Chapter 5 Genetic Analysis of Apomixis - cimmyt

62 Tamara N. Nouma.o and Jeon-Phmppe '(..lIe-Cal.adoThe comparison of early fertilization events inPellllisetllnJ ciliare has identified ultrastructuraldifferences related to the control of egg cellparthenogenesis, an essential event in theapomictic life cycle that is absent from sexualreprod uction; however, the dynamics of pollentube arrival and sperm cell delivery have notbeen investigated. Further ultrastructuralstudies will be necessary to determine if spermcell delivery and movement are equivalent insexual and aposporous female gametophytes.The nature of the signal that activates theaposporous egg cell and the fate of the spermcells after pollen tube delivery remain to beelucidated. Levels of calcium and otherelements have never been measured insynergids of apomictic embryo sacs (Chaubaland Reger 1992). Additional studies shouldinclude three-dimensional reconstruction ofegg cells during the first hours followingpollination and quantitative information onthe pattern of synergid degeneration and onthe specific fate of sperm cells within themegagametophyte. Attempts to follow themovement of fluorochromatically-stainedsperm cells in sexual embryo sacs have beenlimited, and the reliability of such studies usingconventional clearing techniques isquestionable; however, in vitro analysis ofsperm nuclei movement within isolatedembryo sacs has been accomplished (Faure etal. 1994). Isolation and fusion of gametes cannow be used to compare the interaction ofsperm cells with sexual or aposporous eggcells, in order to determine if cell to cellinteractions impede the fusion of sperm nucleiwith an ilpomictically-derived egg cell. Finally,some \'ery enlightening findings may bel'btilined in the near future by following onthe pioneering work of Olga Erdelska(Skwakian Academy of Sciences, Bratislilvil).Recent investigations in Torellia jOllmieri haveshown that, at least in certain species in whichthe megagametophyte can be readily dissectedor is partially uncovered by the integuments,some unique discoveries can be made byobserving the process of double fertilizationin vivo (Higashiyama et at. 2000). Microinjectionand cell ablation technology maysoon be combined with microcinerr:atographyto investigate sexual and apomictic megagametophytedevelopment in vivo.AcknowledgmentsWe thank A. Vassilyev and U. Grossniklausfor their critical reading of the manuscript andstimulating discussions, and J. Osadtchiy fortechnical assistance. Work by TamaraN'lUmova. was partially supported by a grantfrom the lAC (the Netherlands) and byfinancial support from the InternationalScience Foundation (USA).ReferencesAsker, 5.E., and l. Jerling. 1992. Apomixis in Plonts. Boca Rotan, Florida:CRC Press.Bashaw, H., and E.C. Holt. 1958. Megasporogenesis, embryo socdevelopment and embryogenesis in dallisgrass, Paspalum dilatatum.Agran.1. 50: 753-56.Bell, P.R. 1988. Control points in generation cycle of ferns. 1. Plant Physiol.132: 378.--. 1992. Apospory and apogamy: implications for understandingthe plant life cycle. Int. 1. Plant Sci. 153: 123-36.---.1995. Incompatibility in flowering plants: adaptation of onancient response. The Plont (ell 7: S--16.Burson, B.l., PW. Voigt, B.A. Sherman, and c.L. Dewald. 1990. Apomixisand sexuality in eastern gamagrass. (rap. Sci. 30: 81>-89.Campbell, 5.C, WG. Craig, and E.B. ScaN. 1987. Apomixis ond sexuality inthree species of Amelanchier, shodbush (Rosaceae, Moloideael. Am.1. Bal. 74: 321-28.Corman, J.G., C.F. Crone, and O. Riera-lizarazu. 1991. Comparativehislology of cell walls during meiosis and apomicticmegasporogenesis in !wo hexaploid Australasian Elymus species.(rop Sri. 31: '527-32.Corman, J.G. 1997. Asynchronous expression of duplicate genes inangiosperms may couse apomixis, bispory, tetraspory, andpolyembryony. Bioi. 1. linn. Soc 61: 51-94.Chapman, G.P, and N. 8usri. 1994. Apomixis in Pennisefum: onultrastructural study. International 1. of Plonf Sciences 155: 492-97.Chaubal, R., and BJ Reger. 1992. Calcium in the synergid cells and otherregions of pearl millet ovaries. Sex. Plant. Reprod. 5: 34-46.Crone, CF., and J.G. Carmon 1987. Mechanisms 01 apomixis in ElymusreC1isefus from eastern Australia and New Zealand. Ame~ 1. Bot. 74:477-96.Diboll, A.G 1968. Fine structural development of the megagametophyte01 lea mays follOWing fertilization. Am 1. Bal. 55: 787-806.