SELECTION FOR MONOGENIC PEST RESISTANCE TRAITS WITH COUPLING-PHASE AND REPULSION-PHASE RAPD MARKERS

The random amplified polymorphic DNA (RAPD) assay has recently provided a marker system that may be useful for indirect selection and pyramiding of monogenic pest resistance traits in crop species. Our objective was to investigate the efficiency of marker-assisted selection with RAPD markers linked in coupling and in repulsion with a single resistance allele, Common bean (Phaseolus vulgaris L.) near-isogenic lines differing for the recessive bean common mosaic virus (BCMV) resistance allele bc-3 were screened to identify linked RAPD markers. Two cosegregating RAPD markers were identified and scored across an Fz population of 103 individuals segregating for the bc-3 allele. One RAPD marker was linked in coupling (1.9. +/- 1.4 cM) and one in repulsion (7.1 +/- 2.6 cM) with the bc-3 allele. Categorization of the bc-3 genotypes in the F-2 population revealed that selection against the repulsion-phase RAPD, as opposed to selection for the coupling-phase RAPD, provided a greater proportion of homozygous resistant (81.8 versus 26.3%) selections, and a lower proportion of both segregating (18.2 versus 72.5%) and homozygous susceptible (0.0 versus 1.2%) selections. Selection of individuals based on the phenotype of both RAPD markers (i.e., the plus form of the coupling-phase RAPD and simultaneous with the minus form of the repulsion-phase RAPD) was identical to selection based solely on the repulsion-phase RAPD alone. The selection results obtained in our case study were consistent with the theoretical expectations, and both demonstrated that repulsion-phase linkages provided greater selection efficiency than coupling-phase linkages, even when the former have greater linkage distances from the pest resistance allele. Because repulsion-phase RAPD markers are more useful in marker-assisted selection for monogenic pest resistance traits, researchers will want to design their screening experiments in ways that will optimize the discovery of these.