploid plants were not inferior under the conditions of the growth room and may persist in nature. We discuss other factors, such as herbivory, that could prevent hybrid establishment in the field.