FINE STRUCTURAL DEVELOPMENT OF THE MEGAGAMETOPHYTE OF ZEA MAYS FOLLOWING FERTILIZATION

The changes in cytoplasmic fine structure during the interval between pollination and zygotic division in the megagametophyte of Zea mays are reported. The rate of membrane synthesis appears to be low in the mature unfertilized megagametophyte. This megagametophyte is suggested to be in a state of relative metabolic inhibition. The rate of membrane synthesis is high in the zygote and free nuclear endosperm. After fertilization the swelling of the mitochondria in all cytoplasms is interpreted to reflect a presumed increase in metabolic rate relative to growth. In the zygote and endosperm, endoplasmic reticulum (ER) production is associated with the nuclear envelope. In all of the material the ribosomes not associated with the ER are helical polysomes which increase in length after fertilization. On the basis of permanganate staining, the ribonucleoprotein of the endosperm is suggested to be different from that of the other cytoplasms. The specific plastid form of each cell type does not change during the first few hours after fertilization. Dictyosomes are most numerous and active in the antipodals. They are least numerotus in the egg and zygote. The smallest dictyosome vesicles are in the synergids. Dictyosome number and vesicle size increase in the egg and central cell after fertilization. Similarities among the megagametophytes of several angiosperms are enumerated. THE EMBRYO SAC is uniquely the megagametophytic phase in the life cycle of angiosperms. The developmental pathways, final organization of the mature megagametophyte, and the details of fertilization differ greatly among species (Vazart, 1958). Electron-microscopic studies show that this variation in structure extends also to the sub-cellular level even among identical cell types (van der Pluijm, 1964; Jensen, 1965a, b; Diboll and Larson, 1966; Vazart, 1966; Rodkiewicz and Mlikulska, 1967). Among these studies the work on Gossypium hirsutum by Jensen (1964, 1967) is unique for its inclusion of the initial stages of embryogeny and for its comparative application of methods of electron microscopy, physiology and histochemistry. As part of a continuing study of the female reproductive system of Zea mays, the present paper describes the changes in the cytoplasmic fine structure during the interval between pollination and zygotic division (ca. 30 hr). Zea mays was selected because of the large amount of information already available on the species and for the ease with which the material can be handled. A comparison of the data introduced in this paper with those from the above studies should contribute to a better understanding of the angiosperm megagametophyte, structural 1 Received for publication 4 January 1968. Supported by U.S. Public Health Grant No. 1 ROI GM 13517. The author expresses his appreciation to Miss Dagmara Davis for technical assistance. 2 Present address: Department of Biology, Macon Junior College, Macon, Ga. changes relative to fertilization in angiosperms, and to the total knowledge of the corn plant. MATERIALS AND METHODS-Individual plants of commercially available 'Golden Bantam' corn were grown during the summer in 2-gal plastic containers in the greenhouse. The planting mix consisted of soil, peat and vermiculite. The plants were watered daily with a weak Hoagland's solution supplemented weekly with Bandini Gro-Rite commercial fertilizer. Day length was extended to 2:30 AM with 300-w incandescent bulbs producing 25 ft-c of light at bench level. This latter procedure was adopted to satisfy the requirements of continuing other species in a vegetative state. Temperature was maintained above 70 F at all times. Corn ears were bagged before emergence of silk. Unpollinated ears were collected 1 to 2 days after silk emergence. Fertilized material was collected 22, 24, 28, and 32 hr after hand pollination. Approximately 1-mm-thick sagittal sections of ovules containing the megagametophytes were cut from the ear and placed immediately in cold 3% or 6% glutaraldehyde buffered to pH 7.0 with 0.1 M sodium cacodylate. Megagametophytes surrounded by nucellar tissue (megasporangium) were excised from these sections while immersed in the fixative. Following 6 hr of fixation, the tissue blocks were washed in distilled water or buffer; post-fixed in either 5% unbuffered KMnO4 for 12 hr or in buffered 1 % OS04 for 12 hr; washed again, and then stained for 24 hr with 5 % aqueous uranyl acetate. The