Chapter 5 Genetic Analysis of Apomixis - cimmyt

184 Uel Gn...lIDo,To date, the phenotypes of about a dozenmegagametophytic Arabidopsis mutants havebeen described in the literature (reviewed inDrews et al. 1998; Grossniklaus and Schneitz1998; Yang and Sundaresan 2000). InGametophytic factor (Gf) (Redei 1965;Christensen et al. 1997), andarta (Howden etal. 1998), tistrya (Howden et al. 1998),femalegametophyte2 ifem2) and fem3, gametophytefactor4 (gfa4) and gJa5 (Christensen et al. 1998),the functional megaspore does not initiatemegagametogenesis. In prolifera (pr/) (Springeret at. 1995), cdc16 (Yang and Sundaresan 2000),and hadad (hdd) (Moore et al. 1997), thesyncytial mitotic divisions are affected andembryo sacs show an early developmentalarrest. PRL as a member of the MCM2-3-5family and a putative component of DNAReplication Licensing Factor, is an essentialgene required in all dividing cells (Springer etal. 1995). CDC16 is another gene required forthe normal operation of the cell cyclemachinery. In gfa2, gfa3, and gJa7 mutantembryo sacs, the two polar nuclei do not fuse,a phenotype also observed in some of ourmutants 0. Moore and U. Grossniklaus,unpublished results).In maize, megagametophytes carryingindeterminate gametophyte (ig) or the r-X1deficiency undergo abnormal mitotic divisionsand are transmitted through the femalegametophyte at a reduced frequency (Kermicle1971'; Lin 1978, 1981; Weber 1983; Huang andSheridan 1996). We identified an Arabidopsismutant, haumea (hma), sharing some of theseaspects with ig, namely additional divisioncycles during megagametogenesis G. Mooreand U. Grossniklaus, unpublished data).Embryo sacs mutant for lethnl ovule (/01 and102) do not produce viable seeds (Singleton andMangelsdorf 1940; Nelson and Clary 1952).The 102 embryo sacs show a defect in nucleardivision and migration, and arrestpredominantly at the 1- and 2-nucleate stages,although some 102 megagametophytesundergo all three division cycles (Vollbrecht1994; Sheridan and Huang 1997).In hdd embryo sacs, nuclear divisions at themicropylar and chalaza] pole areasynchronous, and some of the mutantmegagametophytes cellularize prematurely.Thus, nuclear division and cellularization areregulated independently. However, these cellsdo not differentiate into a particulargametophytic cell type, suggesting that cellspecification may depend on the correct spatialcontext and lor the presence of neighboringcells. Prema ture cellulariza tion andasynchronous divisions at the poles have alsobeen observed in segmental deletions in maize(Vollbrecht and Hake 1995), suggesting thatseveral loci, including hdd, are involved in thespatial and temporal coordination ofcellularization, nuclear division, andmigration. Whereas these processes arenormally tightly coordinated, they areuncoupled ofeach other in hdd mutant embryosacs, indicating that several independentdevelopmental programs control the differentprocesses during embryo-sac development.They are usually highly coordinated, possiblyby checkpoint mechanisms and regulatoryfeedback loops. Developmental aberrationsmay relax this coordinated control, and thevarious developmental programs can occurindependently of their normal context.and ParthenogenesisFertilization and egg activation have beenstudied extenSively in animal systems at thephysiological, cellular, and molecular level(reviewed in Nucitelli 1991; Whitacker andSwann 1993; Jaffe 1996). Numerous studieshave shown that the adhesion of the sperm tothe egg cell triggers a transient rise in freecalcium ions and initiates a cascade ofdownstream events after fertilization Gaffe1991,1996; Whitacker and Swann 1993; Homaet al. 1993). The release of calcium ions isEgg Acti~ation