bio 130 microsporogenesis gametogenesis 2013
TRANSCRIPT
megasporogenesis
megagametogenesis
Microsporogenesis- formation of spores called microspores
Microgametogenesis-development of microspore into the microgametophyte or the pollen grain containing sperm cells
Ontogeny of the anther
Development and differentiation of sporogenous tissue
Structure of pollen
Events in anther development
Androecium-collective name for all stamensIn 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
Vasculature in theFilament
Both anther and filament traversed by a single vascular bundle –with xylem and phloem
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 tapetum2. Primary sporogenous cells- microsporocytes
Sporangium initiation is restricted to four separated areas corresponding to corners of the developing anthers
hypoderm
microsporocytes
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 siteB. 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
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. tapetum2.Inner primary sporogenous cells. Divides by mitosis or directly function as microsporocytes-Undergo Meiosis
Archesporiallayer
Wall layers microsporocytes
Future inner tapetum
Outer tapetum
ANTHER WALL LAYERS
A. Anther primordiumB. 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
Carthamus tinctorius
Sex Plant Reproduction (2011) 24:307-317
Early stages continuedF. Structural organization of anther wall is complete prior to microspore mother cellformation. 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
meiosis
Meiotic divisions in the microsporangium
Meiosis I
Pair and exchange segments
Chromosomes line up by homologous pairs
Each pair of homologouschromosomes separates
Two haploid cells form, eachchromosome stillconsists of twosister chromatids
Leptotene- chromatin condenses, preceded by DNA replication Zygonema-homologous chromosomes pair form bivalentsPachytene-physical exchange of chromosome parts occurs bet homologous chromosomes
Diplotene- partial separationof each of sister chromatids from their homologouschromatids
Diakinesis- homologs are held together by chiasmata at their tips.
Summary: ist meiotic prophase- replicated homologouschromosomes 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
Chromosomes still composed of twochromatids
Chromosomesat metaphase plate. Due to crossing –over in Meiosis I, each chromosome notgenetically identical.
Anaphase IISister chromatids Separate, move to opposite poles as Individual chromosomes
Telophase II andCytokinesis. Nuclei form.Chromosomes begin decondensing
Meiosis II
Meiotic divisionsIII
A Pachytene, D. MetaphaseB. Diplotene E. Anaphase C. diakinesis F. Telophase (cell plate not formed yet)
A. Late interphase in the dyadB. Metaphase II E.tetradsC. Anaphase II F. Post meiotic D. Telophase II microspore
EF
E
E
D
B
F
DCC
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. ectexine2. endexine-smooth layer surrounds protoplastH. Walls are more prominentI. Large vacuoles in microspores before gametogenesis
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.
Pollen development Continued F. Pollen grain has copious starch grainsG.DAPI-stained pollen reveals location of gen cell nucleus at time of sperm formationH. DAPI stain reveals elongatedsperm cell nuclei close togetherI. Sperm appearing as 1 structure. J. wall is well-defined
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
Glandular or secretory tapetum-cells remain in their the sac and later disintegrate andabsorbed by pollen mother cells
Amoeboid or invasive tapetum.Flows amoeba-likeinto the sac interior after callose dissolves and engulfs the separated microspores
E. Late vacuolate microspores above degenerating tapetumF. Partly engorged pollen with nucleus of vegetative and generative cellsG. Mature engorged pollen in sacs. Tapetum is gone. Endothecium has wall bars.
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
Cells lining the anther lumen – a layer known as the endothecium – secretes materials that are essential for the proper maturation of the pollen grains.
Roles played by tapetum
1.Nourishment of the developing pollen mother cells and microspores
2. Formation of exine3 . Deposition of tryphine on the pollen wall4. Secretes enzymes that dissolves the callose surrounding tetrads . In some species e.g. sweet pepper
Pollen grains
exine
intine
cytoplasm
Pollen from different species, variation in exine morphology
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--tativecell
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.
A. MicrosporeB. 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.appressedto wall
D. Pollen grain and generative cell have enlarged.E. Generative cell in mitosisF. Binucleate generative cell appressed to pollen wall
G. Two sperm cells still attached to each other but free frompollen wall; pollen engorging but central vacuole still present.
H. Mature engorged pollen grain with separated lenticular sperm cells embedded in vegetative cell.
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) testedamong 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
Cross-section of mature lily anther just before it dehisces at the stomium