Characterization of nearly 1,000 cultivated potato accessions with simple sequence repeats (SSRs; also referred to as microsatellites) has allowed the identification of a reference set of SSR markers for accurate and efficient genotyping. In addition, 31 SSRs are reported here for a potato genetic map, including new map locations for 24 of them. A first criterion for this proposed reference set was ubiquity of the SSRs in the eight landrace cultivar groups of the potato, Solanum tuberosum. All SSRs tested in the present study displayed the same allele phenotypes and allele size range in the diverse germplasm set as in the advanced potato cultivar germplasm in which they were originally discovered. Thirteen of 13 SSR products from all cultivar groups are shown to cross-hybridize with the corresponding SSR product of the source cultivar to ascertain sequence homology. Other important SSR selection criteria are quality of amplification products, locus complexity, polymorphic index content, and well-dispersed location on a potato genetic map. Screening of 156 SSRs allowed the identification of a highly informative and userfriendly set comprising 18 SSR markers for use in characterization of potato genetic resources. In addition, we have identified true-and pseudo-multiplexing SSRs for even greater efficiency.

The cultivated potato, Solanum tuberosum, is affected by a variety of diseases with late blight, caused by Phytophthora infestans, being the most severe. Wild potato species have proven to be a continuing source of resistance, sometimes of an extreme type, to this disease. The present study constructs the first late blight linkage map of a member of series Piurana, S. paucissectum, a tuber-bearing relative of potato, using probes for conserved sequences from potato and tomato. Eight probes mapped to unexpected linkage groups, but syntenic differences with prior maps of potato were not supported by any blocks of rearranged chromosome segments. All 12 linkage groups were resolved and significant associations with late blight resistance were found on chromosomes 10, 11 and 12. A major quantitative trait locus (QTL) on chromosome 11 accounts for more than 25% of the phenotypic variance measured in a field trial. Crossing of S. paucissectum with cultivated potato resulted in very few seeds indicating partial reproductive barriers. Differential reactions of accessions of this potential donor species with simple and complex isolates of P. infestans suggest that it carries major resistance genes that are not those previously described from the Mexican species, S. demissum. However, the additivity of the QTL effects argues for the quantitative nature of resistance in this cross.