GENETIC-MAPPING OF PH-2, A SINGLE-LOCUS CONTROLLING PARTIAL RESISTANCE TO PHYTOPHTHORA-INFESTANS IN TOMATO

Late blight caused by the fungal pathogen Phytophthora infestans is on e of the most important diseases of potato (Solanum tuberosum) and tom ato (Lycopersicon esculentum). Genetic analysis of the resistance to t his pathogen was performed on an F-2 progeny of 322 plants derived fro m a cross between the tomato line L. esculentum var, Hawaii7996 suscep tible to late blight and the resistant wild relative L. pimpinellifoli um WVa700. Qualitative and quantitative analyses of the resistance wit h restriction fragment length polymorphism markers spanning the genome showed that this resistance was controlled by a single, incompletely dominant allele, Ph-2, present on the distal part of the long arm of c hromosome 10 in an interval of 8.4 cM flanked by markers CP105 and TG2 33. Genetic analysis of F-2 progeny from a second cross between an L. esculentum introgression line IL10-3 carrying a homozygous L. pennelli i segment spanning the distal part of the long arm of chromosome 10 an d WVa700 confirmed the map location, but high suppression of recombina tion was observed in this cross in the introgressed fragment, A high-r esolution genetic linkage map of the chromosomal region surrounding Ph -2 was initiated to permit future map-based cloning of this gene, Ampl ified fragment length polymorphism markers closely linked to Ph-2 were screened by bulked segregant analysis.