Sporic or diplohaplontic life cycle. A diploid (2n) sporophyte undergoes meiosis to produce haploid (1n) reproductive cells, often called spores. Haploid cells undergo mitosis to produce a gametophyte. The gametophyte produces haploid gametes which fuse to form a diploid zygotic sporophyte.

Alternation of generations is a term applied to a reproductive cycle of certain vascular plants, fungi, and protists. The term is a bit confusing for people familiar only with the life cycle of a typical animal. A more understandable name would be "alternation of phases of a single generation" because we usually consider a generation of a species to encompass one complete life cycle. The life cycle of organisms with "alternation of generations" is characterized by each phase consisting of one of two separate, free-living organisms: a gametophyte (thallus or plant), which is genetically haploid, and a sporophyte (thallus or plant), which is genetically diploid. Image File history File links Drawn by self for Biological life cycle Cut-&-merge halves Image:Zygotic_meiosis. ... Image File history File links Drawn by self for Biological life cycle Cut-&-merge halves Image:Zygotic_meiosis. ... This article or section does not cite its references or sources. ... Divisions Non-seed-bearing plants Equisetophyta Lycopodiophyta Psilotophyta Pteridophyta Superdivision Spermatophyta Pinophyta Cycadophyta Ginkgophyta Gnetophyta Magnoliophyta The vascular plants are plants in the Kingdom Plantae (also called Viridiplantae) that have specialized tissues for conducting water. ... Divisions Chytridiomycota Zygomycota Ascomycota Basidiomycota The Fungi (singular: fungus) are a large group of organisms ranked as a kingdom within the Domain Eukaryota. ... Typical phyla Protists (IPA: ) are a heterogeneous group of living things, comprising those eukaryotes that are not animals, plants, or fungi. ... Generation (From the Greek γιγνομαι), also known as procreation, is the act of producing offspring. ... In biology, a species is one of the basic units of biodiversity. ... A life cycle includes the major sexual stages of a species, especially in regard to its ploidy. ... // Overview A gametophyte is the haploidor possibly diploid structure or phase of life of a sexually-reproducing plant. ... Thallus may mean: Thallus (tissue), an undifferentiated vegetative tissue (without specialization of function) of some non-mobile organisms, which were previously known as the thallophytes. ... Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... In plants that undergo alternation of generations, a sporophyte is the structure, or phase of life, that contains a total complement of chromosomes: The sporophyte produces spores, in a process called meiosis. ... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ...

A haploid plant of the gametophyte generation produces gametes by mitosis. Two gametes (originating from different organisms of the same species or from the same organism) combine to produce a zygote, which develops into a diploid plant of the sporophyte generation. This sporophyte produces spores by meiosis, which germinate and develop into a gametophyte of the next generation. This cycle, from gametophyte to gametophyte, is the way in which plants and many algae undergo sexual reproduction. Light micrograph of a newt kidney cell in early anaphase of mitosis. ... Categories: Biology stubs ... This article or section does not cite its references or sources. ... For the article on the figure of speech, see meiosis (figure of speech). ... Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring. ...

Distinctions

The distinction of "free-living" is important, because all sexually reproducing organisms can be thought to involve alternating phases, at least at the cellular level as meiosis. However, not all biologists agree. It is often stated that alternation of generations refers to both the diploid and haploid stages being "multicellular" and this is more important than "free-living" ^[1]. Such a distinction changes the concept to one separating animals and plants.

All plants have diploid sporophyte and haploid gametophyte stages that are multicellular, and the differences between plant groups are in the relative sizes, forms, and trophic abilities of the gametophyte or sporophyte forms, as well as the level of differentiation in the gametophytes. An example would be comparing pollen and ovules to bisexual gametophyte thalli. Both approaches are discussed in this article. Divisions Land plants (embryophytes) Non-vascular plants (bryophytes) Marchantiophyta - liverworts Anthocerotophyta - hornworts Bryophyta - mosses Vascular plants (tracheophytes) Lycopodiophyta - clubmosses Equisetophyta - horsetails Pteridophyta - true ferns Psilotophyta - whisk ferns Ophioglossophyta - adderstongues Seed plants (spermatophytes) †Pteridospermatophyta - seed ferns Pinophyta - conifers Cycadophyta - cycads Ginkgophyta - ginkgo Gnetophyta - gnetae Magnoliophyta - flowering plants Adiantum pedatum (a fern... Multicellular organisms are those organisms consisting of more than one cell, and having differentiated cells that perform specialized functions. ... SEM image of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), hollyhock (Sildalcea malviflora), lily (Lilium auratum), primrose (Oenothera fruticosa), and castor bean (Ricinus communis). ... An ovule is a structure found in seed plants that develops into a seed after fertilization. ...

