Development for Tri-Genomic Hexaploid Brassica Populations

Synthesis of hexaploid Brassica has been stimulated by the successful crossing of B.carinata and B. rapa followed by chromosome doubling by Prof. Jinling Meng, Huazhong Agricultural University (HAU). Three different approaches are underway at The University of Western Australia (UWA). The first approach is crossing B. napus with B. nigra followed by chromosome doubling and over 2000 putative hybrid seeds were harvested. Evaluation of 283 putative hybrids by microsatellite markers, plant morphology, pollen viability and cytology confirmed triploid hybrids. Cuttings were taken and plants were treated with colchicine to double the chromosomes. The second approach involves a two-step crossing design. Two tetraploid species are crossed to produce an unbalanced tetraploid and this unbalanced tetraploid is crossed with a third tetraploid species. High frequencies of unreduced gametes in the unbalanced tetraploids result, and when these unreduced gametes combine with normal reduced gametes of the third tetraploid species, we expect formation of highly heterozygous hexaploids produced without the aid of colchicine. Characterisation of putative hybrids by flow cytometry confirmed one hexaploid and one pentaploid with the remainder at tetraploid level. The third approach involves crossing B. juncea with B. oleracea followed by chromosome doubling. Putative hybrid seeds were harvested and characterised. An international collaborative project is underway between UWA, HAU and Zhejiang University to further develop higher ploidy level Brassica.