Ultrastructural aspects of pollen ontogeny in an endangered plant species, Pancratium maritimum L. (Amaryllidaceae).

Pollen ontogeny in Pancratium maritimum L. was studied from the sporogenous cell to mature pollen grain stages using transmission electron, scanning electron, and light microscopy to determine whether the pollen development in P. maritimum follows the basic scheme in angiosperms or not. In the course of microsporogenesis and microgametogenesis, special attention was given to the considerable ultrastructural changes that are observed in the cytoplasm of microsporocytes, microspores, and mature pollen grains throughout the successive stages of pollen development. Microsporocyte differentiation concerning number and ultrastructure of organelles facilitates the transition of microsporocytes from the sporophytic phase to the gametophytic phase. However, cytoplasmic differentiation of generative and vegetative cells supports their functional distinctness and pollen maturation. Although microsporogenesis and microgametogenesis in P. maritimum generally follow the usual angiosperm pattern, abnormalities such as formation of unreduced gametes were observed. During normal microsporogenesis, meiocytes undergo meiosis and successive cytokinesis, resulting in the formation of isobilateral, decussate, and linear tetrads. Subsequent to the development of free and vacuolated microspores, the first mitotic division occurs and bicellular monosulcate pollen grains are produced. Pollen grains are shed from the anther at binucleate stage. During pollen ontogeny, three periods of vacuolization were observed: in meiocytes, in mononucleate free microspores, and in the generative cell.