Editing Alternation of generations

{{Short description|Reproductive cycle of plants and algae}}

[[Image:Alternation of generations simpler.svg|thumb|360px|Diagram showing the alternation of generations between a diploid sporophyte (bottom) and a haploid gametophyte (top)]]
'''Alternation of generations''' (also known as '''metagenesis''' or '''heterogenesis''')<ref>{{Cite web|title=alternation of generations {{!}} Definition & Examples|url=https://www.britannica.com/science/alternation-of-generations|access-date=2021-02-25|website=Encyclopedia Britannica|language=en|archive-date=2021-03-04|archive-url=https://web.archive.org/web/20210304193738/https://www.britannica.com/science/alternation-of-generations|url-status=live}}</ref> is the predominant type of [[Biological life cycle|life cycle]] in [[plant]]s and [[algae]]. In plants both phases are [[Multicellular organism|multicellular]]: the [[haploid]] sexual phase – the [[gametophyte]] – alternates with a [[diploid]] asexual phase – the [[sporophyte]].

A mature sporophyte produces haploid [[spore]]s by [[meiosis]], a process which reduces the number of chromosomes to half, from two sets to one. The resulting haploid spores germinate and grow into multicellular haploid gametophytes. At maturity, a gametophyte produces [[gamete]]s by [[mitosis]], the normal process of cell division in eukaryotes, which maintains the original number of chromosomes. Two haploid gametes (originating from different organisms of the same [[species]] or from the same organism) [[fertilisation|fuse]] to produce a diploid [[zygote]], which divides repeatedly by mitosis, developing into a multicellular diploid sporophyte. This cycle, from gametophyte to sporophyte (or equally from sporophyte to gametophyte), is the way in which all land plants and most [[algae]] undergo [[sexual reproduction]].

The relationship between the sporophyte and gametophyte phases varies among different groups of plants. In the majority of [[alga]]e, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In [[liverwort]]s, [[moss]]es and [[hornwort]]s, the sporophyte is less well developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen.<ref name="Thomas-1978"/><ref name="Glime-2007"/> By contrast, in all modern [[vascular plants]] the gametophyte is less well developed than the sporophyte, although their [[Devonian]] ancestors had gametophytes and sporophytes of approximately equivalent complexity.<ref name="Kerp-2003"/> In [[fern]]s the gametophyte is a small flattened [[autotrophic]] [[prothallus]] on which the young sporophyte is briefly dependent for its nutrition. In [[flowering plant]]s, the reduction of the gametophyte is much more extreme; it consists of just a few cells which grow entirely inside the sporophyte.

Animals develop differently. They {{em|directly}} produce haploid gametes. No haploid spores capable of dividing are produced, so generally there is no multicellular haploid phase. Some insects have a [[Arrhenotoky|sex-determining system]] whereby haploid males are produced from unfertilized eggs; however females produced from fertilized eggs are diploid.

Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as '''diplohaplontic'''. The equivalent terms '''haplodiplontic''', '''diplobiontic''' and '''dibiontic''' are also in use, as is describing such an organism as having a diphasic [[ontogeny]].<ref>{{Cite journal|last1=Kluge|first1=Arnold G.|last2=Strauss|first2=Richard E.|date=1985|title=Ontogeny and Systematics|url=https://www.jstor.org/stable/2097049|journal=Annual Review of Ecology and Systematics|volume=16|pages=247–268|doi=10.1146/annurev.es.16.110185.001335|jstor=2097049|issn=0066-4162|access-date=2021-02-25|archive-date=2022-01-27|archive-url=https://web.archive.org/web/20220127121107/https://www.jstor.org/stable/2097049|url-status=live|url-access=subscription}}</ref> Life cycles of animals, in which there is only a diploid multicellular stage, are referred to as '''diplontic'''. Life cycles in which there is only a haploid multicellular stage are referred to as '''haplontic'''.

