Synthetic apomixis: From theoretical conception to heterosis fixation

Synthetic apomixis is referred to use genetic engineering to resemble natural apomictic pathways, enabling the production of clonal offspring through seeds that are genetically identical to the maternal parents. This process bypasses meiosis and fertilization. The critical techniques involve two processes: 1) the established MiMe (Mitosis instead of Meiosis) system, which requires the simultaneous disruption of three genes, each of them abolishes the formation of DNA double-strand breaks for initiating homologous recombination, induces precocious separation of sister chromatids, and suppresses the second meiotic division, thus leading to the generation of diploid gametes that are clonal to the parent; 2) the induction of embryogenesis from these diploid gametes via ectopic expression of parthenogenesis-associated genes such as BBM1, PAR and HUAXU or haploid inducers such as MTL, DMP, or CENH3. The successful coupling of these two modules produces clonal seeds with an identical genotype to the mother plant, thus achieving the fixation of heterosis. To date, the MiMe system has been successfully established in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), tomato (Solanum lycopersicum), maize (Zea mays), rapeseed (Brassica napus), and sorghum (Sorghum bicolor), although the optimal combination of the three meiotic genes exhibits marked species-specific variation. Efficient synthetic apomixis has so far been realized only in rice, most notably through the “Fix” strategy, achieving a clonal seed induction efficiency of up to 99%. However, major challenges remain in other crop species. This review systematically summarizes the molecular basis and species-specific features of the MiMe system, the various routes and efficiencies of parthenogenesis and haploid induction, synthetic apomixis and its applications in major crops. We also discuss critical challenges and offer perspectives on the future application of synthetic apomixis in crop breeding.