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, which is sensitive to Nb. Through a combination of bulked segregant analysis and selective restriction fragment amplification, several amplified fragment length polymorphism (AFLP) markers linked to Nb were identified. 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 suggested 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 chromosome 5 where several other resistance genes-including R1, a resistance gene against Phytophthora infestans, Gpa, a locus that confers resistance against Globodera pallida, and Rx2, a gene that confers extreme resistance against PVX-have previously been identified.