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Potato has about 100 related wild Solanum species growing naturally in the Americas. The US Potato Genebank aims to keep samples useful for research and breeding to improve the crop, often in the form of botanical seed families. A key component of genebank efficiency is assessing diversity within and among populations, and DNA marker sequence diversity is a powerful proxy for trait diversity. We previously reported on three factors which can cause under-estimation of heterozygosity: ascertainment, allele frequency, and ploidy bias. We here report, using GBS data for four diploid potato species, that average percent of apparent heterozygosity increases as data is more complete—the maximum difference was 2% heterozygotes when only a few individuals are called, to 36% when nearly all individuals were called. However, there was evidence that estimates of average heterozygosity based only on loci for which every individual has data can also be biased upward. Implausibly high levels of heterozygosity suggest non-segregating non-homologous SNPs, which occurred as 5–9% of all loci with complete data. We propose that best estimates of average heterozygosity in unselected seedlings should be based on loci with data for all samples after eliminating those loci that appear to be artificially fixed as heterozygous, which reduces observed heterozygote frequency by 16–26%. On that basis, the wild species examined have similar heterozygosity to the cultivated phureja.

Potato has about 100 related wild Solanum species growing naturally in the Americas. Solanum jamesii (jam), native to the southwest USA and Mexico, has many valuable traits for breeding, but making hybrids is extremely difficult. We investigated the approach of using the bridge species Solanum verrucosum (ver). A survey of all ver populations in the US Potato Genebank identified the best females. The standard bridge crossing technique using emasculation of ver and “mentor” double pollination after first pollinating with jam was very inefficient for most ver because: 1) emasculation depressed seedset in ver females, 2) despite very careful emasculation many accidental ver selfs resulted, and 3) mentor pollination produced many unwanted hybrids with the mentor pollen parent. We therefore produced populations of ver by backcrossing five generations into tuberosum cytoplasm. The BC5 lines are very vigorous in growth and flowering, but having very low male fertility and complete self-incompatibility they can be mass crossed with jam pollen without emasculation. Even with no mentor pollen, most pistils of verBC5 pollinated with jam produce a small fruit with an average of less than one normal seed. Resulting seedlings were confirmed ver-jam hybrids by DNA markers.

Potato cultivars are propagated by tubers which are sensitive to damage by freezing. Potato has about 100 related wild Solanum species growing naturally in the Americas. When tubers of a spectrum of these species were slowly cooled, most were killed at a few degrees below 0 C. Only tubers of Solanum jamesii, native to the southwest USA, remained firm, sprouted and grew after one week exposure to freezing temperatures as cold as -15C. Differential Thermal Analysis was performed to detect low temperature exotherms (LTEs) in jamesii and similar-sized tubers of cultivars Russet Burbank and Snowden. LTEs, indicating supercooling, were detected only in the jamesii tubers. Survival of potato tubers at freezing temperatures could be useful in several ways. Even non-lethal cold exposure of potato tubers at harvest reduces quality for eating and for seed, but storage at freezing temperatures without damage could help preserve tuber dormancy and limit diseases.

Useful genetic diversity found in wild potato germplasm is important for progress in potato breeding and science. An effective way to maximize its utilization is through characterization of ex situ collections. For example, marker-based core subsets from large collections are an effective strategy to accelerate evaluation, utilize germplasm, and enhance cost-effectiveness. This project used AFLP markers to create a core subset for the US Potato Genebank’s collection of S. demissum. This species from Mexico and Guatemala is known for excellent levels of late blight resistance and frost tolerance. A total of 149 accessions with diverse geographic origin were characterized with 1403 AFLP markers; these markers were effective on determining genetic associations among the accessions. For the core subset, the AFLP loci were considered as traits, with the presence of the marker as the required state to include in a core. The results of the analyses identified that 10 of the accessions of S. demissum captured 83% of all the markers detected in the whole collection. The inclusion of 28 additional accessions, which had between 1 to 5 unique markers, resulted in a core set of 38 accessions that captured 96% of all the marker diversity. We expect this can benefit the genebank and the potato community by opening opportunities for research, trait screenings, and marker association studies.

Crop genebanks keep a sample of the diversity in wild and cultivated germplasm. Knowing the structure of diversity within and among populations informs best decisions on efficient collecting, preservation and use. The US Potato Genebank has over 90 Solanum species, ranging in representation from only a few populations to several hundred. In a previous study we showed that populations of three species captured nearly all AFLP bands in polymorphic loci when represented by 100 populations that had been added over time to the genebank. In the current study we add analysis of two additional species and propose a simple metric, L100, to compare the expected marker diversity capture of 100 populations among different species. L100 detects when there is less population overlap and more rare markers, indicating that more populations are needed to capture the available diversity in a species.

AFLP markers were used to build a core subset in 144 accessions of the United States Potato Genebank (USPG) ex situ collection of the species formerly known as Solanum phureja (now reclassified as Solanum tuberosum L. subsp. andigenum). A core subset aims to sample the minimum number possible of non-redundant germplasm units capturing the maximum diversity of the entire collection. Characterization and trait evaluation of a core subset is expected to be easier and more effective when compared to the entire germplasm collection. In this study 1534 polymorphic AFLP markers were generated and demonstrated to be useful to determine genetic relationships in the materials used. All the accessions were clearly discriminated from each other with genetic similarity levels ranging from 62 to 89%, and no duplicate germplasm samples were detected. To create the core subset, the presence of the AFLP marker was considered as the genetic trait to retain and the selection of accessions was determined by their rankings on the number of most markers added to the core. The results of this selection process revealed that the 9 top-ranked accessions of phureja, when combined, achieved the benchmark of capturing 85% of the markers detected in the complete collection. Further selection to increase the percentage of marker capture added 17 accessions with unique marker contributions which resulted in a final core subset of 26 accessions capturing 96% of the marker diversity. A Principal Component analysis determined that spatial distribution of diversity in the core was a proportional sampling of the entire diversity of the collection validating the selecting approach used. A review of the public database for valuable traits for the USPG phureja germplasm revealed that the core subset also included accessions with desirable pest/disease resistances and stress tolerances (in 25 out of the 30 traits reported in the database). The USPG expects this core subset would create opportunities for facilitating evaluation and research by using a reduced set of germplasm units with nearly all the diversity of the entire set.

