embryo sac - types, explanation

 

TYPES OF EMBRYO SAC

Monosporic Embryo Sac

The embryo sac in which only one out of four megaspores is involved in the development of the embryo sac is called as Monosporic Embryo Sac. Since all the nuclei in such embryo sac are derived through mitoses of a single nucleus, hence all nuclei are genetically identical.

On the basis of number of nuclei in the mature embryo sac, Maheshwari recognised two types - 8 - nucleate and 4- nucleate.

8- nucleate or Polygonum type:

This type of embryo sac development was first discovered in Polygonum divaricatum for the first time by Strasburger, hence it is called as polygonum type. Since, this is common in Angiosperms(81%), it is called as normal type. Of the four megaspores formed from the megaspore mother cell, only one megaspore situated towards chalazal end remains functional and the remaining three situated towards micropylar end degenerate. The functional megaspore divides and gives rise to 2 nuclei, one of which moves to the micropylar pole and called as primary micropylar nuclei and the other moves to the chalazal pole and is called as primary chalazal nuclei.The second division produces one pair of nuclei at micropylar and chalazal end. The third division results in two groups of 4 nuclei at the opposite poles of the elongated embryo sac. The 4 nuclei towards the micropylar end differentiates into a three celled egg apparatus and the upper polar nucleus. The 4 cells towards the chalazal end differentiates into three antipodal cells and the lower polar nucleus. The two polar nuclei fuse together somewhere in the middle of the embryo sac to give rise to a secondary nucleus. The egg apparatus at the micropylar endconsists of two synergids and an egg cell. The other three nuclei at the chalazal end form the antipodal cells. This type of embryo sac is most common and generally known as the normal type of embryo sac.

4 – nucleate or Oenother Type:

Greets in 1908 first discovered a 4-nucleate monosporic type of embryo sac in Oenothera lamarckiana. Hence it is called as Oenothera type.This type of embryo sac is formed by the micropylar megaspore of the tetrad. The megaspore nucleus undergo mitotic division and form two nuclei which remain at the micropylar end. These two nuclei undergo one mitotic division and form 4 nuclei. Thus megaspore nuclei undergo only two nuclear divisions instead of the three occurring in the Polygonum type of embryo sac. Thus, 4 nuclei are produced which organize to form 4-nucleate embryo sac. Three nuclei remain at the micropylar end and form an egg apparatus consisting of one egg cell and two synergids. The fourth nucleus acts as polar nuclei. Since, the third division is omitted and all the nuclei are situated in the micryopylar end of the embryo sac, there is neither a lower polar nuclei nor any antipodal cells. This type of embryo sac development is found to be a characteristic and constant feature of family Onagraceae.

Bisporic Embryo Sac:

In plants bearing the bisporic embryo sacs the first meiotic division in the megaspore mother cell is accompanied by wall formation, so that a dyad is formed. The upper cell of the dyad is much smaller and soon degenerates. In the functional dyad cell, the nucleus undergoes second meiotic division to form 2 megaspore nuclei. Wall formation does not occur after the second division, and both the megaspore nuclei contribute to the formation of the embryo sac. Since a bisporic embryo sac is derived from two meiotic division their nuclei belong to different genetic constitution.

Allium Type:

The chalazal dyad cell remains functional and gives rise to a 8 nucleate embryo sac. Strasburger described for the first time a bisporic embryo sac in Allium fistulosum.  Each megaspore nucleus undergoes two mitotic divisions forming eight nuclei.The eight nuclei formed organize similar to the polygonum type.

Endymion Type:

In this type, the micropylar dyad cell remains functional and gives rise to a 8 nucleate embryo sac.

Tetrasporic Embryo Sac:

In this type of Embryo sac development, the megaspore mother cell undergoes two meiotic division forming four nuclei. Neither of the meiotic divisions is accompanied by wall formation so that at the end of meiosis all the four haploid nuclei remain in a common cytoplasm forming a coenomegaspore.  All the four nuclei of the coenomegaspore take part in the formation of the embryo sac. A tetrasporic embryo sac is more heterogenous than a bisporic embryo sac because the four nuclei of meiosis are involved in the formation of the embryo sac are genetically different.

Before the onset of postmeiotic mitosis, the manner in which the four nuclei in the coenomegaspore are arranged is of three types:

1)     1+1+1+1 arrangement: one nucleus toward micropyle, one nucleus toward chalaza and other two placed laterally (one on each side), eg., Peperomia, Penaea, Plumbago type.

2)     2+2 arrangement: two nuclei toward micropyle and two  towards chalaza, eg., Adoxa type

3)     1+3 arrangement: one nucleus toward micropylar end and three towards the chalaza, eg., Dursa, Fritillaria and Plumbagella type.

16-nucleate tetrasporic type:

Peperomia type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These for nuclei undergo two mitotic division and form 16 nuclei. Of these 16 nuclei, two nuclei at the micropylar end become cellular and form egg apparatus consisting of one egg cell and one synergid. 6 nuclei at the chalazal end become cellular and form 6 antipodals. The remaining 8 nuclei fuse in the centre of embryo sac and form secondary nucleus with 8n condition.

