Megasporogenesis: It is the process of producing megaspores from megaspore mother cells. Megaspores are formed in the reproductive organs of females. The seeds are formed from the ovules and fruits from the ovary after fertilisation. During fertilisation, the haploid male gamete (present inside the pollen grain) and the haploid female gamete (present inside the ovule) fuse together to form the diploid zygote. 

In this article, we will learn more about what is megasporogenesis, and its structure with the diagram. 

Define Megasporogenesis

Before diving deep, let’s understand the definition of Megasporogenesis in the first place. Megasporogenesis is defined as the process of formation of haploid megaspores by the meiotic division of diploid Megaspore Mother Cells (MMC) inside the megasporangium (ovule). The haploid megaspore undergoes repeated mitotic divisions to form the embryo sac.

Structure of Ovule or Megasporangium

1. Gynoecium represents the female component of a flower. It may consist of one or a number of carpels.
2. Each carpel represents a megasporophyll which has three parts, i.e., stigma, style and ovary.
3. An ovary may contain one or many ovules, and each ovule encloses an embryo sac. Ovule or megasporangium develops from the inner wall of the ovary.
4. As the ovule develops, it gets raised from the ovary wall by a short stalk called a funicle through which food and water are transported to the ovule.

Fig: Structure of Pistil or Carpel

5. The body of the ovule fuses with the funicle in the region called the hilum, which represents the junction between the ovule and the funicle.

6. The multilayered body of the ovule is called nucellus which is enclosed by one or two protective layers called integuments, except for a small pore at one end called the micropyle.

7. Within the nucellus is present one megaspore mother cell (or embryo-sac mother cell).

8. The other end of the ovule, i.e., opposite the micropylar end, where the funicle joins with the nucellus and integument, is called the chalaza or chalazal end. It represents the basal part of the ovule.

Fig: Structure of Pistil or Carpel

Types of Ovules (or Megasporangia)

On the basis of the relative position of the micropyle with respect to funicle and chalaza, the following types of ovules have been recognised:

1. Orthotropous ovule– The micropyle, chalaza and funicle lie in one straight line. For example – Family Polygonaceae, Piperaceae and ovules of most gymnosperms.
2. Anatropous (Inverted) ovule- The body of the ovule is completely inverted so that the micropyle and hilum come to lie very close to each other. Most angiosperms contain this type of ovule. For example- Plants belong to the gamopetalous sub-class.
3. Campylotropous ovule- The ovule’s body is curved so that the micropyle is directed towards the chalaza, and the chalaza is situated at right angles to the funicle. For example- Members of the Cruciferae and Leguminosae families.
4. Amphibious (Transverse) ovule- The curvature of the ovule is more pronounced, and the embryo sac becomes horse-shoe shaped. For example- Families Alismataceae, and Butomaceae.
5. Hemianatropous ovule– When the nucellus and the integuments lie more or less at right angles to the funicle. For examples – Families of Ranunculaceae and Primulaceae.
6. Circinotropous ovule– The funicle forms a complete circle around the ovule as it is exceptionally long, from it except for a small area at the end of the funicle—for example- Opuntia and other members of families Cactaceae and Plumbaginaceae.

Process of Megasporogenesis

1. Megasporogenesis refers to the process of formation of haploid megaspores from the diploid megaspore mother cell (MMC).
2. Megaspore mother cell (MMC) is a large diploid \((2n)\) cell containing dense cytoplasm and a prominent nucleus which undergoes meiotic division to give rise to four haploid megaspores.
3. In the majority of the flowering plants, only one of the megaspores is functional and the rest three degenerate.
4. The functional megaspore grows, and its nucleus undergoes mitotic divisions to produce eight haploid nuclei. The structure that is formed as a result of megasporogenesis is called embryo-sac or megagametophyte.
5. The formation of embryo-sac from a single megaspore is called monosporic development. Similarly, several other types of embryo-sacs are also known in angiosperms which are classified on the basis of the number of megaspore nuclei taking part in development.
6. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to opposite poles forming the two-nucleate embryo sac.
7. Two more sequential mitotic divisions result in the formation of the four-nucleate and later the eight nucleate stages of the embryo sac.
8. These mitotic divisions are strictly nuclear, i.e., nuclear divisions are not followed immediately by cell wall formation.
9. After the eight-nucleated stage, the cell walls are formed, which results in the formation of a typical female gametophyte or embryo sac.

