Chapter 5 Genetic Analysis of Apomixis - cimmyt

from Sexuality t. Apomixis: MoIeatIor ..d Gtto.tk Ajlpr_lIe. 189development in TripsaclIm is normal under awide range of ratios of maternal to paternalgenomes. Similarly, apomictic Paspaillm speciesare insensitive to an imbalanced genome ratioin the endosperm while the sexuals maintainthis requirem~nt (Quarin 1999).Altered modes of fertilization that are expectedto maintain the endopserm balance numberhave been reported in several cases. This canbe achieved if either both sperm cells deliveredby the pollen tube fuse with the central cell, orif only one of the two polar nuclei and a singlesperm nucleus participate in karyogamy(Rutishauser 1954; Reddy and d'Cruz 1969;Nogler 1972, 1984a). Alternatively, unreducedpollen could serve as the male parent (Chao1980). As a very successful alternative torelaxed imprinting requirements, manyapomictic grasses show apospory of thePanicum-type, where 4-nucleated embryosacsare formed, which most often contain only onepolar nucleus that fuses with a single spermnucleus (e.g.,Savidan 1980). Sexual individualsof these agamic complexes usually produce 8­nucleated Polygonum-type embryo sacs withtwo reduced polar nuclei. Thus, fertilizationof both sexually and apomictically derivedcentral cells produces endosperms withbalanced parental genomes (Reddy 1977).Apomictic species may be evolutionarilyderived from predisposed genera that hadrelaxed imprinting requirements or,alternatively, evolved specific adaptations ofthe fertilization mechanism that maintainedthe imprinting requirements (see alsoGrimanelli et aI., Chap. 6). Such predispositionsand adaptations are not thought toexist in most sexual species and imprintingmay pose a serious problem to the introductionof apomixis into sexual crop plants(Grossniklaus et al. 1998a; Spillane et al. 2000;Savidan 2000; Grossniklaus et at. 2001). Indeed,apomictic maize-Tripsawm and pearl millet­Pennisetllm hybrids show a high degree ofseedabortion (Grimanelli et al. 1995; Dujardin andHanna 1989) tha t is likely to result from agenomic unbalance in the endosperm(Grossniklaus et al. 1998a; Morgan et al. 1998).Crosses with pollen donors of higher ploidythat maintained the endosperm balancenumber could possibly restore fertility. Atpresent, our understanding of imprinting andits importance for seed development is verylimited. A sustained effort toward a betterunderstanding of the genetic and molecularbasis governing imprinting is required toovercome the potential constraints to theengineering of apomictic crops.Genetic Screens for MutantsDisplaying Apomictic Traits inSexual Model SystemsIn previous sections, I discussed a number ofgenes that control sexual development thatcould serve as powerful tools for theengineering of apomixis. As an alternative, ascreen for mutants that display apomictic traitsin a sexual species could directly lead to theidentification of key regulatory components(Peacock 1992). This approach has been takenin several laboratories using Arabidopsis as amodel system. Although no apomictic specieshave been described in this genus, the closerelative Arabis holboelli is apomictic (Asker andJerling 1992). While direct experimentationwith Arabis is difficult because of its poorgenetic characterization and long generationtime, its close relationship to Arabidopsis maybe useful for comparative and widehybridization approaches.Arabidopsis Mutants withAutonomous Seed DevelopmentScreens for Arabidopsis mutants that allow seeddevelopment in the absence of fertilizationhave been performed in several laboratories.These screens take advantage of male sterilemutants and aim to identify second sitemutations that pseudo-suppress sterility. In