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Megasporogenesis

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An Introduction to Megasporogenesis

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Vascular plants having stems, roots, and leaves are known as angiosperms. A flower contains the angiosperm's seeds. The vast majority of plants on earth are made up of them. The seeds grow into fruit inside the organs of the plant.


At the base of the megasporophyll, the ovary contains the ovules. Angiosperms are heterosporous, which means they generate both megaspores and microspores (pollen grains). The nucellus retains a single functioning megaspore indefinitely. Pollination facilitates reproduction by transferring pollen grains from the anther to the stigma.


What is Megasporogenesis?

Megasporogenesis is the process of creating haploid megaspores inside the megasporangium by meiotically dividing diploid megaspore mother cells (MMC) (ovule). To create the embryo sac, the haploid megaspore goes through several mitotic divisions through a process called megagametogenesis. Female reproductive organs are where megaspores develop. After fertilisation, the ovules become seeds, and the ovary becomes a fruit. The haploid male gamete, which is found inside the pollen grain, and the haploid female gamete, which is found inside the ovule, combine to form the diploid zygote during fertilisation.


Structure of Megasporangium

  • The gynoecium represents the feminine portion of a flower. There may be one or several carpels in it.

  • A megasporophyll, which consists of the stigma, style, and ovary, is represented by each carpel.

  • Each ovule in an ovary, which can have one or more, contains an embryo sac. The inner ovarian wall is where the ovule or megasporangium forms.

  • A slender stalk known as a funicle, through which food and water are delivered to the ovule, lifts the ovule from the ovary wall as it develops.

  • The hilum, which stands for the intersection of the ovule and the funicle, is where the ovule body merges with the funicle.

  • The nucellus, the multilayered body of the ovule, is protected by one or two layers known as integuments, except for a tiny pore known as the micropyle at one end.

  • One megaspore mother cell can be found inside the nucleus (or embryo-sac mother cell).

  • The chalaza, or chalazal end of the ovule, is the other end, or the end opposite the micropylar end, where the funicle connects with the nucellus and integument. It stands in for the ovule's base.


Process of Megasporogenesis

Megasporogenesis Diagram


Megasporogenesis Diagram


Above is the detailed megasporogenesis flow chart, and the following are the steps:

  • The process of creating haploid megaspores from a diploid megaspore mother cell is known as megasporogenesis (MMC).

  • A large diploid (2n) cell known as the megaspore mother cell (MMC) conducts meiotic division to produce four haploid megaspores. It also contains thick cytoplasm and a conspicuous nucleus.

  • Only one of the four megaspores is viable, and the other three degenerate in the majority of blooming plants.

  • The functioning megaspore develops, and as it does, its nucleus goes through mitosis and divides into eight haploid nuclei. Megagametophyte or embryo-sac are two names for the structure that results from megasporogenesis.

  • Monosporic development is the process by which an embryo sac develops from a single megaspore. The number of megaspore nuclei participating in development is used to categorise a variety of distinct forms of embryo-sacs that are known in angiosperms.

  • The two nuclei that result from the functional megaspore's nucleus splitting during mitosis travel to opposite poles to form the two-nucleate embryo sac.

  • The four-nucleate and later eight nucleate stages of the embryo sac are created by two more consecutive mitotic divisions.

  • These mitotic divisions are strictly free nuclear, meaning that cell wall construction does not immediately follow nuclear divisions.

  • Cell walls begin to form after the eight-nucleated stage, which leads to the development of a normal female gametophyte or embryo sac.


Conclusion

This article gives an insight into the important process of megasporogenesis in angiosperms and the detailed structure of a megasporangium. Apart from this, there are many types of megasporangium or ovules in angiosperms like orthotropous, anatropous, campylotropous, amphibious, hemianatropus, and circinotropous ovule. The nucleus of the functioning megaspore divides during mitosis to produce eight haploid nuclei as it expands through megagametogenesis. Megagametophyte or embryo-sac are the two names for the structure that results from megasporogenesis.

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FAQs on Megasporogenesis

1. What is the difference between microsporogenesis and megasporogenesis?

Two processes that take place in seed plants are microsporogenesis and megasporogenesis. Microsporangia, which are 2n cell-sized structures, are found in the anthers of stamens. Microspores, which are n cells, are produced by the meiosis of the microspore mother cells. Microsporogenesis is the name of this process. Pollen grains, which are male gametes, are produced by the mitotic division of microspores. Ovules are what megasporangia are. Megaspore mother cells are found in ovules. 


Megaspores, which are n cells, are produced when the megaspore mother cells undergo meiosis and divide. Megasporogenesis is the process by which megaspores develop from megaspore mother cells. Megaspores go through mitosis and produce egg sacs. This is how microsporogenesis and megasporogenesis differ from one another.

2. What is the difference between megasporogenesis and megagametogenesis?

Megasporogenesis: The haploid (n) megaspores are produced by the meiotic division of the diploid (2n) megaspore mother cell. The megaspore mother cell first divides transversely into two cells. Four (4) haploid megaspores are created when both cells divide transversely once more.


Megagametogenesis: The initial cell of the female gametophyte is called a megaspore (n). The female gametophyte, or embryo sac, is formed when the functional megaspore enlarges at the expense of the tape rum and the nucellus. The embryo sac is initially uninucleated; as it grows, its nucleus divides three times in succession to generate eight nuclei.

3. What is fertilisation in plants?

When two haploid gametes unite, they produce a diploid zygote, which then grows into an embryo. This process is known as fertilisation in plants. The meeting of the gametes takes place in the following way in gymnosperms (conifers) and angiosperms (flowering plants): the male gametes are encased in pollen grains and are transported to the female reproductive organs by the wind or by insects. The embryo, which is the result of fertilisation, is placed inside the seed after the process is complete.


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