Q.What do you mean by monosporic development of female gametophyte? Explain the development of female gametophyte with help of suitable diagrams. OR What is menstrual cycle? Name the term used for first occurrence of menstruation. Explain the major events of menstrual cycle in human female.
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Start your 14-day free trial to unlock the full solution →In monosporic development (the most common type, seen in Polygonum), only one of the four megaspores formed by meiosis survives; it alone divides mitotically to build the mature 7-celled, 8-nucleate embryo sac.
Formation of megaspores: Within the ovule, a diploid megaspore mother cell (MMC) undergoes meiosis to produce four haploid megaspores, typically arranged in a linear tetrad.
Monosporic development: In this most common mode of embryo sac development (as in Polygonum, hence also called the Polygonum type), only one of the four megaspores remains functional — usually the one situated at the chalazal end of the tetrad, which has better access to nutrients — while the other three megaspores degenerate. Because the resulting embryo sac develops from a single (mono-) surviving megaspore, this pattern is termed monosporic development.
Development of the female gametophyte from the functional megaspore:
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The single functional megaspore enlarges considerably, and its nucleus divides mitotically to produce 2 nuclei, which move to the opposite poles, forming a 2-nucleate embryo sac.
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Each of these two nuclei divides mitotically again, producing a 4-nucleate embryo sac, with 2 nuclei at each pole.
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A further, third mitotic division of both sets of nuclei produces an 8-nucleate embryo sac, with 4 nuclei positioned at each of the two poles (micropylar and chalazal).
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Cellularisation and organisation: Following these three free-nuclear mitotic divisions, cell walls are laid down, organising the 8 nuclei into a 7-celled embryo sac:
- At the micropylar end: three cells form the egg apparatus — a central egg cell flanked by two synergids (which possess a filiform apparatus that helps guide pollen tube entry).
- At the chalazal end: three cells form the antipodal cells, believed to have a nutritive function.
- In the centre: the remaining two nuclei (one from each pole) — the polar nuclei — come to lie together within a large central cell; they usually fuse (or remain closely associated) to eventually take part in triple fusion during double fertilisation, giving rise to the (typically triploid) endosperm.
So, though the embryo sac has 8 nuclei in total, cellularisation groups them into 7 cells (since the large central cell contains two, unfused, polar nuclei) — this characteristic 7-celled, 8-nucleate structure is the mature female gametophyte typical of most flowering plants developing via the monosporic pathway.
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