EARLY CYTOLOGICAL EVENTS AFTER INDUCTION OF CELL-DIVISION IN EGG CELLS AND ZYGOTE DEVELOPMENT FOLLOWING IN-VITRO FERTILIZATION WITH ANGIOSPERM GAMETES

Early events, such as formation of the cell wall, first nuclear divisi on and first unequal division of the zygote, were examined following i n vitro fusion of single egg and sperm protoplasts of maize (Zea mays L.). The time course of these events was determined. The formation of cell wall components was observed 30 sec following egg-sperm fusion an d proceeded continuously thereafter. Within 15 h after fusion most of the organelles became more densely grouped around the nucleus of the z ygote. In the in vitro produced zygote the location of the cell organe lles and of the dividing nucleus showed polarity. Two nucleoli were fi rst observed 18 h after gamete fusion. The zygotic nucleus remained un divided for about 40 h. The first cell division was observed 40-60 h, generally 42-46 h, after egg-sperm fusion. The non-fused egg cell coul d be triggered to sporophytic development in vitro by pulses of high a mounts of 2,4-D. Without such a treatment, cultured egg cells of diffe rent maize lines did not divide. Although nuclear fusion seemed to occ ur, fusion products of two egg cells also did not divide. Cell wall fo rmation was incomplete and non-uniform, showing a polarity of cultured egg cells and fusion products of two egg protoplasts. Cell division w as also induced after fusion of maize egg with sperms of genetically r emote species, such as Coix, Sorghum, Hordeum or Triticum. These gamet ic heterologous fusion products developed to microcalli. Moreover, cel l division occurred in fusion products of an egg and a diploid somatic cell-suspension protoplast from maize.