Genetic variation can be exploited by introgression; genes are moved into the cultivated gene pool by continuously backcrossing with the cultivated parent. While this takes many generations, it is the most widely adopted method for expanding available variation in a breeding programme. However, it can be a slow process.
Example 1: Improved resistance of cultivated European spring barley to mildew has been achieved by introgressing an Ethiopian landrace mlo-11 (J0rgensen, 1992). Norman Borlaug successfully introgressed Rht semi-dwarfing genes from the Japanese variety Norin-10 into elite wheat varieties resulting in the 1960s 'Green Revolution' (Ellis et al., 2007).
'Linkage drag' is the simultaneous introgres-sion of deleterious alleles. Introgression of chromosome regions from landraces or wild relatives can be a slow and complex process because just the minimal region needs to be introduced. Biotechnology, particularly through the use of molecular markers, plays a key role in accelerating this process.
Example 2: Rye chromosome fragment 1RS contains genes responsible for improved grain yield, race-specific rust resistance, improved adaptation and stress tolerance (Zarco-Hernandez et al., 2005). When introgressed into wheat, the 1RS/1BL translocation in wheat negatively impacts gluten strength: it makes bread dough 'sticky'.
Entire chromosomes or even entire genomes can be added as an alternative to introgres-sion. Genetic diversity in intensively bred species, such as wheat, can be exploited.
Example 3: In 1938, the first fertile 'Triticale' (Triticosecale) exhibited improved yield and adaptability to regions not suitable for wheat production (Feuillet et al., 2008). This reproduced a naturally occurring process exploited by early farmers to develop many modern crop species such as hexaploid wheat, tetraploid potatoes and tetraploid cotton. Genome analysis has shown that even maize and rice are ancient tetraploids. 'Synthetic hexaploid wheats' can be generated by hybridizing durum wheat (AABB) with Aegilops tauschii (DD). Breeding programmes have produced more than 1000 'synthetic wheats' and are an important source of genetic diversity (CIMMYT, 2009b).
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