Editing Polyploidy (section)

== Terminology ==

=== Types ===
{{Redirect|Triploid|the human chromosomal disorder (69 XXX, etc.)|Triploid syndrome}}
{{anchor|Polyploid types}}
[[File:Organ-specific patterns of endopolyploidy in the giant ant Dinoponera australis - JHR-037-113-g001.jpg|thumb|upright=1.25|Organ-specific patterns of endopolyploidy (from 2''x'' to 64''x'') in the giant ant ''[[Dinoponera australis]]'']]'''Polyploid''' types are labeled according to the number of chromosome sets in the [[cell nucleus|nucleus]]. The letter ''x'' is used to represent the number of chromosomes in a single set:

*'''haploid''' (one set; 1''x''), for example male [[Myrmica rubra|European fire ant]]s
*'''diploid''' (two sets; 2''x''), for example [[human]]s
*'''triploid''' (three sets; 3''x''<!-- Sic, not "×"-->), for example sterile [[Crocus sativus|saffron crocus]], or [[parthenocarpy|seedless watermelons]], also common in the [[phylum]] [[Tardigrada]]<ref>{{cite journal| vauthors = Bertolani R |year=2001|title=Evolution of the reproductive mechanisms in Tardigrades: a review|journal=Zoologischer Anzeiger|volume=240|issue=3–4|pages=247–252|doi=10.1078/0044-5231-00032|bibcode=2001ZooAn.240..247B }}</ref>
*'''tetraploid''' (four sets; 4''x''), for example, [[Plains viscacha rat]], [[Salmonidae]] fish,<ref name="StouderBisson1997">{{cite book |doi=10.1007/978-1-4615-6375-4_4 |chapter=The Origin and Speciation of Oncorhynchus Revisited |title=Pacific Salmon & Their Ecosystems |date=1997 |last1=McPhail |first1=J. D. |pages=29–38 |isbn=978-0-412-98691-8 }}</ref> the cotton ''[[Gossypium hirsutum]]''<ref>{{cite journal | vauthors = Adams KL, Wendel JF | title = Polyploidy and genome evolution in plants | journal = Current Opinion in Plant Biology | volume = 8 | issue = 2 | pages = 135–141 | date = April 2005 | pmid = 15752992 | doi = 10.1016/j.pbi.2005.01.001 | bibcode = 2005COPB....8..135A }}</ref>
*'''pentaploid''' (five sets; 5''x''), for example Kenai Birch (''[[Betula kenaica]]'')
*'''hexaploid''' (six sets; 6''x''), for example some species of [[wheat]],<ref>{{cite book |doi=10.1016/B978-0-12-817563-7.00028-3 |chapter=Wide hybridization |title=Plant Breeding and Cultivar Development |date=2021 |last1=Singh |first1=Dhan Pal |last2=Singh |first2=Asheesh K. |last3=Singh |first3=Arti |pages=159–178 |isbn=978-0-12-817563-7 }}</ref> [[kiwifruit]]<ref name="kiwifruit">{{cite journal |last1=Crowhurst |first1=Ross N. |last2=Whittaker |first2=D. |last3=Gardner |first3=R. C. |title=THE GENETIC ORIGIN OF KIWIFRUIT |journal=Acta Horticulturae |date=April 1992 |issue=297 |pages=61–62 |doi=10.17660/ActaHortic.1992.297.5 }}</ref>
