The Dragon fruit, known as pitaya or pitahaya, belongs to the Cactaceae family, which originated from the southern and central regions of Mexico and America, and is separated in two genera: Hylocereus and Selenicereus. It is a nutritious and exotic fruit cultivated throughout the tropical and subtropical regions of the world. Pitahaya production has attracted interest in the United States, Australia, Southeast Asia, Israel and other regions [1]. This fruit has gained considerable attention from consumers because it is a unique fruit that can tolerate drought stress and contains a considerable amount of nutrients. It is rich in polyphenols, vitamins, sugar, amino acids and betalain pigments [2]. Furthermore, pitahaya fruit contains substantial amounts of unsaturated fatty acids (linoleic and linolenic) with broad applications in therapeutic and cosmetics preparations [3]. It has drawn worldwide attention because of its new flavor, color and attractive appearance, along with enormous health benefits [4]. It gained commercial potential in different countries as the result of consumer preference for new, exotic, and phytochemically rich fruits and its adaptability to new environments with abiotic stress tolerance, such as droughts and extreme temperatures [5].

Generally, differences in pitahaya germplasm can be easily shown with some distinguishing phenotypic characteristics, such as fruit size, fruit color, and number of spines at the areola that formed on branch/stem segments [6]. Nowadays, it is very difficult to separate species and varieties of the dragon fruit because of high intra and interspecific hybridization that has created some taxonomical confusion worldwide [7]. Morphological and genetic heterogeneity in many fruit characteristics, such as sweetness, size, shape, color, and bracts number, resulting from intra and inter-specific hybridization that makes it difficult to increase quality standards for the exportation market, posing serious problems when determining performance in handling and shelf-life [8].

Internationally, a large collection of pitahaya germplasm accessions is curated at the University of California South Coast Research and Extension Center (SCREC), Irvine, CA. The collection was first established in 2005 and includes seven varieties native to Nicaragua, with approximately 120 individual plants representing four different species (H. undatus, H. polyrhizus, H. costaricensis, and Hylocereus sp. Unnamed), two varieties native to Mexico with 34 individual plants representing two species (H. ocamponis and H. megalanthus), two varieties from San Diego with 34 individual plants representing two species (H. undatus and H. guatemalensis), and seven varieties from Florida, including 120 individual plants representing four species and hybrids (H. undatus, H. guatemalensis, H. megalanthus, and several putative hybrids identified as Hylocereus spp.). Additional accessions have been added intermittently to the collection, including 17 additional accessions from Nicaragua (within the H. polyrhizus/costaricensis group) [9]. These accessions were tentatively labeled H. costaricensis/polyrhizus because both species are commonly found in Nicaragua and throughout Central America. An additional variety was purchased from Mexico (17 individual plants representing H. ocamponis) and several other varieties were also sourced from Florida. Additional material has been obtained from local producers, and two plants per accession have been added to the collection. In total, the collection now includes 378 individual plants, potentially representing 54 varieties and seven different species [10].

Conventionally, morphological traits had been used to differentiate plant germplasm/species and to elucidate their genetic relationship [11]. The great morphological diversity between Hylocereus and Selenicereus species has been reported at the intraspecies and intravarietal levels, as a result of their coevolutionary process with the environment, which makes the production of new varieties extremely difficult. This led the International Union for the Protection of New Varieties of Plants [12] to develop a guide to document how new varieties of pitahaya are determined. However, as mentioned above, intraspecies/intravarietal morphological differences between vegetative clones and hybridization within this group lead to genetic mosaics between new lines and make identification between varieties extremely difficult.

Mainly because pitahaya species have easily hybridized since the late 1980s and early 1990s, breeders in the United States, Israel, and Southeast Asia have developed several hybrids [13], including crosses between either H. guatamalensis or H. megalanthus with H. undatus as the other parent, which have resulted in plants with great adaptability and high fruit quality [9].

In Colombia, the germplasm bank of yellow pitahaya and its wild relatives is located in the facilities of the National University of Colombia, Palmira. It has 300 introductions, both cultivated and wild, of yellow pitahaya (S. megalanthus 238 accessions) and red pitahaya (H. undatus, H. costaricensis, Hylocereus spp. 36 accessions). All of these are properly coded; however, not all of them are characterized [14].

In this country, knowledge of this crop mainly comes from the empirical processes of farmers who are motivated by the prices that fruits can fetch in some of the months of the year; however, increased penetration into international markets requires research on the processes of propagation, obtaining elite material, and resistance to biotic and abiotic factors, among other topics [15]. However, one of the bigger limitations is the broad morphological variation seen in the vegetative structures, which leads to confusion in identifying each species, with a lack of consensus [16], where classification is mainly based on the number of areola ribs, the contour of the stem, the relative firmness of the stem and the size and color of the fruits; in addition, various studies on domesticated cactus species have demonstrated variations in fruit characteristics related to the domestication process, resulting in a lack of a taxonomic database [1].

Studies on morphological characterization of germplasm worldwide have shown great variation in the evaluated accessions that can be conserved and used in the genetic improvement of the species; for example, a study evaluated morphological, biochemical and molecular characterizations of four dragon fruit (Hylocereus spp.) genotypes grown in Andaman and on Nicobar Island and revealed the presence of a considerable amount of genetic variations that could be used as key traits for distinguishing three different species [8]. Four dragon fruit genotypes: H. polyrhizus, H. megalanthus and two H. hybrids resulted in different plant growth and development. H. polyrhizus had the best plant growth; whereas, H. megalanthus had the lowest plant growth. Therefore, the red variety was more suitable for cultivation in Pangandaran [17]. The differences in the anatomical structure and morphology of the plants could cause differences in its optimal growth location. Morphological and agronomic characteristics can be used to determinate genetic variation in a single population.

Despite the productive potential, a limiting factor in the development of this crop in Colombia is the incidence of pest and diseases, low fruit quality, the technological level, the associativity and the lack of cultivated material, generating significant losses in yield [18,19]. Different research institutions and universities have tried to find a solution to these problems [14,18]. These studies have shown that there is no certified planting material and that only a few are grown by farmers, generating vulnerability to different phytosanitary problems, for which it is necessary to carry out genetic studies that lead to the identification of elite materials that meet the needs of the productive chain of yellow dragon fruit in Colombia since the genetic base of germplasm resources is limited.

2. Discussions on Yellow pitahaya