Biologists recognize two categories of alternation: the first if the sporophyte and the gametophye forms are more or less identical, alternation is called isomorphic; and second if the forms have very different appearances, alternation is called heteromorphic. Other terms applied to this kind of life cycle are diplobiontic, diplohaplontic, haplodiplontic, or dibiontic.

Heterogamy is a term used to describe alternation between parthenogenic and sexually reproductive phases that occurs in some invertebrates and vertebrates. Although conceptually similar to "alternation of generations", the genetics of heterogamy is significantly different. Kaguya is one success from 460 attempts at growing embryos. ... Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring. ... Invertebrate is a term coined by Jean-Baptiste Lamarck to describe any animal without a spinal column. ... Classes and Clades Vertebrata is a subphylum of chordates, specifically, those with backbones or spinal columns. ...

Fungi

Fungal mycelia are typically haploid. When mycelia of different mating types meet, they produce two multinucleate ball-shaped cells, which join via a "mating bridge". Nuclei move from one mycelium into the other, forming a heterokaryon (meaning "different nuclei"). This process is called plasmogamy. Actual fusion to form diploid nuclei is called karyogamy, and may not occur until sporangia are formed. Karogamy produces a diploid zygote, which is a short-lived sporophyte that soon undergoes meiosis to form haploid spores. When the spores germinate, they develop into new mycelia. Mycelium is the vegetative part of a fungus consisting of a mass of branching threadlike hyphae that exists below the ground or within another substrate. ...

Protists

Some protists undergo an alternation of generations, including the slime molds, foraminifera, and many marine algae. Orders Allogromiida Carterinida Fusulinida - extinct Globigerinida Involutinida - extinct Lagenida Miliolida Robertinida Rotaliida Silicoloculinida Spirillinida Textulariida incertae sedis    Xenophyophorea    Reticulomyxa The Foraminifera, or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands that branch and merge to form a dynamic net. ...

The life cycle of slime molds is very similar to that of fungi. Haploid spores germinate to form swarm cells or myxamoebae. These fuse in a process referred to as plasmogamy and karyogamy to form a diploid zygote. The zygote develops into a plasmodium, and the mature plasmodium produces, depending on the species, one to many fruiting bodies containing haploid spores. Species Plasmodium berghei Plasmodium brasilianum Plasmodium chabaudi Plasmodium cynomolgi Plasmodium falciparum Plasmodium gallinaceum Plasmodium knowlesi Plasmodium lophurae Plasmodium malariae Plasmodium ovale Plasmodium relictum Plasmodium vivax Plasmodium yoelii etc. ...

Foraminifera undergo a heteromorphic alternation of generations between a haploid gamont and a diploid agamont phases. The single-celled haploid organism is typically much larger than the diploid organism.

Alternation of generations occurs in almost all marine seaweeds. In most red algae, many green algae, and a few brown algae, the phases are isomorphic and free-living. Some species of red algae have a complex triphasic alternation of generations. Kelp are an example of a brown alga with a heteromorphic alternation of generations. Species from the genus Laminaria have a large sporophytic thallus that produces haploid spores which germinate to produce free-living microscopic male and female gametophytes. Possible classes Florideophyceae Bangiophyceae Cyanidiophyceae The red algae (Rhodophyta, pronounced /ˈrəʊdə(ʊ)ˌfʌɪtə/, ancient greek: rhodos phytos = red plant) are a large group of mostly multicellular, marine algae, including many notable seaweeds. ... Divisions Chlorophyta Charophyta The Green Algae are the large group of algae from which the embryophytes (higher plants) emerged. ... Orders Dictyotales Desmerestiales Fucales Laminariales (kelps) etc. ... Families Alariaceae Chordaceae Laminariaceae Lessoniaceae Phyllariaceae Pseudochordaceae Kelp are large seaweeds, belonging to the brown algae and classified in the order Laminariales. ... Species Laminaria japonica . ...

