非洲狼尾草无融合生殖胚囊显微结构特点_英文_

非洲狼尾草无融合生殖胚囊显微结构特点_英文_ Microstructural Aspects of Mature Embryo Sac Derived from Nucellus Cells in Pe nniset um Squa mulat um L IU Lin ()Department of Biology and Agroforestry ,Linyi Teachers’College , Linyi Shandong 276005 Abstract :Microstructural study was made to characterize mature apomictic embryo sac in pennisetum squamulatum Freson. Often several apomeiotic embryo sacs occur in a single nucellus. The egg cell in the mature embryo sac derived from the ordinary nucellar cell is highly vacuolized and contains numerous small vacuoles. Most aomeiotic egg cells reveal morphological polarity , but their nucleus does not always show uneven distribution , mostly located centrally. The polar nucleus in central cell was found almost always in connection with the apomeiotic cell . The synergid becomes degenerated soon after the inflorescence opens. There are several nucellar cells in micropylar region that develop into embellum. Key words : Pennisetum squamulatum , apomeiotic embryo sac ,egg cell ,synergid ,embellum () Article ID :1009 - 6051 200106 - 0052 - 04 CLC number :Q945. 51 Document code :A 1 - 3Pennisetum squamulatum , a number of grasses ,was repeatedly proved to be an obligate apospory species . The sexual tissue , including macrospore mother cells , dyads , and triads , degenerate during their various developmental stages ,and no functional macrospores persist and none develop s into meiotic embryo sacs. During the degeneration of the sexual tissue some of their neighbors ,the ordinary nucellar cells ,become specialized to function as macrospores by enlarging rapidly and vacuolizing prominently. These cells are referred to as aposporous embryo sac initials. The initial undergoes two nucleate divisions to become a mature apomeiotic embryo sac . The mature apomeiotic embryo sac contains four nuclei ,one belonging to egg cell ,another to synergid ,the other two to the central cell . In spite of the rich information on the developmental mechanism of apomixes in this plant ,little is known of the aspects of the egg cell in mature apomeiotic embryo sac . In this study there is a clear attempt to characterize the members of the mature embryo sacs. 1 Material and Methods Pennisetum squamulatum Fresen was grown in the experimental farm of South China Institute of Botany , the Chinese Academy of Sciences. Ovaries were isolated from flowers at different developmental stages , and then were placed in 2 % ( ) gluteraldehyde at p H 6 . 8 0 . 1 M phosphate bufferfor 12 h at room temperature . After washing in buffer , the ovaries were post2fixed in 1 % OsO4 for 12 h at 4oC. Following dehydration in a graded series of alcohol the material was embedded in a mixture of Epon2812 . μSections 1 —3 m thick were made with glass knives , and stained with TBO. Observations and photographs 收稿日期 :2001210215 () 作者简介 :刘林 1963 — ,男 ,山东费县人 ,临沂师范学院副教授 ,理学博士 ,主要研究方向 :植物生殖生物学. were progressed on an Olympus light microscope . 2 Observations ( ) Most nucullus contain two or three apomeiotic embryo sacs Plate I ,1 —2 ; II ,2. In all situations only the most micropylar one becomes mature when the flowers open. In the mature embryo sac can egg cell , synergid ,and ( ) μcentral cell be found Plate I ,1. The egg cell in a mature embryo sac is typically pear2shaped of a size about 50m μ( ) ×70 m and shows a great extent of vacuolation Plate I , 3. Only a very limited region of the egg cell is in contact with the embryo sac boundary , and most of the cell is surrounded with the synergids and the central cell ( ) ( ) Plate I ,1 —4. In some egg cells vacuoles are large and little cytoplasm was detected Plate I ,1. Majority of the ( ) egg cells are filled with small vacuoles ,cytoplasm is press in between the vacuoles Plate I ,2 —4 ; Plate II ,1 —4. ( ) The nucleus is mainly located in the center of the egg cell Plate I , 1 —4, but sometimes located micropylarly ( ) ( ) Plate II ,1or chalazally Plate II ,3 —4. The nucleus of the egg cell shows two domains ,the nucleolus ,showing ( a very positive reaction to