Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ultr••IA,""aI A.aI,.is .1 ApamKtk D...lop..... 47AposporyApospory is a form of apomixis in whichsporophytic cells in the ovule give rise tounreduced female gametophytes (Gustafsson1947). The autonomous division of aposporousegg cells generates viable embryos withoutfertilization; however, the majority ofaposporous species are pseudogamous andrequire fertilization of the polar nuclei forendosperm development (Nogler 1984).An ultrastructural characterization ofaposporous megagametophytes has beenconducted in PaniCllm and Penniselum. In thesetwo grass genera, the mechanism ofaposporous female gametophyte formation isvery similar. Early during megasporogenesis,the orientation of the ovule changes within theovary, and the integuments progressivelyenclose the nucellus. The ovule becomesanatropous as it rotates toward the base of thepistil, leaving the micropyle facing away fromthe style. In PaniCllm maximllm and PenlJiselumciliare (syn = Cel1c}zYlls ciliaris L.), sexualmegagametophyte development ismonosporic; a single meiotically derivedmegaspore gives rise to the embryo sac. Thefunctional megaspore enlarges, and its nucleusdivides mitotically three times to form themegagametophyte. Buth species develop asexual megagametophyte of the PolygolJlImtype: two synergids, the egg cell, a binucleatecen tra I ce II whose n uc lei fuse prior tofertilization, and three antipodals thatproliferate to give a cluster of cells at thechalazal pole. After fertilizatio:1 of both the eggcell and the central cell, the ovuIe develops intoa seed.Whereas sexual ovules only develop ameiotically derived megagametophyte, inaposporous plants one or several nucellar cellsacquire a reproductive fate and are able topursue growth and differentiation whilemegasporogenesis proceeds. The spatial andtemporal patterns of megaspore mother cell(MMC) differentiation and meiosis are thesame during sexual and aposporousdevelopment. In obligately apomicticgenotypes, all meiotically derived megasporesdie and reduced mt'gagametophytes are notformed. Active nucellar cells undergo twomitotic division~. After cellularization, 4­nucleated female gametophytes differentiateinto an egg apparatus and a central cellcontaining usually one or two polar nuclei. Inboth species, the egg cell is able to divideparthenogenetically and give rise to viableembryos. Fertilization of the polar nucleus (ornuclei) is necessary for endosperm formationand seed set. In facultative plants, bothdevelopmental processes are viable; sexual andaposporous female gametophytes can coexistwithin the same ovule, or in different ovulesof the same inflorescence.Differentiation of Aposporous InitialsAposporous development requires thedifferentiation of nucellar cells into unreducedorganized female gametophytes. In mostaposporous species, the differentiation of theMMC appears to take place in a subepidermalla yer of the n ucell us, and followsmorphological characteristics that have beenpreviously described in many sexuallyreproducing angiosperms (Huang and Russell1992). In Panicum maxim11m, the MMC ischaracterized by a centrally located nucleuscontaining a conspicuous nucleolus and densecytoplasm. In contrast, the nucellar cellssurrounding the MMC have thinner cell wallsand numerous plasmodesmata that shareultrastructural similarities with youngmeristematic cells. In aposporous genotypes,meiotically derived dyads and linear tetradsoften degenerate during megasporogenesis.Aposporous initials usually differentiateadjacent to degenerating megaspores or onenucleatedsexual embryo sacs. Chalazallylocatednucellar cells assume vacuolizedspherical shapes and increase in volume