Recessive Resistance to Plasmopara lactucae-radicis Maps by Bulked Segregant Analysis to a Cluster of Dominant Disease Resistance Genes in Lettuce. Rick Kesseli. Department of Vegetable Crops, University of California, Davis 95616 U.S.A.; Biology Department, University of Massachusetts, Boston 02125 U.S.A. Hanneke Witsenboer,(1) Mike Stanghellini,(3) George Vandermark,(3,4) and Richard Michelmore(1). (1)Department of Vegetable Crops, University of California, Davis 95616 U.S.A.; (2)Biology Department, University of Massachusetts, Boston 02125 U.S.A.; (3)Department of Plant Pathology, University of Arizona, Tucson 85721 U.S.A.; (4)Agritope Inc., Beaverton, OR 97005 U.S.A. MPMI 6:722-728. Accepted 6 July 1993. Copyright 1993 The American Phytopathological Society.

The recessive gene (plr) for resistance in lettuce to the fungus Plasmopara lactucae-radicis, was mapped to a cluster of genes for resistance to taxonomically diverse pathogens. Four RAPD (random amplified polymorphic DNA) loci linked to plr were identified by bulked segregant analysis of a previously unanalyzed F₂ population segregating for plr. Two of these RAPD loci had been previously mapped in a second cross and therefore identified a region of many other loci potentially linked to plr. RAPD loci in the region of plr were mapped using F₃ families. Informative F₃ families that showed a recombination event in the region were subsequently scored for resistance to P. lactucae-radicis to locate plr on the genetic map. Ambiguities regarding the precise position of plr caused by dominance or misscorings were resolved by scoring individuals of selected F₃ families to define the genotype of the progenitor plants of the F₂ population. The plr gene and three other disease resistance genes (downy mildew resistance, Dm5/8 and Dm10, and turnip mosaic virus resistance, Tu) are all located on a segment of 12–18 cM. This strategy of quickly identifying linked loci by bulk segregant analysis of the F₂ population followed by precise mapping of the gene with F₃ families proved efficient and accurate. The growing number of disease resistance genes that map into three clusters in lettuce supports the hypothesis that resistance genes are members of multigene families whose members have a common function and origin. The presence of other multigene families associated with these resistance gene clusters suggests that duplications and rearrangements involving these regions may be common.