*'''heptaploid''' or '''septaploid''' (seven sets; 7''x''), for example some cultured [[Siberian sturgeon]]<ref>{{cite journal |last1=Havelka |first1=Miloš |last2=Bytyutskyy |first2=Dmytro |last3=Symonová |first3=Radka |last4=Ráb |first4=Petr |last5=Flajšhans |first5=Martin |title=The second highest chromosome count among vertebrates is observed in cultured sturgeon and is associated with genome plasticity |journal=Genetics Selection Evolution |date=11 February 2016 |volume=48 |article-number=12 |doi=10.1186/s12711-016-0194-0 |doi-access=free |pmid=26867760 |pmc=4751722 }}</ref>
*'''octaploid''' or '''octoploid''', (eight sets; 8''x''), for example ''[[Acipenser]]'' (genus of [[sturgeon]] fish), [[dahlia]]s
*'''decaploid''' (ten sets; 10''x''), for example certain [[Fragaria|strawberries]]
*'''dodecaploid''' or '''duodecaploid''' (twelve sets; 12''x''), for example the plants ''[[Celosia argentea]]'' and ''[[Spartina anglica]]''{{Hair space}}<ref>{{cite journal| vauthors = Aïnouche ML, Fortune PM, Salmon A, Parisod C, Grandbastien MA, Fukunaga K, Ricou M, Misset MT | display-authors = 6 |year=2008|title=Hybridization, polyploidy and invasion: Lessons from ''Spartina'' (Poaceae)|journal=Biological Invasions|volume=11|issue=5|pages=1159–1173|doi=10.1007/s10530-008-9383-2 }}</ref> or the amphibian ''[[Xenopus ruwenzoriensis]]''.
*'''tetratetracontaploid''' (forty-four sets; 44''x''), for example [[Morus nigra|black mulberry]]<ref>
:{{ cite journal | issue = 10 | year = 2017 | volume = 7 | vauthors = Hussain F, Rana Z, Shafique H, Malik A, Hussain Z | pages = 950–956 | publisher = [[Medknow]] | title = Phytopharmacological potential of different species of Morus alba and their bioactive phytochemicals: A review | journal = [[Asian Pacific Journal of Tropical Biomedicine]] | doi = 10.1016/j.apjtb.2017.09.015 | issn = 2221-1691 | doi-access = free }}
:
:{{ cite book | year = 2018 | publisher = [[Springer International Publishing AG]] | vauthors = Al-Khayri JM, Jain SM, Johnson DV | veditors = Al-Khayri JM, Jain SM, Johnson DV | pages = 89–130 | url=| title = Advances in Plant Breeding Strategies: Fruits | volume = 2 | doi = 10.1007/978-3-319-91944-7 | isbn = 978-3-319-91943-0 }}
:
:This review and book cite this research.
:
:{{cite journal | vauthors = Zeng Q, Chen H, Zhang C, Han M, Li T, Qi X, Xiang Z, He N | display-authors = 6 | title = Definition of Eight Mulberry Species in the Genus ''Morus'' by Internal Transcribed Spacer-Based Phylogeny | journal = PLOS ONE | volume = 10 | issue = 8 | pages = e0135411 | date = 2015 | pmid = 26266951 | doi = 10.1371/journal.pone.0135411 | pmc = 4534381 | bibcode = 2015PLoSO..1035411Z | doi-access = free }}
</ref>

