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Carlos A. Urrea, Phillip N. Miklas, James S. Beaver, and Ronald H. Riley

Bean golden mosaic virus (BGMV) is a devastating disease of common bean (Phaseolus vulgaris L.) in tropical America. The disease is effectively controlled by combinations of genetic resistances. The most widely deployed source of resistance to BGMV is a recessive gene (bgm-1) derived from the dry bean landrace cultivar Garrapato (Mexico) that conditions a nonmosaic partial resistance response to the pathogen. To expedite introgression of partial resistance into snap bean for southern Florida and other susceptible dry bean market classes for the Caribbean and Central American regions, a RAPD marker tightly linked to bgm-1 has been identified. Two contrasting DNA bulks, one consisting of five BGMV-resistant and the other five susceptible F6 recombinant inbred lines, were used to screen for polymorphic fragments amplified by 300 decamer primers in the polymerase chain reaction. RAPDs generated between the bulks were analyzed across F2 populations segregating for the marker and the gene. One codominant RAPD marker (R2570/530) tightly linked to the recessive resistance gene bgm-1 was found. The 530-base pair (bp) fragment was linked in repulsion with bgm-1 and the other 570-bp fragment was linked in coupling. No recombinants between R2570/530 and bgm-1 were observed among 91 F2 progeny from one dry bean population, and there were two recombinants (4.2 cM) observed among 48 F2 progeny combined across four snap bean populations. Assays of R2570/530 across susceptible germplasm and lines likely to have the `Garrapato'-derived partial resistance to BGMV have revealed that the codominant marker is gene-pool nonspecific and maintains its original linkage orientation with the recessive bgm-1 gene through numerous meioses. The codominant marker is useful for rapidly introgressing partial resistance to BGMV into susceptible germplasm.

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Scott D. Haley, Phillip N. Miklas, Lucia Afanador, and James D. Kelly

The objective of this study was to evaluate the degree of RAPD marker variability between and within commercially productive market classes representative of the Andean and Middle American gene pools of common bean (Phaseolus vulgaris L.). Six sets of near-isogenic lines were screened with oligonucleotide primers in the polymerase chain reaction-based RAPD assay. Simultaneous analyses with at least three sets of lines enabled us to score RAPD markers between the two major gene pools, races within the same gene pool, and different genotypes of the same race (within race). A “three-tiered” pattern of polymorphism was observed: between gene pools> between races> within races. The overall level of polymorphism between the Andean and Middle American gene pools was 83.4%. The overall level of polymorphism between races within the same gene pool was similar for Andean races (60.4%) and Middle American races (61.7%). The level of polymorphism between related commercial navy bean lines was 39.2% and between related commercial snap bean lines was 53.6 %. The inherent simplicity and efficiency of RAPD analyses, coupled with the number of polymorphisms detectable between related commercial genotypes, should facilitate the construction of RAPD-based genetic linkage maps in the context of populations representative of most bean breeding programs.

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Phillip N. Miklas, Richard Delorme, Valerie Stone, Mark J. Daly, J. Rennie Stavely, James R. Steadman, Mark J. Bassett, and James S. Beaver

Understanding the genomic associations among disease resistance loci will facilitate breeding of multiple disease resistant cultivars. We constructed a genetic linkage map in common bean (Phaseolus vulgaris L.) containing six genes and nine quantitative trait loci (QTL) comprising resistance to one bacterial, three fungal, and two viral pathogens of bean. The mapping population consisted of 79 F5:7 recombinant inbred lines (RILs) derived from a `Dorado'/XAN 176 hybridization. There were 147 randomly amplified polymorphic DNA (RAPD) markers, two sequence characterized amplified region (SCAR) markers, one intersimple sequence repeat (ISSR) marker, two seedcoat color genes R and V, the Asp gene conditioning seed brilliance, and two rust [Uromyces appendiculatus var. appendiculatus (Pers.:Pers) Unger] resistance genes: one conditioning resistance to Races 53 and 54 and the other conditioning resistance to Race 108. These markers mapped across eleven linkage groups, one linked triad, and seven linked pairs for an overall map length of 930 cM (Kosambi). Genes conditioning resistance to anthracnose (Co-2) [Colletotrichum lindemuthianum (Sacc. and Magnus) Lams.-Scrib.], bean rust (Ur-5), and bean common mosaic virus (I and bc-3) (BCMV) did not segregate in this population, but were mapped by inference using linked RAPD and SCAR markers identified in other populations. Nine previously reported quantitative trait loci (QTL) conditioning resistance to a variety of pathogens including common bacterial blight [Xanthomonas campestris pv. phaseoli (Smith) Dye], ashy stem blight [Macrophomina phaseolina (Tassi) Goid.], and bean golden mosaic virus (BGMV), were located across four linkage groups. Linkage among QTL for resistance to ashy stem blight, BGMV, and common bacterial blight on linkage group B7 and ashy stem blight, BGMV, and rust resistance loci on B4 will complicate breeding for combined resistance to all four pathogens in this population.