Megasporogenesis by vimal priya

megasporogenesis is the formation of the megasporesminside the

1. VIMAL PRIYA subramanian MEGASPOROGENESIs

2. Megasporangium The sporangium which produces only megaspores. The megasporangium together with its protective coats, the integuments, is called ovule.

3. STRUCUTRE OF OVULE The  ovule is the structure that gives rise to and the one that contains female reproductive cells. Funicle - The ovule is attached to the placenta by means of a stalk called funicle . Hilum - The body of the ovule fuses with funicle in the region called hilum .( hilum - the junction between ovule and funicle ) Integuments- Each ovule has one or two protective envelopes called integuments.(inner and outer )

4. STRUCTURE OF OVULE

5. - Micropyle - The integuments leave small opening at one end called micropyle .- this end is micropylar end. - Chalaza - Opposite the micropylar end, is the chalaza , representing the basal part of the ovule. - Nucellus - Tissue encloses embryo sac Egg apparatus- at micropylar end. It has 2 synergids and an egg . Filiform apparatus- synergids – special thickening at micropylar end. It guides the pollen tube. Antipodals - 3 in number towards chalazal end. Two Haploid nuclei at polar end below egg apparatus

6. Orthotropous or atropous ovule ( ortho -straight, tropous - turn) The body of the ovule is erect or straight. The hilum , chalaza and the micropyle lie in a straight line e.g.Polygonum . Anatropous ovule ( ana - backward or up, tropous - turn) The body of the ovule becomes completely inverted during the development so that the micropyle lies very close to the hilum ( eg ) Gamopetalae members. Hemi-anatropous or hemitropous ovule The body of the ovule is placed transversely at right angles to the funicle . The micropyle and chalaza lie in one straight line e.g. Ranunculus .

7. Campylotropous ovule ( kampylos - curved) The body of the ovule is curved or bent round so that the micropyle and chalaza do not lie in the same straight line. e.g.  Leguminosae . Amphitropous ovule The curvature of the ovule is very much pronounced and the embryosac also becomes curved e.g. Allismaceae , and  Butomaceae . Circinotropous ovule The nucellus and the axis are in the same line in the beginning but due to rapid growth on one side, the ovule becomes anatropous. The curvature continues further and the micropyle again points upwards (e.g.)  Opuntia .

8. Megasporogenesis Megasporogenesis is the formation of the megaspore. The formation of megaspores inside the ovules. A diploid cell in the ovule, called a megasporocyte or a megaspore mother cell, undergoes meiosis and gives rise to four haploid megaspores.

9. DEVELOPMENT OF FEMALE GAMETOPHYTE 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 the archesporium . This archesporium cell may or may not cut off some parietal cells and then becomes the megaspore mother cell. The megaspore mother cell now undergoes meiosis or reduction division, and, usually, a linear row of four haploid megaspore cells (‘linear tetrad’) is formed. Meanwhile, two integuments develop from the base of the ovule. Of the linear tetrad of megaspores, usually the lower­ most one enlarges and becomes the functional megaspore while the three on top disinte­grate.

10. DEVELOPMENT OF OVULE

11. The functional megaspore now develops into the female gametophyte or the embryo sac. In Angiosperms, the development of the female gametophyte is completely endosporous , i.e., within the megaspore. In a typical case, the nucleus of the embryo sac, which is same as the functional megaspore, divides into two, then four and finally, eight daughter nuclei (four at each pole). Then, one nucleus from each pole moves to the centre of the embryo sac and fuses, there forming the fusion or secondary nucleus. Finally, the embryo sac or the female gametophyte becomes organised . Three nuclei at the base form the antipodal cells. The secondary nucleus remains at the centre. On the top, three cells from the egg apparatus which consists of two flask-shaped synergids and a round egg cell (or ovum or oosphere ) hanging between and below them.

12. The synergids usually are somewhat notched by an indentation and they show striations at the tip (‘ filiform apparatus’). There is prominent vacuolation below the synergid nuclei and above the egg nucleus, showing accumulation of cytoplasm at different regions.

13. Structure of embryosac

14. Types of embryosac Monosporic Embryo sac: It is derived from the one of the four megaspore.ll the nuclei in this embryosac are genetically identical.Because they are derived through mitosis of a single cell. Example Polygonum type: The embryo sac is formed by the chalazal megaspore.The mature embryo sac is formed comprises three cell egg apparatus,three antipodal cells,and a binucleate central cell.

15. Bisporic embryo sac The first meiotic division is accompained by wall formation, so that a dyad is formed only one of the dyad cell undergoes second meiotic division whereas the otherone degenarate both megaspore nuclei contribute to te formation of embryosac . Example: Alisma

16. Tetrasporic embryo sac In this type at the end of the meiosis all the four nuclei are contribute the formation of the embryo sac. Example: Drusa type: The type of embryo sac is also sixteen nucleate the mature embryo sac comprises a a normal egg apparatus ( three celled),two polar nuclei,eleven antipodal cell.

17. Types of embryosac

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