[The development of pollen grains and formation of pollen tubes in higher plants : III. DNA-replication of vegetative and sperm nuclei in mature pollen grains of barley]

Relationship between nuclear DNA-content of sperm cells and the timing of events in the cell cycle of Brassica campestris L. (Brassicaceae)

In this work we conducted a quantitative analysis of the nuclear DNA-content of developing sperm cells of the plant <em>Brassica campestris</em> L. The sperm cells were in young pollen grain, mature pollen grain and pollen tubes. When generative cells, at the pre-anthesis stage, split into two sperm cells, we have established that the newly-formed sperm cells begin to synthesize nuclear DNA in developing pollen grain of <em>B. campestris</em>. We measured this DNA-content during the development of sperm cells. The results indicate that during development, sperm cells of <em>B. campestris</em> have passed the G₁ phase of the cell cycle and entered the S phase, presumably then fusing with egg cells at a level of 2C, as is characteristic of G₂ type fertilization in angiosperms.

Applications of fluorochromes to pollen biology. I. Mithramycin and 4',6-diamidino-2-phenylindole (DAPI) as vital stains and for quantitation of nuclear DNA

The two DNA-specific fluorochromes DAPI and mithramycin have been found to be extremely useful dyes in studies of pollen development and growth. Both fluorochromes stain nuclei brilliantly either in fixed or in living tricellular and bicellular angiosperm pollen, thereby permitting rapid scanning for pollen abnormalities and easy observation of nuclear details. These water soluble dyes can be incorporated into the germination medium for studies of pollen germination in vitro, facilitating observation of the movement of generative, sperm and tube nuclei during pollen growth. In fixed pollen, the fluorochromes bind quantitatively with DNA and thus may be used to quantitate ploidy changes and to study cell cycles during pollen development, germination and fertilization.

Cell-cycle dependency of radiosensitivity and mutagenesis in fertilized egg cells of rice, Oryza sativa L. : 1. Autoradiographic determination of the first DNA synthetic phase

To determine the time and duration of the first and second DNA synthetic phases in fertilized egg cells and central cells of rice, a total of 753 ovules were sampled at 2 h intervals during the first 30 h after pollination and exposed to (3)H-thymidine for 2 h at 25 °C. Autoradiographic observation of labeled nuclei was made for fertilized egg cells, as well as for central and antipodal cells. The first and second DNA synthetic phases in fertilized egg cells were found 8-12 h and 21-25 h after pollination, respectively. The durations of each cell-cycle phase in the egg cell were estimated to be 4-6 h for G1, 4 h vor S and for G2, and 2 h for M. In the central cell, the first DNA synthesis took place at 3-4 h after pollination, i.e., immediately after fertilization, followed by the formation of the primary endosperm nucleus. Antipodal cells also showed labeled nuclei in the early stages after fertilization. The first divisions of fertilized egg cell and primary endosperm nucleus were observed at 16-18h and at 4-6 h after pollination, respectively. The present observations suggest that sperm and egg nuclei participate in fertilization with haploid amount (1C) of DNA and fertilized egg cell originates thus in 2C state.

[The development of pollen grains and formation of pollen tubes in higher plants : V. The division of the generative nucleus in the pollen tube of Tradescantia paludosa]

Fixed pollen tubes of Tradescantia paludosa were investigated cytologically to determine the precise moment of division of the generative nucleus and to measure the length of the pollen tubes at the same time.The exact moment of division of the generative nucleus could be shown to be closely correlated with the length of the pollen tube of Tradescantia. In the majority of the pollen tubes the vegetative nuclei are not yet in the degenerative phase at the time of division of the generative nucleus. In all random samples investigated pollen tubes without nuclei were found. Depending on the random sample the percentage of pollen tubes without nuclei was 11-22%.In the discussion it is pointed out that the data obtained from the cytological investigations show that the findings of the precise moment of division of the generative nucleus agree with earlier published results on the development of pollen grains and formation of pollen tubes. The development of the male gametophyte of the angiosperms is to be divided into a series of stages. The data obtained here from the cyto-morphological studies of pollen tubes of Tradescantia and from cyto- and biochemical investigations of others indicate that the different stages of the pollen grain and the pollen tube can be separated. In the second part of the discussion there is presented an evaluation of the hypothesis that during evolution of the angiosperms species have formed repeatedly, and in most cases probably independently from one another, in which the degenerative phase of the vegetative nucleus begins earlier and earlier during the development of the male gametophyte, or that in some instances DNA replication is either partially or entirely eliminated.