GENETIC-CHARACTERIZATION OF THE PTO LOCUS OF TOMATO - SEMI-DOMINANCE AND COSEGREGATION OF RESISTANCE TO PSEUDOMONAS-SYRINGAE PATHOVAR TOMATO AND SENSITIVITY TO THE INSECTICIDE FENTHION

The Pto locus governs resistance to bacterial speck disease in tomato caused by race 0 strains of Pseudomonas syringae pathovar tomato (Pst) . Large populations segregating for the Pto locus were generated and g enetically characterized. Analysis of the locus has revealed that Pto acts in a semi-dominant manner and cosegegrates with sensitivity to an organophosphorous insecticide, Fenthion, suggesting that Pto may be a complex locus responsible for both phenotypes. We have redefined its map position on chromosome five of the classical genetic map and assig ned its position on the molecular map, thus facilitating the alignment of the two genetic maps of the short arm of chromosome five of tomato . Furthermore, we have screened random amplified polymorphic (RAPD) ma rkers for their ability to differentiate near-isogenic lines that diff er only with respect to Pto and have identified and mapped seven of th ese markers. Our results suggest that Pto may be located in a euchroma tic region on chromosome five which will be advantageous for the cloni ng of this locus by one of several molecular strategies.