==Definition==
Alternation of generations is defined as the alternation of multicellular diploid and haploid forms in the organism's life cycle, regardless of whether these forms are free-living.<ref name="Taylor2005">{{Harvnb|Taylor|Kerp|Hass|2005}}</ref> In some species, such as the alga ''[[Ulva lactuca]]'', the diploid and haploid forms are indeed both free-living independent organisms, essentially identical in appearance and therefore said to be [[Glossary of botanical terms#isomorphic|isomorphic]]. In many algae, the free-swimming, haploid gametes form a diploid zygote which germinates into a multicellular diploid sporophyte. The sporophyte produces free-swimming haploid spores by [[meiosis]] that germinate into haploid gametophytes.<ref>"{{cite web |url= http://www.plantscience4u.com/2014/05/diplohaplontic-life-cycle-in-algae.html#.V3vOW6s7o3R |title= Plant Science 4 U |access-date= 5 July 2016 |archive-date= 18 August 2016 |archive-url= https://web.archive.org/web/20160818104509/http://www.plantscience4u.com/2014/05/diplohaplontic-life-cycle-in-algae.html#.V3vOW6s7o3R |url-status= live }}</ref>

However, in [[Embryophyte|land plant]]s, either the sporophyte or the gametophyte is very much reduced and is incapable of free living. For example, in all [[bryophytes]] the gametophyte generation is dominant and the sporophyte is dependent on it. By contrast, in all [[seed plant]]s the gametophytes are strongly reduced, although the fossil evidence indicates that they were derived from isomorphic ancestors.<ref name="Kerp-2003"/> In [[seed plant]]s, the female gametophyte develops totally within the sporophyte, which protects and nurtures it and the embryonic sporophyte that it produces. The pollen grains, which are the male gametophytes, are reduced to only a few cells (just three cells in many cases). Here the notion of two generations is less obvious; as Bateman & Dimichele say "sporophyte and gametophyte effectively function as a single organism".<ref name="Bateman-1994-2">{{Harvnb|Bateman|Dimichele|1994|p=403}}</ref> The alternative term 'alternation of phases' may then be more appropriate.<ref name="StewartRothwell-1993">{{Harvnb|Stewart|Rothwell|1993}}</ref>

== History ==

=== In animals ===

Initially, [[Adelbert von Chamisso]] (studying [[salp]]s, colonial marine animals between 1815 and 1818<ref>{{cite web |title=Object of the month: The poet and the dolphin skull |url=https://www.kulturtechnik.hu-berlin.de/en/object-of-the-month-the-poet-and-the-dolphin-skull/ |publisher=Hermann von Helmholtz-Zentrum für Kulturtechnik |access-date=4 May 2024 |date=1 July 2023}}</ref>) and [[Japetus Steenstrup]] (studying the development of [[trematode]]s in 1842, and also [[tunicate]]s and [[cnidarian]]s) described the succession of differently organized generations (sexual and asexual) in animals as "alternation of generations".<ref name="Haig-2008"/> Later, the phenomenon in animals became known as [[heterogamy]], while the term "alternation of generations" was restricted to the life cycles of plants, meaning specifically the alternation of haploid gametophytes and diploid sporophytes.<ref name="Haig-2008">{{Citation |last1=Haig |first1=David |date=2008 |title=Homologous versus antithetic alternation of generations and the origin of sporophytes |journal=The Botanical Review |volume=74 |issue=3 |pages=395–418 |doi=10.1007/s12229-008-9012-x |s2cid=207403936 |url=http://dash.harvard.edu/bitstream/handle/1/11148775/Haig_HomologousVersus.pdf?sequence=3 |access-date=2014-08-17 |archive-date=2014-08-19 |archive-url=https://web.archive.org/web/20140819090641/http://dash.harvard.edu/bitstream/handle/1/11148775/Haig_HomologousVersus.pdf?sequence=3 |url-status=live }}</ref>

