Presentation on microsporogenesis and megasporogenesis

1. MICROSPOROGENESIS
AND
MEGASPOROGENESIS
Submitted to :-DR.KAUSHIK PANIGRAHI
Submitted by:-SRADHANJALI DHIR
Roll.no:-48ho/16

2. Megasporogenesis
 In gymnosperms and flowering plants, the megaspore is
produced inside the nucleus of the ovule. During
megasporogenesis, a diploid precursor cell,
the megasporocyte or megaspore mother cell,
undergoes meiosis to produce initially four haploid cells
the megaspores.Angiosperms exhibit three patterns of
megasporogenesis: monosporic, bisporic, and tetrasporic,
also known as the Polygonum type, the Alisma type, and
the Drusa type, respectively. The monosporic pattern
occurs most frequently (>70% of angiosperms) and is
found in many economically and biologically important
groups such as Brassicaceae
(e.g., Arabidopsis, Capsella, Brassica), Gramineae (e.g.,
maize, rice, wheat), Malvaceae (e.g., cotton),
Leguminoseae (e.g., beans, soybean), and Solanaceae
(e.g., pepper, tobacco, tomato, potato, petunia).

3.  This pattern is characterized by cell plate formation
after meiosis 1 & 2, which results in four one-
nucleate megaspores, of which three degenerate.
 The bisporic pattern is characterized by cell plate
formation only after meiosis 1, and results in two
two-nucleate megaspores, of which one
degenerates.
 The tetrasporic pattern is characterized by cell
plates failing to form after either

4.  meiosis 1 or 2, and results in one four-nucleate
megaspore.Therefore, each pattern gives rise to
a single functional megaspore which contains
one, two, or four meiotic nuclei, respectively.
 The megaspore then undergoes
megagametogenesis to give rise to the
female gametophyte.

5. Megagametogenesis
 Plant ovules with megasporocytes before
meiosis: Gymnosperm ovule on left,
angiosperm ovule (inside ovary) on right
 After megasporogenesis, the megaspore
develops into the female gametophyte (the
embryo sac) in a process called
megagametogenesis.The process of
megagametogenesis varies depending on
which pattern of megasporogenesis occurred.

6.  Some species, such as Tridax trilobata, Ehretia
laevis, and Alectra thomsoni, can undergo
different patterns of megasporogenesis and
therefore different patterns of
megagametogenesis. If the monosporic
pattern occurred, the
single nucleus undergoes mitosis three times,
producing an eight-nucleate cell.These eight
nuclei are arranged into two groups of four.

7.  These groups both send a nucleus to the center
of the cell; these become the polar nuclei.
Depending on the species, these nuclei fuse
together before or upon fertilization of the
central cell.
 The three nuclei at the end of the cell near the
micropylar become the egg apparatus, with an
egg cell in the center and two synergids.
 At the other end of the cell, a cell wall forms
around the nuclei and forms the antipodals.
Therefore, the resulting embryo sac is a seven-
celled structure consisting of one central cell,
one egg cell, two synergid cells, and three
antipodal cells.

8.  The bisporic and tetrasporic patterns
undergo varying processes and result in
varying embryo sacs as well.
 In Lilium which has a tetrasporic pattern, the
central cell of the embryo sac is 4N.
 Therefore, upon fertilization the endosperm
will be 5N rather than the typical 3N

9. Microsporogenesis
 The formation of microspores inside the
microsporangia (or pollen sacs) of seed
plants.A diploid cell in the microsporangium,
called a microsporocyte or a pollen mother
cell, undergoes meiosis and gives rise to four
haploid microspores. Each microspore then
develops into a pollen grain (the
microgametophyte).

10.  Structure of Stamen, Anther, Pollen Sac and Pollen Grain in
Plants!
 (a) The Stamen:
 Stamen in a flower consists of two parts, the long narrow
stalk like filament and upper broader knob-like bi-lobed
anther .
 The proximal end of the filament is attached to the
thalamus or petal of the flower. The number and length of
stamens vary in different species.
 (b) Structure of anther:
 A normal bithecous or dithecous anther is made up of two
anther lobes, which are connected by a strip of sterile part
called connective. Two anther lobes contain four elongated
cavities or pollen sacs (microsporangia) (Fig. 2.5B) in
which pollen grains are produced.

11.  (c) Structure of microsporangium (pollen sac):
 Young anther while it is still in flower bud inT.S.
reveals the presence of outermost epidermis.
 The outermost wall layer lying just below the
epidermis is called endothecium or fibrous layer
(Fig. 2.5 C), because wall (two radial and inner)
develop fibrous thickenings on them except at
the junctions of two pollen sacs. Below the
endothecium, there are 1-3 middle layers of
parenchyma cells.

12.  The cells of innermost wall layer are radially elongated
and rich in protoplasmic contents.This layer is called
tapetum.The tapetum forms the nutritive tissue
nourishing the developing microspores.The cells of
tapetum may be multinucleate or may have large
polyploid nucleus.The tapetal cells provide nourishment
to young microspore mother cells either by forming a
plasmodium (amoeboid or invasive type) or through
diffusion (parietal or secretory type).
 The pollen sac wall encloses a number of archesporial
cells that further forms microspore mother cells
(microsporocytes). In the beginning microspore mother
cells are polygonal and closely packed, but as the anther
enlarges, the pollen sac becomes spacious and gets
loosely arranged. A few microspore mother cells become
non- functional and are finally absorbed by developing
microspores

13.  During microsporogenesis the nucleus of each
microspore mother cell undergoes meiosis and
gives rise to four haploid nuclei (microspore
tetrad).
 These four nuclei are arranged in a tetrahedral
manner forming tetrahedral tetrad.
 The four microspores separate from each other,
and each develops a characteristic shape or form
which differs in different species of plants.

14. (d) Structure of microspore (Pollen grain):
 Pollen grains develop from the diploid microspore mother
cells in pollen sacs of anthers. Typically, pollen grain is a
haploid, unicellular body with a single nucleus. Pollen
grains are generally spherical measuring about 25-30
micrometeres in diameter. The outer surface of
microspores may have spines, ridges or furrows which
may vary in other ways in different species.

 There may be oval, ellipsoidal, triangular, lobed or even
crescent-shaped pollen grains. The cytoplasm is
surrounded by a two layered wall. The outer layer exine is
thick and sculptured or smooth. It is cuticularised and the
cutin is of special type called sporopollenin which is
resistant to chemical and biological decomposition. In
insect pollinated pollen grains, the exine is covered by a
yellowish, viscous and sticky substance called pollen kit.

15.  Pollen grains are well preserved as fossils because of
the presence of sporopollenin. At certain places the
exine remains tirin.
 The thin areas are known as germ pores, when they
are circular in outline and germ furrows when they
are elongated.The cytoplasm is rich in starch and
unsaturated oils.
 Uninucleate protoplast becomes 2-3 celled at the
later stages of development.The branch of study of
pollen grains is called palynology.
 In Calotropis and orchids, the pollen of each anther
lobe form a characteristic mass called pollinium.
Each pollinium is provided with a stalk called
caudicle and a sticky base called disc or
corpusculum.

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