Re-evaluation of the prospects of marker-assisted selection for improving insect resistance against Diatraea spp. in tropical maize by cross validation and independent validation

Cross validation (CV) and validation with an independent sample (IV) are ne w biometric approaches in QTL analysis to obtain unbiased estimates of QTL effects and the proportion of the genetic variance explained by the detecte d marker-QTL association (p). Our objective with these methods was to obtai n a realistic picture on the prospects of marker-assisted selection (MAS) f or improving the resistance of maize against the tropical stem borer specie s Diatraea grandiosella (SWCB) and Diatraea saccharalis (SCB). Published QT L mapping studies on leaf-damage ratings (LDR) with populations of F-2:3 li nes and recombinant inbred lines (RIL) from crosses CML131xCML67 and Ki3x C ML139 of tropical maize inbreds were re-analyzed with CV and IV. With CV, t he reduction in p for LDR compared to p obtained with the whole data set va ried between 41.0 and 79.6% in the populations of F-2:3 lines and between 3 0.1 and 65.2% in the two populations of RIL. Estimates of p for SCB LDR wer e similar for CV and IV. For SWCB LDR, p estimates obtained with IV were la rger than those obtained with CV in CML131x CML67. The reverse was observed for Ki3xCML139. Under the assumption of identical selection intensities, a nd based on the re-estimates of p, MAS using only molecular marker informat ion is less-efficient than conventional phenotypic selection (CPS). MAS com bining marker and phenotypic data increases the relative efficiency by only 4% in comparison to CPS. In conclusion, MAS for improving SWCB and SCB LDR seems not-promising unless additional QTLs with proven large effects are a vailable or the costs of marker assays are considerably reduced.