The genus Rheum belongs to the family Polygonaceae and includes ≈55 species with 14 synonyms [U.S. Department of Agriculture (USDA), 2012]. Wang et al. (2005) reported that Rheum originated in the central and northern plateaus of Asia. In addition to its contemporary uses for cooking in a variety of dishes, the medicinal effects of several species in this genus have been described by multiple cultures. Rhubarb species are discussed in traditional Chinese medicine, Shen Nung Pen Ts’ao Ching (Hsu et al., 1986), in Historia Naturalis by Pliny the Elder (Jones, 1956), in De Materia Medica of Dioscorides (English translation by Beck, 2005), and in Al-Qanun fi al-Tibb (Canon of Medicine) by Ibn Sina in Syria (Barney and Hummer, 2012). From Asia and the Mediterranean region, medicinal rhubarb spread to Europe and North America (Beck, 2005; Foust, 1992; Sibly, 1790; Turner, 1938).
Rheum has a base chromosome number of x = 11 and includes species with ploidy ranging from diploid (2n = 2x = 22) through hexaploid (2n = 6x = 66). Culinary rhubarb, Rheum ×rhababarum, is mostly tetraploid, although its origin is complex, likely involving R. undulatum, either diploid or tetraploid (2n = 4x = 44), diploid R. palmatum and tetraploid R. rhaponticum (Chin and Youngken, 1947; Englund, 1983; Foust and Marshall, 1991; Morse, 1901; Ruirui et al., 2010; Turner, 1938).
Rheum species and cultivars are maintained in the U.S. National Plant Germplasm System (NPGS) administered by the USDA, Agricultural Research Service (ARS) Western Regional Plant Introduction Station, Pullman, WA. Before Nov. 2011, and during this study, the rhubarb collection was maintained at the USDA-ARS Arctic and Subarctic Plant Gene Bank (ASPGB) in Palmer, AK. In 2012, this collection was transferred and is now conserved in Pullman, WA. Cost-effective management of genetic resources includes confirmation of clonal identity and elimination of redundancy. Molecular analyses can complement morphological trait assessments in this task.
During 2 years of trials at Palmer, Pantoja and Kuhl (2009) evaluated the effectiveness of 15 morphological characters for identifying cultivars and other clonal rhubarb specimens. Significant variability between the 2 years was reported for petiole epidermis color, petiole internal color, and the number of petioles per plant. The results suggest limited usefulness of these descriptors for identifying rhubarb accessions (Pantoja and Kuhl, 2009).
A few molecular marker studies have been reported in rhubarb. Kuhl and DeBoer (2008) used AFLP markers to investigate the genetic diversity of 41 accessions from the Palmer Rheum collection. Hu et al. (2010, 2011) used intersimple sequence repeat (ISSR) markers to assay genetic diversity in Chinese populations of R. tanguticum, whereas Wang (2011) optimized ISSR–polymerase chain reaction in R. officinale, R. palmatum, and R. tanguticum. Compared with AFLPs and ISSRs, microsatellite or SSR markers are more reproducible and can be shared among laboratories. In addition to being typically codominant and exhibiting Mendelian inheritance, they also provide anchored loci for comparative mapping. These qualities make SSR markers an ideal tool for establishing genetic profiles (Powell et al., 1996). Zhang et al. (2008) previously developed 10 SSRs for R. tanguticum and used seven of them to examine the genetic diversity and population structure in 114 individuals representing 10 geographically separate populations endemic to the Qinghai-Tibetan Plateau (Chen et al., 2009).
Advances in short-read sequencing platforms have considerably decreased the cost of identifying microsatellite repeats and enabled the development of SSR markers for species of little economic importance (reviewed in Zalapa et al., 2012). For example, DNA sequencing using next-generation sequencing technology (Ilumina platform®; Ilumina, San Diego, CA) allowed development of polymorphic SSR markers for alaska yellow cedar [Callitropsis nootkatensis (Jennings et al., 2011)] and mile-a-minute weed [Mikania micrantha (Yan et al., 2011)]. The objective of this study was to develop SSR markers from short-read DNA sequences and use them to fingerprint the NPGS rhubarb collection.
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