MAPPING A NEW NEMATODE RESISTANCE LOCUS IN LYCOPERSICON-PERUVIANUM

Accessions of the wild tomato species L. peruvianum were screened with a root-knot nematode population (557R) which infects tomato plants ca rrying the nematode resistance gene Mi. Several accessions were found to carry resistance to 557R. A L. peruvianum backcross population segr egating for resistance to 557R was produced. The segregation ratio of resistant to susceptible plants suggested that a single, dominant gene was a major factor in the new resistance. This gene, which we have de signated Mi-3, confers resistance against nematode strains that can in fect plants carrying Mi. Mi-3, or a closely linked gene, also confers resistance to nematodes at 32 degrees C, a temperature at which Mi is not effective. Bulked-segregant analysis with resistant and susceptibl e DNA pools was employed to identify RAPD markers linked to this gene. Five-hundred-and-twenty oligonucleotide primers were screened and two markers linked to the new resistance gene were identified. One of the linked markers (NR14) was mapped to chromosome 12 of tomato in an L. esculentum/L. pennellii mapping population. Linkage of NR14 and Mi-3 w ith RFLP markers known to map on the short arm of chromosome 12 was co nfirmed by Southern analysis in the population segregating for Mi-3. W e have positioned Mi-3 near RFLP marker TG180 which maps to the telome ric region of the short arm of chromosome 12 in tomato.