Genebanks aim to maximize the preservation, classification, evaluation and distribution of germplasm in their collections to researchers and breeders. The wild is the source of most of the diversity available to genebanks, so diversity in the wild and how to best capture it also deserves study. The ultimate in valuable information from the wild would be the discovery of a large robust population at a single location on public land that is very easy to visit and contains a large proportion of the total genetic diversity detected in the species, which we have termed a “MegaPopulation”. We here report such a population of the wild potato species Solanum fendleri in the USA (also known as S. stoloniferum) on Mount Lemmon at the top of the Santa Catalina mountains near Tucson, Arizona.

The Potato Introduction Station in Sturgeon Bay Wisconsin has maintained herbarium specimens as part of its operations. These specimens mostly were grown from seeds in the field plots or greenhouses, but also include original specimens made during germplasm collecting expeditions. The herbarium of the Station (herbarium acronym PTIS) has been transferred from the potato genebank in Sturgeon Bay Wisconsin to the Wisconsin State Herbarium (acronym WIS) in Birge Hall, University of Wisconsin-Madison where it is now fully integrated and curated and funded by WIS; PTIS is discontinued and is no longer maintained as a separate herbarium. Separately, maps and taxonomic literature that were used by the potato taxonomist in Madison were transferred to the Potato Station in Sturgeon Bay.

Ploidy manipulation and the resulting need for rapid ploidy screening can be important in potato research and breeding programs. We tested three predictors of ploidy, particularly seeking the quickest, simplest, and most reliable: Chloroplast number per guard cell (C#), guard cell length (GC), and pollen diameter (P), with a total of seven variations in methods of preparation. Time required for each preparation was assessed, and a panel of inexperienced volunteers compared these methods for accuracy using a standard set of coded samples of known ploidy. The common method of counting C# with iodine stain took longer and was no more accurate than observing C# or GC in tap water. GC from tape impressions of the underside of leaves was reliable and has the advantage of permanent slides for later reference. We recommend GC, whether in water, stained, or as tape impressions. GC is significantly different in diploids and tetraploids, but the distributions do overlap, so experience and care in selecting a representative sample of cells contributes to accuracy. The standard measurement of P after staining with aceto-carmine was faster to prep and just as reliable as epidermal methods for some technicians, even with no previous experience. Pursuit of ultra-simplified methods led us to measure P in tap water. Diameters of pollen in plain water are significantly larger, but only for living pollen, suggesting this method might also be developed into a rapid and reliable way to estimate pollen viability.

Potato has a great impact on the economic and nutritional status of people worldwide. To capitalize on its potential, continued improvements in the current crop through breeding, production, and management technology are needed. We also need to expand potato use in diverse market outlets. In this review, we provide background on the challenge, examples of innovation in adapting other foods in response to changes in society, and explore several new possibilities for expansion of potato use.

Meloidogyne chitwoodi (Columbia root-knot nematode, CRKN) can cause serious damage in potato production systems, decreasing tuber value in the fresh market and processing industries. Genetic resistance to CRKN was first identified from the wild diploid potato species Solanum bulbocastanum accession SB22 and was successfully introgressed into tetraploid potato breeding material. To expand the base of genetic resistance, 40 plant accessions representing nine wild potato species were screened for their resistance to M. chitwoodi. Greenhouse screening identified fifteen clones from S. hougasii, one clone from S. bulbocastanum, and one clone from S. stenophyllidium with moderate to high levels of resistance against three isolates of M. chitwoodi. Geographical mapping showed that the resistance sources identified in this and previous studies primarily originated in the states of Jalisco and Michoacan in west-central Mexico. These new sources of resistance will be introgressed into elite potato populations to facilitate the development of potato cultivars with durable resistance to M. chitwoodi.

Potato has about 100 wild species relatives that are multiplied in the form of botanical seed populations by genebanks and distributed for use in research and breeding, so understanding factors that affect seed germination is valuable. This is especially true for problematic seedlots that have slow and low (“trickle”) germination even when subjected to germination techniques that give optimal results in most other seedlots in the genebank. Twelve seedlots with trickle germination even after at least 2 years of storage and 2 robustly-germinating positive control seedlots encompassing 9 species were germinated in petri plates, and subjected to day/night = 10 h/14 h = 20C/6C alternating temperature (AT) trials along with constant room temperature (RT) controls, in the spring of 2015 and 2016. When Relative Area Under the Germination Progress Curve (RAUGPC) was calculated, normal germinating seedlots’ germination in AT was not significantly different from RT control. Some species’ seedlots’ RAUGPC germination in AT was nearly zero, some were about 50% of RT control, and some in AT germination had RAUGPC double or even triple that of RT control. In a separate trial in 2017, four seedlots most responsive to AT were shown to germinate significantly better in constant cold than their RT controls, but not as well as seeds germinated under AT. Also in 2017, two seedlots known to be very dependent on gibberellin pretreatment were tested. RT germination without gibberellin had RAUGPC of only 13% of RT control, but AT without gibberellin pretreatment increased this to 65%, showing AT substantially substitutes for gibberellin pretreatment in these seedlots. AT could be a useful application to all seed germination assays in the genebank if it facilitated a better assessment of the potential viability of trickle-germinating seedlots. But one would need to identify and exclude those seedlots for which alternating temperature greatly depresses germination.

