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Question
Sketch and label stages in the development of the angiospermic female gametophyte from the functional megaspore.
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Solution
In angiosperms, the female gametophyte, also known as the embryo sac, develops from the functional megaspore through a process called megagametogenesis.
Stages of development:
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Functional megaspore (Haploid): Out of the four megaspores formed after meiosis of the megaspore mother cell, only one (usually the chalazal one) remains functional, while the other three degenerate.
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Two-nucleate stage: The nucleus of the functional megaspore divides mitotically to form two haploid nuclei, which migrate to opposite poles of the cell.
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Four-nucleate stage: Both nuclei divide again mitotically to form four nuclei, two at each pole.
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Eight-nucleate stage (Mature embryo sac): A final mitotic division produces eight nuclei, four at each pole.
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The egg apparatus is made up of three nuclei at the micropylar end: one egg cell and two synergids.
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Three nuclei at the chalazal end make up antipodal cells.
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Two nuclei move to the center and fuse to form the secondary (polar) nucleus.
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Thus, the mature embryo sac becomes seven-celled and eight-nucleate.
Development of Female Gametophyte (Embryo Sac)
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