MAPPING AND INTROGRESSION OF A TOMATO YELLOW LEAF CURL VIRUS TOLERANCE GENE, TY-1

The whitefly-transmitted tomato yellow-leaf curl gemini-virus (TYLCV) is a major pathogen of tomatoes. The wild tomato species Lycopersicon chilense, which is resistant to the virus, was crossed to the cultivat ed tomato, L. esculentum. The backcross-l selfed (BC1S1) generation wa s inoculated and a symptomless plant was selected. This plant was anal yzed using 61 molecular markers, which span the tomato genome, to dete rmine which L. chilense chromosome segments were introgressed. A BC2S1 population was cage-inoculated with viroliferous whiteflies (Bemisia tabaci), the natural insect vector of the virus, and subjected to RFLP analysis. Markers on chromosomes 3 and 6 were significantly associate d with the level of tolerance; the association of chromosome-6 markers was further substantiated in two additional BC2S1 populations. A tole rant BC2S1 plant which was homozygous for L. chilense introgressions i n chromosomes 3, 6 and 7 was crossed to generate a BC3S1 population wh ich was planted in an infested field. A TYLCV-tolerance gene with part ial dominance, TY-I, was mapped to chromosome 6; two modifier genes we re mapped to chromosomes 3 and 7. Field and whitefly-mediated cage ino culations of nearly-isogenic lines in BC3S3 supported our conclusion t hat TY-I is the major TYLCV-tolerance locus.