Intraspecific and interspecific crossability in three Ziziphus species (Rhamnaceae)

Three cultivated species of Ziziphus Mill. [Z. jujuba Mill., Z. mauritiana Lam. and Z. spina-christi (L.) Willd.] comprising a total of seventeen cultivars/genotypes were selected for a mating system study. Phenological stage determination showed an overlap in blooming between the three species during June. Intraspecific cross compatibility was evaluated for Z. jujuba and Z. mauritiana, and interspecific crosses between the three species were carried out during two flowering seasons. Following intraspecific hand cross-pollination, pollen tube formation and growth to the ovule 24 h after pollination (HAP) were observed in all three species. Although very low seed set was obtained following intraspecific Z. jujuba crosses, histological studies showed normal embryo formation and endosperm development, suggesting that double fertilization had occurred and that embryo development arrest occurred later. Following interspecific hand pollination, in vivo pollen grain germination and the formation of pollen tubes and their growth to the ovule 24 HAP were observed in compatible crosses, which also set fruit and seed. Viable embryos were obtained from Z. mauritiana × Z. jujuba, Z. mauritiana × Z. spina-christi and Z. spina-christi × Z. mauritiana crosses. Putative hybrids from these crosses were germinated in vitro, and the resultant plantlets were studied using flow cytometric analysis. Indications of true hybrid origin were obtained based on total 2C-DNA content of plantlets, but they died during the hardening-off process. This study showed that gene flow among these species is possible, thus increasing the potential that interspecific crosses can be used for genetic crop improvement.