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Alternation of generations

2007 Schools Wikipedia Selection. Related subjects: General Biology

   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.
   Enlarge
   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 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
   (tissue) or plant), which is genetically haploid, and a sporophyte
   (thallus or plant), which is genetically diploid.

   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.

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" . 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.

   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 animals. Although
   conceptually similar to "alternation of generations", the genetics of
   heterogamy is significantly different.

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.

Protists

   Some protists undergo an alternation of generations, including the
   slime molds, foraminifera, and many marine algae.

   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.

   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 algae. 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.

Plants

Non-vascular plants

   Liverwort gametophyte
   Enlarge
   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.

Vascular plants

   Diagram of alternation of generations in ferns.
   Enlarge
   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.
   The underside of a Dicksonia antarctica frond showing the sori, or
   spore-holding structures.
   Enlarge
   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.
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