Integrated cytological and molecular analysis reveals the seedless mechanism in a 60Co-γ induced low-acid sweet orange mutant 'Wuzihong 1-7' (Citrus sinensis Osbeck).

'Wuzihong 1-7' is a novel seedless low-acid sweet orange mutant developed through 60Co-γ irradiation of 'Youzihong 1-7'. This study investigates its multi-level seedlessness mechanisms through comparative analysis with the parental line. Experiments included modified Carbol-fuchsin staining for pollen meiosis; SEM analysis of pollen morphology/germination; paraffin sectioning for embryo sac development; and ISSR markers for genomic variation detection. Abnormal pollen meiosis resulted in 10.1% viability (vs. 85.7% in control), 89.9% embryo sac development arrest, predominantly at mononuclear (38.2%) and binuclear (42.7%) stages. Post-fertilization embryos showed 71.3% arrest at globular stage, and three specific ISSR markers (UBC-813, UBC-834, and UBC-835) confirmed that 'Wuzihong 1-7' is a genetically stable variant. Seedlessness in 'Wuzihong 1-7' is mainly caused via three primary mechanisms: male sterility (meiotic defects), female gametophyte abortion, and post-pollination developmental barriers. The identified ISSR markers will provide molecular tools for seedless trait selection. This multi-omics analysis advances understanding of radiation-induced seedlessness and facilitates targeted breeding of seedless citrus varieties.