High-resolution genetic and physical map of the Rvi1 (Vg) apple scab resistance locus

Apple scab, caused by the fungal pathogen V. inaequalis, is one of the most prejudicial diseases in apple (Malus x domestica Borkh.). At least 17 monogenic resistances against this pathogen have been identified in the Malus genus, but only two, Rvi6 (Vf) and Rvi15 (Vr2), have been cloned and functionally characterized so far. Here, we describe the first steps of the cloning of a new scab resistance gene, Rvi1 (Vg). This gene is carried by the well-known cultivar 'Golden Delicious' and confers specific resistance against V. inaequalis strains carrying the AvrRvi1 avirulence factor. With a fine mapping approach on a population of 1,983 individuals, we developed SSR markers tightly linked to the Rvi1 gene. Sequencing of two bacterial artificial chromosome (BAC) clones covering, respectively, the Rvi1 apple scab resistant and susceptible allele, allowed the identification of a 110-kilo base pairs (kbps) sequence flanked by two linked markers (Vg12- and Vg15-SSR). Open reading frames were predicted on these BACs: four TIR-NBS-LRR (TNL) putative genes, a putative TNL pseudogene and one serine/threonine protein phosphatase 2A were identified.