Editing Ploidy (section)

==Special cases==

===More than one nucleus per cell===
In the strictest sense, ploidy refers to the number of sets of chromosomes in a single [[nucleus (cell)|nucleus]] rather than in the cell as a whole. Because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. Authors may at times report the total combined ploidy of all nuclei present within the cell membrane of a [[syncytium]],<ref name="D'Amato-2002">Encyclopedia of the Life Sciences (2002) "Polyploidy" Francesco D'Amato and Mauro Durante</ref> though usually the ploidy of each nucleus is described individually. For example, a fungal [[dikaryon]] with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together.<ref>{{cite web |url=http://www.utm.utoronto.ca/~anders38/ms.pdf |title=Dikaryons, diploids, and evolution |author1=James B. Anderson |author2=Linda M Kohn |publisher=University of Toronto |access-date=2012-12-16 |archive-url=https://web.archive.org/web/20130527004555/http://www.utm.utoronto.ca/~anders38/ms.pdf |archive-date=2013-05-27 |url-status=dead }}</ref>

===Ancestral ploidy levels===
It is possible on rare occasions for ploidy to increase in the [[germline]], which can result in [[polyploid]] offspring and ultimately polyploid species. This is an important evolutionary mechanism in both plants and animals and is known as a primary driver of [[speciation]].<ref name="Mable-2004"/> As a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called the '''monoploid number''' (''x''), and is distinct from the haploid number (''n'') in the organism as it now reproduces.{{citation needed|date=May 2023}}

[[Common wheat]] (''Triticum aestivum'') is an organism in which ''x'' and ''n'' differ. Each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). The somatic cells are hexaploid, 2''n''&nbsp;=&nbsp;6''x''&nbsp;=&nbsp;42 (where the monoploid number ''x''&nbsp;=&nbsp;7 and the haploid number ''n''&nbsp;=&nbsp;21). The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor, [[einkorn wheat]].{{citation needed |date=March 2017}}

[[Tetraploidy]] (four sets of chromosomes, 2''n''&nbsp;=&nbsp;4''x'') is common in many [[plant]] species, and also occurs in [[amphibian]]s, [[reptile]]s, and [[insect]]s. For example, species of ''[[Xenopus]]'' (African toads) form a '''ploidy series''', featuring diploid (''[[Western clawed frog|X. tropicalis]]'', 2n=20), tetraploid (''[[African clawed frog|X. laevis]]'', 4n=36), octaploid (''[[De Witte's clawed frog|X. wittei]]'', 8n=72), and dodecaploid (''[[Uganda clawed frog|X. ruwenzoriensis]]'', 12n=108) species.<ref>{{cite journal |last1=Schmid |first1=M |last2=Evans |first2=BJ |last3=Bogart |first3=JP |title=Polyploidy in Amphibia |journal=Cytogenet. Genome Res. |date=2015 |volume=145 |issue=3–4 |pages=315–330 |doi=10.1159/000431388 |pmid=26112701|doi-access=free }}</ref>

Over evolutionary time scales in which [[chromosomal polymorphism]]s accumulate, these changes become less apparent by [[karyotype]] – for example, humans are generally regarded as diploid, but the [[2R hypothesis]] has confirmed two rounds of whole genome duplication in early vertebrate ancestors.{{cn|date=June 2024}}

===Haplodiploidy===
{{main|Haplodiploidy}}

Ploidy can also vary between individuals of the same species or at different stages of the [[biological life cycle|life cycle]].<ref>{{cite journal |vauthors=Parfrey LW, Lahr DJ, Katz LA | year=2008 | title=The dynamic nature of eukaryotic genomes | url= | journal=Mol Biol Evol | volume=25 | issue= 4| pages=787–794 | doi=10.1093/molbev/msn032 | pmid=18258610 | pmc=2933061}}</ref><ref>{{cite journal | author=Qiu Y.-L., Taylor A. B., McManus H. A. | year=2012 | title=Evolution of the life cycle in land plants | journal=Journal of Systematics and Evolution | volume=50 | issue= 3| pages=171–194 | doi=10.1111/j.1759-6831.2012.00188.x| url=https://deepblue.lib.umich.edu/bitstream/2027.42/92043/1/j.1759-6831.2012.00188.x.pdf | hdl=2027.42/92043 | s2cid=40564254 | hdl-access=free }}</ref> In some insects it differs by [[eusociality|caste]]. In humans, only the gametes are haploid, but in many of the [[social insect]]s, including [[ant]]s, [[bee]]s, and [[termite]]s, males develop from unfertilized eggs, making them haploid for their entire lives, even as adults.{{citation needed|date=May 2023}}

In the Australian bulldog ant, ''[[Myrmecia pilosula]]'', a haplodiploid species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes.<ref>{{cite journal |vauthors=Crosland MW, Crozier RH | title=Myrmecia pilosula, an Ant with Only One Pair of Chromosomes | journal=Science | volume=231 | issue=4743 | pages=1278 | year=1986 | pmid=17839565 | doi=10.1126/science.231.4743.1278 | bibcode=1986Sci...231.1278C | s2cid=25465053 }}</ref> In ''[[Entamoeba]]'', the ploidy level varies from 4''n'' to 40''n'' in a single population.<ref>{{cite web |url=https://bcrc.bio.umass.edu/courses/fall2010/biol/biolh100-03/sites/default/files/vazquez_eukaryotic_diversity_2010.pdf |title=Archived copy |access-date=2014-02-18 |url-status=dead |archive-url=https://web.archive.org/web/20140223015031/https://bcrc.bio.umass.edu/courses/fall2010/biol/biolh100-03/sites/default/files/vazquez_eukaryotic_diversity_2010.pdf |archive-date=2014-02-23 }}</ref> [[Alternation of generations]] occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle.{{citation needed|date=May 2023}}

===Tissue-specific polyploidy===
In large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. Because the chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. However, in many situations somatic cells double their copy number by means of [[endoreduplication]] as an aspect of [[cellular differentiation]]. For example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei.<ref>{{cite book |url=https://books.google.com/books?id=VhLOSt1bq-wC&pg=PA234 |title=The Development and Regenerative Potential of Cardiac Muscle |author1=John O. Oberpriller |author2=A Mauro |publisher=Taylor&Francis|isbn=9783718605187 |year=1991 }}</ref>