Correlation of genetic and physical structure in the region surrounding the I 2 Fusarium oxysporum resistance locus in tomato

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Abstract
Summary The dominant gene I 2 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 I 2. 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 I 2 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 centi-Morgan (CM) from I 2. 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 I 2. 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 I 2 by chromosome walking from TG105.