=== Classification ===

==== Autopolyploidy ====
'''Autopolyploids''' are polyploids with multiple chromosome sets derived from a single [[taxon]].

Two examples of natural autopolyploids are the piggyback plant, ''[[Tolmiea menziesii|Tolmiea menzisii]]''<ref>{{Cite journal| vauthors = Soltis DE |date=October 1984 |title=Autopolyploidy in ''Tolmiea menziesii'' (Saxifragaceae)|journal=American Journal of Botany|volume=71|issue=9|pages=1171–1174|doi=10.2307/2443640|jstor=2443640}}</ref> and the white sturgeon, ''[[White sturgeon|Acipenser transmontanum]]''.<ref>{{Cite journal| vauthors = Drauch Schreier A, Gille D, Mahardja B, May B |date= November 2011|title=Neutral markers confirm the octoploid origin and reveal spontaneous autopolyploidy in white sturgeon, ''Acipenser transmontanus''|journal=Journal of Applied Ichthyology|language=en|volume=27|pages=24–33|doi=10.1111/j.1439-0426.2011.01873.x|issn=1439-0426|doi-access=free|bibcode= 2011JApIc..27...24D}}</ref> Most instances of autopolyploidy result from the fusion of unreduced (2''n'') gametes, which results in either triploid (''n'' + 2''n'' = 3''n'') or tetraploid (2''n'' + 2''n'' = 4''n'') offspring.<ref name="Bretagnolle_1995">{{cite journal | vauthors = Bretagnolle F, Thompson JD | title = Gametes with the somatic chromosome number: mechanisms of their formation and role in the evolution of autopolyploid plants | journal = The New Phytologist | volume = 129 | issue = 1 | pages = 1–22 | date = January 1995 | pmid = 33874422 | doi = 10.1111/j.1469-8137.1995.tb03005.x | doi-access = free }}</ref> Triploid offspring are typically sterile (as in the phenomenon of [[triploid block]]), but in some cases they may produce high proportions of unreduced gametes and thus aid the formation of tetraploids. This pathway to tetraploidy is referred to as the ''triploid bridge''.<ref name="Bretagnolle_1995" /> Triploids may also persist through [[asexual reproduction]]. In fact, stable autotriploidy in plants is often associated with [[Apomixis|apomictic]] mating systems.<ref>{{Cite journal| vauthors = Müntzing A |date=March 1936|title=The Evolutionary Significance of Autopolyploidy|journal=Hereditas|language=en|volume=21|issue=2–3|pages=363–378|doi=10.1111/j.1601-5223.1936.tb03204.x|issn=1601-5223|doi-access=}}</ref> In agricultural systems, autotriploidy can result in seedlessness, as in [[watermelon]]s and [[banana]]s.<ref>{{cite journal | vauthors = Varoquaux F, Blanvillain R, Delseny M, Gallois P | title = Less is better: new approaches for seedless fruit production | journal = Trends in Biotechnology | volume = 18 | issue = 6 | pages = 233–242 | date = June 2000 | pmid = 10802558 | doi = 10.1016/s0167-7799(00)01448-7 }}</ref> Triploidy is also utilized in salmon and trout farming to induce sterility.<ref>{{Cite journal| vauthors = Cotter D, O'Donovan V, O'Maoiléidigh N, Rogan G, Roche N, Wilkins NP |date=June 2000|title=An evaluation of the use of triploid Atlantic salmon (''Salmo salar'' L.) in minimising the impact of escaped farmed salmon on wild populations|journal=Aquaculture|volume=186|issue=1–2|pages=61–75|doi=10.1016/S0044-8486(99)00367-1|bibcode=2000Aquac.186...61C }}</ref><ref>{{Cite journal| vauthors = Lincoln RF, Scott AP |year=1983|title=Production of all-female triploid rainbow trout|journal=Aquaculture|language=en|volume=30|issue=1–4|pages=375–380|doi=10.1016/0044-8486(83)90179-5|bibcode=1983Aquac..30..375L }}</ref>

Rarely, autopolyploids arise from spontaneous, somatic genome doubling, which has been observed in apple (''Malus domesticus'') [[Sport (botany)|bud sports]].<ref>{{Cite journal| vauthors = Dermen H |date=May 1951|title=Tetraploid and Diploid Adventitious Shoots: From a Giant Sport of McIntosh Apple|journal=Journal of Heredity|volume=42|issue=3|pages=145–149|doi=10.1093/oxfordjournals.jhered.a106189 }}</ref> This is also the most common pathway of artificially induced polyploidy, where methods such as [[Somatic fusion|protoplast fusion]] or treatment with [[colchicine]], [[oryzalin]] or [[mitotic inhibitor]]s are used to disrupt normal [[Mitosis|mitotic]] division, which results in the production of polyploid cells. This process can be useful in plant breeding, especially when attempting to introgress germplasm across ploidal levels.<ref>{{cite book |doi=10.1002/9780470380130.ch3 |chapter=Enhancing Crop Gene Pools with Beneficial Traits Using Wild Relatives |title=Plant Breeding Reviews |date=2007 |last1=Dwivedi |first1=Sangam L. |last2=Upadhyaya |first2=Hari D. |last3=Stalker |first3=H. Thomas |last4=Blair |first4=Matthew W. |last5=Bertioli |first5=David J. |last6=Nielen |first6=Stephan |last7=Ortiz |first7=Rodomiro |pages=179–230 |isbn=978-0-470-17152-3 |chapter-url=http://oar.icrisat.org/2546/1/Enhancing_Crop_Gene_Pools.pdf }}</ref>