Megasporogenesis Flow-Chart: Female Gametophyte

Structure of Embryo-sac

As illustrated in the step-by-step flow diagram, an embryo sac contains eight nuclei which further undergo various developmental changes as follows:

1. Each end of the embryo-sac has four haploid \((n)\) nuclei. One nucleus from each group of four moves towards the centre to form the polar nuclei that ultimately fuse to form a single diploid nucleus \((2n)\) at the time of fertilisation.
2. A group of three nuclei is present at the micropylar end, and another group of three nuclei is present at the opposite or chalazal end.
3. Thin cell walls form and isolate all of these six nuclei. Hence, six of the eight nuclei are surrounded by cell walls and organised into cells. The remaining two nuclei, called polar nuclei, are present in the central part of the embryo sac.
4. The three cells at the micropylar end constitute the egg apparatus. One of its cells enlarges and forms the egg cell or female gamete, and the cells present on either side are called synergids.
5. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus that plays an important role in guiding the pollen tube during fertilisation.
6. The three cells at the chalazal end are known as antipodals.
7. Before fertilisation, a mature embryo sac contains seven nuclei, out of which six are haploid \((n),\) and one is diploid \((2n).\)
8. Thus, in angiosperms, a typical embryo sac at maturity is \(7\)-celled but \(8\)-nucleated.

Fig: Structure of a mature embryo-sac in angiosperms

Key Takeaways:

Megasporogenesis is defined as the formation of haploid megaspores by the meiotic division of diploid megaspore mother cells (MMC) inside the megasporangium (ovule). The haploid megaspore undergoes repeated mitotic divisions to form the embryo sac. During fertilisation, the haploid male gamete (present inside the pollen grain) and the haploid female gamete (present inside the ovule) fuse together to form the diploid zygote.

The functional megaspore grows, and its nucleus undergoes mitotic divisions to produce eight haploid nuclei. The structure formed as a result of megasporogenesis is called embryo-sac or megagametophyte.

FAQs related to Megasporogenesis

Q.1: What is megasporogenesis?
Ans: Megasporogenesis is defined as the process of formation of haploid megaspores by the meiotic division of diploid megaspore mother cells (MMC) inside the megasporangium (ovule).

Q.2: Where does megasporogenesis take place?
Ans: Megasporogenesis takes place in the gymnosperms and the flowering plants in which a functional megaspore is produced inside the nucellus of the ovule.

Q.3: What do megaspores develop into?
Ans: Megaspores develop into a mature embryo-sac (or female gametophyte) which is \(7\)-celled but 8-nucleated and contains the haploid egg nucleus and diploid polar nucleus.

Q.4: What are megasporogenesis and microsporogenesis?
Ans: Megasporogenesis refers to the formation of haploid megaspores by the meiotic division of diploid megaspore mother cells (MMC) inside the megasporangium (ovule), whereas microsporogenesis refers to the formation of pollen grains (or microspores) inside the pollen sacs (or microsporangium) of flowering plants by meiotic or reduction division. Hope you understand the difference between megasporogenesis and microsporogenesis.

Q.5: What is the megasporogenesis process?
Ans: Megasporogenesis refers to the process of formation of haploid megaspores from the diploid megaspore mother cell (MMC). In the majority of the flowering plants, only one of the megaspores is functional and the rest three degenerate. The functional megaspore grows, and its nucleus undergoes mitotic divisions to produce eight haploid nuclei. The structure that is formed as a result of megasporogenesis is called embryo-sac or megagametophyte.