An isozyme marker for resistance to race 3 of Fusarium oxysporum f. sp. lycopersici in tomato.

The inheritance and the linkage relationship of resistance to race 3 of Fusarium oxysporum f. sp. lycopersici derived from Lycopersicon pennellii (LA 716) were analyzed in an interspecific backcross to L. esculentum. Progeny from each backcross (BC1) individual were in-oculated with race 3 and their response was measured according to a visual rating system; progeny responses were used to calculate a mean disease rating for each BC1 individual. The frequency distribution of the disease ratings was bimodal, indicating that resistance to race 3 was controlled by one major locus. Linkage analysis of this locus proceeded in two steps. Initially, disease ratings were compared between homozygotes and heterozygotes at each of 17 segregating marker enzyme loci. Highly significant differences were detected for the chromosome segment marked by the Got-2 locus on chromosome 7. This indicated that Got-2 was linked to the race 3 resistance gene, designated I-3; this gene accounted for the observed bimodal distribution of BC1 disease ratings. In the second step, the genotype of each BC1 individual at the I-3 locus was determined using cluster analysis of the disease ratings; a test for independent assortment between I-3 and Got-2 revealed strong linkage, with an estimated map distance of 2.5 centiMorgans. Additional evidence for this linkage was obtained from the analysis of five breeding lines previously selected for resistance to race 3 solely on the basis of inoculations; all five showed co-selection for Got-2. The Got-2 locus is proposed as a selectable marker to expedite the transfer of race 3 resistance to commercial tomato cultivars.