The potato crop has much exotic related germplasm, so optimizing the techniques for obtaining hybrid seeds from interspecific crosses is of interest to potato researchers and breeders, especially when the cross of interest is very difficult and few hybrid seeds are expected per pollination. A model plant of Solanum verrucosum (a particularly useful wild bridge species) was selected to have very uniform and robust flower and seed production in natural crosses as a backdrop for detecting effects of different emasculation techniques. For three uniform buds in single inflorescences, one was pollinated naturally (N) after no emasculation, one was emasculated by gently opening the corolla and plucking out only the immature anthers (A) with a tweezer, and one had total (T) removal of the sepals, corolla and anthers with a tweezers. Pollen of a highly fertile diploid tuberosum male was applied uniformly to all three buds within an inflorescence one day later. No pistil abortion occurred, excluding that as a limiting factor for getting hybrid seed. But both average fruit weight and average seeds per fruit were highly significantly reduced by emasculation in the pattern T

Potato has about 100 wild species relatives that are multiplied in the form of botanical seed populations by genebanks, and distributed for use in research and breeding, so factors that affect long term seed germination are of interest. In 1987 the US Potato Genebank conducted routine seed multiplication on 127 populations of 31 species as greenhouse-grown potted plants. Half of the parent plants in each population were given two doses of liquid fertilizer in addition to the standard slow-release granular fertilizer which had been mixed into the medium for all plants. Plants with this extra liquid fertilizer application grew larger, and produced significantly more fruit and seeds. However, seeds from parents fertilized extra did not have improved seed weight or germination after 26 years of storage. We conclude that optimizing the fertilizer regime used on parent plants, while doubling the average seed yield, does not result in higher quality seeds.

Potato has two wild relatives native to the USA, Solanum jamesii (jam) and S. fendleri (fen). Core collections are a useful tool for genebanks, identifying a ranked minimum number of samples that together encompass most of the total genetic diversity. With diversity measured as presence of AFLP bands, we made core collections for each species such that >90 % of diversity was captured in a minimum of populations. For fen, bulks containing about 25 % of populations accomplished that standard. For jam, a single “mega-population” at Mesa Verde, CO, consisting of many thousands of plants, was found to encompass 82 % of the AFLP bands detected in all samples across the entire USA range, and adding three more populations captured a total of >90 %. Core members for both jam and fen with the most diversity originated from the northern part of the range. That suggests that these areas merit more collecting. If resources for germplasm preservation and evaluation are limited, samples in the core collections should take priority.

Potato psyllid, Bactericera cockerelli (Sulc) (Hemiptera: Triozidae), is a key pest of potato and the vector of “Candidatus Liberibacter solanacearum,” the pathogen associated with zebra chip disease. Development of potato cultivars with genetic resistance to potato psyllid would enable cost-effective control of this pest with reduced use of insecticides. To facilitate the development of resistant cultivars, the objective of our study was to screen germplasm accessions of Solanum verrucosum for resistance to potato psyllid. The susceptibility of S. verrucosum germplasm accessions to potato psyllid was highly variable in choice prescreening assays and no-choice performance assays. Compared with the susceptible potato cultivar, ‘Russet Burbank,’ several S. verrucosum populations exhibited strong resistance to potato psyllid. The S. verrucosum accession, PI 195170 was highly resistant to potato psyllid, and is a potential source of genetic resistance for the development of resistant potato cultivars.

Greening of the tuber skin is an undesirable defect in fresh and processed potato. Tubers of 92 Solanum microdontum families represented by 12 individuals each were generated in the winter greenhouse in 2009–2010. These were evaluated in two trials of family bulks after 4 days of exposure to 200 ft candles of fluorescent white light at room temperature, similar to that observed in local grocery stores, and according to preliminary tests, able to turn some microdontum tubers very green. A qualitative score of green (G) or white (W) was assigned to each tuber. Tubers of about 10 % of families were scored as mostly W. In comparison, when 185 named Solanum tuberosum cultivars were illuminated in the same way, none from Europe or North America scored W. Two replicates of G and W individual tubers were selected from a variety of microdontum families and planted to create a new clonal generation of tubers. Those second generation daughter tubers showed a response to illumination which was consistent with that of the previous clonal generation. Finally, W and G clones were selected and mated. All GxG hybrids were uniformly G, but only three WxW crosses produced families that were nearly uniformly W. Thus some selections were shown to have consistent non-greening over trials spaced in time, over a clonal generation, and, when crossed together produced pure non-greening progeny. On the other hand, several very green selections similarly were consistently very green over time, clonal regeneration, and intermating. Tests of tuber tissue for glycoalkaloids showed that some illuminated clones neither green nor accumulate glycoalkaloids. These materials would make good models for further study of inheritance and physiology of the greening trait, and for breeding.

Knowing how genetic diversity is partitioned among and within wild potato species populations is important for efficient sampling for collection, preservation and evaluation. We sought to evaluate the effectiveness of SNPs for assessing germplasm by using the exact set of four model species previously evaluated by RAPDs. To avoid large numbers of SNP samples, population bulks of 25 plants were used, and original RAPD data was adjusted to match SNP genotype data. It was noted that especially for SNPs, it was necessary to examine only loci polymorphic within species to get a realistic view of genetic partitioning within species. This resulted in only a few hundred useful loci for some species. When considering among-population versus within-population partitioning of diversity, both SNPs and RAPDs distinguished the species as expected according to their known breeding system. Primitive wild species were confirmed as very heterogeneous within their populations. Both SNP and RAPD markers can be used to help breeders and genebank managers understand patterns and use in potato germplasm diversity.

