TOWARD AN INTEGRATED LINKAGE MAP OF COMMON BEAN .3. MAPPING GENETIC-FACTORS CONTROLLING HOST-BACTERIA INTERACTIONS

Restriction fragment length polymorphism (RFLP)-based genetic linkage maps allow us to dissect the genetic control of quantitative traits (Q T) by locating individual quantitative trait loci (QTLs) on the linkag e map and determining their type of gene action and the magnitude of t heir contribution to the phenotype of the QT. We have performed such a n analysis for two traits in common bean, involving interactions betwe en the plant host and bacteria, namely Rhizobium nodule number (NN) an d resistance to common bacterial blight (CBB) caused by Xanthomonas ca mpestris pv. phaseoli. Analyses were conducted in the progeny of a cro ss between BAT93 (fewer nodules; moderately resistant to CBB) and jalo EEP558 (more nodules; susceptible to CBB). An RFLP-based linkage map for common bean based on 152 markers had previously been derived in th e F2 of this cross. Seventy F2-derived F3 families were inoculated in separate greenhouse experiments with Rhizobium tropici strain UMR1899 or X. c. pv. phaseoli isolate isolate W18. Regression and interval map ping analyses were used to identify genomic regions involved in the ge netic control of these traits. These two methods identified the same g enomic regions for each trait, with a few exceptions. For each trait, at least four putative QTLs were identified, which accounted for appro ximately 50% and 75% of the phenotypic variation in NN and CBB resista nce, respectively. A chromosome region on linkage group D7 carried fac tor(s) influencing both traits. In all other cases, the putative QTLs affecting NN and CBB were located in different linkage groups or in th e same linkage group, but far apart (more than 50 cM). Both BAT93 and Jalo EEP558 contributed alleles associated with higher NN, whereas CBB resistance was always associated with BAT93 alleles. Further investig ations are needed to determine whether the QTLs for NN and CBB on link age group D7 represent linked genes or the same gene with pleiotropic effects. Identification of the QTLs raises the possibility of initiati ng map-based cloning and marker-assisted selection for these traits.