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

Pyramiding epistatic resistance genes to improve long term disease resistan ce has challenged plant breeders. Indirect selection using tightly linked m arkers will often facilitate the breeding of desired epistatic resistance g ene combinations. In common bean, the most effective strategy for broad spe ctrum 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 li nes 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 speci fic strains of bean common mosaic virus and bean common mosaic necrosis vir us 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 F-3 progeny g enerated from a cross between the contrasting near isogenic lines (I + bc-I /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 alleli c 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 SBD5(1300). Tight linkage (0 recombinants) was confirmed in a se cond population of 58 F-2 progeny co-segregating for SBD5(1300) 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 an d cranberry beans. The SBD5(1300) marker mapped on linkage group B3, reveal ing independence of bc-1(2) from the I gene on B2 and bc-3 gene on B6, whic h supports the opportunity to readily combine genes for broad spectrum and pyramided resistance to bean common mosaic potyviruses in a single bean cul tivar.