The importance of Russet Burbank, the world’s foremost French fry processing cultivar, requires a complete description of its origin. Its maternal lineage included Rough Purple Chili, Garnet Chili, Early Rose, and Burbank. An incorrect but widely disseminated account attributes the origin of Russet Burbank to Colorado potato grower Lou D. Sweet, with 1914 often given as the date of introduction. However, it is likely that Russet Burbank was originally released in 1902 as May’s Netted Gem by L. L. May & Co. (St. Paul MN). The names Netted Gem and Russet Burbank were used synonymously for many decades. Isoenzyme, multiplex PCR, and SNP data confirm Russet Burbank as a mutation of Burbank and do not support a seedling origin. Russet Burbank was found to be similar to Burbank in processing and nutritional characteristics. A goal of this effort is that descriptions of Russet Burbank’s lineage and origins will be corrected by seed companies in lists of potato varieties and at world repositories holding Russet Burbank and its progenitors.

Developing chipping cultivars with improved tuber quality and disease resistance is a major interest for breeders and the potato industry. A popular chipping cultivar ‘Atlantic’, is desired for its high yield and gravity. However, this cultivar suffers from poor internal tuber quality and high scab susceptibility. On the contrary, cultivar ‘Superior’ is known to have excellent tuber internal quality and moderately scab resistance. In addition, this cultivar is known to have high tuber calcium as compared to ‘Atlantic’. The present study intended to generate populations that can be suitable for the genetic study of tuber calcium, internal quality, common scab, and other commercially important traits such as yield, specific gravity and chip quality at the tetraploid level. Two populations obtained by reciprocally crossing the cultivars ‘Atlantic’ and ‘Superior’ were evaluated during 2009 to 2012 at Hancock, Wisconsin. Significant genotype effects and moderately low to high broad-sense heritabilities were identified for all traits evaluated indicating that the observed phenotypic variation has an important genetic component. In addition, the parents differed significantly for all traits across trials, and most genotypes performed in between the two parents but some genotypes were more extreme than the parents. Furthermore, evidence of reciprocal effects was found for some traits. In addition to learning about the genetics of these important traits we were able to identify some genotypes that combined the commercially desired traits of the two cultivars.

The potato psyllid, Bactericera cockerelli, is a key pest of potato and important vector of the pathogen that causes zebra chip disease. Control of zebra chip relies entirely on the use of insecticides to reduce populations of this vector. The development of potato varieties resistant to B. cockerelli would contribute to cost-effective control of this insect. Wild potato germplasm are key sources for desirable traits including pest resistance to develop new potato cultivars. Our objective was to screen Solanum bulbocastanum germplasm for resistance to B. cockerelli. The combined use of choice and no-choice assays demonstrated considerable variability among S. bulbocastanum populations in their susceptibility to psyllids. At least six S. bulbocastanum populations exhibited resistance to B. cockerelli: PI 243512, PI 243513, PI 255518, PI 275194, PI 275197, and PI 283096. The documentation of the variability among S. bulbocastanum germplasm populations in their susceptibility to B. cockerelli can aid the development of potato cultivars that are naturally resistant to the potato psyllid.

Solanum microdontum is a diploid potato species with features that make it a good model for research into management and use of germplasm in the genebank. Its taxonomic status is unambiguous and it is in the taxonomic series of wild species closest to cultivated forms. It is represented by about 100 populations in the genebank—not too many for comprehensive evaluation, yet not too few to make prioritization of the most valuable populations worthwhile. This species is also particularly rich in desirable traits, often exhibiting very broad segregation. We here report use of DNA markers for selection of a core set of populations, and assessing whether that core captures populations with the most desirable evaluation results for economic traits. DNA was extracted from bulks of 27 plants from each of 94 populations to generate AFLPs. A total of 1,741 informative loci were detected. AFLP loci were treated as though they were traits, with the banded condition considered to be the desired state to include in a core set. At least one band unique to a population was present in 45 populations, and these 45 populations together captured 98 % of all bands. Adding another 14 populations for a core of 59 populations captured 100 % of bands. This core set was assessed for whether it encompassed those populations known to have useful traits, including nutritional and quality components; and disease, stress and pest resistances. As with AFLP bands, 25 of 26 of the most desirable phenotypic traits were also found in populations in the core set of 59 populations. The most desirable status of 3 traits is lost by selecting a core of 45 populations. We conclude that these core sets would be a rational starting point when prospecting for new useful traits in microdontum.

Sixty-five Solanum tuberosum group Andigena, Phureja and Stenotomum genotypes from an initial population of 1,500 were analyzed for phenylpropanoids, carotenoids, and antioxidant capacity. Total phenolic content ranged from 3 to 49 mg g−1 DW, total carotenoids from 4.1 to 154 μg/g DW, anthocyanins from 0.27 to 34 mg g−1 DW and antioxidant capacity from 60 to 1,767 μmol TE/g DW. HPLC analysis of phenolic extracts revealed that 5-O-chlorogenic acid (5CGA) was the most abundant polyphenol in all genotypes. Ten genotypes were independently grown out for more in-depth phytonutrient analysis. The Phureja genotypes RN 27.01 had the highest polyphenol, anthocyanin and antioxidant content, while RN 39.05 had the highest carotenoid content. The tuber percentage dry matter varied markedly among the ten genotypes, influencing the phytonutrient values when expressed on a dry weight basis. Chlorogenic acid concentrations ranged from 1.7 to 29.4 mg g−1 DW and kaempferol-3-rutinose was present up to 3 mg g−1 DW. Petunidin-3-O-coum-rutinoside-5-O-glu or pelargonidin-3-O-coum-rutinoside-5-O-glu were the most abundant anthocyanins. The principal carotenoids were lutein, zeaxanthin, violaxanthin, and antheraxanthin, but no one carotenoid was predominant in all genotypes. These findings further support utilization of Phureja group germplasm for phytonutrient enhancement efforts.

