Brassica : Harvesting the Genome, Diversity and Products
vigour. The diploid Brassica species are also comprised of ancestral chromosomal segmental duplications that result in blocks of paralogous genes that may then provide a wider range of developmental or adaptative variation in terms of expression or specificity.
Molecular marker based genetic maps have been established for all Brassica species. In recent years, considerable effort has been placed in developing high-density maps based on DNA sequence-tagged genetic markers (RFLPS, SSRs, CAPS, SNPs etc). The development of genomics tools is dependent on the ability of researchers to navigate amongst different genetic maps that are anchored with common sequence-tagged markers. An increasing amount of information is available that provides links between Brassica genetic maps and the Arabidopsis genome – primarily throught the use of sequence-tagged markers highly orthologous to defined Arabidopsis genes and loci.
Quantitative Trait Loci (QTL) have been identified for a wide range of morphological, physiological and crop traits in the different Brassica crop types. The ability to resolve QTL is dependent upon access to a larger number of recombinant individuals (segregating populations, subsitution or near-isogenic lines), as well as to high density genetic and physical maps. There is a continuing requirement to provide the tools that allow researchers to resolve QTL to the level of physical maps (BAC contigs) and candidate genes. This can be achieved through an integration of physical BAC contigs anchored with genetic markers onto common reference genetic linkage maps, as well as access to comparative genomic data between Brassica and Arabidopsis.
Breeding systems and technologies
Brassica species have been established as the model sporophytic pollen self-incompatibility system for research, with a well-characterised and diverse allelic series of 'S-' alleles in both B. oleracea and B. rapa having strong to weak incompatibility reactions. The S locus has been well characterised at the genomic and sequence level. Alleles of genes at the S-locus display considerable variability, with a very high level of non-synonymous substitution mutation. Comparative studies indicate that this variation appears to have occured prior to the speciation of B. rapa and B. oleracea. Prior to adoption of cytoplasmic male sterility sytems, self-incompatibility was widely used in development of F1 hybrid brassica crops.
Cytoplasmic male sterility (CMS) occurs infrequently in natural populations of Brassica, and provides an excellent tool to study genetic interactions between mitochondria and nucleus during flower development. CMS plants can be selected either following sexual crosses between different species of the same family or by somatic hybridisation between unrelated species. Several systems and sources of CMS have been characterised and used in Brassica, and CMS has increasingly replaced self-incompatibility in hybrid Brassica production. A single restorer locus appears to be capable of restoring different forms of Brassica CMS.
Brassica crops were amongst the first to be targeted for commercial transgenic genetic modification, for traits such as herbicide resistance and modification of male sterility. There have been many research programmes and patents associated with modification of fatty acid pathways to yield novel oils.
Due to the strong self-incompatibility system, most brassicas are outbreeders with a high degreee of heterozygosity in natural populations and open-pollinated crops. Doubled haploid technology is widely applied in brassica crop improvement and research programmes, either via anther culture or microspore culture.
Brassica consumption: Health, Medical and Clinical benefits Cardiovascular disease and cancer are ranked as the leading causes of death in most industrialised countries. Epidemiological studies demonstrate that increased fruit and vegetable intake decrease the risk of cancer and heart disease, with Brassica vegetables appearing to be especially protective against cancer and heart disease. Although the mechanisms are not fully understood, they are likely due to secondary metabolites. Understanding the basis of variation in such metabolites and the detailed regulation of the relevant biochemical pathways will provide a sound basis for more targeted clinical experimentation in the context of human allelic variation. It is important in dietary studies to quantify the ‘metabolic profile’ of the food, as opposed to just the metabolite of particular interest. Brassica vegetables are also known to be beneficial in the prevention of other major
Draft White Paper for Multinational Brassica Genome Project (MBGP); 12/01/2006