THE PLANT CELL, Vol 7, Issue 12 1975-1988, Copyright © 1995 by American Society of Plant Biologists
Double Fertilization in Gnetum gnemon: The Relationship between the Cell Cycle and Sexual Reproduction
J. S. Carmichael and W. E. Friedman
Department of Biology, University of North Dakota, Grand Forks, North Dakota 58202
Gnetum gnemon, a nonflowering seed plant and member of the Gnetales,
expresses a rudimentary pattern of double fertilization that results in the
formation of two zygotes per pollen tube. The process of double
fertilization in G. gnemon was examined with light and fluorescence
microscopy, and the DNA content of various nuclei involved in sexual
reproduction was quantified with 4[prime],6-diamidino-2-phenylindole
microspectrofluorometry.Male and female gamete nuclei pass through the
synthesis phase of the cell cycle and increase their DNA content from 1C to
2C before fertilization. Each of the two zygotes found in association with
a pollen tube is diploid and contains the 4C quantity of DNA at inception.
Based on these results as well as previous studies of nuclear DNA content
in plant sperm, eggs, and zygotes, three fundamental and distinct patterns
of gamete karyogamy among seed plants can be circumscribed: (1) G1
karyogamy, in which male and female gametes contain the 1C quantity of DNA
throughout karyogamy and the zygote undergoes DNA replication; (2) S-phase
karyogamy, in which gamete nuclei initiate fusion at 1C but pass through
the S phase of the cell cycle before completely fusing; and (3) G2
karyogamy, in which male and female gamete nuclei pass through the S phase
of the cell cycle before the onset of fertilization. Our results show
definitively a pattern of G2 karyogamy in G. gnemon.
