Effectiveness of SW13 and ROC11 markers in breeding for resistance to BMCV and BCMNV in common bean
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Bean common mosaic virus (BCMV) and bean common mosaic necrosis virus (BCMNV) are responsible for significant economic losses in bean production worldwide. Necrotic strains are more aggressive than the non-necrotic strains and have become increasingly problematic in bean growing areas of Africa and particularly in the East African Region. Genetic resistance is the most effective strategy for virus control. The I and bc-3 genes are known to confer resistance to BCMV and BCMNV. The combination of these genes provides durable resistance to all BCMV and BCMNV strains. ROC11 and SW13 molecular markers linked to bc-3 and I genes, respectively, were used to track the two genes in crosses involving two Rwandan bean cultivars (CAB19 and RAB487), and TARS-VR-7S, a donor cultivar for bc-3 and I gene. Progenies of BC1F1 were evaluated phenotypically and with molecular markers to determine the effectiveness of the markers. The presence of molecular markers was determined by DNA analysis through polymerase chain reaction, electrophoresis and visualisation of the PCR product. For phenotypic selection, plants were artificially inoculated and evaluated visually based on symptoms. The genes were transferred at varying rate among generations and crosses; observations made deviated significantly from expectations in both crosses. The ROC11 marker did not work well in the RAB487/TARS-VR-7S cross, it was applied only on BC1 progeny, indicating limitations of the marker. Phenotypically the results were as expected in both crosses. Comparison of genotypic and phenotypic data indicated that genotypic selection was not superior to phenotypic selection. However, with the epistatic effect of the recessive bc-3 gene on the dominant I gene, the use of markers was necessary to track transfer of both genes. Deviations observed could be attributed to many factors involving genetic background of the parental lines. Ten Rwandan bean cultivars screened for resistance to BCMNV were all highly susceptible to BCMV and BCMNV with differences in time of symptom appearance (1-4 weeks after inoculation) and infection rate (66.2-100%). No resistance gene to BCMV and BCMNV was detected in the cultivars used. This calls for an immediate breeding program for resistance to the viruses.