Development of female gametophyte

B.Sc.II Sem III
Unit VI
Development of female gametophyte

1. B.Sc.- II, Sem- III Unit – VI Embryology of Angiosperm Development of Female Gametophyte Dr. Swati Pundkar Assistant Professor Departmnet of Botany Shri Shivaji Science College, Amravati

2. Pistil - ovule bearing or seed bearing female reproductive organ of a flower is called the pistil or gynoecium. The gynoecium represents the female reproductive part of the flower. The gynoecium may consist of a single carpel (monocarpellary, e.g. Pea) or may have more than one carpel ( multicarpellary - number of carpels in a gynoecium are 2, 3, 4, 5, they are said to be bi, tri, tetra and penta carpellary , respectively).When there are more than one carpel, the pistils may be fused together (syncarpous)or may be free (apocarpous) Carpel -The pistils of a flower are considered to be composed of carpels. A carpel is a theoretical construct interpreted as modified leaves bearing structures called ovules, inside which the egg cells ultimately form. A pistil may consist of one carpel, with its ovary, style and stigma, or several carpels may be joined together with a single ovary, the whole unit called a pistil. The gynoecium may consist of one or more uni -carpellate (with one carpel) pistils, or of one multicarpellate pistil. GYNOECIUM

3. MEGASPOROGIUM AND MEGASPOROGENESIS Megasporogenesis  refers to the development of megaspores from the megasporocyte , the cell that undergoes meiosis. Meiosis of the megasporocyte nucleus results in the formation of four haploid megaspore nuclei. In most taxa, meiosis is followed by cytokinesis, resulting in four megaspore cells.

4. STRUCTURE OF MATURE OVULE

5. The ovule is part of the makeup of the female reproductive  organ  in seed plants. It’s the place where female reproductive cells are made and contained, and it is what eventually develops into a seed after  fertilization , only for the seed to then ripen and produce a complete adult  plant . Components of Ovules I nteguments Micropyle- Nucellus Hypostase and epistase Obturator

6. Structure of ovule (Megasporangium) : Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum . Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe . The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments . These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus . In the centre of the nucellus is situated a female gametophyte known as embryo sac .

7. Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons. Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons. Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc. Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of Euphorbiaceae . e.g., Castor ( Ricinus ). Ategmic : In some parasites like Loranthus, Viscum, Santalum etc., there is no integument. Such an ovule is called ategmic .   INTEGUMENTS The integument is the tough outer protective layer of the ovule. TYPES OF INTEGUMENTS

8. Micropyle The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the micropyle. The micropyle opening allows the pollen (a male gametophyte) to enter the ovule for fertilization. In angiosperms, only a pollen tube enters the micropyle. During germination, the seedling's radicle emerges through the micropyle. When both the integuments are involved the passage formed by outer integument is called exostome and by the inner integument is called as endostome . Nucellus is part of the inner structure of the ovule, forming a layer of diploid ( sporophytic ) cells immediately inside the integuments. It is structurally and functionally equivalent to the megasporangium. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes sporogenesis via meiosis. When the nucellus is massive and sporogenous cell is deeply embedded in it then such ovule is called as Crassinucellate . Ex.- Polypetalae and Monocotyledons In Gamopetalae tha nucellus is single layered such ovule is called as tenuinucellate. The nucellus is used as nutrition by the embryo sac or endosperm and is consumed almost completely by the time the endosperm matures.

9. Hypostase and epistase Hypostase refers to group of cells present below theembryo sac and above the vascular supply to the funiculus. The modified cells ofthe nucellus present at the top of the embryo sac are called as epistase . Obturator Some uni or multicellualr hairs are present in the basal part of theovule collectively form the obturator. It probably guides the pollen tube towardsmicropyle . Chalaza Located opposite from the micropyle is the chalaza where the nucellus is joined to the integuments. Nutrients from the plant travel through the phloem of the vascular system to the funiculus and outer integument and from there apoplastically and symplastically through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening.

10. TYPES OF OVULES

11. Depending upon the shape and orientation, the ovules of angiosperms are classified into following types : Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight line. This is most primitive type of ovules. e.g., Betel, Piper, Polygonum . Anatropous : The body of the ovule is completely inverted (turn at 180 o angle ) so that micropyle and hilum come to lie very close to each other. e.g., 82% of angiosperm families. Hemianatropous : Ovule turns at 90 o angle upon the funicle or body of ovule is at right angle to the funicle e.g., Ranunculus . Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down slightly. e.g., in members of Leguminosae and Cruciferae . Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe e.g. Lemna , Poppy, Alisma . Circinotropous : The ovule is initially orthotropous but becomes anatropous due to unilateral growth of funicle. The growth continues till the ovule once again becomes orthotropous . As a result funicle completely surrounds the body of the ovule e.g., Opuntia (prickly pear).

