Abstract:

【Objective】This study explored the ultrastructural changes of female gametophyte during its development in the apomictic monosomic addition line M14 of Beta corolliflora in sugar beet. 【Method】 The ultrastructural changes were investigated by electron microscopy. 【Result】 The monosomic addition line M14 is a facultative apomict. The diplosporous female gametophytes are of Antennaria type and Allium odorum type sexuality is of Polygonum type. Antennaria type is the main one and its ultrastructural characteristics are that the development of female gametophyte was rapid, from the functional megaspore until the cellularized embryo sac, the category and amounts of organelles present the following growth trend, i.e. nucleolus which are more big had nucleolus vacuoles, nuclear pores are obvious; there are many ribosome; also a lot of mitochondria in the development of female gametophyte, many tubes appear from the two-nucleate embryo sac, but transform into electron-transparent in eight-nucleate embryo sac, then recovered as former. The number of plastids was stable and their shapes were varied, some of them contained starch grains. The endoplasmic reticulum had many branches or were interwove as ropes near nucleus, vacuoles and cell walls. Dictyosomes were relatively rare in the development of female gametophyte, but abundant dictyosome which often associated with many small electron-transparent vesicles was found in the persistent synergid. Some lipid drops were always present near the vacuoles and mitochondrias. Late period of cellularized megagametophyte, two synergids successively degenerated in the great majority of embryo sacs, very few had only one degenerated synergid, with another persistent. Compared with Antennaria type embryo sac, the development of Allium odorum type and Polygonum type were slowly, there were no obvious changes of category and amounts of organelles until the period of one-nucleate embryo sac. Whether had degenerated cell and deposition of callose during the development of functional megaspores and one-nucleate embryo sacs were the characters distinguishing the Antennaria-type gametophyte of female and Allium odorum-type.【Conclusion】 There were distinct characteristics during the development of Antennaria-type, Allium odorum type and Polygonum type gametophyte. From the functional megaspore until the cellularized embryo sac, Antennaria type gametophyte present high rate of metabolic activity, its volume augmented increased with category and amount of organelles. It is supposed that Antennaria type gametophyte dominated the afterward growth. Whether had degenerated cell and deposition of calluses before the development of one-nucleate embryo sacs were the characters distinguishing the female gametophyte of Polygonum type, Antennaria type or Allium odorum type. From functional megaspores to one-nucleate embryo sacs, volume and cytoplasmic mature degree of Allium odorum type and Antennaria type embryo sac present slow-moving growth trend. It is concluded that most sexual female gametophyte degenerated in later developmental stages.

Key words: monosomic addition lines of apomictic sugar beet, facultative apomixes, Allium odorum type embryo sac, Antennaria type embryo sac, Polygonum type embryo sac, gametophyte, ultrastructure