12. Development of anther
protoderm
Hypoderm-found
beneath protoderm and
becomes
Archesporial layer. Divides
into:L
1.Pri parietal cells
(outer)-
differentiates into
sporangial outer wall-
--endothecium and
tapetum
2. Primary sporogenous
Sporangium initiation is restricted
to four separated areas
corresponding to corners of the
developing anthers
13. Anther still meristematic lobe .Periclinal division
occurs beneath the protoderm, layer called
archesporial layer
14. At maturity, the 2
sporangia of each
side become
confluent due to
breaking down of the
partition between
them
Endothecium lying
immediately beneath
epidermis, hygroscopic-
aids in dehiscence
Microsporocytes-or
microspore mother
cells are derived
from archesporial
cells
15.
16. Parts of ovule
1.nucellus- central body with vegetative cells enclosing the
Sporogenous cells
2. 1 or 2 integuments (unitegmic or bitegmic) enclosing the
Nucellus
3. funuculus-stalk connecting ovule with the placenta.
4. chalaza-where nucellus, integuments and the funiculus
merge
17. X-section of an ovary Ovary contains a cavity
lined with an epidermal
layer. Ovules develop
from the epidermal
cells and are contained
within the cavity of the
ovary, attached to its
inner surface by a
short stalk- funiculus
Ovary bears
ovules on a
ridge on the
ovary wall
called placenta
18. Developing ovules of Lilium. Ovule emerges from the
placenta as conical protuberance with the first
sporogenous cell,called archesporial cell.
Integuments formed by periclinal div. of epidermis
20. A, D. (bel mutant)
exposed nucellus
and a single
integument
F. Larger mutant ovule
Outer integument has
many cells.
21. Megaspore mother cell
differentiates from
surrounding nucellar tissue
and undergoes meiosis.
Inner integ.
Outer integ
Begin as
ridges of
tissue early in
Ovule dev.
C, E,F differential growth of
ovule causes them to curve
so micropyle is bent around
funiculus and placenta (C).
22. Diakinesis- homologs are held
together by chiasmata at their
tips.
Summary: ist meiotic prophase- replicated homologous
chromosomes synapse, usually undergo crossing-over, then
condense as tetrads. Held together at the centromeres, pairs of
Sister chromatids in each tetrad are ready to be distributed to
opposite .poles during the remainder of the first meiotic division
23. a,b) different stages
of pachytene
c.) metaphase with
precocious
migration to the pole
d) Anaphase with
laggard
chromosome
e) Telophase
f) Metaphase II with
precocious
migration
g) Late anaphase
H) Telophase II
i) tetrad
24. Tapetum- originates from
Primary parietal cells.
Dense cytoplasm, may
become polyploid or
multinucleate.
completely invest the
locule, have nutritional
function by transferring
food materials to the
differentiating pollen
grains
25. Meiosis I
Pair and exchange
segments
Chromosomes line
up by homologous
pairs
Each pair of
homologous
chromosomes
separates
Two haploid
cells form, each
chromosome still
consists of two
sister chromatids
26. Leptotene- chromatin condenses, preceded by DNA replication
Zygonema-homologous chromosomes pair form bivalents
Pachytene-physical exchange of chromosome parts occurs bet
homologous chromosomes
Diplotene- partial separation
of each of sister chromatids
from their homologous
chromatids
27. Meiosis I
Leptotene-chromatin condenses ,reveal individual
chromosomes. Already replicated.
Zygotene- separated homologous chromosomes
pair (synapsis)with each other forming bivalents or
chromosome pairs.
Pachytene-Physical exchange of chromosome parts
occur between homologous chromosomes. Arms of the
two pairs of sister chromatids separate from each other
and then each arm becomes closely associated with its
homologous pair.tetrad- group of 4 chromatids
Crossing-over or recombination occurs
Diplotene- partial separation of each pair of sister
chromatids from their homologous counterparts.Sister
chromatids still held together.
Diakinesis-chromatids condense and appear compact
rods that are grouped as tetrads.
28. Chromosomes
still composed
of two
chromatids
Chromosomes
at metaphase
plate. Due to
crossing –
over in
Meiosis I, each
chromosome not
genetically identical.
Anaphase II
Sister chromatids
Separate, move to
opposite poles as
Individual chromosomes
Telophase II and
Cytokinesis. Nuclei form.
Chromosomes begin
decondensing
Meiosis II
29.
30. Meiosis
Requires 2 nuclear divisions
Produces 4 haploid daughter cells
Replication occurs only once.
Daughter cells receive on of each kind of parental
chromosome but in different combinations.
Daughter cells not genetically identical to parent cell
Or to each other
31. Cells lining the
anther lumen – a
layer known as the
endothecium –
secretes materials
that are essential
for the proper
maturation of the
pollen grains.
