Field hybridization experiments are described in which B. juncea, B. napus, and B. oleracea were crossed with B. napus (male), and B. napus were crossed with B. juncea (male). Five of the experiments used chlorsulfuron herbicide-resistant B. napus as the paternal parent, allowing over 98 000 seeds to be easily and efficiently screened for chlorsulfuron resistance to detect hybrid progeny. Two experiments used leaf morphological characters to identify putative hybrids. Intraspecific B. napus crosses produced low percentages (1.83% and 1.79%) of the hybrid progeny. Brassica juncea × B. napus interspecific crosses produced on average 2.1% hybrids, and the B. napus × B. juncea cross produced 0.2% hybrids. No hybrids were detected by chlorsulfuron resistance in the B. oleracea × B. napus cross. Fecundity of the F1 hybrid plants in all of the crosses was low compared with their parents, with hybrids having less than 28% pollen stainability and producing less than 2.4 seeds per flower pollinated when selfed or backcrossed; most of the F1 hybrids studied produced less than one seed per flower pollinated. These results show that low levels of hybridization and gene transfer between B. napus and some relatives could occur in New Zealand when grown in close proximity.