Autopolyploids possess at least three [[homologous chromosome]] sets, which can lead to high rates of multivalent pairing during [[meiosis]] (particularly in recently formed autopolyploids, also known as neopolyploids) and an associated decrease in fertility due to the production of [[Aneuploidy|aneuploid]] gametes.<ref name="Justin_2002">{{Cite journal| vauthors = Justin R |date=January 2002|title=Neopolyploidy in Flowering Plants|journal=Annual Review of Ecology and Systematics|volume=33|issue=1|pages=589–639|doi=10.1146/annurev.ecolsys.33.010802.150437 }}</ref> Natural or artificial selection for fertility can quickly stabilize meiosis in autopolyploids by restoring bivalent pairing during meiosis. Rapid adaptive evolution of the meiotic machinery, resulting in reduced levels of multivalents (and therefore stable autopolyploid meiosis) has been documented in ''Arabidopsis arenosa''<ref>{{Cite journal |last1=Yant |first1=Levi |last2=Hollister |first2=Jesse D. |last3=Wright |first3=Kevin M. |last4=Arnold |first4=Brian J. |last5=Higgins |first5=James D. |last6=Franklin |first6=F. Chris H. |last7=Bomblies |first7=Kirsten |date=November 2013 |title=Meiotic Adaptation to Genome Duplication in Arabidopsis arenosa |url=|journal=Current Biology |volume=23 |issue=21 |pages=2151–2156 |doi=10.1016/j.cub.2013.08.059 |issn=0960-9822 |pmc=3859316 |pmid=24139735|bibcode=2013CBio...23.2151Y }}</ref> and ''Arabidopsis lyrata'',<ref>{{Cite journal |last1=Marburger |first1=Sarah |last2=Monnahan |first2=Patrick |last3=Seear |first3=Paul J. |last4=Martin |first4=Simon H. |last5=Koch |first5=Jordan |last6=Paajanen |first6=Pirita |last7=Bohutínská |first7=Magdalena |last8=Higgins |first8=James D. |last9=Schmickl |first9=Roswitha |last10=Yant |first10=Levi |date=2019-11-18 |title=Interspecific introgression mediates adaptation to whole genome duplication |journal=Nature Communications |language=en |volume=10 |issue=1 |pages=5218 |doi=10.1038/s41467-019-13159-5 |issn=2041-1723 |pmc=6861236 |pmid=31740675|bibcode=2019NatCo..10.5218M }}</ref> with specific adaptive alleles of these species shared between only the evolved polyploids.<ref>{{Cite journal |last1=Marburger |first1=Sarah |last2=Monnahan |first2=Patrick |last3=Seear |first3=Paul J. |last4=Martin |first4=Simon H. |last5=Koch |first5=Jordan |last6=Paajanen |first6=Pirita |last7=Bohutínská |first7=Magdalena |last8=Higgins |first8=James D. |last9=Schmickl |first9=Roswitha |last10=Yant |first10=Levi |date=2019-11-18 |title=Interspecific introgression mediates adaptation to whole genome duplication |journal=Nature Communications |language=en |volume=10 |issue=1 |pages=5218 |doi=10.1038/s41467-019-13159-5 |issn=2041-1723 |pmc=6861236 |pmid=31740675|bibcode=2019NatCo..10.5218M }}</ref><ref>{{Cite journal |last1=Seear |first1=Paul J. |last2=France |first2=Martin G. |last3=Gregory |first3=Catherine L. |last4=Heavens |first4=Darren |last5=Schmickl |first5=Roswitha |last6=Yant |first6=Levi |last7=Higgins |first7=James D. |date=2020-07-15 |editor-last=Grelon |editor-first=Mathilde |title=A novel allele of ASY3 is associated with greater meiotic stability in autotetraploid Arabidopsis lyrata |journal=PLOS Genetics |language=en |volume=16 |issue=7 |pages=e1008900 |doi=10.1371/journal.pgen.1008900 |doi-access=free |issn=1553-7404 |pmc=7392332 |pmid=32667955}}</ref> 

