Relationship of the I-3 Gene for Resistance to Fusarium Wilt Race 3 with Resistance to Races 1 and 2 in Tomato

Fusarium wilt is a major soil-borne disease of tomato in many production regions throughout the world. Disease management is primarily achieved by deployment of the dominant, race-specific resistance genes, I, I-2 and I-3 for control of races 1, 2 and 3, respectively. I-3 was introgressed from Solanum pennellii, and the introgression in early breeding materials conferred resistance to races 1 and 2 as well. However, resistance to these races is reportedly not conferred by the I-3 gene, but by the linked genes, I1 and a putative race 2 resistance gene. Recent studies have shown that the I-3 locus is negatively associated with several horticultural traits, and there is evidence that some of these problems result from linkage drag. Whereas the development and deployment of a reduced introgression may resolve these horticultural issues, it may also have the unintentional effect of dissociating the I-3 gene from race 1 and race 2 resistance. A map-based cloning approach was used to fine-map the I-3 gene to an approximately 30 Kb interval. Four recombinants with reduced introgression sizes, two lacking the S. pennellii introgression above the I-3 interval, and two lacking the introgression below this interval, were backcrossed to the Fusarium wilt susceptible cultivar, Bonnie Best. For each recombinant, BC₂F₁ populations were used to investigate the location of I-1 and the putative race 2 gene relative to I-3. Results from race 1 inoculations indicated that the I1 gene is likely located outside the 4 Mbp introgression which is present in most modern cultivars and spans from 60 to 64 Mbp on the SL2.50 genome assembly. For all four recombined introgressions, Fusarium wilt race 2 inoculations indicated that the putative resistance gene is located within the 30 Kb I-3 interval. This suggests that race 2 resistance is conferred either by I-3 or by a very tightly linked allele. Implications of these findings on future research and breeding strategies will be discussed.