=== In plants ===

In 1851, [[Wilhelm Hofmeister]] demonstrated the morphological alternation of generations in plants,<ref name="Svedelius-1927"/> between a spore-bearing generation (sporophyte) and a gamete-bearing generation (gametophyte).<ref>{{citation |last=Hofmeister |first=W. |date=1851 |url=https://archive.org/details/vergleichendeun00hofmgoog |title=Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildildiung höherer Kryptogamen (Moose, Farne, Equisetaceen, Rhizocarpeen und Lycopodiaceen) und der Samenbildung der Coniferen |location=Leipzig |publisher=F. Hofmeister |access-date=2014-08-17 |language=de }}. Translated as {{citation |first=Frederick |last=Currey |date=1862 |url=https://www.biodiversitylibrary.org/bibliography/37864#/summary |title=On the germination, development, and fructification of the higher Cryptogamia, and on the fructification of the Coniferæ |publisher=Robert Hardwicke |location=London |access-date=2014-08-17 |archive-date=2014-08-19 |archive-url=https://web.archive.org/web/20140819111436/http://www.biodiversitylibrary.org/bibliography/37864#/summary |url-status=live }}</ref><ref>{{Citation |last1=Feldmann |first1=J. |author-link1=Jean Feldmann |last2=Feldmann |first2=G. |year=1942 |title=Recherches sur les Bonnemaisoniacées et leur alternance de generations |language=fr |trans-title=Studies on the Bonnemaisoniaceae and their alternation of generations |journal=Ann. Sci. Natl. Bot. |series=Series 11 |volume=3 |pages=75–175 |url=http://bibdigital.rjb.csic.es/Imagenes/P0044_S11_03/P0044_S11_03_0162.pdf |access-date=2013-10-07 |archive-url=https://web.archive.org/web/20140819125740/http://bibdigital.rjb.csic.es/Imagenes/P0044_S11_03/P0044_S11_03_0162.pdf |archive-date=2014-08-19 |url-status=dead }}, p. 157</ref> By that time, a debate emerged focusing on the origin of the asexual generation of land plants (i.e., the sporophyte) and is conventionally characterized as a conflict between theories of antithetic ([[Ladislav Josef Čelakovský]], 1874) and homologous ([[Nathanael Pringsheim]], 1876) alternation of generations.<ref name="Haig-2008"/>

In 1874, [[Eduard Strasburger]] discovered the alternation between diploid and haploid nuclear phases,<ref name="Haig-2008"/> also called cytological alternation of nuclear phases.<ref name="Feldmann-1972">{{Citation |last=Feldmann |first=J. |author-link=Jean Feldmann |date=1972 |title=Les problèmes actuels de l'alternance de génerations chez les Algues |journal=Bulletin de la Société Botanique de France |volume=119 |pages=7–38 |doi=10.1080/00378941.1972.10839073 |language=fr |doi-access=free }}</ref> Although most often coinciding, morphological alternation and nuclear phases alternation are sometimes independent of one another, e.g., in many [[red algae]], the same nuclear phase may correspond to two diverse morphological generations.<ref name="Feldmann-1972"/> In some [[fern]]s which lost sexual reproduction, there is no change in nuclear phase, but the alternation of generations is maintained.<ref>{{cite book |author=Schopfer, P. |author2=Mohr, H. |title=Plant physiology |publisher=Springer |location=Berlin |year=1995 |pages=[https://archive.org/details/plantphysiology0000mohr/page/288 288–291] |isbn=978-3-540-58016-4 |chapter=Physiology of Development |chapter-url=https://books.google.com/books?id=jTxfMQg38B4C&q=mohr%20physiology&pg=PA288 |url=https://archive.org/details/plantphysiology0000mohr/page/288 }}</ref>

==Alternation of generations in plants==

===Fundamental elements===

The diagram above shows the fundamental elements of the alternation of generations in plants. There are many variations in different groups of plants. The processes involved are as follows:<ref name="Foster-1974">{{Harvnb|Foster|Gifford|1974}},{{pn|date=May 2023}} {{Harvnb|Sporne|1974a}}{{pn|date=May 2023}} and {{Harvnb|Sporne|1974b}}. {{pn|date=May 2023}} </ref>
* Two single-celled haploid gametes, each containing ''n'' unpaired chromosomes, fuse to form a single-celled diploid zygote, which now contains ''n'' pairs of chromosomes, i.e. 2''n'' chromosomes in total.<ref name="Foster-1974"/>
* The single-celled diploid zygote germinates, dividing by the normal process ([[mitosis]]), which maintains the number of chromosomes at 2''n''. The result is a multi-cellular diploid organism, called the ''sporophyte'' (because at maturity it produces spores).<ref name="Foster-1974"/>
* When it reaches maturity, the sporophyte produces one or more '''sporangia''' (singular: sporangium) which are the organs that produce diploid spore mother cells (sporocytes). These divide by a special process ([[meiosis]]) that reduces the number of chromosomes by a half. This initially results in four single-celled haploid spores, each containing ''n'' unpaired chromosomes.<ref name="Foster-1974"/>
* The single-celled haploid spore germinates, dividing by the normal process (mitosis), which maintains the number of chromosomes at ''n''. The result is a multi-cellular haploid organism, called the ''gametophyte'' (because it produces gametes at maturity).<ref name="Foster-1974"/>
* When it reaches maturity, the gametophyte produces one or more '''gametangia''' (singular: gametangium) which are the organs that produce haploid gametes. At least one kind of gamete possesses some mechanism for reaching another gamete in order to fuse with it.<ref name="Foster-1974"/>
The 'alternation of generations' in the life cycle is thus between a diploid (2''n'') generation of multicellular sporophytes and a haploid (''n'') generation of multicellular gametophytes.<ref name="Foster-1974"/>

