722 C.Dumas.P.Rogowsky C.R.Biologies 331 (2008)715-725 6.Development of unfertilized male and female ulation of the genes encoding members of the prc2 gametes is complex.Thu s.MEA does not only auto -regulate its own transcription 86 but is also subject to pater Unfertilized gametes can develop to certain degrees nal imprinting [87].just like certain other members and even give rise to haploid,generally sterile plants of the PRC2 complex.On the other hand,the target Artificial doubling of chromo omes at the seedling Pheres/is one of the rare genes known to be stage pre ents sterility and together with self pollinatior of perfectly homozy ent The de of the s.The fr all is generally very low.e.g.0.1%in maize 731.This fre eles [8ol More r ntly viahle seeds c quency can be dramatically increased by the use of par normal diploid embryo and an unfertilized endosperm ticular male parents (inducing lines)to reach for exam have been obtained by fertilization of prc2 mutants by ple 8.1%in maize [74].While unfertilized sperm cells hemizygous CDKA:/-YFP plants generating only on do not develop into embryos,immature m rospores the prod n hap :d by vanc ing the contribution of the hich ca n and has beer ated int rtile bng,This。 the perm rep ed and and more recently micros erary ciency is strongly species and genotype dependent and can reach over 30%in rapeseed [75].There are no re. References ports on substantial development of unfertilized central cells other than in mutants mu allowing the devel ☒s.G.Naw in.Resultate einer Revi Pol me (PcG i tene.Bul Acad which in wild phyte de d.Sur nent after fer tilization 176.771.In the seed the mplex is com (1899864-871 posed of the SET(Suppressor of variegat on/Enhancer 4]B.M zeste/Trithorax)domain protein MEDEA (MEA) 19g IZATION IN Embryology of Angiosperms.McGraw-Hill. (I [6]J.E Faure,C.Dumas,Fertilization in ring plants:New ap ctio hanisn in:B.R Jordan (Ed.).Th of members of th ment of the tral cell in the absence of fertilization CABI Publishing.2007.pp.269-297 8( In addition,msil mutants also show a few divisions of S.D.Russell.The g loid embryo The Polycomb Repressive Complex. (PRC2)is thougl at K2,gene [10]W.A.Jer en,D.B.Fisher.C tton embrvose is:the sperm on b methy 3K2 for thr a MADS Maize as box transcription factor [831.Fusca3 (Fus3)coding fo Cl( a B3 transcription domain factor [84]and Formin Ho 1337-1348. mology Protein5(FH5)encoding an actin nucleator [13】J.EF involved in endosperm cellularization [851.The reg. 1600722 C. Dumas, P. Rogowsky / C. R. Biologies 331 (2008) 715–725 6. Development of unfertilized male and female gametes Unfertilized gametes can develop to certain degrees and even give rise to haploid, generally sterile plants. Artificial doubling of chromosomes at the seedling stage prevents sterility and together with self pollination allows the production of perfectly homozygous plants, which is of a great interest for plant breeders. The de￾velopment of the non-fertilized egg cell or gynogenesis naturally occurs in most Angiosperms. The frequency is generally very low, e.g. 0.1% in maize [73]. This fre￾quency can be dramatically increased by the use of par￾ticular male parents (inducing lines) to reach for exam￾ple 8.1% in maize [74]. While unfertilized sperm cells do not develop into embryos, immature microspores, the product of male meiosis, can be induced by various stresses to form haploid embryos in vitro, which can be regenerated into fertile plants after chromosome dou￾bling. This process was originally termed androgenesis and more recently microspore embryogenesis. Its effi- ciency is strongly species and genotype dependent and can reach over 30% in rapeseed [75]. There are no re￾ports on substantial development of unfertilized central cells other than in mutants. The best characterized mutations allowing the devel￾opment of unfertilized central cells concern members of the chromatin remodeling Polycomb group (PcG), which in wild type plants plays a major role in the arrest of female