Gametophyte
A gametophyte (
Algae
In some
Land plants
In land plants, anisogamy is universal. As in animals, female and male gametes are called, respectively, eggs and sperm. In extant land plants, either the sporophyte or the gametophyte may be reduced (heteromorphic).[2] No extant gametophytes have stomata, but they have been found on fossil species like the early Devonian Aglaophyton from the Rhynie chert.[3] Other fossil gametophytes found in the Rhynie chert shows they were much more developed than present forms, resembling the sporophyte in having a well-developed conducting strand, a cortex, an epidermis and a cuticle with stomata, but were much smaller.[4]
Bryophytes
In
In some bryophyte groups such as many liverworts of the order Marchantiales, the gametes are produced on specialized structures called gametophores (or gametangiophores).
Vascular plants
All
In
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 the clade that includes Ophioglossaceae and Psilotaceae, the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. Homosporous ferns secrete a chemical called antheridiogen.
Lycophytes
Extant
Seed plants
The
In
The female gametophyte in gymnosperms differs from the male gametophyte as it spends its whole life cycle in one organ, the ovule located inside the megastrobilus or female cone.[14] Similar to the male gametophyte, the female gametophyte normally is fully dependent on the surrounding sporophytic tissue for nutrients and the two organisms cannot be separated. However, the female gametophytes of Ginkgo biloba do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte.[15] The female gametophyte forms from a diploid megaspore that undergoes meiosis and starts being singled celled.[16] The size of the mature female gametophyte varies drastically between gymnosperm orders. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta, the single celled female gametophyte undergoes many cycles of mitosis ending up consisting of thousands of cells once mature. At a minimum, two of these cells are egg cells and the rest are haploid somatic cells, but more egg cells may be present and their ploidy, though typically haploid, may vary.[14][17] In select Gnetophyta, the female gametophyte stays singled celled. Mitosis does occur, but no cell divisions are ever made.[13] This results in the mature female gametophyte in some Gnetophyta having many free nuclei in one cell. Once mature, this single celled gametophyte is 90% smaller than the female gametophytes in other gymnosperm orders.[14] After fertilization, the remaining female gametophyte tissue in gymnosperms serves as the nutrient source for the developing zygote (even in Gnetophyta where the diploid zygote cell is much smaller at that stage, and for a while lives within the single celled gametophyte).[14]
The precursor to the male angiosperm gametophyte is a diploid microspore mother cell located inside the
The female gametophyte of angiosperms develops in the ovule (located inside the female or
Heterospory
In heterosporic plants, there are two distinct kinds of gametophytes. Because the two gametophytes differ in form and function, they are termed heteromorphic, from hetero- "different" and morph "form". The egg-producing gametophyte is known as a megagametophyte, because it is typically larger, and the sperm producing gametophyte is known as a microgametophyte. Species which produce egg and sperm on separate gametophytes plants are termed dioicous, while those that produce both eggs and sperm on the same gametophyte are termed monoicous.
In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct types of sporangia, each of which produces a single kind of spore that germinates to produce a single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants. That is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium; Sphaerocarpos is an example of such a plant.
In seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of several (typically two to five) cells when the pollen grains exit the sporangium. The megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carried by a physical or animal vector) and produces two sperm by mitosis.
In gymnosperms, the megagametophyte consists of several thousand cells and produces one to several
In angiosperms, the megagametophyte is reduced to only a few cells, and is sometimes called the
See also
- Sporophyte – Diploid multicellular stage in the life cycle of a plant or alga
- Alternation of generations – Reproductive cycle of plants and algae
- Archegonium – Organ of the gametophyte of certain plants, producing and containing the ovum
- Antheridium – Part of a plant producing and containing male gametes
References
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- ^ Ralf Reski (1998): Development, genetics and molecular biology of mosses. In: Botanica Acta 111, pp 1-15.
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- ^ a b c d e f Fernando, Danilo D.; Quinn, Christina R.; Bernner, Eric D.; Owens, John N. (2010). "Male Gametophyte Development and Evolution in Extant Gymnosperms". International Journal of Plant Developmental Biology. 4: 47–60.
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Further reading
- Roig-Villanova, Irma; Bou, Jordi; Sorin, Céline; Devlin, Paul F.; Martínez-García, Jaime F. (2006-03-24). "Identification of Primary Target Genes of Phytochrome Signaling. Early Transcriptional Control during Shade Avoidance Responses in Arabidopsis". Plant Physiology. 141 (1). Oxford University Press (OUP): 85–96. PMID 16565297.
- Cucinotta, Mara; Colombo, Lucia; Roig-Villanova, Irma (2014-03-27). "Ovule development, a new model for lateral organ formation". Frontiers in Plant Science. 5. Frontiers Media SA: 117. PMID 24723934.