Chapter 5 Genetic Analysis of Apomixis - cimmyt

F.... s.ualn, " Aponlllis: MoIoalar .d Geeetk ApProMMl 177understood (e.g., Knox and Heslop-Harrison1963; Knox 1%7; Frost and Soost 1%8; Cox andFord 1987; Hussey et al. 1991).The developmental regulation of sexualreproduction appears to be preserved duringapomixis. Although an apomictic gametophyteor embryo has a distinct developmental origin,the sexual developmental program is largelyconserved: megagametophyte development,embryogenesis, and the development of theendosperm and seed coat are identical insexual and apomictic genotypes. At the levelof gene expression, very few differences canbe detected between obligate apomictic andsexual genotypes of Pennisetum ciliare (Vielle­Calzada et al. 1996). In apomixis, the sexualpathway is altered at the transitions betweenthe two phases of the plant life cycle, meiosisand double fertilization (Figure 12.2): (i)meiosis is aberrant or absent leading to theproduction of an unreduced cell acting as thefunctional megaspore; (ii) the egg cell initiatesembryogenesis parthenogenetically orembryos form directly from a sporophyticinitial (adventive embryony); and (iii) theendosperm develops either autonomously or,in pseudogamous species, fertilization of thecentral cell is required for endospermformation and successful seed development.In pseudogamous species, special adaptationsmay be required for successful endospermformation. Thus, apomixis can be viewed as ashort-circuited sexual pathway (Koltunow1993; Vielle-Calzada et al. 1996; Grossniklauset al. 1998a) in which part of the sexualdevelopmental program is initiated at thewrong time or in the wrong cell. Thus,apomixis is characterized by a relaxation of thespatial and temporal constraints on thereproductive developmental process. It islikely that apomictic reproduction results fromthe heterochronic or heterotopic expression ofregulatory factors that control megasporogenesis,as well as egg and cen tral cellactivation in sexual species (Mogie 1992;Peacock 1992; Koltunow 1993; Grossniklausetal. 1998a).Whereas nonreduction and parthenogeneticembryogenesiS as two of the key componentsof apomixis have been discussed extensively,endosperm formation has attracted lessattention. In pseudogamous apospecies,mechanisms preventing the fertilization of theegg cell but allowing fusion of sperm andcentral cells may rely on the formation of acomplete egg cell wall prior to sperm arrival(Savidan 1992; Vielle et al. 1995). However,specific adaptations to maintain theendosperm balance number Oohnston andHanneman 1982; Ehlenfeldt and Ortiz 1995)may be required to ensure normal seeddevelopment. In maize, endosperm formationis strictly dependent on the presence ofmatemal and patemal genomes in a ratio of2m:1 p, due to differential imprinting of theparental genomes (Lin 1984; Kermicle andAlleman 1990). This requirement is likely toexist in many plantspecies, but may be relaxedor absent in some (Haig and Westoby 1991;Messing and Grossniklaus 1999). Sinceapomictic species produce normal pollen, thefertilization of an unreduced central cell witha single reduced sperm cell would violate theendopserm balance number and lead to seedabortion. Endosperm forma tion is animportant process that must be considered forthe tra~sfer of apomixis into sexual species(Grossniklaus et al. 1998a; Spillane et al. 2000;Savidan, 2000; Grossniklaus et al. 2001;Grimanelli et al., Chap. 6).Models for Apomixis: HeterochronicInitiation of DevelopmentA developmental analysis ofapomictic eventsclearly indicates that several developmentalprocesses occur simultaneously orasynchronously. Meiosis and embryo-sacformation may occur at the same time: theapomictic initial initiates embryo-sac