Potential marker-assisted selection for bc-1 2 resistance to bean common mosaic potyvirus in common bean

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Pyramiding epistatic resistance genes to improve long term disease resistance has challenged plant breeders. Indirect selection using tightly linked markers will often facilitate the breeding of desired epistatic resistance gene combinations. In common bean, the most effective strategy for broad spectrum control of the bean common mosaic virus disease is to combine I, bc-u, bc-1 2, bc-2 2, and bc-3 genes. We describe the use of near-isogenic lines and bulked segregant analysis to identify a marker tightly linked with the bc-1 2 gene. The recessive bc-1 2 gene conditions resistance to specific strains of bean common mosaic virus and bean common mosaic necrosis virus and is masked by the bc-2 2 and bc-3 genes. We identified a RAPD marker completely linked (0 recombinants) with bc-1 2, based on 72 F3 progeny generated from a cross between the contrasting near isogenic lines (I + bc-1/I + bc-1 2). Segregation in this I gene background revealed that bc-1 2 was dominant to bc-1 in conferring resistance to top necrosis in the allelic series Bc-1 > bc-1 2 > bc-1. To facilitate marker-assisted selection of bc-1 2 across breeding programs, the RAPD was converted to a SCAR marker, designated SBD51300. Tight linkage (0 recombinants) was confirmed in a second population of 58 F2 progeny co-segregating for SBD51300 and bc-1 2 gene from a different source. Based on a survey of 130 genotypes, the SCAR will be useful for MAS of bc-1 2 in most beans of Middle American origin and snap beans, but will have very limited utility in the case of kidney and cranberry beans. The SBD51300 marker mapped on linkage group B3, revealing independence of bc-1 2 from the I gene on B2 and bc-3 gene on B6, which supports the opportunity to readily combine genes for broad spectrum and pyramided resistance to bean common mosaic potyviruses in a single bean cultivar.