Plants

Non-vascular plants

Liverwort gametophyte

Nontracheophyte plants including the liverworts, hornworts and mosses undergo an alternation of generations; the gametophyte generation is the most common. The haploid gametophyte produces haploid gametes in multicellular gametangia. Female gametangia are called archegonium and produce eggs, while male structures called antheridium produce sperm. Water is required so that the sperm can swim to the archegonium, where the eggs are fertilized to form the diploid zygote. The zygote develops into a sporophyte that is dependent on the parent gametophyte. Mature sporophytes produce haploid spores by meiosis in sporangia. When a spore germinates, it grows into another gametophyte. A horned liverwort (Anthocerotae) photographed on the Island of Hawaii by Eric Guinther and released under the GNU Free Documentation License. ... A horned liverwort (Anthocerotae) photographed on the Island of Hawaii by Eric Guinther and released under the GNU Free Documentation License. ... // Overview A gametophyte is the haploidor possibly diploid structure or phase of life of a sexually-reproducing plant. ... Orders Need to be entered Liverworts are non-vascular plants in the Class Marchantiopsida, formerly known as the Hepaticae. ... Families & Genera Anthocerotaceae Anthoceros Folioceros Leiosporoceros Phaeoceros Sphaerosporoceros Dendrocerotaceae Dendroceros Megaceros Notoceros Notothyladaceae Notothylas Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. ... Subclasses Sphagnidae Andreaeidae Tetraphidae Polytrichidae Archidiidae Buxbaumiidae Bryidae Moss gametophyte generation plants with a single sporophyte. ... A gametangia is an organ or cell in which gametes are produced. ... An archegonium (pl: archegonia) (from the Greek arche = beginning and gonos = born) is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. ... Diagram of antheridium anatomy An antheridium (plural: antheridia) is a structure or organ of the gametophyte phase of certain plants producing and containing the spermatids or male gametes. ... For the article on the figure of speech, see meiosis (figure of speech). ... A sporangium (pl. ...

Vascular plants

Diagram of alternation of generations in ferns.

Ferns and their allies, including clubmoss and horsetails, reproduce via an alteration of generations. The conspicuous plant observed in the field is the diploid sporophyte. This plant creates by meiosis single-celled haploid spores which are shed and dispersed by the wind (or in some cases, by floating on water). If conditions are right, a spore will germinate and grow into a rather inconspicuous plant body called a prothallus. Image File history File links Download high resolution version (816x865, 218 KB) Summary Licensing File links The following pages link to this file: Alternation of generations User:J. Finkelstein User:J. Finkelstein/Sandbox ... Image File history File links Download high resolution version (816x865, 218 KB) Summary Licensing File links The following pages link to this file: Alternation of generations User:J. Finkelstein User:J. Finkelstein/Sandbox ... Classes Marattiopsida Osmundopsida Gleicheniopsida Pteridopsida A fern, or pteridophyte, is any one of a group of about 20,000 species of plants classified in the Division Pteridophyta, formerly known as Filicophyta. ... Families Lycopodiaceae Huperziaceae The Class Lycopodiopsida includes the clubmosses. ... Species The horsetails are vascular plants, comprising 15 species of plants in the genus Equisetum. ... For the article on the figure of speech, see meiosis (figure of speech). ... Spores produced in a sporic life cycle. ... Wind is the roughly horizontal movement of air (as opposed to an air current) caused by uneven heating of the Earths surface. ... This article focuses on water as it is experienced in everyday life. ... Liverwort Prothallus A prothallium, or prothallus; from Latin pro = forwards and Greek θαλλος (thallos) = twig; is a pteridophyte, i. ...

The underside of a Dicksonia antarctica frond showing the sori, or spore-holding structures.

The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte. Image File history File links Download high resolution version (1186x1533, 415 KB) Species Dicksonia antarctica Genus Dicksonia Familia Dicksoniaceae Picture taken by DanielCD on 17 May 2005. ... Image File history File links Download high resolution version (1186x1533, 415 KB) Species Dicksonia antarctica Genus Dicksonia Familia Dicksoniaceae Picture taken by DanielCD on 17 May 2005. ... This article or section does not cite its references or sources. ...

References

1. ^ Taylor, T.N., et al. (2005). "Life history biology of early land plants: Understanding the gametophyte phase". Proceedings of the National Academy of Sciences 102: 5892-5897.