Abstract:Anatomical observations, genetic and cytoembryological studies were conducted to determine the cytological factors in relation to the production of diploid polyembryony within one embryo sac occurring in polyembryonic line SB-1 of O. sativa L. The results showed that the diploid polyembryony in SB-1 was not originated from the fertilization of synergids, but presumably from the megagametophytes with supernumerary egg cells. Potentially polyembryonic megagametophytes at maturity contained either a conventional 3-celled egg apparatus (92.60% of megagametophytes) or the supernumerary eggs forming either a 4-celled (5.2 %) or 5-celled (2.2 %) egg apparatus. In megagametophyte with one additional supernumerary egg cells were almost situated side by side in adjacent to the synergids and recessed from the micropyle, whereas in megagametophyte with two additional supernumerary eggs, one egg cell occupied the conventional possition and the other two lay above the synergids. The arrangement of embryos in polyembryonic caryopsis at various developmental stages was concordant to that of egg cells in megagametophytes with supernumerary eggs. Karyotype analysis showed that most of the twins and triplets were of 2n--2n and 2n-2n-2n respectively. Polyembryony was observed in Fo and F1 caryopsis of cross SB-1 × Malaihong (Malaihong is of non-polyembryony) and in Fl caryopsis of its reciprocal but not in Fo caryopsis. This implies that polyembryony occurring in SB-1 is of a dominant character of embryo sac. Anatomical observations proved that each seedling from twins or triplets was completely independent with no vascular bundle connected with each other. The causes of polyembryony within one embryo sac, the entry of supernumerary sperms and the origin of supernumerary egg cells are discussed.