Male cone morphogenesis, pollen development and pollen dispersal mechanism in Ginkgo biloba L.

Abstract

Ginkgo biloba L. is one of the oldest gymnosperms. Male cone morphogenesis, pollen development and dispersal are important for successful pollination and reproduction. In this study, we investigated the development of male cone, pollen and the sporangial wall in detail. The results indicate that: (1) The primordia of male cones and leaves begin to differentiate in early June and remain open until the following March. The male cones then mature and release pollen in mid-April. The male cones are drooped and approximately perpendicular to the leaves during pollination. (2) The microsporocytes develop from the sporogenous cell and form a tetrahedral tetrad after two simultaneous asymmetrically meioses, then produce a matured four-cell pollen after three polar mitotic divisions. The matured pollen is hemispheric in shape with a large aperture area and three pollen wall layers; once released from the microsporangia, the pollen becomes boat-like in shape. (3) The sporangial walls are eusporangiate and consist of epidermis, endothecium and tapetum. The differentiation of the tapetum occurs separately from that of the epidermis and endothecium, and originates from the outermost layer of sporogenous cells. The sporangial walls exhibit shrinkage of the epidermis, fibrous thickening of the endothecium, and enzymic dissolution of the tapetum during pollen dispersal, which contributes to microsporangia opening. Based on these results, we conclude that there many unique and primitive characteristics of the development of the male cones, pollen and sporangial wall of G. biloba. In addition, we also found that the male cones, pollen and sporangial walls have evolved efficient structural and morphological adaptations to anemophily.