Correlation of genetic and physical structure in the region surrounding the I2 Fusarium oxysporum resistance locus in tomato.

The dominant gene I2 confers on tomato (Lycopersicon esculentum) resistance against the fungus Fusarium oxysporum f. sp. lycopersici race 2. A restriction fragment length polymorphism (RFLP) marker, TG105, has recently been found to be tightly linked to I2. The potential for cloning this gene by a reverse genetics approach prompted us to describe in both genetic and physical detail the region surrounding the I2 locus on chromosome 11. We have analyzed patterns of segregation of RFLP markers on chromosome 11 and Fusarium resistance in 140 F2 plants from a cross between Fusarium-resistant and susceptible parental lines. Marker TG105 mapped 0.4 centiMorgan (CM) from I2. Physical analysis of TG105 and its flanking RFLP markers, TG26 and TG36, by pulsed field gradient gel electrophoresis (PFGE) yielded a restriction map for this region encompassing at least 620 kb of the tomato genome. TG105 and TG26 hybridized to the same 175 kb MluI-NruI restriction fragment. We have therefore linked two genetically distinct RFLP markers. Based on the 4.1 cM distance between them, we have assigned a mean value of 43 kb for each cM recombination distance in the vicinity of I2. This local ratio between physical and genetic distances is more than 10-fold below the average for the tomato genome. It should therefore be possible to clone I2 by chromosome walking from TG105.