Organization of the early post-fertilization megagametophyte ofPopulus deltoides

Abstract

Gynoecia ofPopulus deltoides, 48 to 60 h after pollination, were examined using transmission electron microscopy. The mature megagametophyte contains two synergids, an egg cell, a central cell with two unfused polar nuclei and three antipodals. Pollen tube passage between cells in the styloid, on the placenta and through the micropyle does not induce degeneration of surrounding cells. Pollen tube entry into the megagametophyte occurs through the degenerated synergid immediately adjacent to the filiform apparatus. The tube grows the length of the filiform apparatus 20–30 μm into the synergid, and then discharges its contents-two sperm cells, the vegetative nucleus, and pollen tube cytoplasm. Sperm cells appear progressively hypertrophied during their passage in the pollen tube. In the degenerate synergid, the sperm nucleus is located at one end of the cell and the remaining cytoplasm is diffuse, containing a large vacuole. Following transmission of sperm nuclei into the egg and central cell, one male nucleus fuses with the egg and the other fuses with the micropylar polar nucleus. Enucleate cytoplasmic bodies of apparent sperm origin were observed within the degenerated synergid. The gametic fusion mechanism in this plant appears to involve: (1) eccentric positioning of the nucleus within the sperm cell, (2) separation of cytoplasmic and nuclear portions of the sperm cell, and (3) fusion of the nuclear portion with the egg cell. This represents a previously undescribed mechanism by which male cytoplasmic organelles are eliminated from sperm cells prior to fusion.