Identification of molecular markers linked to resistance quantitative trait loci (QTL) to common bacterial blight in dry bean

Abstract

Common bacterial blight (CBB) is a major disease limiting dry bean production in South Africa. All locally grown commercial cultivars are susceptible to the disease and improvement of the cultivars, by introducing stable resistance, is important. Two resistance sources, GN Nebr.#1 sel 27 and XAN 159 each contribute 2 independent quantitative trait loci (QTL) with major effects on CBB resistance. Sequence characterized amplified region (SCAR) markers linked to these four QTL are available for DNA marker-assisted breed ing (Miklas et al., 2000). Wilk 2 has shown better resistance in greenhouse trials when tested against the other available resistant cultivars. The aim of this study was to determine if there is an additional resistance QTL present in Wilk 2 and to develop flanking markers if possible. The AFLP technique was employed as a molecular tool to identify markers linked to CBB resistance. Twenty primer pair combinations of the EcoRI/Msel-AFLP approach were screened. Seventy-nine putative markers were identified, tested on the segregating population and mapped using MAPMAKER-EXP along with the four available SCARs. One linkage group was obtained with two major and one minor QTL identified. The highest linkages to the resistance trait obtained through STATGRAPHICS were 81 .17% and 76.16%. The available SCARs showed a lower linkage than expected (BC 420 58.44% to SAP 6 5.84%). Six markers were selected for further development of new SCARs. Polymorphic SCAR markers explained between 15.03 to 71.16% of the phenotypic variation for CBB resistance. The SCAR markers developed can be used for MAS, with SN 3-400 (E-AAC/M-CTC 2) being able to distinguish between the homozygotic and heterozygotic progeny, SN 2A-600 (E-AAC/M-CAT 3) and SN 3-300 (E-AAC/M-CTC 2) for selecting for the homozygous resistant plants, which could be highly efficient in a segregating population. The SCAR SN 1-1 (E-AAC/M-CAT 2) was unique to Wilk 2 and can be used in breeding programmes for pyramiding of CBB resistance genes.