Introduction to Megasporogenesis, Steps Involved in Megasporogenesis, Megagametogenesis, Structure of Female Gametophyte, Significance of Megasporogenesis, Megasporogenesis v/s Microsporogenesis, Practice Problems, and FAQs

What comes to your mind when you hear the word ‘egg’? Most of us will immediately relate the word egg with birds. But in biology, an egg cell refers to a female gamete in organisms. Did you know that flowers can also bear eggs within them? Surprised? Well, the egg cell within the flower is nothing but the female gamete which participates in sexual reproduction by fusing with a male gamete and giving rise to a zygote. 

But where does this egg cell originate from? Egg cells develop from female spores known as the megaspores. But where do these megaspores come from in the first place? Megaspores are produced by the process of megasporogenesis. In this article we will discuss how megasporogenesis leads to the formation of megaspores and how the megaspores eventually give rise to the egg cells. Keep reading to have a better understanding of these phenomena.

Table of contents

• Introduction
• Steps involved in megasporogenesis
• Megagametogenesis
• Structure of female gametophyte
• Significance of megasporogenesis
• Megasporogenesis v/s microsporogenesis
• Practice Problems
• FAQs

Introduction

The process through which megaspores are produced from megaspore mother cells is known as megasporogenesis. The process takes place in the reproductive organs of females, that is, the pistil. The pistil consists of three parts - the stigma, the style and the ovary. The ovary consists of ovules which are the sites for synthesis of megaspores. Megasporogenesis involves creation of haploid megaspores inside the megasporangium (ovule) by meiotic division of diploid megaspore mother cells (MMC).  Megaspores further develop into female gametophytes or embryo sacs which bear the female gamete or the egg cell. 

Steps involved in megasporogenesis

The process of megasporogenesis involves the following steps:

• Formation of megaspore mother cell
• Meiosis I
• Meiosis II
• Functional megaspore

Formation of megaspore mother cell

The megaspore mother cell is produced inside the megasporangium or ovule. The megaspore mother cell is formed towards the micropylar region of the ovule. It arises when one of the subepidermal or hypodermal cells in the micropylar region of the nucellus differentiates into an archesporial cell. The cell enlarges and has a prominent nucleus with a thick cytoplasm. The archesporial cell can directly serve as the megaspore mother cell or it can divide periclinally (along the surface) to form an outer parietal cell and an inner sporogenous cell. The parietal cell further divides to form parietal tissue whereas the sporogenous cell acts as the megaspore mother cell. The megaspore mother cell is also known as a megasporocyte and is characterised as a diploid cell in plants. It undergoes meiosis and forms a linear row of four haploid megaspores. 

                                    Fig: Megaspore mother cell

Meiosis I

The diploid megaspore mother cell formed at the micropylar end undergoes the first reductional division of meiosis. As a result, two haploid cells are produced that form the megaspore dyad. 

                                                  Fig: Meiosis I

Meiosis II

There are two cells in the megaspore dyad and each cell undergoes a second meiotic division. This division produces four haploid megaspores that are arranged in a linear tetrad. 

                                             Fig: Meiosis II

Functional megaspore

Out of the four haploid megaspores produced, three disintegrate and only one megaspore present on the chalazal side remains functional and further develops into an embryo sac or female gametophyte. Therefore, we can say that one megaspore mother cell produces one functional megaspore.

                                     Fig: Functional megaspore

Megagametogenesis

Megagametogenesis is the process of development and maturation of megaspores into the female gametophytes. The process of megagametogenesis is the function of mitotic divisions, the formation of new cells, and the fusion of existing nuclei or cells. The functional megaspores undergo three mitotic divisions and mature into a female gametophyte or embryo sac. These mitotic divisions are the free nuclear type which means that nuclear division is not followed by cell wall formation.

• First mitotic division
• Second mitotic division
• Third mitotic division

First mitotic division

The nucleus of the haploid functional megaspore undergoes the first mitotic division. This results in two haploid nuclei. These nuclei move towards the opposite poles. The embryo sac at this stage is known as the binucleate embryo sac.

                                    Fig: Binucleate embryo sac

Second mitotic division

The two haploid nuclei in the binucleate embryo sac undergo a second mitotic division and this results in four haploid nuclei. This stage of the embryo sac is known as the four nucleate or tetranucleate embryo sac. 

                                  Fig: Tetranucleate embryo sac

Third mitotic division

The four nuclei present in the embryo sac now undergo third mitotic division and as a result, eight haploid nuclei are produced. Four nuclei are at the chalazal end and four at the micropylar end. This stage of the embryo sac is known as the eight nucleate embryo sac. 

                                   Fig: Eight nucleate embryo sac

Three nuclei at the micropylar end form the egg apparatus and the fourth one moves to the centre and forms one of the two polar nuclei. Three nuclei at the chalazal end form the antipodal cells and the fourth one moves to the centre to form the second polar nucleus. After this stage, cell wall formation takes place. The polar nuclei fuse together to form the central cell. As a result, a typical female gametophyte or embryo sac is formed which is a 7 celled structure with 8 nuclei.

Structure of female gametophyte

The female gametophyte is also known as embryo sac or megagametophyte. A fully developed female gametophyte is a multicellular haploid structure that is oval in shape. It remains embedded in the nucellus towards the micropylar end. The mature embryo sac is composed of an egg apparatus, antipodal cells and central cell.

Antipodal cells

The formation of three antipodal cells begins towards the chalazal end. These are haploid in nature. The function of the antipodal cells is not established yet. Although, in some plants, they provide nutrition to the developing embryo. These cells usually degenerate after fertilisation as the embryo matures.

                                Fig: Antipodal cells

Egg Apparatus

The egg apparatus is a group of three cells in the micropylar end consisting of one egg cell in the centre and two synergid cells on its sides.

