Tight Physical Linkage of the Nematode Resistance Gene<i>Gpa2</i>and the Virus Resistance Gene<i>Rx</i>on a Single Segment Introgressed from the Wild Species<i>Solanum tuberosum</i>subsp.<i>andigena</i>CPC 1673 into Cultivated Potato

Comparative mapping studies within several plant genera have shown that genes for resistance to taxonomically diverse pathogens are often co-localized on the maps of these genotypes. Here we describe the genetic analysis of such a gene cluster on chromosome 12 of potato. This cluster harbors the resistance loci Gpa2 and Rx, which confer resistance to the potato cyst nematode Globodera pallida and potato virus X (PVX), respectively. By screening two mapping populations with 10 CAPS (cleaved amplified polymorphic sequence) markers derived from AFLP (amplified fragment length polymorphism) markers and the ends of overlapping bacterial artificial chromosome (BAC) clones, it is demonstrated that Gpa2 and Rx are tightly linked. From a recombination fraction of 1/3386, found after screening two different mapping populations, a genetic distance of 0.02 cM between Gpa2 and Rx was calculated. The physical length of this genetic interval does not exceed 300 kb of DNA as determined by the total size of overlapping BAC clones in the Rx/Gpa2 contig. Evidence is provided that both genes have been introgressed from a single wild species clone, Solanum tuberosum subsp. andigena CPC 1673. This suggests that these genes underlying the disease resistance specificities to potato virus X and potato cyst nematodes have a common evolutionary origin.