12. Formation of megaspore : The ovule or the megasporangium develops as a small protuberance of the placental tissue. In the very young ovule a single hypodermal cell is differentiated as archesporium cell. The archesporial cell may directly function as megaspore mother cell ( tenuinucellate ovule ) or may divide periclinally to form an outer parietal cell and an inner sporogenous cell ( crassinucellate ovule ). The sporogenous cell directly behaves as megaspore mother cell (or megasporocyte ). The diploid megaspore mother cell enlarges in size and divides by meiosis to form a linear tetrad of four haploid megaspores. Occasionally T- shaped or inverted T-shaped tetrads are also formed. Megaspore is the first cell of female gametophyte . T he linear tetrad, three megaspores towards the micropyle degenerate. The lowermost, i.e. , the chalazal megaspore enlarges and remains functional. It later produces an embryo sac.

13. Megasporogenesis: The process of formation of megaspores from the megaspore mother cell is called megasporogenesis. Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus. The MMC undergoes meiotic division. Meiosis results in the production of four megaspores. Female gametophyte : The female gametophyte is also called as embryo sac. It is seven celled and eightnucleate structure. There is large central cell with 2 polar nuclei which later on fuse to form secondary nucleus. At the micropylar end there is egg apparatus consist of egg and two synergids. At the chalazal end three antipodal cells are present. The cells of egg apparatus and antipodal cells are uninucleate and haploid where as central cell is binucleate and diploid.

14. Development of Embryo sac : The functional megaspore elongates and enlarges in size. The megaspore nucleus undergoes mitotic division to form 2 nuclei which are pushed towards opposite poles. These 2 nuclei again undergo division twice to form four nuclei at each pole. At this stage all the 8 nuclei are present in common cytoplasm without wall formation. The 8 nuclei now undergo organization. Out of the four nuclei of micropylar end 3 organize to form egg apparatus and forth one left free in the cytoplasm of central as polar nucleus. Three nuclei of chalazal end form 3 antipodal cells and forth one form polar nucleus of central cell. Thus seven celled structure of embryo sac is form.

15. TYPES OF EMBRYO SAC a) Monosporic embryo sac b) Bisporic embryo sac c) Tetrasporic embryo sac   MONOSPORIC EMBRYO SAC Monosporic embryo sac develops from a single megaspore and as such all the nuclei present in this type of embryo sac are genetically similar.  

16. Types of Monosporic Embryo Sac   Polygonum type 8 nucleated Most common type (81% families) First time described in Polygonum divaricatum by Strasburger (1879) Develops from the chalazal megaspore. The nucelus is divides thrice to form eight nuclei embryo sac Oenothera type: 4 nucleated embryo sac (Egg apparatus- 3 cells and Polar nucleus- single cell) Develops from micropylar megaspore Does not have antipodals Example – Onagraceae family

17. BISPORIC EMBRYO SAC Develops from one of the two dyads formed as a result of the first meiotic division (Meiosis I) of Megaspore Mother Cell. Both the nuclei of the functional dyad take part in the formation of embryo sac. Each nucleus undergoes two mitotic divisions and the mature embryo sac is 8 nucleated. The eight nuclei are organised into antipodals, egg apparatus and polar nuclei as in Polygonum type of embryo sac. The 4 nuclei derived from one megaspore nucleus are genetically different from the other four derived from the second megaspore nucleus. On the basis of the position of functional dyad bisporic embryo sacs are of two types ALLIUM TYPE : Develops from the chalazal dyad ENDYMION TYPE : Develops from the micropylar dyad.

18. TETRASPORIC EMBRYO SAC Meiotic division of the megaspore mother cell is not accompanied by cytokinesis and hence all the four haploid nuclei lie in a single cell called Coeno-megaspore . All four nuclei of coeno-megaspore participate in the formation of embryo sac Genetically more heterogeneous than bisporic type of embryo sac. The tetrasporic embryo sacs are further divided on the basis of following criteria- The position of haploid nuclei in the coeno-megaspore The number of times these nuclei divide Organization of nuclei in the mature embryo sac

19. TYPE OF TETRASPORIC EMBRYO SAC No nuclear fusion occurs Adoxa Type Plumbago Type Penaea Type Pepromia Type Drusa Type After the second meiotic division three megaspore nuclei fuse to form triploid nucleus at the chalazal end of the coenomegaspore , The fourth nucleus at micropylar end remains haploid Fritillaria Type Plumbagella Type

20. ADOXA TYPE It has 8 nuclei which are formed by the mitotic division of the four haploid nuclei of the coeno-megaspore. The arrangement of the 8 nuclei in the embryo sac is the same as in Polygonum type. Ex. Adoxa , Tulipa etc.

