RESPONSE TO POTYVIRUS INFECTION AND GENETIC-MAPPING OF RESISTANCE LOCI TO POTYVIRUS INFECTION IN LACTUCA

We have investigated the interaction between two different potyviruses and resistant cultivars of Lactuca sativa. Turnip mosaic virus (TuMV) and lettuce mosaic virus (LMV) were used to inoculate several cultiva rs under different temperature regimes to characterize the resistance reaction. Resistance conferred by the recessive mo locus against LMV i nfection did not provide immunity. Virus accumulated in plant tissues to different levels depending on the genetic background of the cultiva r, suggesting that several genes were involved in the resistance pheno type. Under temperature regimes that enhanced the hypersensitive react ion, resistant cultivars produced necrotic reactions. In contrast, res istance to TuMV infection conferred by the dominant Tu locus resulted in complete immunity in the plant. No virus accumulated in inoculated leaves nor was any necrotic reaction observed. The resistance loci wer e characterized at the genetic level by mapping them relative to molec ular markers. Only weak linkages could be identified to mo, again supp orting the hypothesis that several genes are involved. The Tu locus wa s mapped in two different crosses relative to several markers, the clo sest two linked at less than 1 cM. A high-resolution genetic map of th e Tu locus was constructed by screening 500 F-2 individuals for recomb inants around that locus.