Explain the process of formation of female gametophyte?
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Answer:
The carpels of an angiosperm is known as megasporophyll. It is differentiated into three regions-ovary, style and stigma. The ovary contains ovules or megasporangium. Megasporogenesis is the process of formation of megaspores from the megaspore mother cell (MMC).
Different stages of development of female gametophyte are:
1) In the nucleus of a developing ovule, one diploid cell develops into a diploid megaspore mother cell.
2) This megaspore mother cell divides by meiosis to produce four haploid megaspores which are arranged in a linear tetrad.
3) One of the megaspores is functional, while the other three degenerate in angiosperms. The lower chalazal megaspore produces the female gametophyte (embryo sac).
4) The nucleus of the functional megaspore divides mitotically into two nuclei.
These are called primary micropylar and primary chalazal nuclei.
5) The second division produces one pair of nuclei at the micropylar end and one pair of nuclei at the chalazal end. The third division results in the formation of 4 nuclei at each pole.
6) One nucleus from each pole then migrates towards the centre to become polar nuclei, which fuse together and give rise to the secondary nucleus (2n). Out of these, the central one is the egg cell while the two lateral cells are called synergids.
7) The three nuclei towards the chalazal end also get organized into three thin-walled cells called antipodal cells.
8) The structure containing egg apparatus, secondary nucleus and antipodals are called female gametophyte or embryo sac.
9) Thus, a typical angiosperm embryo sac, at maturity is 8-nucleate and 7-celled.
Answer:
The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the embryo and endosperm of the seed, respectively. Female gametophyte development begins early in ovule development with the formation of a diploid megaspore mother cell that undergoes meiosis. One resulting haploid megaspore then develops into the female gametophyte. Genetic and epigenetic processes mediate specification of megaspore mother cell identity and limit megaspore mother cell formation to a single cell per ovule. Auxin gradients influence female gametophyte polarity and a battery of transcription factors mediate female gametophyte cell specification and differentiation. The mature female gametophyte secretes peptides that guide the pollen tube to the embryo sac and contains protein complexes that prevent seed development before fertilization. Post-fertilization, the female gametophyte influences seed development through maternal-effect genes and by regulating parental contributions. Female gametophytes can form by an asexual process called gametophytic apomixis, which involves formation of a diploid female gametophyte and fertilization-independent development of the egg into the embryo. These functions collectively underscore the important role of the female gametophyte in seed and food production.
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