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Megasporogenesis by Anurag yadav

  3. MEGASPOROGENESIS • Usually a single hypodermal initial known as primary archesporial cell is differentiated at the apex of the nucellus. • The pri archesporial cell divides periclinally into outer primary parietal cell or pri wall cell and inner pri sporogenous cell. • The pri parietal cell may or may not divide.The pri sporogenous cell directly behave as megaspore mother cell. • The MMC undergoes meiotic division to form 4 megaspores. • The 4 megaspores thus formed and are arranged in an axial row forming a linear tetrad
  4. • Usually only one megaspore of the tetrad is functional and grows at the expense of the other three, which degenerate. The functional megaspore enlarges and forms the embryo sac.
  6. INTRODUCTION • The female gametophyte ,also called embryo sac ,is mostly a 7-celled structure . • There is a large central cell with two polar nuclei which later fuse to form the secondary nucleus. The micropylar end of the central cell is occupied by the egg apparatus, comprising an egg cell and two synergids and at its chalazal end three antipodal cells are present
  7. SYNERGID • The synergids are elongated cells present at micropylar end of the embryo sac . • When two synergids are present they lie in contact with each other,and partly embrace the egg. They are pointed or hooked towards the micropyle. • The wall around the synergids is incomplete. there is a distinct wall around the micropylar one-third of the cell which thin towards the chalazal end and finally ,disappears. • The cytoplasm of the synergids is strongly polarized.the chalazal region of the cell is occupied by one large or many small vacuoles.
  8. • Synergids are ephemeral structures.In embryo sacs with two synergids, one degenerates before the entry of the poller tube into the embryo sac,whereas the other one ,often called the persistent synergid,degenerates shortly after the embryo sac has received the pollen tube discharge. • In Cotula australis one of the synergids become swollen and haustorial in nature.synergids haustoria also occur in Sedum sempervivoides.(Crassulaceae). • Extensive synergids haustoria also been reported in Cortaderia selloana a member of the poaceae. • The haustoria arise as finger-like projections from the filiform apparatus and penetrate the
  9. FUNCTION OF SYNERGID • (1) They play an important role in directing the pollen tube growth by secreting some chemotropically active substance. • (2) The degenerating synergids forms the seat for pollen tube discharge in the embryo sac. • (3) Jensen (1965)suggested that the filiform apparatus may be aiding the synergids in the absoption and transportation of materials into the embryo sac from the nucellus. • Although much has been said about the structure and function of synergids ,yet several qestions remain unanswered,such as the nature of metabolic activity of synergids ,their differential behaviour,entrance of pollen tube through FA,and their role in opening of pollen tube
  10. FILIFORM APPARATUS • A prominent structure ,called filiform apparatus is present at the micropylar end of each synergid. • Its differentiation from the synergid wall was described as early as 1906 by Habermann. • The filiform apparatus is a mass of finger-like projections of the wall into the cytoplasm . • Structurally ,each projection of the FA has a core of tightly packed microfibrils(possibly cellulosic) enclosed by a non- fibriller sheath. • They are rich in polysaccharides. • The form of FA IS may be spherical as in Torenia or wedge- shaped between the tips of synergids as in Petunia and helianthus, in Jasione, the FA extends irregularly along the entire length of the synergids
  11. EGG APPRATUS • The three cells of the egg apparatus (an egg and two synergids)are arranged in a triangular fashion .the egg shows common walls with the two synergids and the central cell. • It has a wall similar in structure and extend to that of synergids.the wall is thicker in the micropylar region but become thinner towards the chalazal side. • It is absent at the chalazal end in cotton ,maize, torenia. • At the micropylar end the latteral walls of the egg cell appear to join the central cell wall .actually the end wall of the egg cell is the wall of the central cell.
