UNDERSTANDING MOLECULAR DIVERSITY: TOWARDS STATEGIC SAMPLING OF GENEBANK COLLECTIONS

James M. Bradeen^a, Maria J. Sanchez^a, Ryan L. Syverson^a, Riccardo Aversano^a,b, Dimitre Mollov^a, and Domenico Carputo^b

^aUniversity of Minnesota, Department of Plant Pathology, 495 Borlaug Hall / 1991 Upper Buford Circle, St. Paul, MN 55108 USA

^bUniversity of Naples “Federico II”, Department of Soil, Plant, and Environmental Science, Via Università 100, 80055 Portici (NA) Italy

jbradeen@umn.edu

Genebank collections are genetic treasure-troves for crop improvement. Potato is host to more than 60 diseases and the 200 wild potato species are important sources of resistance genes (R-genes). However, characterizing genebank collections for disease resistance is time-consuming and costly. Here, we seek predictors of total genome, R-gene allelic, and R gene marker diversity that can drive resource-efficient disease resistance characterization. We focus specifically on the USDA Potato Genebank collection of Solanum bulbocastanum, a wild diploid 1EBN potato from Mexico. We established a reference set of 42 genotypes comprising 12 populations, three morphologically-defined subspecies, and the entire geographic distribution of the species. Our reference set includes multiple individuals per population. AFLP (total genome) diversity analysis revealed considerable within population diversity, consistent with the obligate outcrossing nature of S. bulbocastanum. We found no molecular or phenotypic support for established subspecies. Geographic distance between populations correlated poorly with genetic distance. Using long range-PCR, we isolated alleles of the late blight resistance locus RB. In contrast to observed high levels of total genome diversity, analysis of informative RB nucleotide sites reveals low levels of allelic diversity within the species, regardless of geographic origin, subspecies classification, or within vs. between population relationships. Our results imply a strong selective advantage for conservation of RB allelic sequence. Phenotypic assays are ongoing. We isolated S. bulbocastanum BAC clones containing the RB and R1 late blight resistance loci. PCR markers of varying distances from the R loci were developed from BAC ends. Marker analyses of our 42 genotype S. bulbocastanum reference set, aimed at characterizing haplotype diversity near RB and R1, are ongoing. Taken together, our results suggest that geographic distribution, subspecies classification, and within vs. between population relationships may not be reliable predictors of total genome or R-gene allelic diversity for S. bulbocastanum.