A variety of factors including genetic architecture exerts impact on the ease and tempo of evolution. Domestication in many of the angiosperms is driven by changes at small numbers of loci with larger impacts (Cai and Morishima 2002). In contrary to this, domestication of sunflower has been preceded by selection on a large number of loci which confer moderate to small phenotypic impacts (Burke et al. 2002; Wills and Burke 2007). Domestication related genes are often found in clusters (Weeden 2007). It is suggested that species containing favorable genes in clusters were easily and rapidly domesticated, or alternatively the apparent gene clustering may be due to pleiotropic effects of individual genes (Doganlar et al. 2002a).
The domestication-associated traits act both in recessive and non-recessive manner. The recessive alleles presumably evolved because of "loss-of-function", for example, the nonshattering trait in cereals (Li et al. 2006), fruit weight trait in tomato and eggplant (Frary et al. 2000; Doganlar et al. 2002a), flowering time (Foucher et al. 2003) and dwarf habit (Martin et al. 1997) in pea, transition from two-rowed to six-rowed barley (Komatsuda et al. 2007), and other examples. Comparative analysis of domestication traits in three major cereal crops (sorghum, rice and maize) revealed correspondence in these genes/QTLs (Paterson et al. 1995). These genes can be cloned by adopting map-based cloning (Paterson 2002); association mapping (Buckler and Thornsberry 2002); and/or large-scale genomic scan in a crop species and its wild progenitor (Ross-Ibarra et al. 2007) depending on the crop species and available genetic information.
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