a gametophyte is one of two alternating multicellular stages in the life cycle of plants and algae . It is a haploid multicellular organism that develops from a haploid spore that contains a set of chromosomes. The gametophyte is the sexual stage in the life cycle of plants and algae. It develops sex organs that produce gametes , haploid sex cells thatparticipate in fertilization to form adiploid zygote that contains a double set of chromosomes. A new diploid as a result of cell division of the zygote Multicellular organism is formed, the second stage in the life cycle is known as the sporophyte . The sporophyte can produce haploid spores by meiosis which on germination produces a new generation of gametophytes.
algae
In some multicellular green algae ( Ulva lactuca is an example), red algae and brown algae , sporophytes and gametophytes may be externally indistinguishable (isomorphic). In Ulva gametes are isogamous , all of one shape, size and general morphology.
land plants
In ground plants , anisogamy is universal. Like in animals, the female and male gametes are called eggs and sperm respectively . In existing land plants, the sporophyte or gametophyte may be reduced (heteromorphic).
bryophytes
In bryophytes ( mosses , liverworts , and hornworts ), the gametophyte is the most visible stage of the life cycle. The bryophyte gametophyte is long lived, independent of nutrition, and the sporophytes are associated with and dependent on the gametophytes. [3] When a moss spore germinates it grows to produce a filament of cells (called a protonema ). The mature gametophyte of moss develops into leafy shoots that form the sex organs ( gametangia ) that produce gametes . Eggs develop into archegonia and sperm antheridia । [४]
In some bryophyta groups, such as many liverworts from the order Marchantiales , gametes are produced on specialized structures called gametophores (or gametangiophores).
vascular plants
All vascular plants are sporophyte dominant, and the trend towards smaller and more sporophyte-dependent female gametophytes is evident as land plants have developed reproduction by seed. [5] Those vascular plants, such as clubmosses and many ferns, that produce only one type of spore are called homospores. They have exosporic gametophytes – that is, the gametophyte is free-living and develops outside the spore wall. Exosporic gametophytes can be either hermaphrodite, capable of producing both sperm and eggs in a single thallus ( monicous ), or specialized in separate male and female organisms (dioicous).
In heterosporous vascular plants (plants that produce both microspores and megaspores), gametophytes develop endosporically (within the spore wall). These gametophytes are dioecious , which produce either sperm or egg but not both.
Ferns
In most ferns , for example, in the leptosporangiate fern Dryopteris , the gametophyte is a photosynthetic free-living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably that include the clades Ophioglossaceae and Psilotaceae, gametophytes are subsistence by forming underground and mycotrophic relationships with fungi. Homosporous ferns secrete a chemical called antheridiogen.
lycophytes
The present lycophytes produce two different types of gametophytes. In the homosporous families Lycopodiaceae and Huperziaceae, spores are hermaphrodite, free-living, underground and grow in mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella , which are heterosporous, microspores and megaspores are dispersed from the sporangia by either passive or active injection. [6] Microspores produce microgametophytes that produce sperm. Megaspores produce fewer megagametophytes inside the spore wall. At maturity, the megaspore cracks open at the trilate suture, allowing the male gametes to access the egg cells in the archegonia inside. The gametophytes of IsoetesSimilar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus . [7]
seed plants
The seed plant gametophyte life cycle is even shorter than that of the basal taxa (ferns and lycophytes). Seed plants are not gametophyte independent organisms and depend on the dominant sporophyte tissue for nutrients and water. With the exception of mature pollen, if the gametophyte tissue is separated from the sporophyte tissue, it will not survive. Because of this complex relationship and the small size of gametophyte tissue, it can be a challenge in some situations to differentiate between a single cell, human eye or even seed plant gametophyte tissue and sporophyte tissue under a microscope. While seed plant gametophyte tissue is typically composed of mononucleated haploid cells (1 x n), there may be specific circumstances in which ploidy varies widely despite being widely considered to be part of the gametophyte.
