Contents 1 Megasporogenesis 2 Megagametogenesis 3 See also 4 References


Megasporogenesis[edit] In gymnosperms and flowering plants, the megaspore is produced inside the nucleus of the ovule. During megasporogenesis, a diploid precursor cell, the megasporocyte or megaspore mother cell, undergoes meiosis to produce initially four haploid cells (the megaspores).[1] Angiosperms exhibit three patterns of megasporogenesis: monosporic, bisporic, and tetrasporic, also known as the Polygonum type, the Alisma type, and the Drusa type, respectively. The monosporic pattern occurs most frequently (>70% of angiosperms) and is found in many economically and biologically important groups such as Brassicaceae (e.g., Arabidopsis, Capsella, Brassica), Gramineae (e.g., maize, rice, wheat), Malvaceae (e.g., cotton), Leguminoseae (e.g., beans, soybean), and Solanaceae (e.g., pepper, tobacco, tomato, potato, petunia).[2] This pattern is characterized by cell plate formation after meiosis 1 & 2, which results in four one-nucleate megaspores, of which three degenerate. The bisporic pattern is characterized by cell plate formation only after meiosis 1, and results in two two-nucleate megaspores, of which one degenerates. The tetrasporic pattern is characterized by cell plates failing to form after either meiosis 1 or 2, and results in one four-nucleate megaspore. Therefore, each pattern gives rise to a single functional megaspore which contains one, two, or four meiotic nuclei, respectively.[2] The megaspore then undergoes megagametogenesis to give rise to the female gametophyte.


Megagametogenesis[edit] Plant ovules with megasporocytes before meiosis: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right After megasporogenesis, the megaspore develops into the female gametophyte (the embryo sac) in a process called megagametogenesis. The process of megagametogenesis varies depending on which pattern of megasporogenesis occurred. Some species, such as Tridax trilobata, Ehretia laevis, and Alectra thomsoni, can undergo different patterns of megasporogenesis and therefore different patterns of megagametogenesis. If the monosporic pattern occurred, the single nucleus undergoes mitosis three times, producing an eight-nucleate cell. These eight nuclei are arranged into two groups of four. These groups both send a nucleus to the center of the cell; these become the polar nuclei. Depending on the species, these nuclei fuse together before or upon fertilization of the central cell. The three nuclei at the end of the cell near the micropylar become the egg apparatus, with an egg cell in the center and two synergids. At the other end of the cell, a cell wall forms around the nuclei and forms the antipodals. Therefore, the resulting embryo sac is a seven-celled structure consisting of one central cell, one egg cell, two synergid cells, and three antipodal cells.[2][3] The bisporic and tetrasporic patterns undergo varying processes and result in varying embryo sacs as well. In Lilium which has a tetrasporic pattern, the central cell of the embryo sac is 4N. Therefore, upon fertilization the endosperm will be 5N rather than the typical 3N.[4]


See also[edit] Megasporangium Microspore Spore Double fertilization


References[edit] ^ Estrada-Luna, A. A.; W. Huanca-Mamani; G. Acosta-García; G. León-Martínez; A. Becerra-Flora; R. Pérez-Ruíz; J. -Ph. Vielle-Calzada (Mar–Apr 2002). "Beyond Promiscuity: From Sexuality to Apomixis in Flowering Plants". In Vitro Cellular & Developmental Biology – Plant. 38 (2): 146–151. doi:10.1079/ivp2001278. JSTOR 20065024.  ^ a b c Yadegaria, Ramin; Gary N. Drewsb (2004-04-09). "Female Gametophyte Development". The Plant Cell. 16: S133–S141, Supplement. doi:10.1105/tpc.018192. Retrieved 2010-11-28.  ^ Raven, Peter H.; Evert, Ray Franklin.; Eichhorn, Susan E. (2005). Biology of Plants. New York, NY: W.H. Freeman. ISBN 0-7167-1007-2.  ^ Sargent, E. (1900). "Recent work on the results of fertilization in angiosperms". Annals of Botany. 14: 689–712. Retrieved 2010-11-28.  v t e Botany History of botany Subdisciplines Plant systematics Ethnobotany Paleobotany Plant anatomy Plant ecology Phytogeography Geobotany Flora Phytochemistry Plant pathology Bryology Phycology Floristics Dendrology Plant groups Algae Archaeplastida Bryophyte Non-vascular plants Vascular plants Spermatophytes Pteridophyte Gymnosperm Angiosperm Plant morphology (glossary) Plant cells Cell wall Phragmoplast Plastid Plasmodesma Vacuole Tissues Meristem Vascular tissue Vascular bundle Ground tissue Mesophyll Cork Wood Storage organs Vegetative Root Rhizoid Bulb Rhizome Shoot Stem Leaf Petiole Cataphyll Bud Sessility Reproductive (Flower) Flower development Inflorescence Umbel Raceme Bract Pedicellate Flower Whorl Floral symmetry Floral diagram Floral formula Receptacle Hypanthium (Floral cup) Perianth Tepal Petal Sepal Sporophyll Gynoecium Ovary Ovule Stigma Archegonium Androecium Stamen Staminode Pollen Tapetum Gynandrium Gametophyte Sporophyte Plant embryo Fruit Fruit anatomy Berry Capsule Seed Seed dispersal Endosperm Surface structures Epicuticular wax Plant cuticle Epidermis Stoma Nectary Trichome Prickle Plant physiology Materials Nutrition Photosynthesis Chlorophyll Plant hormone Transpiration Turgor pressure Bulk flow Aleurone Phytomelanin Sugar Sap Starch Cellulose Plant growth and habit Secondary growth Woody plants Herbaceous plants Habit Vines Lianas Shrubs Subshrubs Trees Succulent plants Reproduction Evolution Ecology Alternation of generations Sporangium Spore Microsporangia Microspore Megasporangium Megaspore Pollination Pollinators Pollen tube Double fertilization Germination Evolutionary development Evolutionary history timeline Hardiness zone Plant taxonomy History of plant systematics Herbarium Biological classification Botanical nomenclature Botanical name Correct name Author citation International Code of Nomenclature for algae, fungi, and plants (ICN) - for Cultivated Plants (ICNCP) Taxonomic rank International Association for Plant Taxonomy (IAPT) Plant taxonomy systems Cultivated plant taxonomy Citrus taxonomy cultigen cultivar Group grex Practice Agronomy Floriculture Forestry Horticulture Lists Related topics Botanical terms Botanists by author abbreviation Botanical expedition Category Portal WikiProject Retrieved from "https://en.wikipedia.org/w/index.php?title=Megaspore&oldid=805916618" Categories: Plant developmentFertilityReproductionPlant sexualityHidden categories: Pages using div col without cols and colwidth parameters


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