TRANSMISSION OF MALE CYTOPLASM DURING FERTILIZATION IN NICOTIANA-TABACUM

Serially sectioned embryo sacs of Nicotiana tabacum were examined duri ng fertilization events using transmission electron microscopy. After pollen tube discharge, the outer membrane of the sperm pair is removed , the two sperm cells are deposited in the degenerate synergid and the sperm cells migrate to the chalazal edge of the synergid where gameti c fusion occurs. During fertilization, the male cytoplasm, including h eritable organelles, is transmitted into the female reproductive cells as shown by: (1) the cytoplasmic confluence of one sperm and the cent ral cell during cellular fusion, (2) the occurrence of sperm mitochond ria (distinguished by ultrastructural differences) in the zygote cytop lasm and adjacent to the sperm nucleus, (3) the presence of darkly sta ined aggregates which are found exclusively in mature sperm cells with in the cytoplasm of both female cells soon after cell fusion, and (4) the absence of any large enucleated cytoplasmic bodies containing reco gnizable organelles outside the zygote or endosperm cells. The infrequ ent occurrence of plastids in the sperm and the transmission of sperm cytoplasm into the egg during double fertilization provide the cytolog ical basis for occasional biparental plastid inheritance as reported p reviously in tobacco. Although sperm mitochondria are transmitted into the egg/zygote, their inheritance has not been detected genetically. In one abnormal embryo sac, a pair of sperm cells was released into th e cytoplasm of the presumptive zygote, Although pollen tube discharge usually removes the inner pollen-tube plasma membrane containing the t wo sperm cells, this did not occur in this case. When sperm cells are deposited in a degenerating synergid or outside of a cell, this outer membrane is removed, as it apparently is for fertilization.