Male gametophytes in gymnosperms are produced inside microspores within microsporangia located inside male cones or microstrobili. In each microspore, a single gametophyte is produced, consisting of four haploid cells produced by meiosis of a diploid microspore mother cell. [8] Upon maturity, each microspore-derived gametophyte becomes a pollen grain. During its development, the water and nutrients required by the male gametophyte are provided by the sporophyte tissue until they are released for pollination. The cell number of each mature pollen grain varies between orders of gymnosperms. Cycadophyta has 3-celled pollen grains while Ginkgophyta has 4-celled pollen grains. [8]Gnetophyta may have 2 or 3-celled pollen grains, depending on the species, and Coniferophyta pollen grains vary greatly from single-celled to 40-celled. [9] [8] One of these cells is usually a germ cell and other cells may be a single tube cell that grows to form the pollen tube, sterile cells, and/or prothelial cells, which Both are vegetative cells without an essential reproduction. Celebration. [8] After pollination is successful, the male gametophyte continues to develop. If a tube cell was not developed in Microstrobilus, one is created after pollination through mitosis. [8] The tube cell grows in the diploid tissue of the female cone and may branch into the megastrobilous tissue or move directly towards the egg cell. [10]The megastrobilus sporophytic tissue provides nutrients for the male gametophyte at this stage. [10] In some gymnosperms, the tube cell will form a direct channel from the pollination site to the egg cell; in other gymnosperms, the tube cell will burst in the middle of the megastrobilus sporophyte tissue. [10] This is because in some gymnosperm orders, the germ cell is nonmobile and requires a straight passage, however, in the Cycadophyta and Ginkgophyta, the germ cell is mobile due to the presence of flagella and a straight tube from the pollination site. cell path. Eggs are not needed. [10]In most species the germ cell can be described exclusively as a sperm cell that mingles with the egg cell during fertilization, although this is not always the case. In some Gnetophyta species, the germ cell releases two sperm nuclei which undergo a rare gymnosperm double fertilization process that occurs only with the sperm nucleus and not with the fusion of the developed cells. [8] [11] After fertilization is complete in all sequences, the remaining male gametophyte tissue will deteriorate.
Several examples of variation of cell number in mature seed plant female gametophytes before fertilization. Each cell has a nucleus unless otherwise indicated. A: Typical 7 cell, 8 nucleated angiosperm female gametophyte (e.g. Tilia americana ). B: Typical gymnosperm female gametophyte with numerous haploid somatic cells illustrated with a honeycomb grid and two haploid germ cells (e.g. Ginkgo biloba ). C: Abnormally large 10-celled, 16-nucleated angiosperm female gametophyte (e.g. Peperomia dolabriformis ). D: abnormally small 4-celled, 4-nucleated angiosperm female gametophyte (e.g. Amborella trichopoda ), E: Unusual gymnosperm female gametophyte that is single-celled with multiple free nuclei surrounding a painted central vacuole (e.g. Gnetum gnemon ). Blue: egg cell. Dark Orange: Synergid Cell. Yellow: Auxiliary cell. Green: antipodal cell. Peach: Central cell. Violet: Individual nucelli.
