Chapter 5 Genetic Analysis of Apomixis - cimmyt

190 Ud e;,....1Ia..Arabidopsis, unpollinated pistils do notelongate, so that pistil elongation is an easilyscored phenotype correlated with seeddevelopment. Different male sterile mutantshave been used, including pistillata (pi), ahomeotic flower mutant that lacks stamen(Chaudhury and Peacock 1993; Koltunow etal. 1995; Chaudhury et al. 1997); the waxbiosynthetic mutant eceriferum6 (cer6) (Dellaert1979; Preuss et al. 1993; HUiskamp et al. 1995),a conditional male sterile that is fertile underhigh humidity (Ohad et al. 1996); and thetemperature sensitive mutant TH154, isolatedin R. Pruitt's laboratory, which is male sterileat 25°C but fully fertile at 18°C (D. Page, R.Pruitt, S. Lolle, and U. Grossniklaus,unpublished results). Silique elongation underthe restrictive condition indicates an asexualmode of reproduction with full or partial seeddevelopment, or the development of a fruitwithout concomitant seed production(parthenocarpy).Using this type ofscreen with more than 15,000M1 plants, we identified three classes ofmutants (D. Page, R. Pruitt, S. Lolle, and U.Grossniklaus, unpublished results): (i) mutantssuppressing the male sterility defect, which caneasily be identified because they producefunctional pollen (some of these will be truerevertants of TH154); (ii) mutants displayingparthenocarpy; and (iii) mutants displayingapomictic traits with autonomousdevelopment of seed-like structures in theabsence of pol1en production. The latter arerather rare and, to date, mutants in only threeloci have been reported. These have beencharacterized in more detail at the genetic andmorphological level. In the fertilizationindependentendosperm (fie), mutant endospermdevelops autonomously to the free nuclearstage, the seed coat develops normally, but noembryo forms (Ohad et al. 1996). Likewise,fertilization-independent seed mutants (fisl, fis2,fis3) form autonomous endosperm, which wasshown to be diploid and can progress to thecellular stage in fisl and fis2. The seed coatdevelops properly but no embryos form(Chaudhury et al. 1997). Structures thatresemble an elongated zygote have beenobserved infisl andfis2 at a low frequency. Infieffis plants, the mutant allele is either verypoorly transmitted or not transmitted at al1through the female gametophyte and can onlybe recovered through the pol1en, becausepol1inated seeds derived from a mutantmegagametophyte abort (Ohad et al. 1996;Chaudhury et al. 1997). Thus,fie/fis mutantsdisplay a gametophytic maternal effect on seeddevelopment, which is similar to thephenotype observed for mea that also showsfertilization-independent endospermdevelopment (Grossniklaus et al. 1998b;Grossniklaus and Vielle-Calzada 1998).Indeed,jisl and two other mutants were foundto be allelic to mea (Kiyosue et al. 1999; Luo etal. 1999), as werefis3 and fie (Chaudhury et al.1997; Ohad et al. 1999).MEA and FIE encode members of thePolycomb group, proteins thought to regulategene expression by modulating higher orderchromatin structure (Grossniklaus et al. 1998b;Ohad et al. 1999). FIS2 encodes a putative DNAbinding protein with a Zn-finger domain (Luoet aI.1999). As with their animal counterparts,the MEA and FIE proteins interact directly ina protein complex (Luo et al. 2000; Spillane etal. 2000; Yadegari et al. 20(0). The function andregulation of MEA, FIE, and FIS2 have beenextensively reviewed (Goodrich 1998; Ma 1999;Preuss 1999; Mora-Garcia and Goodrich 2000;Russinova and de Vries 2000; Grossniklaus etal. 2001) and will be summarized only verybriefly. MEA was shown to be regulated bygenomic imprinting, thus explaining itsmaternal effect on seed development (Vielle­Calzada et al. 1999; Kinoshita et al. 1999). WhileFIS2 also seems to be regulated by genomicimprinting (Luo et al. 2000), FIE appears to beexpressed biparentally later during seeddevelopment (Luo et al. 2000; Yadegari et al.