[[File:Onoclea sensibilis 4 crop.jpg|thumb|Gametophyte of the fern ''[[Onoclea sensibilis]]'' (flat thallus, bottom) with a descendant sporophyte beginning to grow from it (small frond, top) ]]

The situation is quite different from that in animals, where the fundamental process is that a multicellular diploid (2''n'') individual {{em|directly}} produces haploid (''n'') gametes by meiosis. In animals, spores (i.e. haploid cells which are able to undergo mitosis) are not produced, so there is no asexual multicellular generation. Some insects have haploid males that develop from unfertilized eggs, but the females are all diploid.<ref name="Foster-1974"/>

=== Variations ===

The diagram shown above is a good representation of the life cycle of some multi-cellular algae (e.g. the genus ''[[Cladophora]]'') which have sporophytes and gametophytes of almost identical appearance and which do not have different kinds of spores or gametes.<ref name="Guiry-2008">{{Harvnb|Guiry|Guiry|2008}}</ref>

However, there are many possible variations on the fundamental elements of a life cycle which has alternation of generations. Each variation may occur separately or in combination, resulting in a bewildering variety of life cycles. The terms used by botanists in describing these life cycles can be equally bewildering. As Bateman and Dimichele say "[...] the alternation of generations has become a terminological morass; often, one term represents several concepts or one concept is represented by several terms."<ref>{{Harvnb|Bateman|Dimichele|1994|p=347}}</ref>

