Tetraploid turnips: towards autopolyploid Brassica crops

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Natural polyploid species often show greater environmental tolerances than their progenitor diploid species. As well, polyploids usually have larger cell and organ sizes than diploids. Hence, induced polyploidy, where chromosome numbers within a species are artificially doubled, has great potential for plant breeding, particularly of root and vegetable crops. In turnip (diploid Brassica rapa) and related vegetable species, targeted breeding efforts in the 1970s and 80s resulted in the successful release of a number of tetraploid crop types. However, these polyploid induction breeding strategies were hindered by problems such as poor fertility and vigour, and were subsequently abandoned. I propose to use modern genotyping, sequencing and cytogenetics technologies to identify the factors responsible for success and failure of induced polyploidy breeding in these crops, investigating successful tetraploid cultivars and creating novel tetraploid material from different genotypes and with different degrees of homozygosity and divergence between subgenomes. I will test the hypotheses that 1) greater genetic divergence between subgenomes in a tetraploid results in greater stability and hybrid vigour, 2) that genetic variation exists for genomic stability at the tetraploid level, and 3) that “double-cross” hybrids (resulting from an F1 by F1 cross at the tetraploid level) will show greater hybrid vigour than normal F1 hybrids. Such hybrids have immense potential for yield improvement as a result of increased heterosis, if stable tetraploid crop types can be created on demand.