Breeding, Genetics and Models
2 BREEDING, GENETICS AND MODELS The classic triangle of U (1935) (Fig. 1.2) shows the inter-relationships of brassicas based on their chromosome numbers and 2n genome descriptors. These species can be inter-crossed using embryo rescue, fusion and other methods. In addition, massive opportunities are emerging from studies of model brassicas such as thale cress (Arabidopsis thaliana) and Wisconsin Fast PlantsTM that identify genes and their products which can be applied in crop species. Further, some breeders are now working with the less well known species in Brassicaceae, such a Brassica carinata, to extract valuable genes for resistance to pathogens, pests and other economic characters. The Brassicaceae is one of the most flexible plant families in terms of interspecifc and intergenomic crosses. Rapid progress is being made in our understanding of their component genes, genomic interactions, protein products and resultant phenotypic characteristics, leading eventually to even wider and more diverse crosses. Concurrently, restriction fragment length polymorphism (RFLP) and linkage maps are being made for most of the major species, and these have shown many common chromosome linkage groups occurring across these species. This is encouraging the research into the potential of single gene transfer into economic crops from the model types. GENOMIC CHARACTERS AND TAXONOMY The following six Brassica species, plus Raphanus sativus, radish 2n = 18, have been inter-crossed, with varying levels of difficulty requiring embryo culture or fusion to obtain hybrids: Brassica nigra Koch, black mustard, 2n = 16; Brassica carinata Braun, Ethiopian mustard, 2n = 34; Brassica juncea L. Coss, brown mustard, 2n = 36; Brassica napus, swede or rutabaga, rape or oilseed rape (canola) 2n = 38; Brassica rapa, turnip and Chinese cabbage, 2n = 20; and Brassica oleracea, cole crops, 2n = 18.
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