Possible variations are:
* ''Relative importance of the sporophyte and the gametophyte.''
** ''Equal'' ('''homomorphy''' or '''isomorphy''').<br />Filamentous algae of the genus ''[[Cladophora]]'', which are predominantly found in fresh water, have diploid sporophytes and haploid gametophytes which are externally indistinguishable.<ref name="Shyam-1980">{{Harvnb|Shyam|1980}}</ref> No living land plant has equally dominant sporophytes and gametophytes, although some theories of the evolution of alternation of generations suggest that ancestral land plants did.
** ''Unequal'' ('''heteromorphy''' or '''anisomorphy''').[[Image:Mnium hornum 2005.04.02 14.55.41.jpg|thumb|Gametophyte of ''[[Mnium hornum]]'', a moss ]]
*** ''Dominant gametophyte'' ('''gametophytic''').<br />In liverworts, mosses and hornworts, the dominant form is the haploid gametophyte. The diploid sporophyte is not capable of an independent existence, gaining most of its nutrition from the parent gametophyte, and having no chlorophyll when mature.<ref>{{Harvnb|Watson|1981|p=2}}</ref>[[Image:Blechnum discolor - crown fern.jpg|thumb|Sporophyte of ''[[Lomaria discolor]]'', a fern ]]
*** ''Dominant sporophyte'' ('''sporophytic''').<br />In ferns, both the sporophyte and the gametophyte are capable of living independently, but the dominant form is the diploid sporophyte. The haploid gametophyte is much smaller and simpler in structure. In seed plants, the [[Gametophyte#Seed plants|gametophyte]] is even more reduced (at the minimum to only three cells), gaining all its nutrition from the sporophyte. The extreme reduction in the size of the gametophyte and its retention within the sporophyte means that when applied to seed plants the term 'alternation of generations' is somewhat misleading: "[s]porophyte and gametophyte effectively function as a single organism".<ref name="Bateman-1994-2"/> Some authors have preferred the term 'alternation of phases'.<ref name="StewartRothwell-1993"/>
* ''Differentiation of the gametes.''
** ''Both gametes the same'' ('''isogamy''').<br />Like other species of ''[[Cladophora]]'', ''C. callicoma'' has flagellated gametes which are identical in appearance and ability to move.<ref name="Shyam-1980" />
** ''Gametes of two distinct sizes'' ('''anisogamy''').
*** ''Both of similar motility''.<br />Species of ''[[sea lettuce|Ulva]]'', the sea lettuce, have gametes which all have two flagella and so are motile. However they are of two sizes: larger 'female' gametes and smaller 'male' gametes.<ref name="Kirby-2001">{{Harvnb|Kirby|2001}}</ref>
*** ''One large and sessile, one small and motile'' ('''oogamy'''). The larger sessile megagametes are eggs (ova), and smaller motile [[microgamete]]s are sperm (spermatozoa, spermatozoids). The degree of motility of the sperm may be very limited (as in the case of flowering plants) but all are able to move towards the sessile eggs. When (as is almost always the case) the sperm and eggs are produced in different kinds of gametangia, the sperm-producing ones are called '''antheridia''' (singular antheridium) and the egg-producing ones '''archegonia''' (singular archegonium).[[Image:Pellia epiphylla IMG 1610.jpg|thumb|Gametophyte of ''[[Pellia epiphylla]]'' with sporophytes growing from the remains of archegonia]]
**** ''Antheridia and archegonia occur on the same gametophyte,'' which is then called '''[[monoicous]]'''. (Many sources, including those concerned with bryophytes, use the term 'monoecious' for this situation and 'dioecious' for the opposite.<ref>{{Harvnb|Watson|1981|p=33}}</ref><ref>{{Harvnb|Bell|Hemsley|2000|p=104}}</ref> Here 'monoecious' and 'dioecious' are used only for sporophytes.)<br />The liverwort ''Pellia epiphylla'' has the gametophyte as the dominant generation. It is monoicous: the small reddish sperm-producing antheridia are scattered along the midrib while the egg-producing archegonia grow nearer the tips of divisions of the plant.<ref>{{Harvnb|Watson|1981|pp=425–6}}</ref>
**** ''Antheridia and archegonia occur on different gametophytes,'' which are then called '''dioicous'''.<br />The moss ''Mnium hornum'' has the gametophyte as the dominant generation. It is dioicous: male plants produce only antheridia in terminal rosettes, female plants produce only archegonia in the form of stalked capsules.<ref>{{Harvnb|Watson|1981|pp=287–8}}</ref> Seed plant gametophytes are also dioicous. However, the parent sporophyte may be monoecious, producing both male and female gametophytes or dioecious, producing gametophytes of one gender only. Seed plant gametophytes are extremely reduced in size; the archegonium consists only of a small number of cells, and the entire male gametophyte may be represented by only two cells.{{sfn|Sporne|1974a|pp=17–21}}
* ''Differentiation of the spores.''
** ''All spores the same size'' ('''homospory''' or isospory).<br />Horsetails (species of ''[[Equisetum]]'') have spores which are all of the same size.<ref name="Bateman-1994-1">{{Harvnb|Bateman|Dimichele|1994|pp=350–1}}</ref>
** ''Spores of two distinct sizes'' ('''heterospory''' or anisospory): larger '''megaspores''' and smaller '''microspores'''. When the two kinds of spore are produced in different kinds of sporangia, these are called '''megasporangia''' and '''microsporangia'''. A megaspore often (but not always) develops at the expense of the other three cells resulting from meiosis, which abort.
*** ''Megasporangia and microsporangia occur on the same sporophyte,'' which is then called '''monoecious'''.<br />Most flowering plants fall into this category. Thus the flower of a lily contains six stamens (the microsporangia) which produce microspores which develop into pollen grains (the microgametophytes), and three fused carpels which produce integumented megasporangia (ovules) each of which produces a megaspore which develops inside the megasporangium to produce the megagametophyte. In other plants, such as hazel, some flowers have only stamens, others only carpels, but the same plant (i.e. sporophyte) has both kinds of flower and so is monoecious.[[Image:Hollyflowers.jpg|right|thumb|Flowers of European holly, a dioecious species: male above, female below (leaves cut to show flowers more clearly)]]
*** ''Megasporangia and microsporangia occur on different sporophytes,'' which are then called '''dioecious'''.<br />An individual tree of the European holly (''[[Ilex aquifolium]]'') produces either 'male' flowers which have only functional stamens (microsporangia) producing microspores which develop into pollen grains (microgametophytes) or 'female' flowers which have only functional carpels producing integumented megasporangia (ovules) that contain a megaspore that develops into a multicellular megagametophyte.

