RAPID MAPPING OF 2 GENES FOR RESISTANCE TO DOWNY MILDEW FROM LACTUCA-SERRIOLA TO EXISTING CLUSTERS OF RESISTANCE GENES

Two resistances to downy mildew derived from Lactuca serriola were cha racterized genetically and mapped using molecular markers. Classical g enetic analysis suggested monogenic inheritance; however, the presence of multiple, tightly-linked genes in each case could not be eliminate d. Therefore, they were designated resistance factors R17 and R18. Ana lysis with molecular markers known to be linked to clusters of resista nce genes quickly revealed linkage of R18 to the major cluster of resi stance genes and provided six linked markers, three RAPD (Random Ampli fied Polymorphic DNA) markers and three codominant SCAR (Sequence Char acterized Amplified Region) markers. The mapping of R17 required the s creening of arbitrary RAPD markers using bulked segregant analysis; th is provided five linked markers, three of which segregated in the basi c mapping population. This demonstrated loose linkage to a second clus ter of resistance genes and provided additional linked markers. Two RA PD markers linked to R17 were converted into SCARs. The identification of reliable PCR-based markers flanking each gene will aid in selectio n and in combining these resistance genes with others.