Chapter 5 Genetic Analysis of Apomixis - cimmyt

From Suoolily to Ap.....i1: MoIet.lar ..dGtto.,k Appr-N' 175Naumova 1993; Crane, Chap. 3). Anultrastructural characterization of apomicticdevelopment is discussed by Naumova andVielle-Calzada (Chap. 4). Here, I will brieflydescribe the main developmental features ofapomixis in order to facilitate a comparisonwith the sexual pathway.Two fundamentally different classes ofapomictic development can be distinguished(Gustafsson 1947; Nogler 1984a; Koltunow1993) (Figure 12.2). In sporophytic apomixis,an embryo forms directly from a nucellar orintegumentary cell in the ovule (adventiveembryony). Although adventive embryos arenot derived from gametophytic cells, theirdevelopment depends on the presence of amegagametophyte, because they usually relyon sexually derived nutritive endospermtissue. Sporophytic apomixis will not beconsidered further in this chapter because anengineered switch between sexuality andgameotphytic apomixis appears moreattractive for breeding purposes. However, itshould be kept in mind that in sporophyticapomixis only embryo initiation is affectedand, thus, it may be easier to tackle sporophyticapomixis at the molecular level.In gametophytic apomicts, the embryo resultsfrom the parthenogenetic development of anegg cell produced by an unreduced embryosac. The unreduced gametophyte can originateeither directly from nucellar cells (apospory)or from a megaspore mother cell that hasundergone no or an aberrant meiosis resultingin the formation of one (mitotic diplospory) ortwo unreduced megaspores (meioticdiplospory). Aposporous embryo sacs formmitotically from nucellar cells that developduring or after megasporocyte differentiationand are similar in appearance to the megasporemother cell. Often several aposporic embryosacs are present in a single ovule in addition tothe sexually derived one. In diplosporousdevelopment, a variety ofcytologically distinctprocesses lead to a failure in meiosis; themegasporocyte switches from a meiotic to anGametophytic ApomixisLR~)-e-du-ce-d-~o-Iar~ ~"'" §)~'"'~Apomictic embryo I Endosperm I I Endosperm II~Se-xu-a-1e-m-bry-o-"( Fertiialion(I EndospermIIApomictic embryo IFigure 12.2 The main developmental features of apomixis in relationship to the sexual pathway.Unreduced cells are in rectangular boxes, reduced cel~ are in oval boxes and key events are in dark~ shaded boxes. In sexual plants, themegaspore mother cell undergoes meiosis and one of the reduced megaspores forms the embryo soc. Embryo and endosperm are formed bydouble fertilizotion. In gamelophylic apomixis, reduction is avoided and embryo soc development initiated from on unreduced diplospore oron aposporic initial cell. The embryo develops parthenogenetically from the unreduced egg while the endosperm forms eilher aUlonomous~or through fertilization of the central cell (pseudogamy). In sporophytic apomixis, the embryo forms directly from on unreduced nucellorinitioIcell. The opomictic embryo relies on sexually produced endosperm.