Gametic Fusion in Plumbago

Abstract

In flowering plants, fertilization may be separated into three specific events: gametic delivery, gametic fusion, and finally, the fusion of gametic nuclei. Angiosperms differ from other organisms in that two sperm are required for normal embryo development: one is required for fusion with the egg to form the embryo and a second for fusion with the surrounding central cell to form the nutritive endosperm. Although behavior of pollen tubes in delivering male gametes is known in detail (Jensen 1974, Russell 1982), the behavior and fate of the sperm cell in this process has been seriously neglected. The arrival of the sperm cells results from discharge of the pollen tube within the female gametophyte. This coincides with removal of the inner pollen tube plasma membrane from the sperm and exposes the sperm cell surface within the female gametophyte. Exposure of the male gametic surface prior to fusion may represent part of a capacitation phase necessary for ensuing membrane-based fusion events. At the same time the future site of gametic fusion is modified in the female reproductive cells; certain cell wall components in the egg are correspondingly removed, leaving female gametic cell membranes also directly exposed. Male gametes are deposited between the female reproductive cells, with speim membranes touching both the egg and central cell. Gametic fusion is a cellular fusion event, occurring near the site of sperm deposition (Figure 1). The participation of sperm cytoplasm is evidenced by the presence of transmitted sperm organelles in both the egg and central cell (Figure 1). Both sperm plastids and mitochondria may be distinguished by differences in structure, shape, and size from