水稻PDER品系的特点和二倍体孢子生殖胚囊的形成和发育

对水稻（ＯｒｙｚａｓａｔｉｖａＬ．）早发生胚ＰＤＥＲ（ｐｒｅ－ｄｅｖｅｌｏｐｅｄｅｍｂｒｙｏｏｆｒｉｃｅ）品系的特点和细胞胚胎学研究表明，ＰＤＥＲ是二倍体植物２ｎ＝２４，约有５０％胚囊的卵细胞未经受精能自行发育形成胚，成熟种子的萌发和生长速度较常规正常水稻快。ＰＤＥＲ的大孢子母细胞经有丝分裂产生未减数的胚囊，即无融合生殖中的二倍体孢子生殖类型。在胚囊形成和发育过程中有如下几个特点：（１）孢原细胞至大孢子母细胞分裂前的过渡期持续时间较长，孢原细胞和大孢子母细胞的细胞质比周围的珠心细胞质稀淡。（２）大孢子母细胞经二次有丝分裂后形成直线排列的三个细胞（三分体），珠孔端的两个解体，合点端的一个发育为功能细胞，有少数胚囊的三个细胞全部解体形成败育胚囊。（３）功能细胞经三次连续核分裂形成具八核七个细胞的成熟胚囊，它的结构与常规正常水稻基本相同，但助细胞呈长形而没有回抱着卵细胞。

THE CHARACTERISTICS OF APOMICTIC PDER RICE VARIETY AND THE DEVELOPMENT OF DIPLOID EMBRYO SAC

Studies on the formation of the pre-developed embryo of rice(Oryza sativa L.)rariety(PDER)indicate that PDER is diploid,with chromosome number 2n=24. About 50% of the embryo sacs of this variety can give rise to embryo from unfertilized egg cells.The seeds so formed show higher germination rate and produce more vigorous plants.Some of the megasporocytes do not undergo meiosis but divide mitotically to give rise to diploid embryo sacs indicating that PDER is an apomictic type of plant showing diploid pargthenogenesis(diplospory).The apomictic embryo sacs have the following characteristics:1.The transition period from the archesporial cell to the megaspore mother cell is much prolonged.Thecytoplasm of the archesporial cell and the megaspore mother cell appears less dense in comparison with that of the surrounding nucellus cells.2.After the second mitotic division the megasporocytes give rise to three uniserially arranged cells.Two cells near the micropylar end degenerate.The cell near the chalazal end becomes the functional megaspore.In some embryo sacs all three cells degenerate resulting in the formation of degenerated embryo sacs.3.The functional megaspore divides three more times(mitotically)to give rise to an 8nucleate embryo sac.The 8-nucleate embryo sac produces seven cells similar to those seen in normal(i.e.non-apomictic) embryo sacs.The morphology of the synergids is,however,slightly different.In the apomictic embryo sacs the synergids appear much elongated and they do not tightly surround the egg cell.