Mapping and DNA sequence characterisation of the Ry.sub.sto locus conferring extreme virus resistance to potato cultivar 'White Lady'

Virus resistance genes carried by wild plant species are valuable resources for plant breeding. The Ry.sub.sto gene, conferring a broad spectrum of durable resistance, originated from Solanum stoloniferum and was introgressed into several commercial potato cultivars, including 'White Lady', by classical breeding. Ry.sub.sto was mapped to chromosome XII in potato, and markers used for marker-assisted selection in breeding programmes were identified. Nevertheless, there was no information on the identity of the Ry.sub.sto gene. To begin to reveal the identification of Ry.sub.sto, fine-scale genetic mapping was performed which, in combination with chromosome walking, narrowed down the locus of the gene to approximately 1 Mb. DNA sequence analysis of the locus identified six full-length NBS-LRR-type (short NLR-type) putative resistance genes. Two of them, designated TMV2 and TMV3, were similar to a TMV resistance gene isolated from tobacco and to Y-1, which co-segregates with Ry.sub.adg, the extreme virus resistance gene originated from Solanum andigena and localised to chromosome XI. Furthermore, TMV2 of 'White Lady' was found to be 95% identical at the genomic sequence level with the recently isolated Ry.sub.sto gene of the potato cultivar 'Alicja'. In addition to the markers identified earlier, this work generated five tightly linked new markers which can serve potato breeding efforts for extreme virus resistance.