TBO ,and the nuclear substance ,less positive to the dye than the nucleolus Plate I ,1 — ) 4 ; Plate II ,1 —4. Plate I 1. Two unreduced embryo sacs ,the micropylar one containing Plate II 1. Showing an egg cell and a a synergid , an egg cell ,and a central cell with a secondary nucleus. The ×200 ; degenerated synergid on its ventral side . synergid located on the dorsal side of the egg cell . ×300 ; 2. Three 2. Two unreduced embryo sacs. The secondary unreduced embryo sacs ,an egg cell recognizable in the most micropylar polar nucleus located on the ventral side of egg ( ) one . Two elongated cells can be seen in miroplar region arrow. × cell . ×100 ; 3. Showing the egg cell in embryo sac and an elongated cell in miropylar region. × 150 ; 3. Showing an egg cell with a secondary polar nucleus on its dorsal ( ) side. A elongated cell in the micropylar region arrow. ×120 ; 4. 120 ; 4. Showing the egg cell in embryo sac and an ( Showing an egg cell with secondary polar nucleus lying on its dorsal elongated cell in miropylar region. ×190 E egg () side . A elongated cell recognizable in miropylar region arrow. ×180 )cell ,S synergid ,PN polar nucleus ( )E egg cell ,S synergid ,PN polar nucleus The central cell is a highly vacuolized cell ,the most remarkable among its constituents being the nucleus and the large vacuole ,the later taking up almost all the cell . The polar nuclei fused into one secondary nucleus ,and the process is very short . The secondary nucleus is always in connection with the egg cell . The secondary nucleus was ( ) ( found mainly on the dorsal side of the egg cell Plate I ,3 ,4,or on the opposite ,the ventral side Plate I ,1 ; Plate ) II ,2. Mostly there is only one synergid in an embryo sac . The synergid is located on the dorsal side of the egg cell ( ) ( ) Plate I ,1or on the ventral side Plate II ,1. It is a short life cell ,and becomes collap sed no soon after the flower ( ) blooms. The degenerated cell is very positive to TBO and strongly OsO42phil Plate II ,1. ( ) There are some elongated nucellar cells in the micropylar region Plate I ,2 —4 ; II ,3 —4. These specialized nucellar cells form embellum. 3 Discussion Reduced embryo sacs ,come from macrospore mother cells by meiosis. The egg cell combines with a sperm to develop into a zygote . This is sexual reproduction. The most typical embryo sac derived from macrospore mother cell contains 8 nuclei or 7 cells. And there is only one embryo sac in a single nucellus. In Pennisetum squamulatum , however ,the embryo sacs are derived from the ordinary nucellar cells with mitosis instead of meiosis. Such embryo sacs are unreduced ones ,called apomeiotic embryo sacs. And usually several embryo sacs develop simultaneously. This is a useful criterion to recognize apomixes. The egg cells in these sacs translate into embryos without the partition of the sperm ,and this process is called parthenogenesis ,or apomixis. The embryo sac shares only 4 nuclei , 2 and usually gives birth to an egg cell ,a synergid ,and a central cell with 2 polar nuclei,this organization of embryo 4 sac following Panicum type. In Pennisetum squamulatum ,only a limited region of the egg cell is in contact with the embryo sac boundary , and most of the cell is surrounded with the synergids and the central cell . In fact , the egg cell is buried in the central cell . This arrangement reveals that synergid and central cell plays a great role in the future development of unreduced egg cell . Undoubtedly they offer a support of nutrients. In most species the egg cell is highly vacuolated. The micropylar two2thirds of the cell is almost completely filled with a single large vacuole ,while the cytoplasm is restricted to a thin layer along the plasma membrane . The chalazal one2third is filled with cytoplasm containing the egg nucleus and , sometimes , some small additional 5 vacuoles. In stipa elmeri the egg cell is also vacuolated ,but there are many small vacuoles randomly distributed in 6 () the cytoplasm. Such situation was found in Pennisetum squqmulatum Freson