Chapter 5 Genetic Analysis of Apomixis - cimmyt

176 U.IGm..lln.apomictic pathway with the net result ofproducing an umed uced functionalmegaspore (see Crane, Chap. 3).Unlike in sexual development, the megasporemother cell of diplosporous spe;:ies is notsurrounded by callose. In apospoTOUS species,callose deposition around the sexualmegaspoTOcyte can be abnormal (Crane andCarman 1987; Carman et al. 1991; Leblanc etal. 1993; Naumova et al. 1993; Naumova andWillemse 1995; Peel et al. 1997). As is typicalfor the functional megaspore in sexual~,aposporic initials, which will form apomicticembryo sacs, are devoid of callose (Naumovaand Willemse 1995). The marked difference ofcallose deposition between sexual anddiplospoTOus species is intriguing, but mostlikely is not causal, instead being theconsequence of a more fundamental lesion(Crane and Carman 1987; Carman et al. 1991;Carman, Chap. 7).The unreduced megaspore of diplosporousapomicts divides mitotically to give rise to amature embryo sac. Usually, only onemegaspore of the dyad initiates embryo sacdevelopment and the other degenerates, butbisporic development, in which twounreduced nuclei are present in the same spore,also occurs (Ixeris-type). In some apomicts, thedeveloping megagametophyte undergoes onlytwo mitoses to form a 4-nucleated embryo sacwhere no antipodals form (Panicum-type)(Gustafsson 1947; Nogler 1984a, b; Crane,Chap. 3). For instance, sexual megagametophytesin Pennisetum ciliare are of the typicalseven-celled Polygonum·type, whereasaposporic embryo sacs carry only four nucleiand typically form one egg cell, two synergidsand one polar nucleus (or a variation thereof)but no antipodals (Taliaferro and Bashaw 1966;Vielle et al. 1995). The egg cell of an apomicticembryo sac initiates embryogenesisautonomously in the absence of fertilization.In Pennisetum, the aposporic egg cell iscompletely covered by a cell wall (Vielle et al.1995) whose presence may prevent the fusionof the apomictic egg with a sperm cell (Askerand Jerling 1992; Savidan 1992). Someapomictic species are truly autonomous anddo not require fertilization at all; both embryoand endosperm develop autonomously. Incontrast, seed development in most apomictsdepends on the fertilization of the central cellto produce the nutritive endosperm, which isrequired for successful seed production(pseudogamy) (Nygren 1967; Asker 1979,1980;Nogler 1984a; Richards 1986; Bashaw andHanna 1990; Asker and Jerling 1992). Unlikein sexual species, the apomictic egg cell ofteninitiates embryogenesis before the firstendosperm division occurs.Interrelationship of Sexual and ApomicticReproductionSexual and apomictic reproduction aredevelopmentally and evolutionarily related.Apomixis can be viewed as a developmentalvariation of the sexual pathway. Apomictic andsexual modes of reproduction are not mutuallyexclusive. Whereas obligate apomicts produceexclusively clonal progeny, both forms ofreproduction coexist in facultative apomicts(Asker 1980; Richards 1986; Bashaw andHanna 1990). They form both reduced egg cellsthat are fertilized to produce geneticallydiverse progeny as well as apomictic embryosacs that give rise to clonal offspring.Apomictic and sexual embryo sacs occur in thesame plant or even within the same ovule(Asker 1980; Nogler 1984a; Vielle et al. 1995).Facultative apomicts, benefiting from theadvantages of both modes of reproduction,may have an evolutionary advantage and aremore common than obligate apomicts (Nogler1984a; Richards 1986; Asker and Jerling 1992).The degree of sexuality versus apomixis infacultative apomicts is influenced by a varietyof environmental factors, the effects of whichon the reproductive system are not well