Penaea type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These for nuclei undergo two mitotic division and form 16 nuclei. These 16 nuclei organise into 4 groups; one group toward micropyle, one group toward chalaza and two groups lateral (4+4+4+4) arrangement. One nucleus from each group migrate towards the centre of the embryo sac. They give rise to tetraploid secondary nucleus. The three nuclei at the micropylar side form egg apparatus by the formation of walls. It consists one egg cell and two synergids. The remaining 9 nuclei organize into 3 groups of antipodal cells by formation of septa which are situated on 3 sides. These are called as triads.

Drusa type:

In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed. These are arranged in 1+3 manner. One nucleus at the micropylar end and the remaining 3 nuclei at chalazal end. These four nuclei undergo two mitotic division and form 16 nuclei. 4 nuclei are present at the micropylar end and 12 nuclei at the chalazal end. Of the 4 nuclei at the micropylar end 3 nuclei organize and form egg apparatus. The fourth nuclei function as upper polar nucleus. Among the 12 nuclei at the chalazal end one function as lower polar nuclei. Both upper and lower polar nuclei move in the centre of embryo sac and form diploid secondary nucleus. The remaining 11 nuclei at the chalazal end organise into antipodals.

8 -  Nucleate Tetrasporic Embryo sac:

Adoxa Type:

It was first studies in Adoxa moschatellina by Jonsson. Embryo sac is 8 nucleate formed after a single post meiotic mitosis and has Polygonum type of arrangement. It is also found in Sambucus, Tulipa, Ulmus etc. In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed, of these two nuclei are present at each pole. The four nuclei undergo one mitotic division and form 8 nuclei. Of these 4 nuclei from the micropylar end 3 nuclei organsie into egg apparatus by the formation of cell wall and one function as upper polar nuclei.The four nuclei from the chalazal end 3 nuclei organize into antipodals and one remaining nucleus function as lower polar nuclei. The two polar nuclei migrate towards the centre and give rise to a secondary nucleus.

Fritillaria type:

This type of embryo sac was first observed in Fritillaria bulbiferum. In this type, as a result of two meiotic divisions in the megaspore mother cell 4 nuclei are formed, which are arranged in 1+3 manner i.e., one nuclei at micropylar end and 3 towards the chalazal end. The three nuclei at chazal end fuse together and form a triploid nucleus. Now the embryo sac shows only 2 nuclei one haploid nuclei at the micropylar end and one triploid nuclei at chalazal end. Each nuclei undergo one mitotic division forming 4 nuclei out of which 2 are haploid nuclei and two are triploid nuclei. These four nuclei undergo second mitotic division and form 8 nuclei – 4 towards micropylar end are haploid and 4 towards chalazal end are triploid. Of the four nuclei at  micropylar end, 3 organize into egg apparatus and one functions as upper polar nuclei. At the chalazal end, 3 nuclei organize into triploid antipodals and one functions as triploid lower polar nuclei. The two polar nuclei( one haploid and one triploid move towards the centre of the embryo sac and form a tetraploid secondary nuclei.

Plumbago type:

This type of embryo sac was first observed in Plumbago capensis.In this type, as a result of two meiotic divisions in the megaspore mother cell 4  haploid  nuclei are formed, which are arranged in 1+1+1+1 manner i.e., one nuclei at micropylar end and 1 towards the chalazal end and remaining two on lateral side, one at each side. These 4 nuclei undergo mitotic division and form 8 nuclei. Out of 2 nuclei at micropylar end, 1 nuclei function as egg cell. One nucleus from each side migrate towards the centre and form a tetraploid secondary nucleus. The remaining 3 nuclei which lie on the sides forms cell and later degenerate.

 

 4- nucleate Embryosac or Plumbagella type:

It was discovered in Plumbagella micrantha. Four megaspore nuclei are formed as a result of meiosis in megaspore mother cell. These 4 nuclei are arranged in 1+3 manner – one nucleus at the micropylar end and the other 3 at the chalazal end. The three nuclei at chalazal end fuse and form a triploid nucleus. So, there are two nuclei – one haploid nuclei at micropylar end and one triploid nuclei at chalazal end. Now, these two nuclei present at opposite poles undergo mitotic division and form 2 haploid nuclei at micropylar end and two triploid nuclei at chalazal end. Of the two haploid nuclei at the micropylar end one nucleus forms egg cell and other nucleus acts as upper polar nucleus. Of the two triploid nucluei at the chalazal end one acts as lower polar nucleus and other forms an antipodal cell. In the centre of embryo sac the two polar nuclei fuse and form an tetraploid (4n) secondary nucleus. The important characteristic of this type of embryo sac are the presence of one triploid antipodal cell, one egg cell and one tetraploid secondary nucleus. The synergids are absent.