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  • 1. megasporogenesis megagametogenesis
  • 2. Microsporogenesis- formation of spores called microspores Microgametogenesis- development of microspore into the microgametophyte or the pollen grain containing sperm cells
  • 3. Ontogeny of the anther Development and differentiation of sporogenous tissue Structure of pollen Events in anther development
  • 4. 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
  • 5. Vasculature in the Filament Both anther and filament traversed by a single vascular bundle –with xylem and phloem
  • 6. 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
  • 7. 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
  • 8. 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
  • 9. 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
  • 10. Carthamus tinctorius Sex Plant Reproduction (2011) 24:307-317
  • 11. 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
  • 12. meiosis
  • 13. Meiotic divisions in the microsporangium
  • 14. 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
  • 15. 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
  • 16. 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
  • 17. 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
  • 18. Meiotic divisionsI 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
  • 19. 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
  • 20. 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.
  • 21. Pollen development Continued F. 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
  • 22. A. Sporogenous or archesp cells, after last mitotic division, each secretes callose, B.Four sacs of one anther to show mmc surrounded by callose T C. Before cytokinesis. Coenocytic tetrads during furrowing D. Microspores separated but still retained as tetrad for some time
  • 23. 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
  • 24. 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.
  • 25. 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
  • 26. Cells lining the anther lumen – a layer known as the endothecium – secretes materials that are essential for the proper maturation of the pollen grains.
  • 27. 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
  • 28. Pollen grains
  • 29. exine intine cytoplasm Pollen from different species, variation in exine morphology
  • 30. 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
  • 31. 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.
  • 32. 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
  • 33. D. Pollen grain and generative cell have enlarged. E. Generative cell in mitosis F. Binucleate generative cell appressed to pollen wall
  • 34. 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.
  • 35. 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
  • 36. Cross-section of mature lily anther just before it dehisces at the stomium