  12. • The egg cell becomes highly polarized early in its development. The micropylar end of the cell is occupied by a large vacuole. thus the distribution of the vacuole and cytoplasm in the egg cell is just opposite of that in the synergids. • The egg may contain small or large amounts of starch ,that is consumed during fertilization or early embryogeny. • The egg cytoplasm is rich in ribosomes. The egg of Nicotiana rustica contains two strongly RNA- positive bodies that multiply and persist till the 4-celled stage of proembryo.
  13. ANTIPODALS • Antipodal cells are present at the chalazal end. • The antipodals exhibit the greatest variation amongst all the cells of the embryo sac.useally they degenerate before or soon after fertilization ,without any appreciable enlargement. • In many plants the antipodals are persistent and show some structural and cytological features suggesting their possible role in the nutrition of the embryo sac. • In Caltha palustris they persist up to the octant stage of the proembryo. • In grasses they undergo a series of mitotic divisions leading to the formation of a large number of antipodal cells. • The highest number of antipodal cells known is 300 in Sasa paniculata. Zea mays has about 20 antipodal cells ,each with 1-4 nuclei. Multinucleate condition of antipodals cells also occurs in Stackhousia, Tagetes
  14. • The antipodals are rich in ascorbic acid ,oxidases,sulfhydril compounds. Starch ,lipids and proteins also occur in aboundance. However they show very low concentrations of RNA and polysaccharides. • Haustorial behaviour of antipodal cells is known in many plant • (Grindelia, Haplopappus, Quinchamalium
  15. FUNCTION • (1) Antipodals are nutritive in function it nourishes the embryo sac. • (2) The antipodal cells may also store large quantities of starch , lipids, and proteins which are utilized by the developing endosperm and embryo. • (3) Antipodal cells is to produce and secerte substances that control the growth and development of endosperm
  16. CENTRAL CELL • It is the largest cell of the embryo sac, and the mother cell of the endosperm. • The enlargement of the embryo sac after the last nuclear division is largely due to the inflation of the large central vacuole of the central cell.the vacuole in the central cell is the reservoir of sugars, amino acids,and inorganic salts. • The nuclei of the central cell, called polar nuclei, are very large,and each possesses a conspicuous nucleolus. • The two polar nuclei fuse,before or during double fertilization,to form the secondary nucleus
  17. • Unlike the egg cell the cytoplasm of the central cell is rich in all cell organelles and appears to be the centre of intense synthetic activity. There are plastid containing starch sometimes proteins and phytoferretin. • In Capsella the central cell possesses numerous sphaerosomes associated with glyoxysomes that probably convert fat into sugar. • There are numerous ribosomes usually present as single ribosomes or small polysomes. • The wall of the central cell is highly variable from one part of the embryo sac to the other. The central cell is connected with egg, synergids, and the antipodals through plasmodesmatal connections.
  18. FUNCTION • The presence of cell wall projections in the micropylar or chalazal region shows that central cell draws nutrition from the surrounding nucellus or integument. • The haustorial extensions with parietal prolongation enter the ovular tissue. In addition to absorption,these extensions secrete enzymes that help in the digestion of the nucellar tissue.(rifot,1973).
  19. EMBRYO SAC MAJOR TYPES Depending on the number of megaspore nuclei taking part in the development, the female gametophytes of angiosperms may be classified into three main types MONOSPORIC BISPORIC TETRASPORIC
  20. MONOSPORIC EMBRYO SAC • The chief charecterstic of a monosporic embryo sac is that derived from only one of the four megaspores, as in polygonum type. • All the nuclei in such an embryo sac are genetically identical because they are derived through mitossis of a single nucleus . • There are two type of monosporic embryo sac Polygonum type Oenothera type
  21. 1 .POLYGONUM TYPE • The embryo sac is formed by the chalazal megaspore of the tetrade and eight nucleate. • The mature embryo sac comprises a 3-celled egg appratus, three antipodal cell, and a binucleate central central cell.