Egg cell

The egg cell is the haploid female gamete which fuses with the male gamete to form a zygote which eventually grows into a new plant. The egg cell is polar in nature and has a vacuolated cytoplasm towards the micropylar end and a nucleus at the chalazal end.  

                            Fig: Egg cell

Synergids

Synergids are two specialised cells that are present adjacent to the egg cell in the female gametophyte. At the micropylar tip of the synergids, finger-like projections having microfibrils are present which are known as filiform apparatus. The filiform apparatus plays an important role in regulating and guiding the pollen tubes inside the embryo sac during fertilisation. Some of the functions of the synergids are -

• Absorption of nutrients from the nucellus
• Secreting active substances to attract the pollen tube
• Form a seat for pollen tube discharge

Synergids degenerate after the entry of the pollen tube or shortly before it.

                                    Fig: Egg apparatus

Central cell

In the middle of the embryo sac, there are two nuclei, known as the polar nuclei present in the large central cell. The central cell is diploid (2n) in nature because it contains two nuclei. It participates in double fertilisation and initiates endosperm development, which is a characteristic feature of angiosperms. 

                                        Fig: Central cell

In the end, we can conclude that an embryo sac contains seven cells and eight nuclei and therefore, a mature embryo sac is also known as a 7-celled 8-nucleated stage. In this, six cells are haploid (n) and one cell is diploid (2n). 

Significance of megasporogenesis

The significance of megasporogenesis is that it helps to maintain the ploidy of the offspring as it leads to the development of a haploid megaspore that develops into a female gametophyte bearing a haploid female gamete or egg cell. The production of haploid egg cells is crucial to maintain diploid number of chromosomes in the offspring. This is because the offspring develops from the diploid zygote which is formed due to fusion of the haploid egg cell with the haploid male gamete.

Megasporogenesis v/s microsporogenesis

Practice Problems

Q 1. Identify the number of cells and nuclei present in a mature embryo sac.

a. 8 cells, 16 nuclei
b. 8 cells, 8 nuclei
c. 7 cells, 16 nuclei
d. 7 cells, 8 nuclei

Solution: A mature embryo sac consists of four types of cells. This includes two haploid synergids, three haploid antipodal cells, one haploid egg cell and a diploid central cell which contains two nuclei. Therefore, a mature embryo has 7-cells and 8 nuclei. Hence, the correct option is d.

Q 2. Identify the cells present in the egg apparatus of a mature embryo sac.

a. One egg cell and one central cell
b. One egg cell and the two synergids
c. One egg cell and the two polar nuclei
d. One egg cell and the antipodal cells

Solution: The egg cell is the female gamete which is haploid in nature and fuses with the male gamete to form an embryo. Synergids are two specialised cells that are present adjacent to the egg cell in the female gametophyte. They are located at the micropylar end. They are grouped with the egg cell and constitute the egg apparatus. Hence, the correct option is b.

Q 3. Determine the correct statement.

a. In the middle of the embryo sac, polar nuclei are present.
b. At the chalazal end, the filiform apparatus is present.
c. The embryo sac is located in the integuments.
d. The nucellus is the protective covering around the ovule.

Solution: In the middle of the embryo sac, there are two haploid nuclei, known as the polar nuclei which fuse and form a diploid central cell. It participates in double fertilisation and develops into an endosperm. 

The filiform apparatus is present in the synergids which lie at the micropylar end.

The embryo sac remains embedded in the nucellus of the ovule.

The integuments are the protective covering around the ovule. The nucellus stores food for the growing embryo.

Hence, the correct option is a.

Q 4. Megasporangium corresponds to which of the following structures?

a. Megaspore mother cell
b. Ovule
c. Egg cell
d. Megaspore

Solution: The megasporangium is equivalent to the structure of the ovule. An ovule is described as an integumented megasporangium found in seed plants. They develop into seeds after fertilisation. Hence, the correct option is b.

Q 5. If a plant has 36 chromosomes in the endosperm, then how many chromosomes are there in the megaspore mother cell?

Answer: A megaspore mother cell is a diploid cell which undergoes meiotic division to form haploid megaspores which will eventually undergo repeated mitotic divisions to form a 8 nucleated, 7 celled haploid female gametophyte or embryo sac. The embryo sac consists of three haploid antipodal cells, 2 haploid synergids, one haploid egg cell and one diploid central cell formed by the fusion of two haploid polar nuclei. 

The endosperm is a triploid tissue that develops from the triploid primary endosperm nucleus formed due to fusion of the diploid central cell and one of the haploid male gametes. 

Thus, if the triploid endosperm (3n) has 36 chromosomes, then the haploid (n) cells will contain - 3n/3 = 36/3 = 12 chromosomes. 

This indicates that the diploid (2n) megaspore mother cell will have - 2 x 12 = 24 chromosomes.

FAQs

Q 1. Why do three megaspores degenerate and only one becomes functional?
Answer: The three megaspores degenerate because of the diminishing calcium content. Only one megaspore remains functional because it accumulates calcium granules in the cytoplasm during development.

Q 2. What is the function of the endosperm?
Answer: An endosperm is a triploid structure that forms due to the fusion of one of the haploid male gametes with the diploid central cell formed due to the fusion of two polar nuclei. It is a nutritious tissue that provides nutrients to the developing embryo.

Q 3. What is a coenomegaspore?
Answer: A coenomegaspore is the one which consists of four haploid nuclei as a result of meiosis. It develops into a tetrasporic embryo sac.

Q 4. What is a monosporic embryo sac?
Answer: An embryo sac that develops due to multiple mitotic divisions of a single megaspore is called a monosporic embryo sac.

Youtube link: https://www.youtube.com/watch?v=DVbLRuXGlko&t=2143s