The female gametophyte in gymnosperms differs from the male gametophyte in that it spends its entire life cycle in one organ, the megastrobilus or ovule located inside the female cone. [12] Similar to the male gametophyte, the female gametophyte is normally completely dependent on the surrounding sporophytic tissue for nutrients and the two organisms cannot be distinguished. However, select female gametophytes have chlorophyll and can produce some of their energy, however, not enough to support themselves without being supplemented by the sporophyte. [13] The female gametophyte is formed from a diploid megaspore that undergoes meiosis and becomes a single cell. [14]The size of the mature female gametophyte varies greatly in the order gymnosperms. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta, the single-celled female gametophyte undergoes several cycles of mitosis and ends up consisting of thousands of cells once mature. At least two of these cells are egg cells and the rest are haploid somatic cells, but more egg cells may be present and their ploidy, although usually haploid, may vary. [12] [15] In select Gnetophyta, the female gametophyte remains single-celled. Mitosis occurs, but no cell division ever occurs. [11]This results in the mature female gametophyte in some Gnetophyta having many free nuclei in a single cell. Once mature, this single-celled gametophyte is 90% smaller than female gametophytes in other gymnosperm orders. [12] After fertilization, the remaining female gametophyte tissue in gymnosperms serves as a nutrient source for the developing zygote (even in Gnatophyta where the diploid zygote cell is then very small, and for some time lives within a single celled gametophyte). [12]
The precursor of the male angiosperm gametophyte is a diploid microspore mother cell located inside the anther. Once the microspore undergoes meiosis, 4 haploid cells are formed, each of which is a single-celled male gametophyte. The male gametophyte will develop inside the anther through one or two rounds of mitosis. It forms a 2 or 3 celled male gametophyte which once dehiscing is known as a pollen grain. [16] One cell is the tube cell, and the rest of the cell/s are sperm cells. [17] The three-celled male gametophyte has evolved several times prior to dehissing and is present in about a third of angiosperm species, allowing rapid fertilization after pollination. [18]Once pollination occurs, the tube cell increases in size and if the male gametophyte has only 2 cells at this stage, the single sperm cell undergoes mitosis to form the second sperm cell. [19] Like in gymnosperms, the tube cell in angiosperms receives nutrients from the sporophytic tissue, and may branch into pistil tissue or grow directly toward the ovule. [20] [21] Once double fertilization is complete, tube cells and other vegetative cells, if present, are all remnants of the male gametophyte and soon degenerate. [21]
The female angiosperm gametophyte develops into the ovule (located inside the female or hermaphrodite flower). Its precursor is a diploid megaspore that undergoes meiosis to produce four haploid daughter cells. Three of these independent gametophyte cells degenerate, what remains is the gametophyte mother cell which normally consists of a nucleus. [22] In general, it will divide by mitosis until it has 8 nuclei divided into 1 egg cell, 3 antipodal cells, 2 synergistic cells, and a central cell containing two nuclei. [22] [19] In select angiosperms, there are special cases in which the female gametophyte is not 7-celled with 8 nuclei. [15]At the shorter end of the spectrum, some species have mature female gametophytes with only 4 cells, each with a nucleus. [23] In contrast, some species have a mature female gametophyte with 10 cells containing 16 total nuclei. [24] Once double fertilization occurs, the egg cell becomes the zygote which is later considered to be the sporophyte tissue. Scholars still disagree as to whether the fertilized central cell is considered a gametophyte tissue. Some botanists consider this endospore to be the gametophyte tissue that usually consists of 2/3 female and 1/3 male, but before double fertilization the central cell can be from 1n to 8n in special cases, fertilized central cells 2n ( 50%). male/female) to 9n (1/9th male, 8/9th female). [19]However, other botanists consider the fertilized endospore to be sporophyte tissue. Some believe it is neither.
heteromorphy
In plants with heteromorphic gametophytes, there are two different types of gametophytes. Because the two gametophytes differ in form and function, they are called heteromorphic , from hetero – “different” and morph – “form”. The gametophyte that produces the egg is known as the megagametophyte , as it is usually larger, and the gametophyte that produces the sperm is known as the microgametophyte . The gametophytes that produce eggs and sperm on different plants are called dioecious.
In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct sporangia, each of which produces a single type of spore and a single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants. That is, some plants have separate egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same type of spore inside the same spore; Sphaerocarpos is an example of such a plant.
In seed plants, the microgametophyte is called pollen. Seed plant microgametophytes have many (usually two to five) cells that form when pollen grains exit the sporangium. The megagametophyte develops within the megaspore of existing seedless vascular plants and within the megasporangium in the cone or flower in seed plants. In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carrying by a physical or animal vector), and produces two spermatozoa by mitosis.
In gymnosperms the megagametophyte consists of several thousand cells and produces one to many archegonia, each with an egg cell. The gametophyte becomes the food storage tissue in the seed.
In angiosperms, the megagametophyte is reduced to only a few nuclei and cells, and is sometimes called the embryo sac. A typical embryo sac consists of seven cells and eight nuclei, one of which is an egg cell. The two nuclei combine with a sperm nucleus to form the endosperm, which becomes the food storage tissue in the seed.
