Abnormal megagametogenesis results in seedlessness of a polyembryonic ‘Meiguicheng’ orange (Citrus sinensis) mutant created with gamma-rays

•The reduction of pollen viability in the mutant might not be caused by meiotic disturbances.•Seedlessness was caused by strong female sterility due to the abortion of megagametophyte.•Nucellar embryos could develop independently to produce seeds in the seedless mutant.