Interspecific–Interploid Crosses among Hylocereus Species (Cactaceae) for the Selection of Improved Cultivars

Hylocereus species, night blooming vine cacti endemic to the Americas, use the Crassulacean acid metabolism (CAM) photosynthetic pathway, and bear large, attractive and edible fruits. Taken together, these characteristics of Hylocereus species indicate their high economic potential as perennial fruit crops in arid and semi-arid regions where water is scarce. To improve Hylocereus fruit traits and yields, therefore, a breeding program was begun two decades ago at Ben-Gurion University of the Negev, Israel. Initial homoploid- and interploid-interspecific crosses produced improved diploid and allopolyploid hybrids. A second cycle of crosses was carried out using two elite, semi-fertile allotriploid hybrids (named S-75 and 12-31) as the female parent. The crosses included a first back-cross (BC₁), interploid crosses between the allotriploid S-75 and the diploid H. undatus (F₁), and selfing of the allotriploid 12-31 (F₂). A total of 109 putative hybrids were studied. Fruit traits (including, peel and flesh color, fruit shape, potential yield, and self-compatibility) were evaluated. Ploidy level was estimated using flow cytometric analysis. Molecular tools, such as nrDNA internal transcribed spacers (ITS) and simple sequence repeats (SSR), were employed to establish the genetic relationships and to define the similarity/dissimilarity between the hybrids and the parental lines. The resultant hybrids showed variety in peel color, which was dark pink, red, purple, or yellow, and their flesh color varied from white, and light-purple to purple. Moreover, fruit shapes constituted a range from round to elongated–ellipse. The weights of hybrid fruits, intermediate between the fruit weights of the parental lines, were from 119 to 273 g. About 19% of the hybrids were self-compatible. A comparison between the hybrids and the parental lines indicated that co-dominance was observed in most of the traits studied.  Flow cytometric analysis revealed that the hybrids were diploid, triploid, tetraploid or hexaploid. The sequence variation in ITS samples from a subset of hybrids showed a single, well-supported clade with only one exception, a hybrid that was similar to the male parent. The SSR analyses showed wider polymorphism among the hybrids studied. Of the 109 putative hybrids studied, six belonging to the F₁—a cross between the allotriploid S‑75 as the female parent and the diploid H. undatus as the male parent—showed the best performances in terms of fruit quality and potential yields. Those six hybrids are currently being evaluated in several commercial orchards under different environmental conditions.