Apomixis in plants: structural and functional aspects of diplospory in Poanemoralis and P-palustris

Data on structural and functional aspects of mitotic diplospory and later s tages of apomictic seed formation are reported for Poa palustris and P. nem oralis. In this study, the plant material of two Russian populations of P. nemoralis and P. palustris were used for transmission electron microscope o bservations. Seed formation was investigated by phase contrast microscopy i n two populations of P. nemoralis collected in The Netherlands. The process es of transformation of the megasporocytes to the megaspore mother cells of diplosporous embryo sacs, and thereafter to one- and two-nucleate diplospo rous embryo sacs (Antennaria type) were characterized by an increase of cel l size, structural and functional reorganization of the nucleus, nucleolus, and cytoplasm, and cell isolation as a result of thickening of the cell wa ll. These were accompanied by an increase in the cell metabolic activity in ferred from visual evidence of the activation of nucleus, nucleolus, endopl asmic reticulum, dictyosomes, mitochondria, and from the appearance of a de nse population of ribosomes and polysomes. The diplosporous embryo sac of t he Antennaria type was characteristic for both P. nemoralis and P. palustri s. No signs of the presence of synaptonemal complexes were observed during the process of diplosporous-embryo-sac megaspore mother cell differentiatio n and division, About 90-95% of the diploid egg cells of diplosporous embry o sacs were able to produce apomictic embryos. These embryos developed befo re anthesis. However, many of them degenerated at the globular stage becaus e of lack of endosperm. The ultrastructural events occurring during the pro cess of diplospory of apomictic species, and meiosis and megagametogenesis of sexually reproduced plants are discussed.