Bio 130 microsporogenesis gametogenesis 2013

2. megasporogenesis
megagametogenesis

3. Microsporogenesis-
formation of spores
called microspores
Microgametogenesis-
development of
microspore into the
microgametophyte or
the pollen grain
containing sperm cells

4. Ontogeny of the anther
Development and differentiation of sporogenous tissue
Structure of pollen
Events in anther development

5. Androecium-collective
name for all stamens
In a flower.
Anther- for pollen
development
Filament- support, nutrient
transport , pollen dispersal
Wind-pollinated species-
filaments forms a flexible
swivel joint, causes anther
to flutter and shake out
pollen
Longitudinal cutaway view
of a cherry flower

6. Vasculature
in the
Filament
Both anther
and filament
traversed by
a single
vascular
bundle –with
xylem and
phloem

7. Development of anther
protoderm
Hypoderm-found beneath protoderm and
becomes archesporial layer. Divides into:
1. Pri parietal cells
(outer)- differentiates into sporangial
outer wall- --endothecium and tapetum
2. Primary sporogenous cells- microsporocytes
Sporangium
initiation is
restricted to four
separated areas
corresponding to
corners of the
developing anthers
hypoderm
microsporocytes

8. Stamen initiation
and emergence
diagrams
The cellular events of stamen
initiation involves
•contributions from hypodermal
layer in some
•with accompanying anticlinal
activity from protoderm and a
little from outer corpus.
A.Before emergence.periclinal div. of corpus cells at stamen site
B.An emergent stamen. Periclinal div of corpus cells but not the hypd.
C.Tangential div. of emergent stamen.
D.Emergent stamen with recent anticlinal div. of hypodermal cells and
anticli. and periclinal div of corpus. E. adjacent section in hypodermal
cells, F. predominance of anticl div. in hypodermis-derived cells
corpus
hypo

9. Anther divides. Periclinal division takes place in the first
layer called archesporial layer) beneath protoderm.
Archesporial layer gives rise:
1.Outer primary parietal layer: gives rise to 2 or 3 layers
A. future endothecium b. middle layer c. tapetum
2.Inner primary sporogenous cells. Divides by mitosis or directly function as
microsporocytes-Undergo Meiosis
Archesporial
layer
Wall layers
microsporocytes
Future
inner
tapetum
Outer tapetum
ANTHER WALL LAYERS

10. A.Anther primordium
B.Archesporial layer next to
the epid.
C.Mitotic div. in archesp layer
forms primary parietal
layer (PPL)and
sporogenous cells
D.Division in the PPL (see
arrow) gives rise to 2
additional layers.
E. Inner PPL differentiates
into outer Tapetum. Outer PPL
differentiates into the sec.
parietal layer.
Additional mitotic division of
outer PPL gives rise to
Endothecium and Middle
layer.
Stamen growth and
Differentiation- early stages

11. Carthamus tinctorius
Sex Plant Reproduction (2011) 24:307-317

12. Early stages continued
F. Structural organization of
anther wall is complete prior
to microspore mother cell
formation. Ep epidermis, Ed
endothecium, M middle layer,
To outer tapetum, Ti inner
tapetum, S sporogenous cells
G. Microsporocyte begin to
differentiate and enclosed by
tapetal cells. Tapetal cells
divide anticlinally and
periclinally. Most have 2
nuclei.
H. Microsporocyte at pre-
prophase stage

13. meiosis

14. Meiotic divisions in the microsporangium

15. 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

16. 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

17. 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

19. 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

24. Meiotic divisions
I
II
A Pachytene, D. Metaphase
B. Diplotene E. Anaphase
C. diakinesis F. Telophase
(cell plate not formed yet)
A.Late interphase in the dyad
B.Metaphase II E.tetrads
C.Anaphase II F. Post meiotic
D.Telophase II microspore
E
F
E
E
D
B
F
DC
C

25. Pollen development before
gametogenesis
A.Microsporocytes prior to meiosis. Clear
boundary is callose.
B. Pitlike structures within callose wall
C. Karyokinesis prior to cytokinesis..thus
haploid nuclei. Callose remains distinct
D. Primexine (note protrusions) surrounds the
protoplast of each tetrad microspore
E. Tetrad of microspores enveloped in thick
callose wall
F.Microspores within tetrads round up,
numerous vacuoles present. Future aperture
developed. Nucleus centrally located.
G. Cell wall continues to thicken.
Outermost portion of wall called exine : has 2
wall layers:1. ectexine
2. endexine-smooth layer surrounds protoplast
H. Walls are more prominent
I. Large vacuoles in microspores before gameto
genesis

26. Pollen development Continued
A.Highly vacuolated
microspore,nucleus near wall
B. Ist pollen mitosis shows
generative cell (arrow)
C. veg. cell moves next to gen.
nucleus
D. Generative cell detaches from
wall and moves into cytoplasm
of veg cell.
E. Cytoplasm of veg cell is
dense with prominent nucleus.
Generative cell enclosed by its
own membrane, cell has
vacuoles. Aperture is a
prominent feature.

27. Pollen development ContinuedF. Pollen grain has copious
starch grains
G.DAPI-stained pollen reveals
location of gen cell nucleus at
time of sperm formation
H. DAPI stain reveals elongated
sperm cell nuclei close together
I. Sperm appearing as 1
structure. J. wall is well-defined

28. A.Sporogenous or archesp cells, after
last mitotic division, each secretes
callose, B.Four sacs of one anther
to show mmc surrounded by callose
under fluorescence microscopy
T
C. Before cytokinesis. Coenocytic
tetrads during furrowing
D. Microspores separated but
still retained as tetrad for some
time

29. Glandular or secretory tapetum-
cells remain in their
the sac and later disintegrate and
absorbed by pollen
mother cells
Amoeboid or invasive
tapetum.
Flows amoeba-likeinto the sac
interior after callose dissolves
and engulfs the separated
microspores

30. E. Late vacuolate microspores above degenerating tapetum
F. Partly engorged pollen with nucleus of vegetative and generative cells
G. Mature engorged pollen in sacs. Tapetum is gone. Endothecium has wall bars.

31. In A tapetum is still intact and microspores embedded in callose,
in B the tapetum intrudes into the sac , c. microspores surrounded by
invasive tapetum. In D. microspores engulfed by tapetum,
In E, invasive tapetum disappears.
tapetum

32. Cells lining the anther lumen
– a layer known as the
endothecium – secretes
materials that are essential
for the proper maturation of
the pollen grains.

33. Roles played by tapetum
1.Nourishment of the developing pollen mother cells
and microspores
2. Formation of exine
3 . Deposition of tryphine on the pollen wall
4. Secretes enzymes that dissolves the callose surrounding
tetrads . In some species e.g. sweet pepper

34. Pollen grains

35. exine
intine
cytoplasm
Pollen from different
species, variation in exine
morphology

36. Telophase of microspore mitosis in African lily. Most
organelles are unequally segregated. Plastid is dividing
adjacent to the chromatin of the future vegetative cell but no
plastids occur between cell plate and chromatin of the future
generative cell
.
Cell plate
Dividing
plastid
Generative
cell
Vege--
tative
cell

37. 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 thevegetative 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.

40. 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
V.CG.C Vacuole
G.C
.appressed
to wall

41. D. Pollen grain and generative cell have enlarged.
E. Generative cell in mitosis
F. Binucleate generative cell appressed to pollen wall

42. 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.

43. 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

47. Cross-section of mature lily anther just before it dehisces
at the stomium