Chapter 5 Genetic Analysis of Apomixis - cimmyt

Chapter 4Ultrastructural Analysis of ApomicticDevelopmentTAMARA N. NALIMOVA AND JEAN-PHILIPPE VIELLE-CALZADAIntroductionSince the first applications of electronmicroscopy to the observation of plant tissues,numerous studies have focused on theultrastructural characteristics of sexualreproduction in flowering plants (Jensen1965; Jensen and Fischer 1968). In manyspecies, a wealth of information has beenpresented on the morphology of the femalegametophyte (megagametophyte or embryosac) before and after pollen tube arrival(Russell 1985; Mogensen 1988; Russell 1992;Huang and Russell 1992). Valuablecontributions to the understanding of doublefertilization have also emerged from studiesthat combine light and electron microscopywith technological advances on the in vitroisolation and fusion of gametes (Kranz et al.1991; Faure et al. 1994). Nevertheless, thefundamental mysteries involving cell to cellsignals and interactions duringme,gagametogenesis and fertilization insexual and apomictic species are stillunsolved. These enigmas include the natureof the movement of sperm cells within theegg apparatus, the specific recognition ofmale and female gametes, the molecularnature of plasma membrane adherence, andthe association, fusion, and activation ofparental nuclei (Gerassimova-Navashina1957; Russell 1992).Despite the increasing scientific interest inapomixis (Asker and Jerling 1992; Vie lle­Calzada et al. 1996a), few studies havecharacterized the fine structure ofmegagametophytes involved in apomicticreproduction. Cytological studies based onlight microscopy have identified thefundamental differences existing between thegeneral organization of sexual and apomicticfemale gametophyes in different genera(Bashaw and Holt 1958; Voigt and Bashaw1972; Philipson 1978; Campbell et al. 1987;Crane and Carman 1987; Burson et al. 1990).The fine structure of nucellar andintegumentary embryony has beeninvestigated in some detail for a limitednumber of species (Naumova and Willemse1982; Wilms et al. 1983; Naumova 1993), andultrastructural studies of diplospory have beeninitiated in only a few apomicitic species ofPoa (Naumova et al. 1999). In addition, a fewultrastructural characterizations ofmegagametophyte development andfertilization have been conducted in someaposporous members of the Poaceae (Chapmanand Busri 1994; Naumova and Willemse 1995;Vielle et al. 1995),The occurrence of apomixis in species thathave conserved the ability to reproducesexually provides unique opportunities forcomparative ultrastructural studies. Theformation of sexual and apomicticmegagametophytes can simultaneously occurin the same genotype or in different genotypesof the same population. Ultrastructuralcomparisons of megagametogenesis and earlyfertilization events in sexual and apomictic