present study, Coix lacryma2jobi , [7 - 8 ] [9 ] [10 ] 11 12 - 13 barley Hordeum vulgare , Zea mays , Triticum , Oryza , and Arundo. This condition is characteristic of the mature eggs of many grasses. Perhap s it is a safe morphological feature to distinguish a grass from other angiosperms. In most species and embryo types the egg cell shows a similar organization , characterized by distinct 14 polarity. The establishment of cytoplasmic polarity along the micropylar2chalazal axis of embryo sac is also a general characteristic of ovule development in angiosperms. This polarity can be reconvized by certain cytological characters such as nuclear position , organelle distribution , distribution of plasmodesmata , spindle orientation , and callose deposition. As in other angiosperms ,in Pennisetum squamulatumthe egg apparatus locating in the micropylar end and the chalaza end absent of antipodal cells forms the polarity of the ovule . Typically ,in most angiosperms the nucleus is more chalazal in the egg and more micropylar in the synergids , whereas the large vacuoles are more micropylar in the egg and more chalazal in the synergids. The relative position of the nucleus and vacuoles is 15 established during the primary expansion of egg apparatus. In Pennisetum squamulatum , however ,the egg cell establishes the polarity only by the shape and the connection with other cells. Unlike in general plants ,the nucleus in this species is mainly located in center ,with small vacuoles around it . Strikingly ,in some cells the nucleus lies in a opposite site ,the micropylar end. References : 1 L IU Lin. Ultrstructural study on aposporous embryo sac and embellum in Pennisetum squamulatum . Dissertation for Doctoral Degree , 2000 ,South China Institute of Botany ,the Chinese Academy of Sciences , Guangzhou. 2 WEN Xin 2shan , YE Xiu2lin ,L I Yuan2qing ,et al . Embryological studies on apomixes in Pennisetum squamulatum J . Acta Botanica () Sinica ,1998 ,40 7: 598 —604. 3 Dujardin A K , Hanna W. microsporogenesis , reproductive behavior and fertility in five Pennisetum squamulatum J . Theor Appl Genet ,1984 ,67 : 197 —210. ( ) 4 MA San 2Mei . Facultative Apomixis in Apluda mutica Gramineae. Dissertation for Doctoral Degree ,2000 ,South China Institute of Botany ,the Chinese Academy of Sciences , Guangzhou. 5 Maze J ,Lin Sch. A study of the mature megagametophyte of Stipa elmeri J . Can J Bot ,1975 ,55 ; 1768 —1782. 6 DONG Jian 2Hua ,XI Xiang2Yuan. Mcirosporogenesis ,megasporogenesis and the formation of male and female gametophyte in Coix () lacryma2jobi L . acta Botanica Sinica ,1992 ,3412: 925 —930. () 7 Cass D D ,J ensen W A. Fertilization in barley. Am J Bot ,1970 ,57 1: 62 —70. () 8 Mogensen H L . Quantitative observations on the pattern of synergid degeneration in barley. Am J Bot ,1984 ,71 10: 1448 —1451. 9 Diboll A G ,Larson D A. An electron microscopic study of the mature megagametophyte in Zea mays J . Am J Bot , 1966 , 53 : 391 —402. ( ) 10 You R ,J ensen W A. Ultrastructure observations of the mature megagametophyte and the fertilization in wheat Triticum aestivum. Can J Bot ,1985 ,63 : 163 —178. () 11 Maeda E ,Maeda K. Ultrastructure of egg apparatus of rice Oryza sativafter anthesisJ . Jpn J Crop Sci ,1990 ,59 : 179 —197. 12 J ane W N. The ultrastructure of the embryo sac before fertilization in Arundo f ormosana Hack. Taiwania ,1992 ,37 : 85 —103. () 13 J ane W N. Ultrastructure of the maturing egg apparatus in Arundo f ormosana Hack Poaceae J . Int J Plant Sci ,1997 ,158 6: 713 —726. 14 Willemse M T M ,Went J L V. The female gametophyte . In Jori B M ,ed. Embryology of angiosperms. 1984 ,Berlin : Springer 2 Verlag ,158 —196. 15 Wilms H J . Ultrastructure of the developing embryo sac of spinachJ . Acta Bot Neerl ,1981 ,30 : 75 —99. 非洲狼尾草无融合生殖胚囊显微结构特点 刘 林 ()临沂师范学院 生物与农林科学系 ,山东 临沂 276005 ( ) 摘 要 :对非洲狼尾草 pennisetum squamulatum Freson 无融合生殖胚囊做了显微结构观察. 一个珠 心中常有几个无融合生殖胚囊 ,卵细胞高度液泡化 ,含大量小液泡 ,细胞呈现极性 ,但多数情况下细胞核 位于中心位置 ,很少显示不均匀分布. 中央细胞的极核总是与卵细胞相连接. 珠孔区域有伸长的珠心细 胞 . 关键词 :非洲狼尾草 ; 无融合生殖胚囊 ; 卵细胞 ; 助细胞 ; 胚珠附器 责任编辑 :袁兆岭