Transfer of Self-Fruitfulness to Cultivated Almond from Peach and Wild Almond

Abstract

The almond [Prunus dulcis (Mill.) D.A. Webb] is normally self-sterile, requiring orchard placement of pollinizer cultivars and insect pollinators. Honeybees are the primary insect pollinators utilized, but climate change and the higher frequency of extreme weather events have reduced their availability to levels insufficient to meet the demands of current and anticipated almond acreage. The incorporation of self-fruitfulness may eliminate the need for both pollinizers and pollinators and allow the planting of single cultivar orchards that facilitate orchard management and reduce agrochemical inputs. Self-fruitfulness requires self-compatibility of self-pollen tube growth to fertilization, as well as a high level of consistent self-pollination or autogamy over the range of anticipated bloom environments. The Italian cultivar Tuono has been the sole source of self-compatibility for breeding programs world-wide, leading to high levels of inbreeding in current almond improvement programs. Both self-compatibility and autogamy have been successfully transferred to commercial almonds from cultivated peaches (Prunus persica L.), as well as wild peach and almond species. Self-compatibility was inherited as a novel major gene, but was also influenced by modifiers. Molecular markers developed for one species source often failed to function for other species’ sources. Autogamy was inherited as a quantitative trait. Breeding barriers were more severe in the early stages of trait introgression, but rapidly diminished by the second to third backcross. Increasing kernel size, which was similarly inherited as a quantitative trait, was a major regulator of the introgression rate. Self-fruitfulness, along with good commercial performance of tree and nut traits, was recovered from different species sources, including Prunus mira, Prunus webbii, P. persica, and the P. webbii-derived Italian cultivar Tuono. Differences in expression of self-fruitfulness were observed, particularly during field selection at the early growth stages. Introgression of self-fruitfulness from these diverse sources also enriched overall breeding germplasm, allowing the introduction of useful traits that are not accessible within traditional germplasm.