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[[Image:Alternation of generations simpler.svg|thumb|350px300px|Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top)]]
[[File:Ceratodon purpureus.jpeg|thumb|300px|Typical moss: green [[haploid]] body and brown [[diploid]] sporophyte]]
 
The term '''alternation of generations''' is used to describe an alternation of forms in the [[life cycle]] of [[plant]]s (and some [[protist]]s).
The term '''alternation of generations''' is used to describe a process in the [[life cycle]] of some [[eukaryote]]s. It describes an alternation in forms which occurs in [[plant]]s and some [[protist]]s. One form is [[diploid]], with 2''n'' [[chromosomes]]: the [[sporophyte]]. The other form is [[haploid]] with only one set of chromosomes: the [[gametophyte]]. Both forms are [[multicellular organism|multicellular]].<ref name=King>"Alternation of generations: reproductive cycles in which a haploid phase alternates with a diploid phase". King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics''. 7th ed, Oxford University Press, p18. ISBN 0-19-530761-5</ref>▼
 
▲The term '''alternation of generations''' is used to describe a process in the [[life cycle]] of some [[eukaryote]]s. It describes an alternation in forms which occurs in [[plant]]s and some [[protist]]s. One form is [[diploid]], with 2''n'' [[chromosomes]]: the [['''sporophyte]]'''. The other form is [[haploid]] with only one set of chromosomes: the [['''gametophyte]]'''. Both forms are [[multicellular organism|multicellular]].<ref name=King>"Alternation of generations: reproductive cycles in which a haploid phase alternates with a diploid phase". King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics''. 7th ed, Oxford University Press, p18. {{ISBN |0-19-530761-5}}</ref>
In [[sexual reproduction]], organisms have a [[haploid]] phase with one set of [[chromosome]]s and a [[diploid]] phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is only the [[gamete]]s. In other eukaryotes [[:wikt:alternate|alternation]] of generations may occur. It means that both the diploid and the haploid phases are multi-cellular organisms. The classic example is the [[moss]]es, where the green plant is a haploid '''gametophyte''', and the reproductive phase is the diploid '''sporophyte'''.<ref name=King/> The two forms often occur together, as shown in the illustration on the right▼
 
In [[sexual reproduction]], organisms have a [[haploid]] phase with one set of [[chromosome]]s and a [[diploid]] phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is only the [[gamete]]s.
 
▲In [[sexualeukaryotes reproduction]], organisms have a [[haploid]] phase with one set of [[chromosome]]s and a [[diploid]] phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is only the [[gamete]]s. In other eukaryotesan [[:wikt:alternate|alternation]] of generations may occur. ItThis means that both the diploid and the haploid phases are multi-cellular organisms. The classic example is the [[moss]]es, where the green plant is a haploid '''gametophyte''', and the reproductive phase is the diploid '''sporophyte'''.<ref name=King/> The two forms often occur ''together'', as shown in the illustration on the right
The term ''alternation of generations'' refers only to the sexual cycle;: organisms oftenmay have [[asexual reproduction]] as well. The term should not be confused with life cycle stages in animals, which may ''look'' very different, but where all cells have two sets of chromosomes.
 
== Plants ==
[[File:Aspidium - prothallium (8244200348).jpg|thumb|250px|In ferns, the gametophyte is smaller than the sporophyte. Figs 1–3 the gametophyte (prothallus) grew from a spore. Fig 4 the fully grown gametophyte is a small flat plant mostly just one layer of cells. Fig 5 after an egg cell near the centre of the gametophyte was fertilized, it grows into a fern plant with leaves and roots.]]
 
The alternation of generations is an important concept in the evolution of plants.<ref>Thomas B.A. and Spicer R.A. 1987. ''The evolution and palaeobiology of land plants''. Croom Helm, London.</ref> All land plants have alternation of generations.
 
In mosses and their relatives (Bryophytes), the [[haploid]] gametophyte is the dominant generation, and the [[diploid]] sporophytes are sporangium-bearing stalks growing from the gametophytes. In ferns, the diploid sporophyte is much larger, butand the haploid gametophyte is also a little plant that can grow for a long time.
 
For [[flowering plant]]s (''Angiosperms''), the sporophyte generation is almost the whole life cycle (the green plant, roots etc.) except the small reproductive structures ([[pollen]] and [[ovule]]).
 
