Chapter 5 Genetic Analysis of Apomixis - cimmyt

From s.x...."Y 10 Apomixis: MoI."Ia, ond G...tk App.-Ile. 187the developmental program for endospermdevelopment is activated in the absence offertilization.It is possible that the same genetic control isresponsible for autonomous endosperm andegg activation. This would be in agreementwith the hypothesis that the endosperm isevolutionary and derived from a secondembryo, as first proposed by Sargant (1900).This hypothesis is supported by morphologicalanalyses of fertilization in nonflowering seedplants of the genera Ephedra and Gnetum(Friedman 1990, 1992; Carmichael andFriedman 1995). In addition, molecular andgenetic investigations have shown that thereis a large overlap in gene activity between theendosperm and embryo. For instance, amongthe 855 characterized defective kernel mutantsin maize (representing about 285 loci), the vastmajority affect both the embryo andendosperm and very few are potentiallyendosperm-specific (Neuffer and Sheridan1980). Similar results were obtained in a studyof defective kernel mutants in barley (Bosnes etal. 1987), suggesting that a very largepercentage ofseed-specific genes are expressedin both embryo and endosperm, despite theirdifferent development and physiology. Incontrast to this extensive overlap in geneexpression between embryo and endosperm,studies on several recently isolated Arabidopsismutants that allow fertilization-independentendosperm formation but not embryogenesissuggest that endosperm activation may becontrolled by different developmentalprograms (Ohad et al. 1996; Chaudhury et al.1997; Grossniklaus and Vielle-ealzada 1998;Luo et al. 1999; Kiyosue et al. 1999; D. Page, R.Pruitt, S. Lolle, and U. Grossniklaus,unpublished data).The importance of the endosperm for seeddevelopment varies among species dependingon its developmental pattern. In some speciesof the Orchidaceae the endosperm undergoesonly a few division cycles or does not divideat all. In many dicotyledonous species,including Arabidopsis, the endosperm formsbut is essentially degraded by the time seedmaturation is initiated. In contrast, theendosperm ofcereals is persistent and ofgreateconomic value. Therefore, an engineeredapomixis in grain crops will have to allow fornormal development of the endosperm. In anideal situation, endosperm formation inengineered apomictic crops could be inducedautonomously. However, successful formationof endosperm in cereals depends on thespecialized cytoplasm of the central cell andrequires contributions from both maternal andpaternal genomes. This may be because somegenes are imprinted, that is their activitydepends on their parental origin (Kermicle1970; Kermicle and Alleman 1990; Messingand Grossniklaus 1999). Thus, engineeredapomictic grain crops are likely to require thefertilization of the central cell. The vastmajority of apomictic Gramineae arepseudogamous; possible autonomousapomixis has been observed in very fewspecies, including Calamagrostis, Poa nervosaand Nardus stricta Oohri et al. 1992).2. Genomic imprinting. Imprinting in plantsis usually regarded as specific to theendosperm (Kermicle and Alleman 1990;Walbot 1996; Alleman and Doctor 2000).Forma~ion of androgenetic and gynogenetichaploids in many species (Kimber and Riley1963; Sarkar and Coe 1966; Kermicle 1969) andof asexually derived embryos in apomictssuggest that imprinting does not playa crucialrole for embryogenesis in these species,although it may exist in ·'Jhers. Thedevelopment of embryos from somatic tissue(Zimmerman 1993; Mordhorst et al. 1997) andthrough anther culture (Zaki and Dickinson1990) is taken as further evidence thatimprinting is not involved in plantembryogenesis. However, the initial