Editing Alternation of generations (section)

{{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'''.