The sporophyte produces [[spore]]s (hence the name), by [[meiosis]]. These develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they have half as many chromosomes. Later, the mature gametophyte produces male or female gametes (or both) by [[mitosis]]. The fusion of male and female gametes ([[fertilization]]) produces a diploid [[zygote]] which develops into a new sporophyte. This is the cycle which is known as alternation of generations or alternation of phases.
 
=== As a factor in plant evolution ===
In the landmark work ''Variation and evolution in plants'', Stebbins discussed how alternation of generations related to the overall evolution of plants. He began:
:"The most striking difference between the sexual cycle of animals and those found in plants is that, with the exception of a few Protozoa,<ref>which are not now regarded as animals</ref> animals are diploid at all stages, while nearly all plants possesshave a haploid stage of greater or lesser duration. Furthermore, theThe sequence of types of alternation of generations... is one of the best-known features of plant evolution... The diploid generation has undoubtedly evolved independently many different times".<ref>Svedelius N. 1929. An evaluation of the structural evidence for genetic relationships in plants. ''Algae Proc. Int. Congr. Sci''. '''1''' 457{{ndash}}471.</ref>
 
Later Stebbins comments:
:"The diploid condition brings about an increase in flexibiityflexibility because it makes possible the condition of genetic [[dominance (genetics)|dominance]] and recessiveness. In a haploid organism every new [[mutation]] is immediately exposed to the action of [[natural selection|selection]]... In a diploid organism, on the other hand, each new mutant arises as a [[heterozygote]] and, if recessive, is sheltered from selection".<ref>Stebbins, G. Ledyard 1950. ''Variation and evolution in plants''. New York: Columbia University Press. Chapter V Genetic systems as factors in evolution: p157, 174.</ref>
The point is that, in diploids, new [[alleles]] are sheltered and (collectively) they are a reservoir of potential variation in the population.
 
=== Algae ===
Most [[algae]] have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar (isomorphic).
 
=== Bryophytes ===
Line 34 ⟶ 38:
 
=== Vascular plants ===
[[Image:Acer palmatum BotGartenMuenster Faecherahorn 6691.jpg|250px|thumb|In flowering plants, the sporophyte comprisesis the whole multicellular plant body except the pollen and megagametophyte withininside the [[ovule]]]]
 
An independent sporophyte is the dominant form in all [[clubmoss]]es, [[horsetail]]s, [[fern]]s, [[gymnosperm]]s, and [[Flowering plants|angiosperms]] (flowering plants) that have survived to the present day.
 
=== Earlier evolution ===
Early land plants had sporophytes that produced identical spores: they looked the same whichever sex they developed into. The ancestors of the gymnosperms evolved complex heterosporous life cycles: the spores producing male and female gametophytes were of different sizes. The female ''megaspores'' tending to be larger, and fewer in number, than the male ''microspores''.
 
During the [[Devonian]], several plant groups independently evolved ''heterospory'' and later ''endospory'', in which single megaspores were kept inside the sporangia of the parent sporophyte. These endosporic megaspores had a miniature multicellular female gametophyte with female sex organs and [[Ovum|egg]] cells. The ova were fertilised by free-swimming [[sperm]] produced by windborne miniatuarisedminiaturised male gametophytes in the form of pre-[[pollen]].
 
The resulting [[zygote]] developed into the next sporophyte generation while still inside the single large female megaspore in the [[sporangium]] of the parent sporophyte. The evolution of heterospory and endospory werewas among the earliest steps in the evolution of [[seed]]s of the kind produced by [[gymnosperm]]s and [[Flowering plants|angiosperms]].<ref>{{citation |author=Kenrick P. & Crane P.R. |year=1997 |title=The origin and early evolution of plants on land |journal=Nature |volume=389 |issue=6646 |pages=33-3933–39 |doi=10.1038/37918 |bibcode=1997Natur.389...33K |s2cid=3866183 |url=http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%207/land%20plants.pdf}}</ref><ref>Taylor T.N. Kerp H. & Hass H. 2005. Life history biology of early land plants: deciphering the gametophyte phase. ''Proceedings of the National Academy of Sciences'' '''102''', 5892-5897.</ref><ref>Bell P.R. & Helmsley A.R. 2000. ''Green plants: their origin and diversity.'' Cambridge University Press {{ISBN |0-521-64673-1}}</ref>
 
