RFLP mapping on potato chromosomes of two genes controlling extreme resistance to potato virus X (PVX).

Two different chromosomal locations of major genes controlling extreme resistance to potato virus X (PVX) were found by restriction fragment length polymorphism (RFLP) analysis of two populations segregating for the resistance. The resistance gene Rx1 mapped to the distal end of chromosome XII, whereas Rx2 was located at an intermediate position on linkage group V in a region where reduced recombination and segregation distortion have also been observed. These linkage anomalies were due to abnormal behaviour of the chromosome contributed by the resistant parent P34. The results presented were obtained using two different strategies for mapping genes of unknown location. One approach was the use of probes revealing polymorphic loci spread throughout the genome and resulted in the mapping of Rx1. The second approach was based on the assumption of possible linkage between the resistance gene and clone-specific DNA fragments introduced from a wild potato species. Rx2 was mapped by adopting this strategy.