An Introduction to Megasporogenesis
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
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.
FAQs on Megasporogenesis
1. What is megasporogenesis as per the Class 12 Biology syllabus?
Megasporogenesis is the biological process of forming haploid megaspores from a diploid megaspore mother cell (MMC) through meiosis. This process occurs inside the ovule (megasporangium) of a flowering plant and is a crucial step in female gamete formation, leading to the development of the embryo sac.
2. What are the key structures within the ovule involved in megasporogenesis?
Several key structures are involved in megasporogenesis:
- Nucellus: The central mass of tissue inside the ovule where the megaspore mother cell (MMC) develops.
- Megaspore Mother Cell (MMC): A large, diploid cell with a prominent nucleus and dense cytoplasm that undergoes meiosis.
- Micropyle: A small opening in the integuments at one end of the ovule.
- Chalaza: The basal part of the ovule, opposite the micropylar end.
- Integuments: The outer protective layers of the ovule.
3. What is the main difference between microsporogenesis and megasporogenesis?
The main difference lies in the type of spore produced and the subsequent development. Microsporogenesis is the formation of haploid microspores (pollen grains) from a pollen mother cell in the anther, which develop into the male gametophyte. In contrast, megasporogenesis is the formation of haploid megaspores from a megaspore mother cell in the ovule, leading to the development of the female gametophyte (embryo sac).
4. How does megasporogenesis differ from megagametogenesis?
These are two sequential processes. Megasporogenesis is the formation of the haploid megaspore from the diploid MMC via meiosis. Megagametogenesis is the subsequent development of this single functional megaspore into the mature female gametophyte (the embryo sac) through a series of mitotic divisions. Essentially, megasporogenesis creates the spore, while megagametogenesis develops that spore into the gamete-bearing structure.
5. Why do three of the four megaspores formed during meiosis typically degenerate?
In most flowering plants (following monosporic development), three of the four megaspores degenerate to provide nourishment and space for the development of the single functional megaspore. This ensures that one healthy, well-nourished megaspore has sufficient resources to develop into a viable embryo sac, increasing the chances of successful fertilisation and seed formation. The functional megaspore is usually the one located at the chalazal end.
6. What is the significance of monosporic development in angiosperms?
Monosporic development, where the embryo sac develops from a single functional megaspore, is significant because it represents an efficient reproductive strategy. By channelling all the nutritional resources into one developing gametophyte, the plant ensures the formation of a robust and healthy embryo sac. This conserves energy and increases the probability of producing a viable seed after fertilisation, which is a key evolutionary advantage for angiosperms.
7. What is the ploidy level of the nucellus, the functional megaspore, and the endosperm?
The ploidy levels of these structures differ based on their roles in reproduction:
- Nucellus: It is a part of the parent sporophyte, so it is diploid (2n).
- Functional Megaspore: It is a product of meiosis from the megaspore mother cell, making it haploid (n).
- Endosperm: It is formed after the fusion of a male gamete (n) with the two polar nuclei (n+n) in the central cell, making it triploid (3n).
8. How does the process of megasporogenesis ensure genetic variation in the female gamete?
Megasporogenesis ensures genetic variation primarily because it is a meiotic process. During Meiosis I, crossing over occurs between homologous chromosomes in the megaspore mother cell (MMC), which shuffles genetic material. Additionally, the independent assortment of chromosomes during anaphase I further randomises the combination of genes passed into the resulting megaspores. This means the single functional megaspore that develops into the embryo sac is genetically distinct from its parent plant.