== Similar processes in other organisms ==
Some [[Chromalveolata]], some [[fungi]] and some [[slime moulds]] have what seems to be genuine alternation of generations. These groups include such a wide range of different types that ltit is difficult to say how common the phenomenon is. Certainly slime moulds are not a [[monophyletic]] group, and that may be true of the other two groups,
 
== Animals ==
Line 52 ⟶ 56:
[[File:Chrysaora jelly.jpg|thumb|The medusa form of a jellyfish]]
[[File:Polypen einer Gorgonie.jpg|thumb|Polyps of a jellyfish]]
Alternation of generations between a multicellular diploid and a multicellular haploid generation does not exist in animals.<ref name=Barnes2001> {{Citation |last=Barnes |first=R.S.K. |last2=Calow |first2=P. |last3=Olive |first3=P.J.W. |last4=Golding |first4=D.W. |last5=Spicer |first5=J.I. |year=2001 |title=The Invertebrates: a synthesis |publication-place=Oxford; Malden, MA |publisher=Blackwell |isbn=978-0-632-04761-1 |p=321}}</ref> ▼
 
▲Alternation of generations between a multicellular diploid and a multicellular haploid generation does not exist in animals.<ref name=Barnes2001> {{Citation |lastlast1=Barnes |firstfirst1=R.S.K. |last2=Calow |first2=P. |last3=Olive |first3=P.J.W. |last4=Golding |first4=D.W. |last5=Spicer |first5=J.I. |year=2001 |title=The Invertebrates: a synthesis |publication-place=Oxford; Malden, MA |publisher=Blackwell |page=321|isbn=978-0-632-04761-1 |p=321}}</ref>
In some animals, there is a [[life cycle]] with different diploid stages. This has sometimes mistakenly been called "alternation of generations",<ref name=Scott1996>{{Citation |last=Scott |first=Thomas |year=1996 |title=Concise Encyclopedia Biology |publication-place=Berlin |publisher=Walter de Gruyter |isbn=978-3-11-010661-9 }}</ref> but is quite different from what happens in plants and some protists. The most common case is that there are two distinct generations, where only one has [[sexual reproduction]]. Animals where it has been found include the [[Cnidaria]] and the [[Tunicate|Tunicates]]. The images on the right show the case of [[jellyfish]]: The medusa looks different from the polyp; they are different [[phenotype|phenotypes]]. Only the medusa reproduces sexually. ▼
 
▲In some animals, there is a [[life cycle]] with different diploid stages. This has sometimes mistakenly been called "alternation of generations",<ref name=Scott1996>{{Citation |last=Scott |first=Thomas |year=1996 |title=Concise Encyclopedia Biology |publication-place=Berlin |publisher=Walter de Gruyter |page=35|isbn=978-3-11-010661-9 }}</ref> but is quite different from what happens in plants and some protists. The most common case is that there are two distinct generations, where only one has [[sexual reproduction]]. Animals where it has been found include the [[Cnidaria]] and the [[Tunicate|Tunicates]]s. The images on the right show the case of [[jellyfish]]: The medusa looks different from the polyp; they are different [[phenotype|phenotypes]]s. Only the medusa reproduces sexually.
Other animals with life cycle forms include [[parasite|parasites]] such as certain [[Trematode|flukes]], which live in different [[snails]] for one part of their life, and in [[vertebrates]] for another part. ▼
 
▲Other animals with life cycle forms include [[parasite|parasites]]s such as certain [[Trematode|flukes]], which live in different [[snails]] for one part of their life, and in [[vertebrates]] for another part.
In some cases, the cycle includes more than two generations. If this is the case, only one stage uses sexual reproduction. In [[aphid|aphids]], for example, there is one generation that reproduces sexually, and up to forty that use [[parthenogenesis]], a type of [[asexual reproduction]].▼
 
▲In some cases, the cycle includes more than two generations. If this is the case, only one stage uses sexual reproduction. In [[aphid|aphids]]s, for example, there is one generation that reproduces sexually, and up to forty that use [[parthenogenesis]], a type of [[asexual reproduction]].
 
== References ==
{{Reflist}}
 
 
[[Category:Botany]]
[[Category:Plant anatomy]]
[[Category:Classical genetics]]
[[Category:Biological reproduction]]
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