Female gametophytic and early zygotic mutant phenotypes highlight the essential role of corresponding genes for reproductive development. (a) A cartoon showing the ontogeny of the wild-type female gametophyte in Arabidopsis and the early transition to seed development. A haploid functional megaspore (FM) develops from a diploid megaspore mother cell (MMC) upon two meiotic divisions (1). Three syncitial mitotic divisions (2) convert the FM into an eight-nuclear cell. Upon nuclear migration, cellularization, nuclear fusion and differentiation (3), a cellularized seven-celled embryo sac forms. It contains an egg cell (EC) and two synergid cells (SC) at the micropylar pole, three antipodals (AP) at the chalazal pole, and one vacuolated homo-diploid central cell (CC) in the middle. Subsequently, the AP cells degenerate. Degeneration of one SC precedes the entry of one pollen tube (PT), and two sperm cells (SP) independently fertilize the egg and central cell, leading to the development of a diploid embryo (EM) and triploid endosperm (EN) respectively. SUS, suspensor, VN, vegetative nucleus. (b-f) Morphology of wild-type ovules corresponding to representative events described above is depicted (ii indicates inner integuments, and oi indicates outer integuments). Both synchronous and asynchronous free nuclear mitotic divisions (as shown in panel e; arrows) lead to development of the free nuclear endosperm (FNE) as shown in panel f. The insert in panel e depicts a developing zygote (ZY). (g) In kerridwin (ken-1), two polar nuclei in the central cell fail to fuse. (h) Female gametophyte development did not initiate beyond the one-nucleate embryo sac stage (arrows) in frigg (fig-1). (i-l) Anomalies in early endosperm and zygotic development in hog1 (homology dependent gene silencing 1) mutants. The zygote did not develop beyond single cell stage, and subsequent divisions and cytokinesis did not occur (panel i, j, and k). The arrows in panels i and j identify the irregular nature of free nuclear mitotic divisions in hog-1 endosperm. The endosperm nuclei were irregular in size and they were often clustered. Compare the large and small irregular endosperm nuclei in hog1-6 (panel l) with the regular free nuclear endosperm nuclei in (m) the wild type. Scale bars: 20 μm for panels d to k, and the insert of panel e; and 50 μm in panels b, c, l, and m.