  22. 2. OENOTHERA TYPE • Geerts, in 1906, found that in Oenothera lamarckiana the embryo sac is usually formed by micropylar megaspore of the tetrad, which undergoes only two nuclear divisions instead of the usual three occuring in Polygonum type of embryo sac. In this way 4 nuclei are produced which organize into the two synergids, the egg and a single polar nucleus
  23. • Since the third division is omitted and all the nuclei are situated in the micropylar part of the embryo sac, there is neither a lower polar nucleus nor any antipodal cells. • Also Found in Epilobium, Circaea and others with few exception
  24. BISPORIC EMBRYO SAC • The bisporic embryo sac developed from one of the two dyads formed as a result of first meiotic division or meosis one of megaspore mother cell one of two dyads degnerate both nuclei of the function of dyad take part information of embryo sac withaout wall formation each nucleus undergoes two mitotic division so the mature embryo sac 8 nucleate the 8 nuclei are arranged in to antipodal egg apparatus and polar nuclei as in polygonal type of embryo sac bisporic type embryo sac are two type- 1.Allium type 2. Endymion type
  25. 1. ALLIUM TYPE • The embryo sac is derived from the chalazal dyad cell.
  26. 2. ENDYMION TYPE • The embryo sac is formed by the micropylor dyad cell. • Also described in Agraphis patula, and appears to be quite characteristics of certain families, Podostemonaceae, Butomaceae( Butomus). Alimaceae and the tribe Viscoideae of the Loranthaceae. And also found in several members of Lileaceae, Balanophoraceae, Amaryllidaceae and Orchidaceae.
  27. TETRASPORIC EMBRYO SAC • The meotic divison of the megaspore mother cell is not a complete by wall formation (cytokinesis) hence all the 4 nuclei like in a single cell called coenomegaspore all the 4 nuclei of coenomegaspore particepate in the formation of embryo sac is genetically more hetrogenious then the tetrasporic embryo sac are of many type . • Under two categories-  Where no nucleare fusion occure  Where nuclear fusion occure
  28. WHERE NO NUCLEAR FUSION OCCURE 1. Adoxa type - The embryo sac is eight nucleate ,formed after a single post –meiotic meosis ,and its organisation is similor to the Polygonum type. 2. Plumbago type- In this type only one post –meiotic mitosis occure , but the organization of the mature embryo sac is very defferent from that in Adoxa type.The mature embryo sac comprises an egg cell and a four nucleate central cell .the other three nuclei are cut-off as peripharal cell. 3. Penaea type – As a result of two post meiotic mitosis in coenomegaspore 16 nuclei are formed . The mature embryo sac comprises 4 group of 3 cell each, one group is at the micropylor end , one at the chalazal end and two arreanged laterally . The remaining four nuclei behave as polar . The micropylor trid usually function as the egg apparatus.
  29. 4. Drusa type- This type of embryo sac is also 16 nucleate . The mature embryo sac comprises a normal egg appratus ,two polar nuclei and 11 antipodal cells.
  30. WHERE NUCLEAR FUSION OCCURE After the second meotic divison 3 megaspore nuclei are of fast fuse to nucleus of the chalazal and 4 nucleus and micropylor end remains haploid  Fritilaria type  Plumbagella type 1.Fritilaria type- After the nuclear fusion both the triploid and haploid nuclei divide twice forming 4 nuclei each pole .the mature embryo sac consist of an egg appratus and a central cell one is haploid and other is triploid. 2. Plumbagella type- After nuclear fusion the triploid nucleus at the chalazal end and haploid nucleus of micropylor end undergoes a single meiotic division to form two group and and 2 nuclei. 2 nuclei each 2 diploid and
  31. 2 haploid nuclei the embryo sac conssist of a haploid egg. A triploid antipodal and one haploid and one triploid .rarely plant species may show more than one kind of embryo sac in Delosperma coopri the 3 kinds mono, bi, and tetrasporic type have been regarded the tempereture is also known to influenc the embryo sac development most angiosperm as 8 or 7 celled embryo sac synergid are not present in Plumbago and Plumbagella type no antipodal are present.