Types of Embryosacs as observed in angiosperms

1. TYPES
OF
EMBRYO SACS
Dr Kavita Rambal

2. Introduction
Embryo sac also known as female gametophyte is formed
as a result of megasporogenesis within the
megasporangium also known as megasporangium.
As a result of megasporogenesis four megaspores are
formed from megaspore mother cell or primary
archesporial cell within the megasporangium also known
as ovule.
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3. TYPES
Three different type of embryo sacs have been reported on the basis of
number of megaspores involved in the development of embryo sac.
 Monosporic
 Bisporic
 Tetrasporic
Basis for classification
 The number of megaspores taking part in the development of embryo sac
 The number of divisions occurring in the nucleus of the functional
megaspore .
 Organization of nuclei in the mature embryo sac
Presentation title 3

4. Monosporic
• 8-nucleate or
Polygonum type
• 4-nucleate or
Oenothera type
Bisporic
• Allium type
• Endymion type
Tetrasporic
• Adoxa type
• Plumbago type
• Penaea type
• Peperomia type
• Drusa type
• Fritillaria type
• Plumbagella type
Presentation title 4

5. Monosporic
(One Megaspore)
• This type of embryo sac
develops from the chalazal
megaspore.
• Its nucleus divides thrice to
form eight nuclei.
• This type is generally
referred to as normal type
of embryo sac.
• It is also called Polygonum
type as it was first time
described in Polygonum
divericatum.
Monosporic
8-nucleate
or
Polygonum
type.
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6. Monosporic
(One Megaspore)
• This type of embryo sac
usually develops from the
micropylar megaspore.
• The megaspore nucleus
divides twice and forms only
four nuclei; of these three
organize into egg apparatus
and the fourth functions as
polar nucleus.
• Thus Oenothera type of
embryo sac does not have any
antipodals.
• This type is characteristic of
the family Onagraceae.
Monosporic
4-nucleate
or
Oenothera
type.
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7. Bisporic embryo sacs
(Two Megaspores)
The Bisporic embryo sac develops from one of the two dyads formed as a
result of the first meiotic division (meiosis I) of megaspore mother cell.
One of the dyads degenerates. Both the nuclei of the functional dyad take part
in the formation of embryo sac.
Each nucleus undergoes two mitotic divisions and as such the mature embryo
sac is 8-nucleate.
The eight nuclei are organized into antipodals, egg apparatus and polar nuclei
as in Polygonum type of embryo sac.
In this type of embryo sac the 4-nuclei derived from one megaspore nucleus
are genetically different from the other four derived from the second megaspore
nucleus. 7

8. Bisporic Embryo sacs
(Two Megaspores)
On the basis of the position of
functional dyad Bisporic embryo
sacs are of two types
ALLIUM TYPE: Develops from the
chalazal dyad
ENDYMION TYPE: Develops from
the micropylar dyad.

9. Tetrasporic Embryo sacs
Meiotic division of the megaspore mother cell is not
accompanied by cytokinesis and hence all the four haploid
nuclei lie in a single cell called Coeno-megaspore.
 All four nuclei of coeno-megaspore participate in the
formation of embryo sac
Genetically more heterogeneous than Bisporic type of embryo
sac.
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10. Tetrasporic Embryo sacs
The Tetrasporic embryo sacs are further divided on the basis of
following criteria-
 The position of haploid nuclei in the coeno-megaspore
 The number of times these nuclei divide
 Organization of nuclei in the mature embryo sac
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11. Tetrasporic Embryo sacs
No nuclear fusion occurs
Plumbago Type
Adoxa Type
Peperomia type
Penaea type
Drusa type
Nuclear fusion occurs
Fritillaria type
Plumbagella type 11

