Gnetales and angiosperms represent the only extant seed plant groups that possess bisexual reproductive structures. Many species of Gnetales consist of female plants which produce only ovule-bearing strobili and male plants with structurally bisexual strobili. Interestingly, the ovules formed on male strobili are reported to be nonfunctional, but this phenomenon is poorly understood. To better understand the development of male ovules and potential mechanism of ovule sterility, male (but structurally bisexual) strobili of Gnetum gnemon were collected and prepared for various histological techniques. From a structural perspective, nucellar tissue is comparable to that in ovules found on female strobili. In addition, female gametophyte structures were observed in male ovules and show many similarities to normal functional female gametophytes such as: (1) initiation of several tetrasporic female gametophytes, (2) free nuclear phase of development, (3) nuclei located in a parietal band of cytoplasm, (4) most chalazal gametophyte becomes dominant, and (5) enlarged micropylar region and constricted chalazal region. Despite these similarities, the developmental pattern of female gametophytes in male ovules departs from that in female ovules in several respects including: (1) fewer nuclei produced during free nuclear phase, (2) lack of dense cytoplasm and nuclei in the chalazal region, and (3) smaller size at the end of free nuclear development. In addition, it is hypothesized that a potential mechanism of sterility may be an inability of megasporocytes to undergo meiosis. Therefore, analyses of DNA/ploidy levels of female gametophyte structures will be discussed. Knowledge of the reproductive development of male ovules in Gnetum will likely increase our understanding of the evolution of bisexuality in seed plants and functional dioecy in Gnetales.

Key words: bisexual, gametophyte, Gnetales, Gnetum