gametophyte development after fer￾tilization [76,77]. In the seed, the complex is com￾posed of the SET (Suppressor of variegation/Enhancer of zeste/Trithorax) domain protein MEDEA (MEA) [78], the VEFS domain protein FERTILIZATION IN￾DEPENDENT SEED2 (FIS2) [79], the WD40 re￾peat proteins FERTILIZATION INDEPENDENT EN￾DOSPERM (FIE) [80] and MULTICOPY SUPPRES￾SOR OF IRA1 (MSI1) [81]. Loss-of-function mutants of members of the complex show autonomous develop￾ment of the central cell in the absence of fertilization. In addition, msi1 mutants also show a few divisions of the egg cell leading to early arrested non-viable hap￾loid embryos[82]. The Polycomb Repressive Complex2 (PRC2) is thought to repress gene expression by methy￾lation of histone H3 at K27 (H3K27). Correlation of gene repression with the presence of the H3K27 marks has been demonstrated for three direct targets of the seed PRC2, namely Pheres1 (Phe1) coding for a MADS box transcription factor [83], Fusca3 (Fus3) coding for a B3 transcription domain factor [84] and Formin Ho￾mology Protein5 (FH5) encoding an actin nucleator involved in endosperm cellularization [85]. The reg￾ulation of the genes encoding members of the PRC2 is complex. Thus, MEA does not only auto-regulate its own transcription [86] but is also subject to pater￾nal imprinting [87], just like certain other members of the PRC2 complex. On the other hand, the target gene Pheres1 is one of the rare genes known to be subject to maternal imprinting by the PRC2 [88]. Ara￾bidopsis Glauce (Glc), whose molecular identity is not yet known, counterbalances the action of the PRC2 by promotion of fertilization-independent endosperm development and expression of paternally inherited al￾leles [89]. More recently, viable seeds consisting of a normal diploid embryo and an unfertilized endosperm have been obtained by fertilization of PRC2 mutants by hemizygous CDKA;1-YFP plants generating only one sperm cell. This result suggests a more general role of the PRC2 complex in balancing the contribution of the paternal genome in the triploid endosperm and has been used as an additional argument that the endosperm rep￾resents in evolution an extension of female gametophyte development rather than a supernumerary embryo [90]. References [1] C. Dumas, Reproduction et développement des plantes à fleurs, C. R. Acad. Sci. Paris, Ser. III 324 (2001) 517–521. [2] S.G. Nawashin, Resultate einer Revision der Befruchtungsvor￾gaenge bei Lilum Martagon und Fritillaria tenella, Bul. Acad. Imp. Sci. St. Petersburg 9 (1898) 377–382. [3] M.L. Guignard, Sur les anthérozoïdes et la double copulation sexuelle chez les végétaux angiospermes, C. R. Acad. Sci. 128 (1899) 864–871. [4] B.M. Johri, K.B. Ambegaokar, P.S. Srivastava, Comparative Em￾bryology of Angiosperms (vols. 1 & 2), Springer-Verlag, 1992. [5] P. Maheshwari, Embryology of Angiosperms, McGraw-Hill, New York, 1950. [6] J.E. Faure, C. Dumas, Fertilization in flowering plants: New ap￾proaches for an old story, Plant Physiol. 125 (2001) 102–104. [7] T. Gaude, I. Fobis-Loisy, C. Miège, Control of fertilization by self-incompatibility mechanisms, in: B.R. Jordan (Ed.), The Molecular Biology and Biotechnology of Flowering, Oxford, CABI Publishing, 2007, pp. 269–297. [8] C. Dumas, T. Gaude, Fertilization in plants: Is calcium a key player?, Sem. Cell Dev. Biol. 17 (2006) 244–253. [9] S.D. Russell, The egg cell: development and role in fertilization and early embryogenesis, Plant Cell 5 (1993) 1349–1359. [10] W.A. Jensen, D.B. Fisher, Cotton embryogenesis: the sperm, Protoplasma 65 (1968) 277–286. [11] C. Dumas, R.B. Knox, C.A. McConchie, S.D. Russell, Emerging physiological concepts in fertilization, What’s New Plant Phys￾iol. 15 (1984) 168–174. [12] C. Dumas, H.L. Mogensen, Maize as a model system for exper￾imental embryogenesis in flowering plants, Plant Cell 5 (1993) 1337–1348. [13] J.E. Faure, C. Digonnet, C. Dumas, An in vitro system for ad￾hesion and fusion of maize gametes, Science 253 (1994) 1598– 1600