Cells of the endothecium are so densely cytoplasmic
that no vacuoles at all are visible; it is even difficult
to distinguish one cell from another in most of the
endothecium. The dark red dots in the both the
endothecium and the pollen grains are nucleoli, with
the actual nuclei being just barely visible around a
few of them
33. Telophase I cell plate will
disperse without forming
cell wall
Early prophase i Mid prophase I
Metaphase II
34. Typical anther has
four elongated microsporangia.
At maturity the two sporangia
become confluent owing to the
breaking down of the
partition between them
36. Pollen development and maturation
The end of meiosis in the microsporocyte or microspore
mother cell marks a turning point in microsporogenesis .
Results in the production of 4 microspores, each with its
own callose envelope.
A candidate gene for separation of microspores from
the tetrad in Arabidopsis anthers is designated as
QUARTET (QRT).
Outcome of microsporogenesis affected by this
mutation is release of microspore in tetrads
Failure of microspore separation in qrt mutants
Traced to the fusion of exine layer of adjacent
microspores Failure of protein degradation
37. Pectin is absent in primary wall of wild type
microspores at the time of release from tetrad
Pectin remains as integral part of the
microspore wall of mutant
QRT gene functions in degradation of
pectin in order to separate the microspore
from tetrad.
Model of primary cell wall
38. In the wild types, after release from the tetrad, the microspore
as the first cell of the gametophyte generation further
differentiates to produce sperm cells.
Internally, microspore increase in size and externally forms a
double-layered wall, outer sculptured exine and inner smooth
intine.
First element of exine is called primexine- detected in the
microspores while still in tetrad stage.
Exine consists of sporopollenin, synthesized in the tapetum,
substance resistant to chemicals and biodegradation.
Exine formation is programmed by the diploid genome of
microsporocyte.
Intine is programmed by haploid genome of microspore, made
up of cellulose microfibrils embedded in a matrix of pectin and
hemicellulose
39. Pollen grain has two cells: from first mitotic division
Vegetative cell- develops into pollen tube.
Contains most cytoplasmic organelles
Generative cell- small, produces the sperm
-cytoplasm partitioned unequally during
mitotic division of microspore
-lacks mitochondria and chloroplast
-at some point in pollen dev., divides by
mitosis, each daughter cell differentiates into sperm
cells, will lack also chloroplasts and mitochondria.
This is the basis of for the maternal inheritance of
chloroplast and mitochondrial genomes which
occurs in ca 90% of all angiosperm species.
40.
41. Development of embryo sac and female gamete (in
an anatropous ovule)
A hypodermal cell of the nucellus enlarges and
becomes differentiated into a megaspore mother cell
or megasporocyte. This diploid megaspore mother
cell increases in size and undergoes meiosis to form
a linear tetrad of 4 haploid megaspores, 3 of which
degenerate and the 4th becomes the functional
megaspore in monosporic types, all 4 become
functional in tetrasporic types
Female Gametophyte
The nucleus of the megaspore undergoes three
successive mitotic divisions forming eight nuclei. The
megaspore enlarges into an oval shaped structure
called the embryo sac. The eight nuclei of the
embryo sac arrange themselves in 3 groups.
42.
43. Micropyle
Inner integument
Outer integument
placenta
funuculus
Outer and inner integument completely overgrow the nucellus
Except for the micropyle.
--Begins with elongation of the functional megaspore,
usually at chalazal end.
-- initially megaspore is non-vacuolate but later small
vacuoles appear which may fuse to form large vacuole.
Development of
embryo sac
44. A. First megaspore mitosis yields binucleate embryo sac.
Spindle of first nuclear div oriented along the long axis of
the cell. Wall formation Does Not follow the nuclear
division. Both nuclei divide 2x, forming 4 in B then 8 in C
B. Large vacuole appears between the two daughter nuclei.
As cell expands, nuclei are pushed toward opposite poles
of the cell. Both nuclei from each pole divide twice
45. D. The 8 nuclei arrange themselves in two clusters of 4 nuclei
one at each opposite ends. One nucleus from each end
migrates towards the middle, called polar nuclei (named for
where they came from, not where they end up).
C.8-nucleate state . All
8 nuclei are present in
a common cytoplasm,
they move around
probably from
remnants of spindle
fibers from earlier
divisions.
46. Chalazal trio called antipodals
( Latin “against the foot”) at
opposite end of the egg and
antipodals
Egg apparatus consists of larger
egg flanked by two smaller cells
called synergids (greek for
“helpers” or cooperators
The large binucleate
49. Megaspore mother cell devs. from surrounding nucellar
tissue and undergoes meiotic division to form megaspore.
Nucellus considered as a megasporangium
funiculus
nucellus
chalaza- region where integuments fuse with funiculus
54. Mutants in ovule determination
1. bell (bel1)- ovule lacks inner integument
2. Aberrant testa shape (ats)- no clear distinction between
inner and outer integument
3. Extreme types of integument mutations:
aintegumenta (ant)
huellenhos (hll)
Do not develop integument and
embryo is disrupted.
