Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ullra.'nd"aI Analy.i. of Apolllidic D...lop....' 57aposporous embryo sacs in the progeny of aPel1l1isetl/ni glal/cum (sexual) x Pennisetllmsqllaml/latl/m (obligate apomict) interspecificcross. In facultative apomictic genotypes, theycompared sexual and aposporous embryo sacsin the same ovule. Particular attention wasgiven to the distribution of internal cell wallingrowths (transfer walls) in the central cell.Many wall projections were observed in themicropylar region, but few ingrowths werefound in the chalazal region of both sexual andapomictic megagametophytes. Plasmodesmatawere present in the cell wall that separatesthe central cell from the antipodals, butappeared to be absent at the chalazal pole ofaposporous embryo sacs.Parthenogenesis and FertilizationIn Pennisetl/m ci/iare, most genotypes reproduceby obligate apospory. Breeding efforts arebased on the identification of rare genotypesthat have completely lost the ability todifferentiate aposporous megagametophytesand only form sexually functional, reduced(haploid) embryo sacs of the Polygonum-type.The comparison of the egg apparatus in sexualand aposporous megagametophytes of P. ci/iareoffers an opportunity to analyze the cellulardynamics of fertilization in apomictic plants.Such a comparison is particularly valuable ifconducted where independent genotypes ofapomictic and sexual germplasm are availablewithin a population segregating for methodof reproduction.In P. ci/iare, fertilization occurs 3-4 hours afterpollination. The examination of thE' eggapparatus of buffelgrass at several timeintervals after pollination has provided someinformation on the structural and functionalfeatures that distinguish sexual and apomicticdevelopment (Vielle et a!. 1995). Compared tothe synergids of the sexually derived eggapparatus, the degenerative process in one orboth aposporous synergids appears to beaccelerated. Prior to pollination, the cytoplasmof both cells contains small vacuoles, and feworganelles can be identified. In general, thecells are similar in appearance to the sexualsynergids during the first two hours followingpollination, but before pollen tube arrival.During the first two hours followingpollination, the cytoplasm of one of thesynergids becomes electron-dense, the plasmamembrane appears disrupted, organellescannot be identified, and the nucleus isirregularly shaped and pressed to the plasmamembrane. Increased amounts of vesiculartraffic are observed. Later, the cytoplasm of thesecond synergid also shows signs ofdegeneration. Two to three hours afterpollination, the degenerated synergid hasentirely collapsed and its remnants appear asan electron-dense fringe closely associatedwith the egg cell. The time of initiation of thedegenerative process is variable in both sexualand aposporous embryo sacs, and in somecases, highly degenerated synergids arepresent in unpollinated ovules during or justafter embryo-sac cellularization.Significant differences are also observedbetween the aposporous and the sexual eggcell. After cellularization but before pollination,the aposporous egg cell is characterized by aconspicuous centrally located nucleus thatcontains a single nucleolus (Figure 4.3e). Thenucleus of the sexually derived egg cell isgener-ally centrally located, but has a smallernucleolus. The chalazal vacuole present in thesexually derived egg cell is replaced by severalsmaller vacuoles that restrict the cytoplasm toa region located around the nucleus. In contrastto the sexual egg cell, the cytoplasm containsabundant mitochondria and polysomes, butfew Golgi bodies and endoplasmic reticulum(ER) can be observed. At the micropylar region,both sexual and aposporous eggs sharecommon cell walls with both synergids, butthese walls disappear in the middle portions