Two additive QTLs conferring broad-spectrum resistance in potato to Globodera pallida are localized on resistance gene clusters

Broad-spectrum resistance in potato to the potato cyst nematode (PCN) is co mmonly regarded as a complex inherited trait. Yet, in this paper we show th at, by use of a selected set of PCN test populations, broad-spectrum resist ance to the species Globodera pallida can be fully ascribed to the action o f two loci: Gpa5 and Gpa6. These loci were readily mapped by means of a str ategy based on mio steps. Firstly, the chromosomal localization of both loc i was assessed by use of an online catalogue of AFLP markers covering a sub stantial part of the potato genome (http://www.spg.wau.nl/pv/aflp/catalog.h tm). Subsequently the chromosomal regions of both loci were identified by m eans of CAPS markers based on RFLP insert sequences. Locus Gpa5 explains at least 61% of the genetic variation. This locus maps to chromosome 5 on a r egion which has previously been shown to harbor resistance factors to viral (Nb, Rx2), fungal (R1) and nematodal (Gpa, Grp1) pathogens. The Gpa6 locus exhibits a minor effect on the resistance (24%) and acts additively to Gpa 5. Interestingly, the Gpa6 locus maps to a region on chromosome 9 where, in the homoeologous tomato genome, the virus resistance gene Sw-5 resides as part of a resistance gene cluster. in potato, resistance to potato virus X has been reported in the vicinity of this region. The map location of Gpa6 indicates the presence of a resistance gene cluster at the end of the long arm of chromosome 9 of potato.