DEFICIENCY ANALYSIS OF FEMALE GAMETOGENESIS IN MAIZE

Plants produce female gametes through mitotic division in the multicel lular, meiotically reduced (haploid) megagametophyte phase. In floweri ng plants, the megagametophyte is the embryo sac; female gametogenesis or megagametogenesis comprises the ontogeny of the embryo sac. As a s tep toward understanding the role of embryo sac-expressed genes in meg agametogenesis, development of normal, haploid embryo sacs in maize wa s compared with development of embryo sacs deficient for various small , cytologically defined chromosomal regions. This analysis allowed us to screen 18% of the maize genome, including most of chromosome arms 1 L and 3L, for phenotypes due specifically to deletion of essential, em bryo sac-expressed genes. Confocal laser scanning microscopy of whole developing embryo sacs confirmed that normal megagametogenesis in maiz e is of the highly stereotyped, bipolar Polygonum type common to most flowering plants examined to date. Deficiency embryo sac phenotypes we re grouped into three classes, suggesting each deficient region contai ned one or more of at least three basic types of haploid-expressed gen e functions. In the first group, three chromosome regions contained ge nes required for progression beyond early, free-nuclear stages of embr yo sac development. Maintaining synchrony between events at the two po les of the embryo sac required genes located within two deficiencies. Finally, three chromosome regions harbored loci required for generatio n of normal cellular patterns typical of megagametogenesis. This analy sis demonstrates that the embryo sac first requires postmeiotic gene e xpression at least as early as the first postmeiotic mitosis. Furtherm ore, our data show that a variety of distinct, genetically separable p rograms require embryo sac-expressed gene products during megagametoge nesis, and suggest the nature of some of those developmental mechanism s. (C) 1995 Wiley-Liss, Inc.