There are some correlations between these variations, but they are just that, correlations, and not absolute. For example, in flowering plants, microspores ultimately produce microgametes (sperm) and megaspores ultimately produce megagametes (eggs). However, in ferns and their allies there are groups with undifferentiated spores but differentiated gametophytes. For example, the fern ''[[Ceratopteris]] thalictrioides'' has spores of only one kind, which vary continuously in size. Smaller spores tend to germinate into gametophytes which produce only sperm-producing antheridia.<ref name="Bateman-1994-1"/>

=== A complex life cycle ===

[[File:Alternation of generations complex.svg|upright=2.5|thumb|Alternation of generations in a species which is heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious]]

Plant life cycles can be complex. Alternation of generations can take place in plants which are at once heteromorphic, sporophytic, oogametic, dioicous, heterosporic and dioecious, such as in a [[willow]] tree (as most species of the genus ''Salix'' are dioecious).<ref>{{cite EB1911|wstitle=Willow|volume=28|pages=688–689}}</ref> The processes involved are:
* An immobile egg, contained in the archegonium, fuses with a mobile sperm, released from an antheridium. The resulting zygote is either male or female.
** A male zygote develops by mitosis into a microsporophyte, which at maturity produces one or more microsporangia. Microspores develop within the microsporangium by meiosis.<br />In a willow (like all seed plants) the zygote first develops into an embryo microsporophyte within the ovule (a megasporangium enclosed in one or more protective layers of tissue known as integument). At maturity, these structures become the [[seed]]. Later the seed is shed, germinates and grows into a mature tree. A male willow tree (a microsporophyte) produces flowers with only stamens, the anthers of which are the microsporangia.
** Microspores germinate producing microgametophytes; at maturity one or more antheridia are produced. Sperm develop within the antheridia.<br />In a willow, microspores are not liberated from the anther (the microsporangium), but develop into pollen grains (microgametophytes) within it. The whole pollen grain is moved (e.g. by an insect or by the wind) to an ovule (megagametophyte), where a sperm is produced which moves down a pollen tube to reach the egg.
** A female zygote develops by mitosis into a megasporophyte, which at maturity produces one or more megasporangia. Megaspores develop within the megasporangium; typically one of the four spores produced by meiosis gains bulk at the expense of the remaining three, which disappear.<br />Female willow trees (megasporophytes) produce flowers with only carpels (modified leaves that bear the megasporangia).
** Megaspores germinate producing megagametophytes; at maturity one or more archegonia are produced. Eggs develop within the archegonia.<br /> The carpels of a willow produce ovules, megasporangia enclosed in integuments. Within each ovule, a megaspore develops by mitosis into a megagametophyte. An archegonium develops within the megagametophyte and produces an egg. The whole of the gametophytic generation remains within the protection of the sporophyte except for pollen grains (which have been reduced to just three cells contained within the microspore wall).{{-}}

==Life cycles of different plant groups==

{{further|Marchantiophyta#Life cycle|Moss#Life cycle|Hornwort#Life cycle|Fern#Life cycle}}

The term "plants" is taken here to mean the [[Archaeplastida]], i.e. the [[glaucophyte]]s, [[red algae|red]] and [[green algae]] and [[land plants]].

Alternation of generations occurs in almost all multicellular red and green algae, both freshwater forms (such as ''[[Cladophora]]'') and seaweeds (such as ''[[Ulva (genus)|Ulva]]''). In most, the generations are homomorphic (isomorphic) and free-living. Some species of red algae have a complex triphasic alternation of generations, in which there is a gametophyte phase and two distinct sporophyte phases. For further information, see [[Red algae#Reproduction|Red algae: Reproduction]].

Land plants all have heteromorphic (anisomorphic) alternation of generations, in which the sporophyte and gametophyte are distinctly different. All [[bryophyte]]s, i.e. [[Marchantiophyta|liverwort]]s, [[moss]]es and [[hornwort]]s, have the gametophyte generation as the most conspicuous. As an illustration, consider a monoicous moss. Antheridia and archegonia develop on the mature plant (the gametophyte). In the presence of water, the biflagellate sperm from the antheridia swim to the archegonia and fertilisation occurs, leading to the production of a diploid sporophyte. The sporophyte grows up from the archegonium. Its body comprises a long stalk topped by a capsule within which spore-producing cells undergo meiosis to form haploid spores. Most mosses rely on the wind to disperse these spores, although ''[[Splachnum sphaericum]]'' is [[entomophilous]], recruiting insects to disperse its spores.