Our previous study examining the effects of agrichemicals on the reproductive capacity of potato species revealed that the pesticide carbofuran negatively influenced flowering duration and pollen viability. These changes could limit reproductive ability non-randomly, modify allelic frequencies, and cause genetic drift. This study utilized Simple Sequence Repeat (SSR) markers to examine that possibility by assessing the genetic structure of progenies derived from potato populations exposed to two levels of carbofuran. A total of eight populations of five potato species (acl, buk, hcr, med and rap) were evaluated. An untreated, uncontaminated population was included as control for comparison. The results revealed that most of the 116 SSR allele frequencies assessed did not differ significantly between control and progenies of pesticide-contaminated parents. A few markers showed a significant frequency shift in some species, specifically buk and med. However, this study reveals that although the pesticide reduces reproduction in wild potato species populations, changes in population genetics are minor and do not significantly threaten diversity.

Gibberellin deficient mutants in potato have been published as ga1 (from S. tuberosum ssp. tuberosum and andigena), pito (from the tuberosum cultivar Pito), and ga2 (from the phureja haploid inducing clone “1.22”). We conducted crossing experiments to investigate genetic similarities. When the cultivar Pito was testcrossed with ga1 4x dwarfs, 1/6 of progeny were dwarf, exactly the expectation if Pito is duplex for ga1. Cultivars Pito and known ga1-carriers Superior and Eramosa all have ga1-carrier Early Rose as a common parent, supporting a hypothesis that the mutant described as pito is actually the same allele as ga1 by common descent. When reputed ga2 heterozygous carrier 1.22 was ga1 testcrossed, only one of 418 progeny was dwarf, suggesting that ga1 and ga2 are not allelic. We then attempted to create mutant ga2 in homozygous form. Since 1.22 does not self, it was outcrossed to individuals from S. bukasovii and S. microdontum populations devoid of dwarfs, then backcrossed to 1.22. No dwarfs were found in over 30,000 BC progeny of 44 different F1 hybrids. A possible explanation is that the ga2 mutant may be present only in a particular clonal sport of 1.22 in which it was reported, and not in the original 1.22 clone tested at the US Potato Genebank. Like pito, ga2 may be identical or allelic to ga1, since mutation from the normal to ga1 dwarf allele in 1.22 pollen was evident in the single dwarf recovered in the ga1 testcross progeny of 1.22. Some special genetic mechanism for relatively frequent mutation at this locus must be present, since normal shoots that occasionally arise on dwarf plants show that the recessive ga1 can also spontaneously revert to the dominant functional allele, as confirmed by testcrosses. However, this phenomenon does not appear to explain spontaneous clonal sports of cv Russet Norkotah selected for agronomic characters, which one might expect to result from the presence of more gibberellin-producing alleles.

The authors collected samples of 97 populations of the wild potato S. stoloniferum (previously fendleri) in the following seven mountain ranges of the southwest USA over 7 years, 2004–2010: Chiricahua (CHI), Huachuca (HUA), Rincon (RIN), Guadalupe (GUA), Pinaleno (PIN), Santa Catalina (CAT), and Santa Rita (RIT). These and previous samples were compared with AFLP markers to determine which ranges have the most genetic richness. A total of 2,079 bands were polymorphic over all populations, and 1,256 were polymorphic among ranges. Of these 1,256 bands, 279 occurred in only one range (= unique). All mountain ranges had some unique bands, but not equally, as PIN with 30%, and CHI with 22% together accounted for over 50% of the unique bands in the seven ranges. An examination of populations within ranges showed that one localized area was the source of over 50% of the unique bands from PIN. This study demonstrates that DNA markers can identify collecting locations like PIN and CHI with particular genetic richness, and thus these locations have high priority for further sampling for the genebank. Similarly, these populations already present in the genebank could be prioritized for evaluation and preservation based on the relative number of unique bands they contribute from their range. The discovery of substantial new diversity in each range implies that other ranges not yet fully explored are likely to also contain novel valuable germplasm, and that collecting in the USA is not complete.

Collecting germplasm from even a fraction of the potential geographic range of wild potato species requires substantial time, money, and effort, so efficiency could be increased if spots particularly rich in unique alleles could be predicted and prioritized. A previous experiment that used AFLP markers to compare “remote” versus “easy” collection sites for Solanum stoloniferum, (previously S. fendleri) within three mountain ranges identified the Santa Catalina Mountains (CAT) of SE Arizona as making a particularly large contribution of unique alleles, despite existing CAT collections being few and all close to roads. This situation motivated a collecting expedition in September 2009 to more thoroughly collect CAT. That expedition resulted in samples of 16 populations, most from new sites never previously described or collected. An analysis was done with 871 AFLP loci, comparing populations from the same three mountain ranges and including the 2009 CAT collections. Results confirmed the prediction of unique allele density of certain location categories within mountain ranges. The new CAT collections captured three times as many unique alleles as contained in the previous collections from that location, and 24 of these were new (unique) among all mountain ranges. One particular new collection, PI 658180, accounted for 46% of the new unique alleles collected in CAT. This study demonstrates the power of DNA markers to empirically identify locations with genetic richness, guiding the most efficient allocation of resources for collecting, preservation, and evaluation of germplasm.

Black dot is caused by the fungus Colletotrichum coccodes (Wallr.) S.J. Hughes. The disease is prevalent in potato fields and can be of economic concern by itself, or as a part of the potato early dying syndrome. Little is known about resistance to this disease. In the present study resistance to potato black dot was tested in Solanum tuberosum Group Andigena. Forty accessions were chosen randomly from the core collection and screened. The accessions originated from Peru, Mexico, Bolivia, Columbia, Ecuador, Argentina and Costa Rica. Five accessions had less (P ≤ 0.05) disease on roots and stems than a set of commercial standards. These accessions were: PIs 189473, 230475, 161683, 243367 and 230470. They were retested to validate their partial resistance. Fifteen plants, each a unique genotype, were selected from each accession and were propagated into multiple clones that were inoculated with the pathogen. Three genotypes were identified with less (P < 0.05) disease on roots and stems than the industry standard. These genotypes originated from the accessions PI-243367 and PI-230475. Clones of the resistant genotypes were added to the potato germplasm collection located at the Potato Improvement Laboratory of the USDA-ARS near Prosser, WA, to be used in the development of black dot resistant commercial varieties.

