RECESSIVE RESISTANCE TO PLASMOPARA-LACTUCAE-RADICIS MAPS BY BULKED SEGREGANT ANALYSIS TO A CLUSTER OF DOMINANT DISEASE RESISTANCE GENES IN LETTUCE

The recessive gene (plr) for resistance in lettuce to the fungus Plasm opara lactucae-radicis, was mapped to a cluster of genes for resistanc e to taxonomically diverse pathogens. Pour RAPD (random amplified poly morphic DNA) loci Linked to plr were identified by bulked segregant an alysis of a previously unanalyzed F-2 population segregating for pit. Two of these RAPD loci had been previously mapped in a second cross an d therefore identified a region of many other loci potentially licked to plr. RAPD loci in the region of plr were mapped using F-3 families. informative F-3 families that showed a recombination event in the reg ion were subsequently scored for resistance to P. lactucae-radicis to locate plr on the genetic map. Ambiguities regarding the precise posit ion of plr caused by dominance or misscorings were resolved by scoring individuals of selected F-3 families to define the genotype of the pr ogenitor plants of the F-2 population. The plr gene and three other di sease resistance genes (downy mildew resistance, Dm5/8 and Dm10, and t urnip mosaic virus resistance, Tu) are all located on a segment of 12- 18 cM. This strategy of quickly identifying linked loci by bulk segreg ant analysis of the F-2 population followed by precise mapping of the gene with F-3 families proved efficient and accurate. The growing numb er of disease resistance genes that map into three clusters in lettuce supports the hypothesis that resistance genes are members of multigen e families whose members have a common function and origin. The presen ce of other multigene families associated with these resistance gene c lusters suggests that duplications and rearrangements involving these regions may be common.