Brassica : Harvesting the Genome, Diversity and Products
Brassica biodiversity and crop improvement
The genepool and genetic resources
Brassica crops are derived from wild species which still exist in nature, and which they are able to be inter-crossed. There are concerns that in some crop types there has been progressive genetic erosion and reduction in genetic diversity within the crop gene-pool. The wild relatives thus have the potential to provide access to a much wider range of allelic variation.
In situ genetic variation exists for the diploid Brassica species, in the centres of diversity. For B. oleracea (and C genome species) this is centred around the Mediterranean and coastal regions of Europe. For B. rapa, natural populations exist near waterways in Europe.
Brassica crops appear to have been under different selection pressures reflecting the period over which they have been domesticated. For leafy vegetable crops such as kale, there is a wide genetic diversity and eco-geographic distribution. More specialised leafy, root or floral organ based vegetables have been progressively selected and refined as mutations have accumuated through domestication, and hence may have more restricted eco-geographic centres of diversity. The B. napus oil crops appear to be relatively recent crop types that have been selected since stable hybridisation between domesticated diploid progenitors. A further restriction of the genepool is likely to have taken place following development of oil-extraction technologies and more recent selection of varieties possessing low glucosinolate and low erucic acid content (double lows).
Ex situ genetic resources are available in the public and private sectors. The European Brassica database (Bras-EDB) compiles information on genetc resource accessions held in public collections. Similar information is held in the USDA/ARS Grin system. The pressure for more intensive agriculture has resulted in an overall reduction of the natural genetic diversity of crop plants, a process called 'genetic erosion'. In some cases to a significant loss in the adaptability and sustainability of crops, particularly in combating natural pests and diseases. Traditionally, most crops were based on local selections ('landraces') developed over generations in particular farming communities and locations.
Characterising and using crop biodiversity is essential to provide the range of alternative crop varieties needed to meet the changing needs of the world. A large international effort over the past few decades has ensured (ex situ) conservation of many crop genetic resources. This has involved collecting traditional varieties and landraces from around the world, and in particular from centres of genetic diversity for specific crops. Modern crops contain a surprisingly narrow genetic base, despite the existence of extensive allelic variation within germplasm collections. Although breeders exploit the existing natural variation for particular target traits within crop species, there has been considerable reduction in biodiversity through canalisation of alleles.
Core Collections have been developed for Brassica spp in the past, within EU GenRes and other programmes. These were based on accessions held within public genetic resource collections. Seed were multiplied and accessions screened for a number of fungal and other resistance traits. Whilst valuable in identifying 'hotspots' of variation within the relevant gene-pools, these collections consisted of heterogeneous and heterozygous materlal. This limits their long-term use for correlating detailed genetic studies
Diversity Fixed Foundation Sets (DFFS) are defined as: "an informative set of genetically fixed lines representing a structured sampling of diversity across a genepool". Recent molecular analysis have shown that there is considerable genotypic variation within individual extant genetic resource accessions. This is present both as within-line heterogeneity and within-plant heterozygosity. As such, any co-ordinated trait and genetic studies will be very difficult to interpret unless material with acceptable levels of homozygosity is produced and made publicly available. To address this issue and provide researchers with relevant resources, 'Diversity Fixed Foundation Sets' (DFFS) are being constructed. These are designed to represent the majority of allelic variation within respective gene pools in a form suitable for long term exploitation by researchers and end users.
Draft White Paper for Multinational Brassica Genome Project (MBGP); 12/01/2006