Megaspore is the first cell of female gametophyte or
embryo sac. Megaspores in turn are formed by the megaspore mother cell after
its meiotic division. The development of female gametophyte in angiosperm is
completely endosporous i.e., within the megaspore.
The formation of the female gametophyte from one or
more megaspore nuclei is called megagametogenesis.
Maheshwari formulated a classification of different types of embryo
sac development based on the following characters: i) the number of megaspores
or megaspore nuclei which participate in the formation of embryo sac, ii) the
total number of divisions which are found during the formation of the megaspore
and the female gametophyte and iii) the number, arrangement of the nuclei and
their chromosome number in the mature embryo sac.
Depending on the number of megaspores taking part in
the development, the embryo sac of angiosperms may be classified into three
main categories - monosporic, bisporic and tetrasporic. Each main group has
more than one type of embryo sac named after the genus in which it was first
described.
In monosporic type, only one of the four megaspores
takes part in the development of the embryo sac. In bisporic type, two
megaspore nuclei take part in the development of the embryo sac. In tetrasporic
type, all the four megaspore nuclei take part in the development of the 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.
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.
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.
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.
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.
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