<gallery class=center mode=nolines widths=220 heights=220>
File:Liverwort life cycle.jpg|Alternation of generations in liverworts
File:Lifecycle moss svg diagram.svg|Moss life cycle
File:Hornwort life cicle svg diagram.svg|Hornwort life cycle
</gallery>

The [[Fern#Life cycle|life cycle of ferns]] and their allies, including [[clubmoss]]es and [[horsetail]]s, the conspicuous plant observed in the field is the diploid sporophyte. The haploid spores develop in [[sorus|sori]] on the underside of the fronds and are 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 class=center mode=nolines widths=220 heights=220>
File:Alternation of generations in ferns.svg|{{center|Alternation of generations in ferns}}
File:Dixonia prothallus.jpg|{{center|A gametophyte (prothallus) of ''Dicksonia ''}}
File:Dicksonia antarctica Cultivated GardenEngland.jpg|{{center|A sporophyte of ''Dicksonia antarctica''}}
File:SoriDicksonia.jpg|{{center|''Dicksonia antarctica'' frond with spore-producing structures}}
</gallery>

In the [[spermatophyte]]s, the seed plants, the sporophyte is the dominant multicellular phase; the gametophytes are strongly reduced in size and very different in morphology. The entire gametophyte generation, with the sole exception of pollen grains (microgametophytes), is contained within the sporophyte. The life cycle of a dioecious flowering plant (angiosperm), the willow, has been outlined in some detail in an earlier section ([[#A complex life cycle|A complex life cycle]]). The life cycle of a [[gymnosperm]] is similar. However, flowering plants have in addition a phenomenon called '[[double fertilization]]'. In the process of [[double fertilization]], two sperm nuclei from a pollen grain (the microgametophyte), rather than a single sperm, enter the archegonium of the megagametophyte; one fuses with the egg nucleus to form the zygote, the other fuses with two other nuclei of the gametophyte to form '[[endosperm]]', which nourishes the developing embryo.

==Evolution of the dominant diploid phase==

It has been proposed that the basis for the emergence of the diploid phase of the life cycle (sporophyte) as the dominant phase (e.g. as in vascular plants) is that diploidy allows masking of the expression of deleterious mutations through [[Complementation (genetics)|genetic complementation]].<ref>{{Citation |last1=Bernstein |first1=H. |last2=Byers |first2=G.S. |last3=Michod |first3=R.E. |year=1981 |title=Evolution of sexual reproduction: Importance of DNA repair, complementation, and variation |journal=The American Naturalist |volume=117 |issue=4 |pages=537–549 |name-list-style=amp |doi=10.1086/283734 |s2cid=84568130 }}</ref><ref>{{Citation |last1=Michod |first1=R.E. |last2=Gayley |first2=T.W. |year=1992 |title=Masking of mutations and the evolution of sex |journal=The American Naturalist |volume=139 |issue=4 |pages=706–734 |name-list-style=amp |doi=10.1086/285354|s2cid=85407883 }}</ref> Thus if one of the parental genomes in the diploid cells contained [[mutation]]s leading to defects in one or more [[gene product]]s, these deficiencies could be compensated for by the other parental genome (which nevertheless may have its own defects in other genes). As the diploid phase was becoming predominant, the masking effect likely allowed [[genome size]], and hence information content, to increase without the constraint of having to improve accuracy of DNA replication. The opportunity to increase information content at low cost was advantageous because it permitted new adaptations to be encoded. This view has been challenged, with evidence showing that selection is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms.<ref name="Szövényi-2013">{{Citation |last1=Szövényi |first1=Péter |last2=Ricca |first2=Mariana |last3=Hock |first3=Zsófia |last4=Shaw |first4=Jonathan A. |last5=Shimizu |first5=Kentaro K. |last6=Wagner |first6=Andreas |year=2013 |title=Selection is no more efficient in haploid than in diploid life stages of an angiosperm and a moss |journal=Molecular Biology and Evolution |volume=30 |issue=8 |pages=1929–39 |doi=10.1093/molbev/mst095|pmid=23686659 |doi-access=free }}</ref>