A basic question in germplasm collecting is whether the in situ genetic diversity in a given geographic range has been adequately sampled. While one would ideally sample all diverse sites with appropriate habitat, there is usually a practical bias against visiting relatively inaccessible sites. For wild potato in the USA, mountain habitats often include easy access locations (near roads, usually at lower altitudes), and relatively remote locations (usually high altitude crests that can be accessed only by trail hiking and camping). This work used AFLP markers to compare three southeastern Arizona mountain ranges for which multiple "easy" and "remote" Solanum fendleri populations had been collected. Of the total markers detected, 24%, 6% and 3% were unique to the "remote" locations, and 3%, 21% and 34% were unique to "easy" locations. This case study demonstrates that populations at such locations are not identical, but the most unique alleles are sometimes captured at the remote location, sometimes at the easy. The practical conclusion is that both locations need to be sampled and compared empirically in the lab for unique allele richness to identify locations with highest priority for additional collecting.

Variation in tuber pH has been previously reported in cultivated potato and shown to be associated with economic traits. We here report a broad survey of potato germplasm for tuber pH, including wild Solanum species. Cultivar pH ranged from about 5.5 to 6.2, while six wild species were significantly lower. Solanum microdontum had the lowest pH average of the wild species examined at 5.17 with the lowest observed reading of 4.99. This species also exhibited a strong environmental effect, in which field tubers (5.28) were much lower than greenhouse tubers (5.63). Long storage generally did not have a significant effect on wild species tubers, except for S. jamesii, which reduced from 5.78 to 5.54. Various types of sampling were tested. Skins were generally lower than flesh, with a high correlation between the tissues. Samples that were frozen, cooked, or reconstituted from dried powder had different absolute pH values, but these were highly correlated, so germplasm would be ranked for pH similarly regardless of method. Although not tested here, physiological links between pH and economic traits like disease resistance, nutrition and tuber quality are plausible. If so, pH testing might serve as a rapid and inexpensive screening tool in breeding programs. The association of pH and useful traits might also provide clues for exploring underlying genetic and physiological mechanisms in potatoes.

Although the potato of commerce, the autotetraploid Solanum tuberosum is a clonal crop, genebank maintenance of wild and cultivated relatives is commonly done as populations of true seeds. In order to help equalize gamete contribution and prevent genetic drift over seed regenerations, an equal number of seeds from each mother plant may be bulked as a source of parents for future regenerations. To determine if this extra sample is beneficial, we selected two S. andigena populations with great variation of seed production among mother plants. Seed production of each mother plant was recorded over 12 standard seed regenerations and genetic indicators were developed in the form of 105 polymorphic RAPD bands. For each polymorphic band, the variation of seed production among mother plants was used to estimate the potential impact on band phenotypic frequency due to an over-all seed bulk. Most bands were fixed or nearly fixed within at least one of the populations, such that the variation in maternal seedset observed would not have put them at signficant risk of being lost in an over-all bulk. For balanced bulks to provide a significant preservation advantage, a band must be rare in all populations and associated with mother plants that produce significantly fewer seeds. Of over 100 loci assessed here, none met these conditions. We therefore conclude that the duplication of documentation and storage space invested in making balanced seed bulks probably return little genetic benefit to the genebank.

Wild potatoes reproduce in the wild clonally by tubers or sexually by seeds. This case study examined the genetic consequences of sampling in situ clones or in situ seeds for model populations of two indigenous potato species of the USA, Solanum stoloniferum (formerly S. fendleri) PI 564039 and Solanum jamesii PI 605371. Solanum stoloniferum is a selfing disomic tetraploid while S. jamesii is a diploid that reproduces by outcrossing. Genetic diversity of in situ clonal collections and in situ seed collections of these species were compared with RAPDs. More diversity (i.e., RAPD polymorphism) was found within the tuber collection than seed collection for S. stoloniferum but for S. jamesii, the opposite was true, with seed collection capturing significantly more diversity than tubers. It has generally been assumed that collecting in situ seeds will result in capture of more genetic diversity. However, this work indicates that clonal collections may capture more genetic diversity, perhaps depending on the breeding behavior of the species.

Decisions that improve genebank management depend on understanding patterns of diversity in the genetic resources being conserved. In potato, populations of the wild Mexican species S. verrucosum (ver) collected from the north (N) of the natural range had been shown to be genetically distinctive from those in the south (S). But this geographic association with genetic diversity was noted to be confounded with proximity of other wild potato species, suggesting introgression as a possible cause. RAPD bands that distinguished N and S ver were generated. When their association with nearby populations of other wild potato species was assessed, the hypothesis of introgression was not supported. We conclude that the apparently distinctive genetics of N ver are not explained by common alleles introgressed from proximal populations of other species. Thus, N ver germplasm populations, although less numerous, should be represented in samples of species ver with regard to collection, preservation and evaluation.

