MARKER-ASSISTED DISSECTION OF THE OLIGOGENIC ANTHRACNOSE RESISTANCE IN THE COMMON BEAN CULTIVAR, G-2333

Two independently asserting dominant genes conditioning resistance to bean anthracnose were identified in an F-2 population derived from the highly resistant bean differential cultivar, 'G 2333'. One gene was a llelic to the Co-4 gene in the differential cultivar 'TO' and was name d Co-4(2), whereas the second gene was assigned the temporary name Co- 7 until a complete characterization with other known resistance genes can be conducted. Two RAPD markers linked to the Co-4(2) allele were i dentified. One RAPD, OAS13(950), co-segregated with no recombinants in two segregating populations of 143 F-2 individuals, whereas the secon d RAPD, OAL9(740), mapped at 3.9 cM from the Co-4(2) allele. Two 24-me r SCAR primers (SAS13), developed from the OAS13(950) RAPD marker, wer e dominant and polymorphic, similar to the original RAPD, and supporte d the tight linkage between the marker(s) and the Co-4(2) allele. The markers were present in germplasm with known resistance alleles at the Co-4 locus. The presence of the markers in two other differential cul tivars not previously characterized and in four navy bean cultivars su ggests the existence of a gene family for anthracnose resistance at or near the Co-4 locus. Since the Co-7 gene was present only in germplas m which also possessed the Co-4(2) and Co-5 genes, the SAS13 markers w ere used in combination with standard inoculation techniques to identi fy F-3 lines in which the Co-7 gene was homozygous and the Co-4(2) all ele was absent. A similar strategy of marker-assisted dissection is pr oposed to identify resistant lines in which the Co-5 gene is absent an d the Co-7 gene is present by selecting against the OAB3(450) marker, which has been shown previously to be linked to the Co-5 gene. These g enes cannot be distinguished using traditional screening methods since all current races of the pathogen virulent to the Co-5 gene are aviru lent to the Co-4(2) and Co-7 genes. We describe the use of molecular m arkers tightly linked to resistance genes to facilitate the identifica tion of an uncharacterized resistance gene for which no discriminating race of the pathogen is known.