Selection for resistance to southwestern corn borer using marker-assisted and conventional backcrossing

Two maize (Zea mays L) lines, susceptible and resistant to first-generation southwestern corn borer (SWCB), Diatraea grandiosella Dyar, were hybridized then backcrossed to the susceptible parent to form a population that was selected over three backcross generations by either marker-assisted or conventional selection for resistance to first generation SWCB leaf feeding. Quantitative trait loci (QTL) mapping was done by BC1F1 genotyping and BC1F2 infestation with SWCB. Three putative QTL were identified on chromosomes 7,9, and 10 accounting for 28% of the phenotypic variance. Marker-assisted selection (MAS) proceeded by selecting plants heterozygous at the QTL regions and homozygous for the recurrent parent genotype outside the QTL regions in the BC1F1 and BC2F1 generations. BC2F2 individuals were selected for the homozygous donor genotype in the QTL regions. Conventional selection initiated from the most resistant 30 BC1F2 lines. Conventional trials of BC2F2 and BC2F3 families were infested with SWCB and based on leaf damage ratings selected selfed progeny of the former generation formed the subsequent trial entries. A comparative trial of BC2F3 lines, selected by the two methods, was evaluated under SWCB infestation at three locations. Leaf damage ratings were taken at all locations and larvae weight was taken at one location. No significant differences for leaf damage ratings or larvae weight were found between lines selected by the two methods. Both methods produced lines significantly improved over the susceptible parent for SWCB leaf feeding damage indicating that the methods were equivalent as conducted in this experiment.