Dragon fruit, strawberry pear, pitahaya, and pitaya are all common names for the fruit of vine cacti from species of Hylocereus (Berger) Britton and Rose and Selenicereus (Berger) Britton and Rose. For the purposes of this article, we refer to all of them collectively as pitahaya. These cacti are native to northern South America, Central America, and Mexico but have been propagated and distributed to many countries for agricultural production (Lobo, 2007). They are commercially being grown in the United States (Crane and Balerdi, 2009; Merten, 2003; Thomson, 2002; Valdivia, 2000), Central and South America (Instituto Nicaraguense de Reforma Agraria, 1994; Lobo, 2007), Asia (Komang, 2010; Raveh et al., 1993), Africa (Joubert, 2012), and Australia (Dimitri, 1999; Luders and McMahon, 2006). There has been an increasing interest in these fruits because they can be grown in areas that are prone to drought and heat where other fruit production is not possible (Nobel and De LaBarrera, 2004; Raveh et al., 1998). Moreover, this fruit has gained more attention in recent years as a result of the increasing demand for new and exotic fruits, which makes the look and attractive colors of the skin and flesh of pitahaya very appealing to consumers. Pitahaya are full of essential nutrients such as complex carbohydrates, minerals, vitamins, fibers, and antioxidants, which may also explain their increase in demand for health purposes (LeBellec et al., 2006). In fact, current demand and market prices indicate that consumer preference is shifting away from white-fleshed varieties in favor of colored-flesh varieties (pink or red) because they have more complex flavors and are perceived to have higher nutritional value.
The four most common species commercially grown are H. undatus, H. polyrhizus, H. costaricensis, and S. magalanthus (syn. H. malanthus) (Ortiz and Carrillo, 2012). With the exception of southeast Asia, the cultivation of each of these species follows their geographic origin. As a result, H. polyrhizus and H. costaricensis have been cultivated in Central America (Nicaragua); H. megalanthus has been primarily cultivated in South America (Colombia and Ecuador); H. undatus has been primarily cultivated in southeast Asia (Vietnam, Thailand, Taiwan, and the Phillipines) and some areas of southern Mexico (Castillo-Martinez et al., 2005). However, pitahaya species hybridize quite easily and since the late 1980s and early 1990s amateur breeders in the United States, Israel, and southeast Asia have developed a number of hybrids (Thomson, 2002). These hybrids generally include crosses between either H. guatemalensis or H. megalanthus with H. undatus as the other parent. These purposeful hybridizations have resulted in plants with great adaptability to the local environment and which produce fruit of exceptional quality. Many of these hybrids are available as commercial varieties and have become quite popular among growers in the United States (i.e., ‘Physical Graffiti’, ‘Halley's Comet’, ‘Delight’, etc.). Recent efforts by private breeders in the United States and Israel have also resulted in newer hybrids, which are also commercially available as proprietary varieties, but the adoption of these has been limited (Lobo, personal communication).
There is considerable taxonomic confusion surrounding these cacti. In addition to the relative ease of hybridization, pitahaya is easily propagated by cuttings, which make it a popular and commonly exchanged plant among backyard growers in the United States. Superior clones or hybrids are distributed quite rapidly and in many cases individual growers will assign their own names to previously named varieties, thus creating duplicate names and exacerbating the taxonomic confusion. Botanists have added to the confusion as well. This is the case with H. magalanthus, which was originally placed in a newly erected genus, Mediocactus (Britton and Rose, 1920), because it appeared to be morphologically intermediate between Selenicereus and Hylocereus. Subsequently, the species was transferred to the genus Selenicereus by Morgan (1953) but was later treated as Hylocereus by Bauer (2003). This intermediate morphology, along with the finding that H. magalanthus has double the number of chromosomes found in related species (2n = 44 vs. 2n = 22), led to the hypothesis that this species was in fact a polyploid and potentially a hybrid between unknown species from the two genera (e.g., Plume et al., 2013).
This taxonomic confusion is also exacerbated by the fact that intraspecies/intravarietal morphological differences often occur, likely as a result of environmental conditions (Castillo-Martinez et al., 2005), which makes erecting and evaluating “new” varieties or cultivars extremely difficult. Collectively, this led to the International Union for the Protection of New Varieties of Plants (UPOV, 2011) to propose a guideline to document how new varieties of pitahaya are established. However, as previously mentioned, intraspecies/intravarietal morphological differences among vegetative clones and hybridization within this group leads to genetic mosaics among new lines and makes identification among varieties extremely difficult. A more practical and less subjective approach is to use molecular markers to differentiate among the various species and/or cultivars, but this approach has only been attempted in three studies of pitahaya based on limited sampling (Legaria Solano et al., 2005; Tao et al., 2014; Tel-Zur et al., 2004).
A large collection of pitahaya germplasm accessions are curated at the University of California South Coast Research and Extension Center (SCREC), Irvine, CA. The collection was first established in 2005 with the goal of evaluating pitahaya varieties for commercial production in southern California. It started with seven varieties originating from Nicaragua including an estimated 120 individual plants representing four different species (H. undatus, H. polyrhizus, H. costaricensis, and unnamed Hylocereus sp.); two varieties originating from Mexico included 34 individual plants representing two species (H. ocamponis and H. megalanthus); two varieties sourced from San Diego and included 34 individual plants representing two species (H. undatus and H. guatemalensis); and seven varieties sourced from Florida including 120 individual plants representing four species and hybrids (H. undatus, H. guatemalensis, H. megalanthus, and a number of putative hybrids identified as Hylocereus spp.) Additional accessions have been intermittently added to the collection, including 17 additional accessions from Nicaragua (within the H. polyrhizus/costaricensis group). These accessions were tentatively labeled as H. costaricensis/polyrhizus because both of these species are commonly found in Nicaragua and throughout Central America but the distinction between them is not clearly determined or easily established. One additional variety from Mexico (17 individual plants representing H. ocamponis) was acquired and several other varieties were also sourced from Florida. Additional material has been sourced from local producers and two plants per accession have been added to the collection. In total, the collection now includes 378 individual plants, representing potentially 54 varieties and seven different species.
However, as the collection was being established, the close examination of morphological traits among the various accessions indicated significant similarities among plants from different varieties and significant differences among plants within the same varieties (Lobo, personal observation). This suggested that some plants within each of the designated varieties were different and that some of the designated cultivars with different names may indeed be the same clone. Therefore, the objectives of this work were to use molecular data to genotype a large number of plants in the collection to test this hypothesis and to obtain preliminary data regarding how much genetic diversity is present within the collection.
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