Abstract
Putative gamete-derived progenies from two Hylocereus species, the diploid H. monacanthus and the tetraploid H. megalanthus, were studied with the dual aims to confirm their gamete origin and to evaluate their potential use as genetic resources. An additional goal was to determine the origin (allotetraploid vs. autotetraploid) of H. megalanthus by exploring morphological variations in the di-haploid (2x) H. megalanthus progeny. Gamete origin was proved in all five H. monacanthus lines obtained and in 49 of the 70 H. megalanthus lines by using flow cytometry and simple sequence repeat (SSR) markers. The five double-haploid (2x) H. monacanthus lines showed low vigor and abnormal flower development, with malformed ovules and aborted pollen grains. Only one flower set fruit, giving several viable seeds. For H. megalanthus, both abnormal ovules and defective anthers were observed in the di-haploid (2x) and double di-haploid (4x) lines. Among the 46 di-haploid lines, only 14 set fruit. Another 13 di-haploid lines formed flower buds that abscised before anthesis or soon after pollination. The severe sterility of the double-haploid H. monacanthus and the reduced fertility of all the di-haploid and double di-haploid H. megalanthus lines can be linked to their reduced heterozygosity, which drastically affected the development of normal female and male organs. We thus concluded that chromosome doubling, as occurred spontaneously in the double-haploid H. monacanthus and the double di-haploid H. megalanthus, is not sufficient to restore fertility in Hylocereus. We also observed very low gametoclonal variation among the di-haploid (2x) H. megalanthus lines, a finding that supported an autotetraploid, rather than an allotetraploid, origin of this species. Nonetheless, despite the above-described challenging limitations, these gamete-derived lines are currently being bred as the seed parent, offering unique possibilities for genetic research and additional breeding.
Highlights
The genus Hylocereus (Cactaceae) of the vine cacti, known as dragon fruit or pitayas, comprises 15 perennial species (Bauer, 2003)
For H. monacanthus, no statistically significant differences were observed between the nuclear DNA content of the donor parent and that of the five androgenic lines (Table 1)
Since Garcia et al (2009b) had previously reported a haploid status for these androgenic H. monacanthus lines during early development, we assumed that spontaneous chromosome doubling had subsequently occurred, and simple sequence repeat (SSR) markers were used to confirm the gamete origin of these five lines
Summary
The genus Hylocereus (Cactaceae) of the vine cacti, known as dragon fruit or pitayas, comprises 15 perennial species (Bauer, 2003). Those cultivated worldwide include: the diploids H. undatus and H. monacanthus, which have white, purple or red flesh and a red peel; Characterization of Gamete-Derived Dragon-Fruit Lines the tetraploid H. megalanthus, with white flesh and a yellow spiny peel (Lichtenzveig et al, 2000); and the spontaneous (natural) or induced (artificial) interspecific hybrids of these three species (Tel-Zur et al, 2012a; Pagliaccia et al, 2015) Interest in these species has increased markedly in recent years, since they are regarded as exotic fruit crops with high economic potential and as a rich source of natural micronutrients (Wu et al, 2006) and phytochemicals with therapeutic activities (Song et al, 2016). These cactus species are a valuable commodity for dryland farmers in regions with increasing water scarcity and land degradation
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