Increased Zygote-Derived Plantlet Formation through In Vitro Rescue of Immature Embryos of Highly Apomictic Opuntia ficus-indica (Cactaceae)

O. ficus-indica (prickly pear cactus) is an important forage and food source in arid and semiarid ecosystems and is the most important cactus species in cultivation globally. The high degree of apomixis in the species is a hindrance in plant breeding programs where genetic segregation is sought for the selection of superior genotypes. To understand if in ovulo embryo rescue could increase the proportion of zygotic seedlings, we compared the mature seed-derived seedlings with those regenerated from in vitro embryo rescue at 20, 25, 30, 35, and 40 post-anthesis days (PADs) in four Italian cultivars. The seedlings were classified as apomictic or zygotic based on molecular marker analysis using inter-sequence single repeat (ISSR) primers. Multiple embryos were recovered from all the cultured immature ovules, and plantlets were regenerated and acclimatized to the field post hardening, with success rates ranging from 62% (‘Senza spine’) to 83% (‘Gialla’). The level of polyembryony differed among cultivars and recovery dates, with the highest being ‘Rossa’, producing 4.8 embryos/ovule at 35 PADs, and ‘Gialla’, the lowest, with 2.7 at 40 PADs. The maximum number of embryos observed within a single ovule was 14 in ‘Trunzara bianca’. ISSR analysis revealed that ovule culture at 35 PADs produced the highest percentage of zygotic seedlings in all the cultivars, from 51% (‘Rossa’) to 98% (‘Gialla’), with a high genotype effect as well. Mature seeds produced much fewer seedlings per seed, ranging from 1.2 in ‘Trunzara bianca’ to 2.0 in ‘Rossa’ and a lower percentage of zygotic seedlings (from 14% in ‘Rossa’ to 63% in ‘Gialla’). Our research opens a pathway to increase the availability of zygotic seedlings in O. ficus-indica breeding programs through in ovulo embryo culture.