21. PLUMBAGO TYPE This type of embryo sac is characterized by the absence of synergids and antipodals. Out of four haploid coeno-megaspore one migrates to the micropylar end, one at chalazal end and two at the lateral sides. Each nuclei divides again and formed four groups of two nuclei. One of the nucleus from each group moves to the center of the cell and form four polar nuclei. The remaining nucleus at the micropylar is cut off by a membrane and form the egg. There are no synergids. The other three nuclei usually d isappear but occasionally they too may be cut off by membranes and appear as accessory egg cells. Example – Plumbaginaceae family

22. PENAEA TYPE The four haploid nuclei of the coeno-megaspore undergo two successive mitotic divisions forming 16 nuclei. These nuclei arrange themselves in four groups of four each, one at the micropylar end, one at chalazal end and one each on the two lateral sides. Now one nucleus from each groups migrates to the centre , and these four nuclei in the centre form polar nuclei. The three nuclei at the micropylar end are cut off by membranes and form the egg apparatus. The remaining three groups of nuclei (one chalazal and two lateral) degenerate at maturity. Highly polyploid (5x) primary endosperm nucleus is formed after double fertilization. Ex. – Family Penaeaceae , Malpighiaceae and Euphorbiaceae .

23. PEPEROMIA TYPE The egg apparatus of Peperomia type is characterized by a single synergid. The four haploid nuclei of coeno-megaspore undergo two successive mitotic divisions forming 16 nuclei. Two nuclei at the micropylar end form egg and a synergid, eight fuse in the centre of the cell to form a polar nucleus and the remaining six at the chalazal end formed antipodals

24. DRUSA TYPE 16 nucleate embryo sac This type of embryo sac is characterised by large number of antipodals In the mature embryo sac three nuclei form egg apparatus. Two act as polar nuclei and the remaining 11 nuclei are cut off by membrane and form antipodal cells.   The number and organization of nuclei may vary due to irregularity in the divisions. Example – Drusa , Rubia, Chrysanthemum, Ulmus etc.    

25. FRITILLARIA TYPE The four haploid nuclei of the coeno-megaspore arrange themselves in two groups – three at the chalazal end in the form of a triploid nucleus and one haploid at the micropylar end.   The triploid chalazal as well as the haploid micropylar nucleus undergo two mitotic divisions and as a result four trploid nuclei are formed at the chalazal end and four haploid at the micropylar end.   In mature embryo sac three haploid nuclei organize into egg apparatus, three triploid into antipodal and remaining one haploid and one triploid nuclei move to the centre where they fuse to form a tetraploid polar nucleus. Example – Fritillaria, Lilium, Piper and Gaillardia

26. PLUMBAGELLA TYPE The initial development is similar to Fritillaria type and a triploid nucleus is formed at the chalazal end and a haploid at the micropylar end. Each of these nuclei undergoes a single mitotic division and form two groups of two nuclei each. One triploid nuclei from chalazal end and one haploid nucleus from the micropylar end fuse at the centre and form tetraploid polar nucleus. One haploid nucleus at the micropylar end forms the egg and one triploid nucleus at the chalazal end the single antipodal. There is no synergids

27. Structure of Mature Embryo Sac Egg Cell is located at the micropylar end of the embryo sac and ultimately fuses with a sperm nucleus to produce a zygote. The egg cell lies adjacent to the two synergids, separated from them by either partial cell walls or the plasmalemma alone. The distribution of cytoplasm within the egg cell is highly polarized, due to the presence of a large vacuole at the micropylar end that restricts the nucleus and most of the cytoplasm to the chalazal end

28. Synergids are located on either side of the egg cell, play an important role in fertilization . The pollen tube discharges its contents into one of the synergids prior to incorporation of the sperm nuclei into the egg and central cells. Central Cell this cell contains two nuclei, a large vacuole, and many cytoplasmic organelles. The polar nuclei originate at both the micropylar and chalazal ends of the coenocytic megagametophyte and migrate to the center after cellularization. The polar nuclei may partially fuse with each other before they are fertilized by a single sperm nucleus, generating the triploid primary endosperm nucleus .The mature endosperm will provide nutrients for the developing embryo or seedling . Antipodal Cell -are located opposite the egg at the chalazal end of the embryo sac. No specific function during reproduction has been attributed to the antipodals, but they are thought to be involved in the import of nutrients to the embryo sac .

29. POLLEN GERMINATION AND POLLEN TUBE GROWTH

30. ENTRY OF POLLEN TUBE INTO OVULE Mesogamy