Wild potato species are maintained primarily as botanical seed populations, so tuber trait studies require conversion of germplasm to tuber form. Such tubers may be obtained from seedlings produced directly from botanical seed or from plants grown from tuber propagules (tuberlings). Since most wild species require short days for tuberization, it is not possible to generate field tubers of these species in most USA locations. Historically, tubers for research evaluations have been generated in the artificial conditions of the winter greenhouse. Since the potato crop is normally grown from tubers produced from tuberlings grown in the field, it is important to know how much results differ when evaluating tubers that were produced from seedlings grown in the greenhouse. We compared antioxidant activity and phenolic content of tubers generated in the greenhouse at Sturgeon Bay, WI, from seedlings, and tubers generated from both seedlings and tuberlings in the field at the Kula Experiment Station, Kula, Maui, HI. A ‘mini-core’ set of 75 PIs representing 25 wild and primitive cultivated species was used. Differences in means of propagule types and growing environments were significant but not large. Average amount of antioxidant activity and phenolic content of tubers from field-grown seedlings was higher than that of tubers from tuberlings. These values were also higher in field-grown tubers than in greenhouse-grown tubers. Relative performance was similar regardless of environment or propagule type, with some important exceptions. Tubers of Solarium pinnatisectum and Solanum jamesii were high regardless of treatment. In contrast, tubers of certain Solanum microdontum, canasense, stenotomum and commersonii (species much more amenable to breeding) exhibited high antioxidant levels when produced in the field from tuberlings, but not when produced from greenhouse-grown seedlings. Thus, some germplasm may not exhibit useful antioxidant potential when tubers are produced in artificial greenhouse conditions.

A sample ofSolanum stoloniferum Schltdl. (reclassified fromSolanum fendleri A. Gray), PI 620874, was collected by the authors from a previously undocumented site in the Patagonia Mountains of southern Arizona in September 2001. When original seed was grown at the US Potato Genebank, two of 25 seedlings produced white flowers. Upon closer inspection they were found to be completely without purple pigmentation on the tubers, stems, and underside of leaves—a characteristic previously unknown in this species and rare in wild potato overall.Solanum stoloniferum, like other species of series Longipedicellata (LON), is a disomic tetraploid. Results of crossing trials were consistent with the most simple genetic explanation for this phenotype: a recessivep allele at the P locus, making the mutant (pp pp), or “Purple-less” abbreviated “P-less.” P-less plants were testcrossed with nearly 200 other LON populations originating across the U.S.A. and Mexico to make BC1 generations that had either one-half or one-quarter P-less seedlings. Surprisingly, about one-third of the populations, and from widely distributed origins, produced one-half P-less progeny, indicating that they also possess the recessive allele on one genome. Since this mutant has not been confirmed as an allele at the original P locus, its designation with the same allele symbols in this report (P andp) is provisional. This discovery of the widespreadp allele in LON and P-less mutants could provide a simple, unambiguous seedling marker for study of gene flow and dispersion, pollinator behavior in the wild, gene expression interactions between homeologous genomes, and chromosome pairing control.

Gibberellins (GAs) are involved in internode elongation and other important processes such as seed germination, flowering, maturation, tuberization, and tuber dormancy. The discovery of GA-deficient mutants enabled further study of the role of these hormones in many plant processes. GA-deficient mutants lack the ability to produce adequate amounts of gibberellin for normal growth, resulting in a rosette type growth and short internodes. Thega 1 mutant allele was introduced into various genetic backgrounds including differentSolanum species and ploidies. Diploid GA-deficient genotypes were obtained by crossing haploidSolanum tuberosum ssp.andigena withSolanum chacoense. The progeny was then bulked and intermated to produce F2 individuals. Tetraploid GA-deficient genotypes were obtained by crossingS. tuberosum ssp.andigena withSolanum sucrense and withSolanum gourlayi. The two resulting progenies were then bulked and intermated. Diploid and tetraploid GA-deficient genotypes were grown on MS media containing different levels of gibberellin (GA3). Plant height and visual observations were made as a way to assess the response of these genotypes to GA3. Concentration of 0.1 µM GA3 and lower failed to restore normal plant height in both diploid and tetraploid genotypes. Normal plant height was restored in most of the GA-deficient genotypes when concentrations between 0.8 and 1.2 µM GA3 were used. We found some important differences between these genotypes: (1) the level of GA3 to restore normal plant height varies among the GA-deficient genotypes, some needed more GA3 than others to grow normally; (2) the time to respond to the presence of GA3 in the media differs between the GA-deficient genotypes, (3) tetraploid genotypes exhibited normal growth and internode length in response to GA3, while diploid genotypes tended to show a rosette-type growth at the apical end. These results suggest thatga 1 mutants can be affected by a series of modifier genes and/or iso-alleles. The importance of variable response to GA among dwarf individuals is two fold: (1) experiments measuring GA response should choose and clonally multiply one genotype to ensure uniform optimal response to GA application; and (2) variation betweenga 1 mutant phenotypes could be used to characterize GA-response modifier genes.

A common effort among members of the Association of Potato Inter-Genebank Collaborators (APIC) has yielded a global inventory of wild potato genetic resources that is freely accessible to researchers and breeders. In that database there are a number of accessions that originated from distributed progeny of a single original germplasm collection. The logical assumption has been that although these samples are in different locations, they should be genetically equivalent. This study tests this hypothesis by comparing 17 pairs of accessions of 16 different potato species, which are reputed duplicates preserved in the potato genebanks of The International Potato Center (CIP) in Peru and of the U.S.A. (USPG). The RAPD marker analysis revealed that even though the average genetic similarity of reputed duplicates was quite high, there were a few with significant differences. Similarly, SSR markers identified three reputed duplicates that were genetically different. SSRs revealed a loss of markers for some inter-genebank comparisons, a probable indication of genetic drift. Duplicate potato collections between CIP and USPG are in most cases genetically identical. The few exceptions merit further investigation regarding causes and the impact on useful traits.

Solanum commersonii Dun. (cmm) is the most frost hardy wild potato species known, being able to tolerate an acute freezing episode to about -5 C and further acclimate to tolerate −10 C after being exposed to chilling temperatures for several days. Breeding with this species to incorporate its frost-hardiness traits can be accomplished by standard sexual hybridization or protoplast fusion. These methods can result in hybrids that vary in contribution of the cmm plastome. To test the effect of cmm cytoplasm, cytoplasmic substitution backcross hybrids were made with three very frost-sensitive species,S. brachistotrichum, S. cardiophyllum, andS. pinnatisectum, by usingS. commersonii as the female to make an F1, then performing repeated backcross (BC) using the sensitive species as males. Relative freezing tolerance (RFT) of all genotypes was assessed by measurement of ion leakage of excised terminal leaflets subjected to a controlled ice nucleation and simulated freeze-thaw stress. Even against the background of these very sensitive species’ genomes, the cmm cytoplasm of substitution hybrids promoted insignificant improvement in frost hardiness or ability to acclimate. We conclude that either (1) cmm cytoplasm does not contribute to frost hardiness, or (2) if cmm cytoplasmic frost hardiness genes do exist, they must be epistatic to (depend on the presence of) nuclear hardiness genes for expression.

