marker-assisted selection in wheat - ictsd

270Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 6Rapid decay of intragenic linkage disequilibrium estimated as r 2 in the cinnamyl-alcoholdehydrogenase (cad) gene in two different Eucalyptus species10.90.80.70.60.50.40.30.20.1E. urophyllar 2 000 500 1000 1500 2000 2500 3000Distance in basepairs10.9E. grandis0.80.7r 20.60.50.40.30.20.10 500 1000 1500 2000 2500Distance in basepairsSource: Faria et al., 2006.LD markers, i.e. polymorphisms that aresufficiently close to the functional mutation(Neale and Savolainen, 2004). The challenge,however, is considerable, as LD inoutcrossing forest trees such as pines decaysvery rapidly, in general within 1 500 to 2 000bp (Neale and Savolainen, 2004), and similarbehaviour has been seen in the few Eucalyptusgenes analysed to date with significant LDextending for only a few hundred base pairs(Thumma et al., 2005; Kirst, Marques andSederoff, 2005; Faria et al., 2006) (Figure 6).Genome-wide association studies for LDmarker-trait discovery in tress will requirevery high SNP marker densities that are currentlystill impracticable (but see below), sothat the only alternative left is a candidategene approach. Finally, direct markers (i.e.polymorphisms that code for the functionalmutations) would be the most valuable and