Chapter 5 Genetic Analysis of Apomixis - cimmyt

170 lie. Gros..iklalSgametes and embryogenesis (Walbot 1996).The meiotic products ofplants undergo severaldivision cycles to form a multicellular haploidorganism. The gametes differentiate later ingametophyte development. In angiosperms,double fertilization concludes thegametophytic phase and marks the beginningof the next sporophytic generation. Inangiosperm apomixis, the plant life cycle isshort-circuited and an unreduced cell lineagegives rise to a megagametophyte (gametophyticapomixis) or directly to an embryo(sporophytic apomixis). Because of the closedevelopmental and evolutionary relationshipbetween apomictic and sexual reproduction,a better understanding of the fundamentalbiological principles governing female gametogenesisand seed development will provideinvaluable tools for the manipulation of thesexual reproductive system towards apomixis.Sexual Model SystemsTwo well-established sexual model systems,Arabidopsis tlwliana and Zea mays (maize), anda rapidly emerging system, Oryza siltiva (rice),are of particular interest for genetic andmolecular investigations. All three are wellcharacterize~at the genetic level and offer avast array of powerful genetic and moleculartechniques (Freeling and Walbot 1993;Meyerowitz and Somerville 1994; Tanksley andMcCouch 1997; McCouch et al. 1997). Versatiletransposon systems for insertionalmutagenesis and gene tagging are availableand offer the opportunity for reverse geneticapproaches (Walbot 1992; Dellaporta andMoreno 1994; Feldmann etal. 1994; Shimamotoet al. 1993; Shimamoto 1995; McKinney et al.1995; Sundaresan 1996; Parinov et al. 1999;Speulman et al. 1999; Tissier et al. 1999;Meissner et al. 1999; Krysan et al. 1999).Maize, as an agriculturally important memberof the grass family (http://www.agron.missouri.edu), has some advantages forapomixis research. It can be hybridized withits apomictic relative Tripsacum dactyloides (e.g.,Mangelsdorf and Reeves 1931; Harlan and deWet 1977; Petrov et al. 1984; Savidan, Chap.11), and the extensive synteny among thegrasses (Bennetzen and Freeling 1993; Mooreet al. 1995; Gale and Devos 1998; Keller andFeuillet 2000) allows for comparative genomicanalyses between sexual and apomictic grassspecies. The Mutator transposon system offershighly efficient methods for insertionalmutagenesis (Chomet 1994) and for sitespecifictransposon mutagenesis by reversegenetic approaches (Das and Martienssen1995; Bensen et al. 1995; Mena et al. 1996;Rabinowtiz and Grotewold 2000), originallypioneered in the fruit fly Drosophilamelanogaster (Ballinger and Benzer 1989).The small plant Arabidopsis tlwliana, a memberof the Brassicaceae, has been widely adoptedas a model system for the developmentalbiology and genetics of flowering plants(Meyerowitz 1989). The small size of the plant,its rapid life cycle, and the large number ofseeds it produces make it ideal for the isolationand study of mutants that affect biochemicaland developmental pathways. The smallgenome size (-125 Mb), high percentage ofsingle copy DNA (Pruitt and Meyerowitz1986), large number of molecular markers(http://www.arabidopsis.com). and thecomplete genome sequence (ArabidopsisGenome Initiative 2000) make Arabidopsis apowert'ul system for molecular studies. Highlyefficient T-DNA-based transformationmethods (Bechthold et al. 1993) andheterologous transposon systems for targetedgene tagging, genome wide insertionalmutagenesis, and reverse genetics areavailable (Feldmann et al. 1994; McKinney etal. 1995; Sundaresan 1996; Parinov et al. 1999;Speulman et al. 1999; Tissier et al. 1999;Meissner et al. 1999; Krysan et al. 1999). TheArabidopsis genome is the first plant genometo be completely sequenced (Arabidopsis