12. Tetrasporic Embryo sacs
• The megaspore nuclei arrange themselves in
a cross-like manner, one lies at the
micropylar end and the other lies at the
chalazal end.
• The other two are present at each side of the
embryo sac.
• Each nucleus divides once. Thus pairs of four
nuclei are formed.
• One nucleus from each pair migrates to the
centre.
• They fuse to form tetraploid secondary
nucleus.
• The nucleus at micropylar end form the egg
cell.
• The rest of the three nuclei degenerate.
• There are no antipodal cells and synergids.
Plumbago
Type
(8-
Nucleate):

13. Tetrasporic Embryo sacs
• Earlier known as “Lilium
type,” is characterized by all 4
megaspore nuclei undergoing
just one more division to form
an 8-nucleate embryo sac.
• Having a normal egg
apparatus, three antipodal
cells, and two polar nuclei.
• It was described for the first
time by Jonsson (1879-1880)
in Adoxa moschatellina .
Adoxa
type

14. Tetrasporic Embryo sacs
• The four haploid nuclei of coeno-
megaspore undergo two successive
mitotic divisions forming 16 nuclei.
• Of these, two nuclei at the micropylar
end form egg and a synergid, eight
fuse in the center of the cell to form a
polar nucleus and the remaining six
which are cut off by membranes at
the chalazal end may be compared
with antipodals.
• Thus the egg apparatus of Peperomia
type is characterized by a single
synergid.
• This type of embryo-sac occurs in
members like Peperomia.
Pepero
mia
type.

15. Tetrasporic Embryo sacs
The four haploid nuclei of the coeno-
megaspore undergo two successive mitotic
divisions forming 16 nuclei.
These nuclei arrange themselves in four groups
of four each, one at the micropylar end, one at
chalazal end and one each on the two lateral
sides.
Now one nucleus from each groups migrates to
the centre, and these four nuclei in the center
form polar nuclei.
The three nuclei at the micropylar end are cut off
by membranes and form the egg apparatus.
The remaining three groups of nuclei (one
chalazal and two lateral) degenerate at maturity.
Thus at maturity the Penaea type of embryo sac
has an egg apparatus and four polar nuclei. There
are no antipodals.
Highly polypoid (5x) primary endosperm
nucleus is formed after double fertilization.
Penaea
type

16. Tetrasporic Embryo sacs
• A 16-nucleate embryo sac of a different
nature was recorded by Hakansson
(1923) in Drusa oppositifolia, a
member of the family Umbelliferae.
• After the meiotic divisions are over,
three of the megaspore nuclei pass
down to the basal end of the embryo
sac, and only one remains at the
micropylar end.
• This 1+3 arrangement is followed by a
2+6 and then a 4 + 12 stage.
• The four micropylar nuclei give rise to
the egg apparatus and upper polar
nucleus, and the 12 chalazal nuclei to a
lower polar nucleus and 11 antipodal
cells.
Drusa
type

17. Tetrasporic Embryo sacs
• The four haploid nuclei of the coeno-
megaspore arrange themselves in two
groups – three at the chalazal end in
the form of a triploid nucleus and one
haploid at the micropylar end.
• The triploid chalazal as well as the
haploid micropylar nucleus undergo
two mitotic divisions and as a result
four trploid nuclei are formed at the
chalazal end and four haploid at the
micropylar end.
• In mature embryo sac three haploid
nuclei organize into egg apparatus,
three triploid into antipodal and
remaining one haploid and one triploid
nuclei move to the centre where they
fuse to form a tetraploid polar nucleus.
Fritillaria
type.

18. Tetrasporic Embryo sacs
• The initial development is similar to
Fritillaria type triploid nucleus at the
chalazal end and a haploid at the micropylar
end.
• Each of these nuclei undergoes a single
mitotic division and form two groups of
two nuclei each.
• One haploid nucleus from the chalazal end
fuse at the centre of the sac to form a
tetraploid polar nucleus.
• One haploid nucleus at the micropylar end
forms the egg and one triploid nucleus at
the chalazal end the single antipodal. There
are no synergids.
• Thus in the mature embryo sac of
Plumbagella type the egg apparatus consists
of a single cell which represents the egg.
Plumbagella
type.

19. Thank you