Types of Embryo Sac in Angiosperms

A megaspore is the first cell of an embryo sac. The single cell divides and differentiates to form the various parts of an embryo. The main parts of an embryo sac are the antipodals, egg apparatus- synergids and egg cell, and central polar nuclei.

Table of Contents

Different Types of Embryo Sac

There are different types of embryo sacs depending on the number of megaspores involved in embryo sac development.

Monosporic
Bisporic
Tetrasporic

In most plants, the embryo sac development is monosporic. It means that the embryo sac is developed from a single megaspore and thus every nuclei inside the embryo sac is identical. Bisporic embryo sacs are formed from two megaspores while the tetrasporic embryo sac development involves all the four megaspores from the megaspore mother cell.

I Monosporic Embryo Sac

One of the best examples of monosporic embryo sac development is in Polygonum which is also known as Polygonum type of embryo sac development. In Polygonum, the ovule or the megaspore mother cell undergoes meiotic division to form a linear tetrad of four haploid megaspores.

Of these four megaspores, three megaspores near the micropylar region perish and the single megaspore in the chalazal region survives. The embryo sac is developed from this single surviving megaspore and thus the name monosporic embryo sac development.

1. Polygonum Type of Embryo Sac Development

The megaspore at the chalazal end enlarges, elongates, and undergoes vacuolation.
Its single nucleus divides mitotically into two and each migrates in the opposite direction to form a micropylar and primary chalazal nucleus.
These nuclei will then be surrounded by cytoplasm.
A large vacuole is formed in the center.
Both the single nuclei divide to form a four-nucleate stage of the embryo sac.
All the four nuclei divide once more to form 8 nuclei.
These 8 nuclei will be evenly distributed at each end having 4 nuclei at each end.
They will not divide further, instead will rearrange themselves for further development of the embryo sac.
Of the four nuclei at the micropylar end, three nuclei form the egg apparatus while the fourth one remains as the upper polar nucleus.
One nucleus in the egg apparatus forms the central large nucleus called the egg and the other two become synergids.
Of the four nuclei at the chalazal end, three nuclei form the antipodals and the last nucleus becomes the lower polar nucleus.
In the next step, the upper and lower polar nuclei move towards the center and fuse to form a diploid secondary nucleus.
The remaining nuclei inside the embryo sac remain haploid.
The megaspore will then develop a cell wall for the embryo sac that now has 7 nuclei- 6 haploids and one diploid, inside.
Later, these nuclei will be surrounded by cytoplasm to function as individual cells.

2. Oenothera Type of Embryo Sac Development

Here, the single megaspore nucleus undergoes one mitotic division to form a 4-nucleate megaspore. One nucleus forms the polar nuclei while the other three become the egg cell and two synergids. There are no antipodals in Oenothera. This type of embryo sac is mainly seen in the Onagraceae family.

II Bisporic Embryo Sac

The bisporic embryo sac is developed from the dyad of cells that is developed after the first meiosis of the megaspore mother cell. One of the cells remains functional and divides its nucleus to develop the embryo sac. Examples of such embryo sacs is seen in Allium and Endymion but they differ slightly from one another.

In Allium, the dyad of megaspore cells that form the embryo sac is from the chalazal end while in Endymyon, the dyad of cells comes from the micropylar end. Otherwise, the embryo sac development remains the same in both cases.

Here, the megaspore undergoes mitotic division twice to form the 8-nucleate stage.
Two nuclei from each end migrate to the center to form the polar nuclei.
The remaining three nuclei at the ends form the antipodals and egg apparatus.

III Tetrasporic Embryo Sac Development

In a tetrasporic embryo sac development, the megaspore mother cell undergoes meiotic division without cell wall formation. This results in four haploid nuclei in the cell without any cell wall around them.

Such megaspores having four haploid nuclei without any cell wall between are called coeno-megaspores. Each of these four nuclei takes part in the embryo sac formation and thus it is called tetrasporic embryo sac formation.

Depending on different criteria, tetrasporic embryo sacs can be categorized into three. These criteria are,

Position of the haploid nuclei in the megaspore mother cell
How many mitotic divisions do they undergo
How they organize inside the mature embryo sac

1. Adoxa Type of Embryo Sac Development (8-nucleate)

The four haploid nuclei inside the megaspore mother cell are distributed in such a way that the microplyar and chalazal end receives two nuclei each.

Each of these nuclei undergoes mitotic division to form 8 nuclei.
One nucleus from each end migrates towards the center to form polar nuclei.
The three nuclei at the micropylar end form the egg apparatus having an egg cell and two synergids.
The three nuclei at the chalazal end form the antipodals.

2. Plumbago Type of Embryo Sac Development (8-nucleate)

The embryo sac of Plumbago lacks synergids and antipodals.
The four haploid nuclei in the megaspore mother cell move towards the four corners- micropylar end, chalazal end, and two lateral sides.
Each of these nuclei divides mitotically to form a pair of nuclei at each corner.
One nuclei from each corner move towards the center and form the polar nuclei.
A membrane separates the single nuclei at the micropylar end which will form the egg.

The other haploid nuclei at the other three corners will either perish or form a membrane around them to form accessory egg cells.

3. Peperomia Type of Embryo Sac Development (16-nucleate)

Embryo sac development in Peperomia is an example of a tetrasporic method of embryo sac development.

Each of these 4 nuclei undergoes mitotic divisions twice.
They form 8 nuclei after the first mitosis and 16 nuclei at the end of the second mitotic division. These 16 nuclei distribute themselves differently.
8 of these 16 nuclei migrate to the center and aggregate themselves.
The remaining 8 nuclei are equally distributed to the four corners of the cell, each having 2 nuclei.
Two haploid nuclei at the micropylar end form the egg apparatus forming an egg and a synergid cell.
The other 6 peripheral nuclei migrate to the chalazal end to form the antipodals.
The 8 aggregate nuclei in the center fuse to form the polar nucleus.

4. Penaea Type of Embryo Sac Development (16-nucleate)

The four haploid nuclei in the coeno-megaspore migrate towards the foru corners of the cell.
Each haploid nuclei divide mitotically twice.
This results in each corner having 4 nuclei each.
One nuclei from each corner move towards the centre and these 4 nuclei will form the polar nuclei.
The three nuclei at the micropylar end form the egg apparatus- one egg cell and two synergids on either side.
The three nuclei at the chalazal end and the two lateral sides will eventually perish as the embryo sac matures.
The four polar nuclei fuse to form a polyploid which will become 5x polyploid after double fertilization.
Such type of tetrasporic embryo sac development is seen in members of Euphorbiaceae, Penaeaceae, etc.

5. Drusa Type of Embryo Sac Development (16-nucleate)

In Drusa, the 16-nucleate stage of the embryo sac shows a difference in their distribution, when compared to other types.

Here, three nuclei among the 16, move towards the micropylar end to form the egg apparatus- one egg cell and two synergids.
Two nuclei move towards the center to form the polar nuclei.
The 11 remaining nuclei move towards the chalazal end from 11 antipodals.

References

https://mgcub.ac.in/pdf/material/202004300509276c475952aa.pdf
Sukumaran O R. Pre-Degree Botany. Murali Publications.
Abraham P C. Anatomy, Embryology & Microtechnique. 1999. St. Mary’s Books & Publications.

Types of Embryo Sac in Angiosperms