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