<gallery class=center mode=nolines widths=220 heights=220>
File:Angiosperm life cycle diagram-en.svg|Angiosperm life cycle
File:Tulip Stamen Tip.jpg|Tip of tulip stamen showing pollen (microgametophytes)
File:Ovule-Gymno-Angio-en.svg|Plant ovules (megagametophytes): gymnosperm ovule on left, angiosperm ovule (inside ovary) on right
File:Double_Fertilization.jpg|Double fertilization
</gallery>

==Similar processes in other organisms==

=== Rhizaria ===

[[File:Foraminifera life cycle.png|thumb|upright=2|Life cycle of [[Foraminifera]] showing alternation of generations]]

Some organisms currently classified in the clade [[Rhizaria]] and thus not plants in the sense used here, exhibit alternation of generations. Most [[Foraminifera]] undergo a heteromorphic alternation of generations between haploid '''''gamont''''' and diploid '''''agamont''''' forms. The diploid form is typically much larger than the haploid form; these forms are known as the ''microsphere'' and ''megalosphere'', respectively.

=== 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.

=== Slime moulds ===
The life cycle of [[slime mould]]s 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.

=== Animals ===
Alternation between a multicellular diploid and a multicellular haploid generation is never encountered in animals.<ref name="Barnes-2001">{{Harvnb|Barnes|Calow|Olive|Golding|2001|p=321}}</ref> In some animals, there is an alternation between [[parthenogenesis|parthenogenic]] and [[sexual reproduction|sexually reproductive]] phases ([[heterogamy]]), for instance in [[salp]]s and [[Doliolida|doliolids]] (class [[Thaliacea]]). Both phases are diploid. This has sometimes been called "alternation of generations",<ref name="Scott-1996">{{Harvnb|Scott|1996|p=35}}</ref> but is quite different. In some other animals, such as [[hymenoptera]]ns, males are haploid and females diploid, but this is always the case rather than there being an alternation between distinct generations.

==See also==
* {{annotated link|Apomixis}}
* {{annotated link|Evolutionary history of plants#life cycles}}: Evolutionary origin of the alternation of phases
* {{annotated link|Ploidy}}

==Notes and references==
{{Reflist|2|refs=

<ref name="Glime-2007">{{citation |last=Glime |first=J.M. |year=2007 |title=Bryophyte Ecology: Vol. 1 Physiological Ecology |publisher=Michigan Technological University and the International Association of Bryologists |access-date=2013-03-04 |url=http://www.bryoecol.mtu.edu/chapters/5-9Sporophyte.pdf |archive-date=2013-03-26 |archive-url=https://web.archive.org/web/20130326072218/http://www.bryoecol.mtu.edu/chapters/5-9Sporophyte.pdf |url-status=live }}</ref>

<ref name="Kerp-2003">{{citation  |last1=Kerp |first1=H. |last2=Trewin |first2=N.H. |last3=Hass |first3=H. |year=2003 |title=New gametophytes from the Lower Devonian Rhynie Chert |journal=Transactions of the Royal Society of Edinburgh: Earth Sciences |volume=94 |issue=4 |pages=411–428  |doi=10.1017/S026359330000078X  |s2cid=128629425 |name-list-style=amp }}</ref>

<ref name="Svedelius-1927">{{Citation |last=Svedelius |first=Nils |year=1927 |title=Alternation of Generations in Relation to Reduction Division |journal=Botanical Gazette |volume=83 |issue=4 |pages=362–384 |jstor=2470766 |doi=10.1086/333745|s2cid=84406292 }}</ref>

<ref name="Thomas-1978">{{citation |last1=Thomas |first1=R.J. |last2=Stanton |first2=D.S. |last3=Longendorfer |first3=D.H. |last4=Farr |first4=M.E. |year=1978 |title=Physiological evaluation of the nutritional autonomy of a hornwort sporophyte |journal=Botanical Gazette |volume=139 |issue=3 |pages=306–311 |name-list-style=amp |doi=10.1086/337006|s2cid=84413961 }}</ref>

}}

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{{Clear}}
{{Botany}}

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{{DEFAULTSORT:Alternation Of Generations}}
[[Category:Plant reproduction]]
[[Category:Reproduction]]