Cell cycle changes in the nuclear concentration of DNA were measured in male and female gametes of tobacco (Nicotiana tabacum L.) using 4',6-diamindino-2-phenylindole (DAPI) labeling and microfluorimetry. Mitotic generative nuclei were used as a reference for 2C DNA concentrations. After division of the generative nucleus, the two sperm cells remained at a 1C DNA concentration in pollen tubes growing in the style. Sperm cells deposited within the synergid, however, had DNA concentrations approaching 2C. Egg nuclei are formed at a 1C state and approach 2C at 48 h after pollination, approximately 4 h before expected karyogamy. If pollination was withheld, the onset of DNA synthesis in the egg nucleus was delayed by 12 to 24 h. Sperm appear to be retained within the synergid until they reach cell cycle synchrony with the female nuclei at 2C. Zygote nuclei display a 4C concentration of DNA soon after fusion and up to 84 h after fertilization. These data provide evidence for gamete fusion at 2C in dicotyledons with bicellular pollen. This differs from some modern studies on monocotyledons and is consistent in the fusion condition with Arabidopsis thaliana.

Key words: cell cycle, DNA microfluorimetry, egg cell, fertilization, Nicotiana, sperm