Identification of restriction fragment length polymorphism and random amplified polymorphic DNA markers linked to downy mildew resistance genes in lettuce, using near-isogenic lines.

Near-isogenic lines were used to identify restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers linked to genes for resistance to downy mildew (Dm) in lettuce. Two pairs of near-isogenic lines that differed for Dm1 plus Dm3 and one pair of near-isogenic lines that differed for Dm11 were used as sources of DNA. Over 500 cDNAs and 212 arbitrary 10-mer oligonucleotide primers were screened for their ability to detect polymorphism between the near-isogenic lines. Four RFLP markers and four RAPD markers were identified as linked to the Dm1 and Dm3 region. Dm1 and Dm3 are members of a cluster of seven Dm genes. Marker CL922 was absolutely linked to Dm15 and Dm16, which are part of this cluster. Six RAPD markers were identified as linked to the Dm11 region. The use of RAPD markers allowed us to increase the density of markers in the two Dm regions in a short time. These regions were previously only sparsely populated with RFLP markers. The rapid screening and identification of tightly linked markers to the target genes demonstrated the potential of RAPD markers for saturating genetic maps.