56. Embryo sac cells
1.Egg- highly vacuolate, strongly polarized. In Arabidopsis,
a large vacuole aligned toward micropylar end and an
aggregation of cytoplasmic organelles and nucleus at
chalazal end. Ultrastructural simplicity of cytoplasm
characterize egg cells.
amount of cytoplasm is limited
cytoplasm spread as a thin layer surrounding vacuole
cytoplasm contains very little ER, limited no. of plastids
mitochondria, dictyosomes but high ribosomes which
are randomly distributed rather than aggregated as
polyribosomes
cell wall does not extend over the entire cell but wall
shows various attenuation toward chalazal pole
57. 2 Synergids- limited life span, wilt after fertilization.
Probably involved in nutrition of egg.
has extensive wall ingrowth at micropylar region
called filiform apparatus
metabolically active
3 Antipodals-transient existence , minimal cytoplasmic
organelle show nuclear abnormalities like
endoreplication
2 polar nuclei-metabolically active, extensive ER,
numerous plastids, mitochondria, dictyosomes and
polysomes, has large quantities of starch, proteins and
lipids
60. Telophase of microspore mitosis in African lily. Most
organelles are unequally segregated. Plastid is dividing
adjacent to the chromatin of the future generative cell, but no
plastids occur between cell plate and chromatin of the future
generative cell.
Cell plate
Dividing
plastid
Generative
cell
Vege
-
tative
cell
61. Post-meiosis: internal microspore/pollen events
After a microspore enlarges in volume, unequal
partitioning of cytoplasm takes place, it divides
mitotically to form
small lens to spheroidal shaped generative cell
pressed against the vegetative cell membrane
The generative cell moves away from the wall and into
the interior of the vegetative cell after callose dissolves.
Thus, one cell is completely surrounded by another cell.
Generative cells typically become ovate to elongate
while in the pollen grain. Lack plastids, before
microspore mitosis, the plastids usually migrate to
an area of the vegetative cell away from where the
future generative cell will form.
62. A. Microspore
B. Post-mitotic pollen grain with vegetative cell and
newly-formed generative cell.
C. Large central vacuole and generative cell appressed
to wall
63. D. Pollen grain and generative cell have enlarged.
E. Generative cell in mitosis
F. Binucleate generative cell appressed to pollen wall
64. G. Two sperm cells still attached to each other but free from
pollen wall; pollen engorging but central vacuole
still present.
H. Mature engorged pollen grain with separated lenticular
sperm cells embedded in vegetative cell.
65. Plastids in generative cell or sperm cells are uncommon.
No plastids in 18 grass species (includes common cereal
grasses.
None in any of the 7 crucifers (Brassicaceae) tested
Among 39 legumes, 9 species had plastids.
Pollen of most species shed from the anther with just
generative and a vegetative cell.
A sample of 2,000 dicots and monocots showed 30%
were 3-celled
67. The exine protects the spore against dessication,
mechanical pressure and ultraviolet radiation. Sometimes
the exine layer is covered by sticky substances (pollenkitt,
tryphine, elastoviscin and sporopollenin viscin threads),
which are also produced by the tapetum.
Pollenkitt- an adhesive material facilitates the attachment of
pollen grains to insects, and in this way also zoophilic
pollination.
It also plays an important role in the adhesion of pollen
grains to the female stigma and in the recognition between
pollen and pistil. Also substances responsible for pollen
allergy are often products originating from the tapetum.
Pollen grains
68. Germination of pollen tube
Pollen tubes
extend up to sev.
cm to reach
embryo sac. Cell
wall lacks
cellulose but has
another
polysaccharide-
callose- , a glucan
Callose –synthesized by Golgi and transported
to the extreme tip of pollen tube by Golgi-derived
vesicles .Fusion of vesicles with plasma
membrane expand the cell membrane of
elongating tube
Content of vesicles expand the wall of elongating tube
69. In angiosperms, to effect fertilization, the pollen grains
germinate on the stigma by putting forth tubes
(pollen tubes) which grow thru the style and find their way
into the ovules where they discharge the sperms in the
vicinity of the egg.
70. Page 114
Tube enters at the apex of the filiform apparatus and after
growing thru it arrives in the cytoplasm of the synergid. The
penetrated synergid starts degenerating before the arrival of
the Pollen tube, but after pollination. The process of
discharge takes place in seconds.
71. In cotton, the contents of the tube are discharged thru a
subterminal pore which faces the chalaza.
72. Pollen tube discharge: includes 2 sperms, the veg, nucleus
and a fair amount of cytoplasm. A portion of cytoplasm is
retained in the pollen tube.
No mixing between cytoplasm released by the pollen tube and
that of the synergid. They remain as two separate entities.
73.
74. Secretory vesicles originate from the Golgi network
and are transported over the actin cytoskeleton into the
growing tip where they fuse with the tip membrane
expanding the wall and the plasma membrane.
Besides pollen tube wall proteins, these transport
vesicles contain mainly methyl-esterified pectins as
part of the primary tube tip wall.
Germinating
pollen
Editor's Notes
Figure 13.8 The meiotic division of an animal cell
