A POTATO HYPERSENSITIVE RESISTANCE GENE AGAINST POTATO-VIRUS-X MAPS TO A RESISTANCE GENE-CLUSTER ON CHROMOSOME-5

The dominant Nb gene of potato confers strain-specific hypersensitive resistance against potato virus X (PVX). A population segregating for Nb was screened for resistance by inoculating with PVX strain CP2, whi ch is sensitive to Nb. Through a combination of bulked segregant analy sis and selective restriction fragment amplification, several amplifie d fragment length polymorphism (AFLP) markers linked to Nb were identi fied. These were cloned and converted into dominant cleaved amplified polymorphic sequence (CAPS) markers. The segregation of these markers in a Lycopersicon esculentum x L. pennellii mapping population suggest ed that Nb is located on chromosome 5. This was confirmed by examining resistant and susceptible potato individuals with several tomato and potato chromosome-5-specific markers. Nb maps to a region of chromosom e 5 where several other resistance genes-including R1, a resistance ge ne against Phytophthora infestans, Gpa, a locus that confers resistanc e against Globodera pallida, and Rx2, a gene that confers extreme resi stance against PVX-have previously been identified.