Alternation of generations 66535 220021425 2008-06-17T22:25:42Z DOI bot 6652755 Citation maintenance. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. [[Image:Sporic meiosis.png|right|thumb|400px|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 [[zygote|zygotic]] sporophyte.]] The '''Alternation of phases''' (or '''generations''')<ref name=StewartRothwell>Stewart, W.N. and Rothwell, G.W. 1993. ''Paleobotany and the evolution of plants'', Second edition. Cambridge University Press, Cambridge, UK. ISBN 0-521-38294-7</ref> describes the life cycle of [[plant]]s, [[fungi]] and [[protist]]s. A multicellular [[diploid]] phase alternates with a multicellular [[haploid]] phase. The term can be 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 [[Biological life cycle|life cycle]]. The life cycle of organisms with "alternation of generations" is characterized by each phase consisting of one of two distinct 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) [[fertilisation|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 all land plants and many algae undergo [[sexual reproduction]]. ==Distinctions== It is often stated that 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, alternation of generations implies that both the diploid and haploid stages are multicellular and this is more important than "free-living".<ref name="ref2005">{{cite journal | last=Taylor | first=T.N. | coauthors=et al. | year=2005 | title=Life history biology of early land plants: Understanding the gametophyte phase | journal=[[Proceedings of the National Academy of Sciences]] | volume=102 | pages=5892–5897 | url=http://www.pnas.org/cgi/reprint/102/16/5892.pdf | doi=10.1073/pnas.0501985102 | pmid=15809414}}</ref> Such a distinction changes the concept to one separating animals and plants. The gametophyte and sporophyte are usually separate, independent organisms in basal algae such as ''[[Ulva lactuca]]'', where the gametes are free-swimming, and the zygote is formed in the water. By contrast, in land plants the sporophytes are to a greater or lesser extent dependent on the gametophytes, and ''vice-versa'', especially in the [[gymnosperms]] and [[angiosperms]]. Indeed it is a defining characteristic of the land plants, or [[embryophyte]]s (and hence the name), that a developing multicellular sporophyte is, for at least the first stages of its development, nurtured by the gametophyte, as can be seen most clearly in the [[bryophytes]]. All [[plant]]s have diploid sporophyte and haploid gametophyte stages that are [[multicellular organism|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 [[ovule]]s to bisexual gametophyte thalli. Both approaches are discussed in this article. Life cycles in which there is no multicellular diploid phase are referred to as '''haplontic'''. Life cycles with alternating haploid and diploid phases are '''diplohaplontic''', but the equivalent terms '''diplobiontic''', '''haplodiplontic''', or '''dibiontic''' are also in use. Two main types of diplohaplontic (alternating) life-cycles are recognized: if the sporophyte and the gametophyte generations are more or less identical in form, the life cycle is said to be '''isomorphic''', meaning "same form". If the generations have very different morphology, the life cycle is called '''heteromorphic''' meaning "different forms". '''Heterogamy''' is a term used to describe alternation between [[parthenogenesis|parthenogenic]] and [[sexual reproduction|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 [[seaweeds|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=== [[Bryophyte]] plants including the [[Marchantiophyta|liverwort]]s, [[hornwort]]s and [[moss]]es 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. They are also seedless. <gallery> Image:Liverwort life cycle.jpg|<center>Diagram of alternation of generations in liverworts.</center> Image:Liverwort.jpg|<center>A liverwort gametophyte</center> Image:Porella SPT.jpg|<center>A liverwort sporophyte.</center> </gallery> ===Vascular plants=== [[Fern]]s and their allies, including [[clubmoss]] and [[horsetail]]s, reproduce via an alternation of generations. The conspicuous plant observed in the field is the diploid sporophyte. This plant creates by [[meiosis]] single-celled haploid [[spore]]s 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 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. <gallery> Image:Alternation of generations in ferns.png|<center>Diagram of alternation of generations in ferns.</center> Image:Dixonia prothallus.jpg|<center>A gametophyte (prothallus) of ''Dicksonia sp.''</center> Image:Dicksonia antarctica Cultivated GardenEngland.jpg|<center>A sporophyte of ''Dicksonia antarctica''.</center> Image:SoriDicksonia.jpg|<center>The underside of a ''Dicksonia antarctica'' frond showing the sori, or spore-producing structures.</center> </gallery> In seed plants, the sporophyte is the dominant multicellular phase, and the gametophytes are both strongly reduced in size and different in morphology. Female gametophytes occur only in the seeds, and male gametophytes only in the pollen. A gymnosperm seed, growing on the diploid sporophyte parent, initially contains a haploid female gametophyte bearing a haploid egg cell enclosed in a cup-shaped structure known as the [[archegonium]]. The egg is fertilised by a sperm nucleus from a [[pollen]] grain, that contains a miniature male gametophyte. The resulting diploid [[zygote]] develops into the seed embryo, which is the diploid sporophyte of the next generation. During its development, which in gymnosperms may take 2-3 years, the offspring sporophyte and its parent gametophyte are both nurtured by the 'grandparent' sporophyte until the seed is ripe enough to be released. ==See also== *[[Evolutionary history of plants#phases]]: Evolutionary origin of the alternation of phases ==References== <references /> {{-}} {{Botany}} <!-- both "reproduction" categories necessary --> [[Category:Plant reproduction]] [[Category:Reproduction]] [[cs:Rodozměna]] [[de:Generationswechsel]] [[es:Alternancia de generaciones]] [[he:חילוף דורות]] [[ja:世代交代]] [[no:Generasjonsveksling]] [[pl:Przemiana pokoleń]] [[pt:Alternância de gerações]] [[fi:Sukupolvenvuorottelu]] [[zh:世代交替]]