Development of sterile ovules on bisexual cones of Gnetum gnemon (Gnetaceae)

Angiosperms and Gnetales (Ephedra, Gnetum, Welwitschia) represent the only seed plants that regularly produce bisexual cones. Unfortunately, the fertility and function of ovules formed on bisexual cones of Gnetales have remained unclear. Some reports indicate that the ovules are sterile while others indicate that they may develop into seeds. This study demonstrates three different developmental patterns of ovules formed on bisexual cones of Gnetum gnemon. Type I ovules did not develop at all after pollination and represented the majority of ovules on each cone. Type II ovules enlarged slightly after pollination due to the enlargement of nucellar tissue. Type III ovules were typically found on the terminal whorl and developed into seed-like structures. The enlargement was due to proliferation of megagametophyte tissue. Sectioned material revealed that megagametophytes show altered development compared to those found in functional female ovules. None of the ovules studied contained embryos, and thus all were sterile. Densitometry of 4',6-diamidino-2-phenylindole (DAPI)- stained sections revealed that megagametophyte nuclei formed in the sterile ovules are unreduced (diploid) and thus do not form viable female gametes.