SOURCES AND GENETIC-STRUCTURE OF A CLUSTER OF GENES FOR RESISTANCE TO3 PATHOGENS IN LETTUCE

The second largest cluster of resistance genes in lettuce contains at least two downy mildew resistance specificities, Dm5/8 and Dm10, as we ll as Tu, providing resistance against turnip mosaic virus, and plr a recessive gene conferring resistance against Plasmopara lactucae-radic is, a root infecting downy mildew. In the present paper four additiona l genetic markers have been added to this cluster, three RAPD markers and one RFLP marker, CL1795. CL1795 is a member of a multigene family related to triose phosphate isomerase; other members of this family ma p to the other two major clusters of resistance genes in lettuce. Seve n RAPD markers in the region were converted into sequence characterize d amplified regions (SCARs) and used in the further analysis of the re gion and the mapping of Dm10. Three different segregating populations were used to map the four resistance genes relative to molecular marke rs. There were no significant differences in gene order or rate of rec ombination between the three crosses. This cluster of resistance genes spans 6.4 cM, with Dm10 1.2 cM from Dm8(.) Marker analysis of 20 cult ivars confirmed multiple origins for Dm5/8 specificity. Two different Lactuca serriola origins for the Du5/8 specificity had previously been described and originally designated as either Dm5 or Dm8. Some ancien t cultivars also had the same specificity. Previously, due to lack of recombination in genetic analyses and the Same resistance specificitie s, it was assumed that Dm5 and Dm8 were determined by the same gene. H owever, molecular marker analysis clearly identified genotypes charact eristic of each source. Therefore, Dm5/8 specificity is either ancient and widespread in L. serriola and some L. sativa, or else has arisen on multiple occasions as alleles at the same locus or at linked loci.