Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ultra,t..""aI AIlaly,l, af Apamktk O...Ia,...., 45ovules of the same species can be used toobtain mechanistic information on specificcellular differences that distinguish thesedevelopmental processes. Although thenumber of species in which ultrastructuralstudies have been conducted remains limited,recent electron microscopy studies in someaposporous grasses have provided newinformation on megasporogenesis,aposporous initiation, the organization of thedifferentiated megagametophyte, and theautonomous division of unreduced egg cells.Nucellar and IntegumentaryEmbryonyDuring integumentary or nucellar embryony(also called adventive embryony), asexualembryos are formed from inner integumentaryor nucellar cells that aredifferentiating in tissues external to themeiotically derived megagametophyte.Nucellar or integumentary cells that give riseto adventive embryos are called embryocytes.A comprehensive review of adventitiveembryony provides significant ultrastructuralinformation about this process (Naumova1993) by summarizing light and electronmicroscopy observations and reviewingembryological information on representativesof more than 250 species of flowering plants.The first morphological evidence ofembryocyte differentiation is usually observedafter the initiation of megagametogenesis. Asthe nucellar cells divide, they invade thecentral cell of the sexually functional embryosac. The formation of viable seed from nucellarembryos usually requires fertilization of thepolar nuclei and subsequent development ofthe endosperm; however, autonomousendosperm formation sporadically occurs insome species. Nucellar embryos are usuallyinitiated independently of pollination.Polyembryony, the formation of severalembryos in a single ovule, is characteristic ofadventive embryony. Several ultrastructuralinvestigations have improved ourunderstanding of embryocyte differentiationand early adventive embryogenesis (Naumova1978; Naumova and Wi1Iemse 1982; Wilms etal. 1983).Embryocytes are generally characterized by adense cytoplasm, an irregularly shapednucleus, and a larger volume than most of thenucellar cells present in the ovule. Theabundance of polysomes, free ribosomes,mitochondria, and plastids suggests highphYSiological activity in embryocytes. Theircell walls are significantly thickened andlacking plasmodesmata. In the genusSarcococca, the number of plasmodesmatagradually decreases during embryocytedifferentiation (Naumova and Willemse 1982).Using light microscopy, Kol tunow et al (1995)found that the nucellar initials form a thick cellwall in Citrus sinensis. They also found alocalized degeneration of the nucellar tissueat the chalazaI pole, a region where nucellaradventive initials are confined. In Sarcoccoca,the general organization of the embryocytecytoplasm is modified during consecutivestages of mitosis, undergoing events similarto those characterizing the dedifferentiation ofpre-meiotic megaspore mother cells (MMC) insexual species (Dickinson and Potter 1978). InEuonymus macroptera, integumentary embryoformation occurs in tenuinucellate ovules.Asexual embryos originate from two or threecell layers enveloping the micropylar regionof the sexually functional megagametophyte(Figure 4.1a,b,c; Naumova 1990). Integumentaryembryos coexisting in a single ovuleusually develop asynchronously (Figure4.1d,e). Plasmodesmata are not present in theembryocyte cell wall or in the integumentarywall that is in direct contact with the centralcell (Figure4.1f,g). The transversal cell wall thatseparates sister integumentary cells varies inthickness (Figure 4.1h,j).