Chapter 5 Genetic Analysis of Apomixis - cimmyt

46 T.""". N. Naomo•• ..dJ...·P~iipp. Y~l..."'''.d.The ultrastructure of both cells contained in atwo-cellular adventive embryo is extremelysimilar: their external cell wall remains thickand lacks plasmodesmata. Young adventiveembryos also lack a distinct polarity, and noregular cell divisions can be discerned; asuspensor is frequently missing. Nucellarembryos do not arise synchronously, and theirearly development can be severely delayed.Whereas some embryos can appear to becomposed of only a few cells, others in thesame ovule may already be undergoingorganogenesis. Adventive embryos are able topenetrate into the adjacent central cell of asexually derived megagametophyte.DiplosporyIn diplosporous species, the megagametophyteis formed from an aberrant meiotic cyclethat prevents reduction and recombination. Anul trastructuraIcharac-terization of d iplosporyhas only been initiated in two apomicts: Poapa!lIslris and Poa nemoratis (Naumova et al.1999). In these two species, diplospory ischaracterized by a complete omission ofmeio~is; the unreduced megasporocytedevelops into an embryo sac after gradualvacuolation and three consecutive mitoticdivisions. In the young developing ovule, thearchesporial cell progressively differentiatesinto a diplosporous precursor or diplosporousembryosac megaspore mother cell (DMC).Subsequently, after three mitotic divisions, theDMC develops into an 8-nucleate embryo sacof the Antennaria-type.In contrast to nucellar cells present within thedeveloping ovule, the archesporial cell of bothspecies is characterized by a large nucleus withdecreased chromatin contraction in regionshaving close contact with the surface of thenucleolus. Numerous poorly differentiatedmitochondria and plastids are uniformlydistributed in the enlMged cell. Interestingly,isolilted enclil\"es of cytoplasm containingmultiple membranes are often observed,whereas the endoplasmic reticulum (ER) ispoorly developed. Compared to the DMC,vacuoles are also abundant but small andlocated mainly at the micropylar and chalazalpoles of the cell.Significant cellular elongation marks thetransition from archesporial cell to DMC.Whereas the archespore is 160 J.lm on average,the DMC is about 350-380 J.lm long, with littleor no changes in width. This increase in size isassociated with a reduction in chromatincondensation and nuclear volume. In theDMC, the nucleus is positioned at themicropylar end of the cell and becomeselongated and irregular in shape. There is asubstantial increase in the population ofribosomes, polysomes, dictyosomes. At thisstage, the vacuoles fuse into two large oneslocated at the micropylar and chalazal poles,and there is an obvious increase in thethickness of the cell wall. The transition fromDMC to a noncellularized embryo sac is alsocharacterized by a gradual increase in celllength to about 500 J.lm. The nucleus becomesirregular and lobbed, with numerousprotuberances in the nuclear envelope. Severalnucleoli differing in size and shape can besimultaneously observed. There is also amarked increase in the population ofmitochondria and plastids that often formclusters heterogeneously distributedthroughout the cytoplasm. Contacts betweenthe external membrane of the nuclear envelopeand granular ER cisternae are often observed.Additionally, series of concentric membranousstructures were also observed in the cytoplasm.The cell wall became slightly thick during thechange of the DMC into a one nucleate embryosac. Nucellar cells adjacent to the developingdiplosporous embryo sac showed signs ofdegeneration, with an electron-opaquecytoplasm and highly condensed nuclei.