marker-assisted selection in wheat - ictsd

Chapter 5 – Marker-assisted selection for improving quantitative traits of forage crops 63Figure 2Strategy applied for marker selectionCo-dominant scoring ofmarkers associated withQTLsGenotypeNumber of plus allelesQTL1QTL2QLT3QTL4QTL5SumSelectioncriterionF 21 2 1 0 0 2 52 1 1 0 0 2 43 0 2 2 1 1 64 2 2 2 1 1 85 2 2 1 1 1 7...195 1 1 2 2 2 8196 1 1 2 0 2 6197 0 2 0 1 2 5198 0 2 0 1 1 4199 2 2 1 1 1 7200 2 1 2 2 1 8fragments) turned out to be straightforward.Figure 3 shows the F 2 frequency distributionfor the number of “plus alleles”.The population mean is somewhat belowthe expected number of five owing to thefact that the AFLP marker on LG1 showeda skewed segregation. This is likely dueto gametophytic selection in favour of thenegative QTL allele, perhaps due to linkagewith an incompatibility locus.The intensities of selection were setat about 25 percent, representing about50 genotypes per selection (Figure 3). Theselection pressure was kept fairly lowbecause of the need to have sufficient seedsfor measuring selection responses. In thisway, the influence of genetic drift accompanyingmarker selection was minimized. Thecut-off point for the top selection was sixpositive alleles and three for the oppositeselection (Figure 3). The frequency of theplus alleles was on average 0.66 and 0.27,respectively. Selection showed a positiveresponse for all NUE loci. However, thebetween-selection difference in allele frequencyof the loci ranged from 0.18 to 0.77,showing that index selection did not affectall NUE loci to the same degree. The differenceswere probably mainly due to chance.Indirect response to markerselectionThe selections were then multiplied usinga polycross scheme (after vegetative propagation)to obtain sufficient seeds forevaluation on hydroponics and under variousfield conditions. The marker selectionswere evaluated for NUE in a replicated trialwith two cuts on hydroponics at two Nlevels, being 2.5 and 5 percent N in leaves(van Loo et al., 2003). The same set of plantcharacteristics as in the original mappingstudies was monitored after each cut. Leafarea expansion rate, leaf length and width,as well as tiller number, were determinedone week after cutting. The determinationof shoot and root dry weight followedthree weeks later. The indirect responsesto marker selection are summarized inFigure 4. At low N supply, the NUEplus