Chapter 5 Genetic Analysis of Apomixis - cimmyt

188 UtI GrOSllilaot.development of such embryos is distinctlydifferent from sexually derived embryos andit is not clear whether the same developmentalprograms control embryogenesis in thesedifferent contexts. It is possible, for example,that imprinting requirements are suppressedunder certain culture conditions. Furthermore,it is likely that the importance of imprintingfor embryo and endosperm will differ amongspecies depending on the respective roles ofthese tissues in the production and acquisitionof nutrients (Messing and Grossniklaus 1999).For instance, the Arabidopsis gametophyticmaternal effect mutation medea (mea)drastically affects cell proliferation in embryoand endosperm, resulting in seed abortion(Grossniklaus et al. 1998b). Genetic andexpression studies suggest that MEA isregulated by genomic imprinting andexpressed in both embryo and endosperm atearly stages of seed development (Vielle­Calzada et al. 1999). At later stages, the imprintat the mea locus occasionally breaks down, butreports as to which tissues are affected differ(Kinoshita et al. 1999; Luo et al. 2000). That thegenetic background has strong effects on themea phenotype suggests that these differencesmay be ecotype dependent (Vielle-Calzada etal. 1999; Grossniklaus et al. 2001). Currently,however, it is not clear which fertilizationproduct is primarily affected, but it is likelythat MEA is required in both the embryo andendosperm. Genetic interactions of mea andsimilar mutants with mutants that affect DNAmethylation and/or chromatin remodelinghave been reported (Vielle-Calzada et al. 1999;Luo et al. 2000; Vinkenoog et al. 2000;Grossniklaus et al. 2001). We are currentlyusing mea as a starting point to isolateadditional genes involved in the genomicimprinting process through second-sitemodifier screens.Genomic imprinting has not been studied inmany plant species, but it has beenunequivocally demonstrated in theendosperm of maize at the genomic,chromosomal, and individual gene levels(Kermicle and Alleman 1990; Messing andGrossniklaus 1999). In maize, properdevelopment of the endosperm is strictlydependent on the presence of maternal andpaternal genomes in a ratio of 2m:1p (Lin 1982,1984; Birchler 1993). Any deviation from thisratio leads to a failure in endosperm formationand consequently to seed abortion.Interspecific and interploidy crosses suggestthat this is likely to be true for other speciesincluding most agriculturally important graincrops (Nishiyama and Yabuno 1978; Johnstonet al. 1980; Haig and Westoby 1991). In contrast,endosperm development in Arabidopsis doesnot require a 2m:1 p ratio because interploidycrosses involving diploid and tetraploid plantsproduce v iable seeds (Redei 1964;Grossniklaus et al. 1998b), but there are distinctparent-of-origin dependent effects on seed sizein interploidy crosses (Scott et al. 1998).3. Imprinting barriers to the introduction ofapomixis into sexual species. Imprintingphenomena may be behind the high degree ofsterility observed in hybrids between sexualand apomictic genera, and so, should be givenconsideration in efforts to introduce apomixisinto sexual species. In gametophytic apomixisthe female reproductive cells are unreducedwhereas microsporogenesis is unaffected andthe male gametophytes are reduced. Thus,fertiliz~tionof the central cell generates a ratioof 4m:1p, which is expected to result in seedabortion. However, apomictic species do notshow strongly reduced fertility, suggesting (i)that the constraints for imprinting are relaxedin apospecies or (ii) that the mechanisms offertilization have been modified. Apomicts doindeed show a relaxed requirement forimprinting, which is supported by the findingthat the ploidy level of the endosperm inapomictic species can be quite variable Oohriet al. 1992). A recent study by Grimanelli et al.(1997) clearly demonstrates that endosperm