Twelve accessions ofSolarium fendleri collected in the United States and Mexico were inoculated with 5,000 eggs of host-races 1 and 2 ofMeloidogyne chitwoodi, Columbia root-knot nematode. The test seedlings that were derived from plant introduction true seed lots were maintained for 55 days before harvest and egg count. The results revealed that two accessions, PI 275162 and PI 275165, were non-hosts (final egg count/initial egg inoculation < 0.1) forM. chitwoodi race 1. The results were uniformly confirmed in the second experiment. No resistance was found to race 2. The tetraploidS. fendleri accessions were crossed to a nematode-susceptible cultivated diploid potato clone from aS. phurejastenotomum population. The triploid hybrids expressed resistance to race 1 at the non-host level. After somatic doubling, the resulting hexaploids also expressed non-host-level resistance to race 1. The two resistant accessions had been collected in southeastern Arizona, one each from the Huachuca and Chiricahua Mountains. Each range is an isolated island of high-elevation mesic flora surrounded by typical lowerelevation Sonora-type desert habitat. These accessions are the only known sources of resistance toM. chitwoodi from wildSolanum species in the USA.

Most wild potato germplasm in genebanks is collected, preserved, and evaluated as botanical seed populations that may be highly heterozygous and heterogeneous. However, some species are selfers so potentially very homozygous, perhaps also homogeneous. Intrapopulation heterogeneity increases sampling error that can undermine consistency in seed regeneration in the genebank, screening results, germplasm collecting, and estimates of taxonomic relationships. Thus, knowledge of genetic heterogeneity (GH) can predict the need to commit more resources for larger sample sizes or replication when populations of a given species are being regenerated, evaluated, collected, and classified. This study investigated withinpopulation GH in 32 potato populations comprising four different breeding systems observed inSolatium species:S. fendleri (2n=4x=48, disomic selfer),S. jamesii (2n=2x=24, outcrosser),S. sucrense (2n=4x=48, tetrasomic outcrosser), andS. verrucosum (2n=2x=24, selfer). RAPD markers were used to estimate heterogeneity among 24 individuals per population. Populations ofS. verrucosum were quite homogeneous with an average GH of 6.0%. Similarly low heterogeneity was detected among the eight populations ofS. fendleri (average GH=7.1%). In contrast,S. jamesii andS. sucrense had a much higher GH of 29.4% and 44.1%, respectively. These results demonstrate and quantify the great difference in intrapopulation heterogeneity among wild potato species. Calculations based on intrapopulation heterogeneity indicate that samples should be composed of 25 to 30 random plants for low sample variation that is uniform for all species.

Our previous studies demonstrated that an increase in 18:2 (linoleate) in the purified plasma membrane fraction during cold acclimation is associated with genetic variations in cold acclimation capacity. This increase was found only in genotypes that are able to cold acclimate and was reversible on deacclimation suggesting a link between the accumulation of 18:2 and acquisition of freezing tolerance. The present study was aimed at understanding the association between the induction of stearoyl-ACP (acyl carrier protein) (Δ9) desaturase and the ability to cold acclimate. Our approach was to study the induction of Δ9 desaturase at the transcript level using potato Δ9 desaturase gene-specific primers and reverse transcription polymerase chain reaction (RT-PCR). For this purpose, total RNA fromSolanum tuberosum (cold sensitive, unable to acclimate) andSolanum commersonii (cold tolerant, able to cold acclimate) was extracted before and after cold acclimation. RT-PCR produced a single band and sequence analysis confirmed that the amplified band was Δ9 desaturase. While the cold acclimating species,Solanum commersonii, exhibited an increase in Δ9 desaturase transcript levels after cold acclimation, the cold non-acclimating species,Solanum tuberosum, exhibited no change. Our results show that the increase in Δ9 desaturase gene transcripts during cold acclimation is associated with the cold acclimation response in potato.

Random amplified polymorphic DNA (RAPD) and simple sequence repeat (SSR) markers were used to construct a partial genetic linkage map in a potato backcross population. The population, derived from two diploid wildSolatium species (frost tolerant, able to cold acclimateS. commersonii; frost sensitive, unable to cold acclimateS. cardiophyllum), was used to map quantitative trait loci (QTL) of non-acclimated relative freezing tolerance (NARFT) and cold acclimation capacity (CAC). Precise assessment of these traits allowed distinction of small but significant differences among 35 backcross genotypes. NARFT and CAC were not correlated in the segregating population, suggesting independent genetic control for these two major components of freezing tolerance. The linkage map spanned 479.4 cM and included 77 RAPD markers and two SSR markers, with 38 RAPD and 10 SSR unassigned markers. Two QTLs for NARFT were detected in two different linkage groups, accounting for 44.0% of the phenotypic variation for this trait. Two QTLs for CAC were detected, accounting for 24.9% of the phenotypic variation for this trait. QTLs for NARFT and CAC were detected at separate genomic regions, in support of the independent genetic control of these two traits. QTLs for NARFT and CAC were detected in a linkage group identified as part of chromosome V, suggesting that such chromosome constitutes a prime candidate for fine-mapping. Due to the relatively small progeny size evaluated in this study, additional QTLs for NARFT and CAC could have been involved but not identified. Therefore, the conclusions derived from this study should be considered preliminary.