Towards the creation of a trigenomic hexaploid 'super Brassica' from interspecific hybridisation between B. napus and B. nigra

[Truncated abstract] Brassica napus (canola, oilseed rape) is a tetraploid (4x) species, the highest natural ploidy in the genus Brassica. Hexaploid crops (6x), such as bread wheat, kiwifruit, triticale and oat, benefit from broader adaptation, greater vigour and high yield potential compared to their lower ploidy ancestral species. However, no species in the genus Brassica are natural hexaploids. There is increasing interest in developing a trigenomic Brassica hexaploid species with genome complement AABBCC containing three basic genomes originating from the natural diploids B. rapa (AA), B. nigra (BB) and B. oleracea (CC). The approach tested in this thesis was to perform interspecific hybridisation between allotetraploid canola (B. napus, AACC, 2n = 38) and diploid black mustard (B. nigra, BB, 2n = 16) to produce triploid F1 hybrids with the genome composition ABC, which were then subjected to chromosome doubling to produce a new fertile hexaploid Brassica species (AABBCC). Brassica napus has a relatively narrow gene pool as a result of several genetic bottlenecks in its evolution, including the intensive breeding for high quality oil and meal traits. In contrast, B. nigra is a relatively undeveloped wild or landrace oilseed crop and is rich in genetic diversity. Black mustard is hardy, suitable as a rainfed crop under varied climatic conditions and yields well under extreme conditions. The B genome of B. nigra is an important source of useful genes including disease and pest resistance and other biotic and abiotic stress tolerances. The breeding target of this research is to combine the complementary characteristics of B. napus and B. nigra in a new hexaploid Brassica species that is both productive and tolerant to range of biotic and abiotic stresses. Successful interspecific crosses are highly dependent on diverse genotype selection to combine desirable traits and enhanced performance through heterosis.