The high degree of [[Homology (biology)|homology]] among duplicated chromosomes causes autopolyploids to display [[polysomic inheritance]].<ref>{{cite journal | vauthors = Parisod C, Holderegger R, Brochmann C | title = Evolutionary consequences of autopolyploidy | journal = The New Phytologist | volume = 186 | issue = 1 | pages = 5–17 | date = April 2010 | pmid = 20070540 | doi = 10.1111/j.1469-8137.2009.03142.x | doi-access =  }}</ref> This trait is often used as a diagnostic criterion to distinguish autopolyploids from allopolyploids, which commonly display disomic inheritance after they progress past the neopolyploid stage.<ref name="Le Comber_2010">{{cite journal | vauthors = Le Comber SC, Ainouche ML, Kovarik A, Leitch AR | title = Making a functional diploid: from polysomic to disomic inheritance | journal = The New Phytologist | volume = 186 | issue = 1 | pages = 113–122 | date = April 2010 | pmid = 20028473 | doi = 10.1111/j.1469-8137.2009.03117.x | doi-access =  }}</ref> While most polyploid species are unambiguously characterized as either autopolyploid or allopolyploid, these categories represent the ends of a spectrum of divergence between parental subgenomes. Polyploids that fall between these two extremes, which are often referred to as segmental allopolyploids, may display intermediate levels of polysomic inheritance that vary by locus.<ref>{{Cite book| vauthors = Stebbins GL |title=Types of polyploids; their classification and significance|year=1947|isbn=9780120176014|series=Advances in Genetics|volume=1|pages=403–429|language=en|doi=10.1016/s0065-2660(08)60490-3|pmid=20259289}}</ref><ref>{{Cite book| vauthors = Stebbins GL |title=Variation and Evolution in Plants|publisher=Oxford University Press|year=1950}}{{page needed|date=March 2019}}</ref>

About half of all polyploids are thought to be the result of autopolyploidy,<ref>{{Cite journal| vauthors = Ramsey J, Schemske DW |date= January 1998 |title=Pathways, Mechanisms, and Rates of Polyploid Formation in Flowering Plants|journal=Annual Review of Ecology and Systematics|volume=29|issue=1|pages=467–501|doi=10.1146/annurev.ecolsys.29.1.467 }}</ref><ref>{{cite journal | vauthors = Barker MS, Arrigo N, Baniaga AE, Li Z, Levin DA | title = On the relative abundance of autopolyploids and allopolyploids | journal = The New Phytologist | volume = 210 | issue = 2 | pages = 391–398 | date = April 2016 | pmid = 26439879 | doi = 10.1111/nph.13698 | doi-access = free }}</ref> although many factors make this proportion hard to estimate.<ref>{{cite journal | vauthors = Doyle JJ, Sherman-Broyles S | title = Double trouble: taxonomy and definitions of polyploidy | journal = The New Phytologist | volume = 213 | issue = 2 | pages = 487–493 | date = January 2017 | pmid = 28000935 | doi = 10.1111/nph.14276 | doi-access = free }}</ref>

==== Allopolyploidy ====
'''Allopolyploids''' or '''amphipolyploids''' or '''heteropolyploids''' are polyploids with chromosomes derived from two or more diverged taxa.

As in autopolyploidy, this primarily occurs through the fusion of unreduced (2''n'') gametes, which can take place before or after [[Hybrid (biology)|hybridization]]. In the former case, unreduced gametes from each diploid taxon – or reduced gametes from two autotetraploid taxa – combine to form allopolyploid offspring. In the latter case, one or more diploid [[F1 hybrid|F<sub>1</sub> hybrids]] produce unreduced gametes that fuse to form allopolyploid progeny.<ref name="Ramsey_1998">{{Cite journal| vauthors = Ramsey J |date=January 1998|title=Pathways, Mechanisms, and Rates of Polyploid Formation in Flowering Plants|journal=Annual Review of Ecology and Systematics|volume=29|issue=1|pages=467–501|doi=10.1146/annurev.ecolsys.29.1.467 }}</ref> Hybridization followed by genome duplication may be a more common path to allopolyploidy because F<sub>1</sub> hybrids between taxa often have relatively high rates of unreduced gamete formation – divergence between the genomes of the two taxa result in abnormal pairing between [[homoeologous]] chromosomes or [[nondisjunction]] during meiosis.<ref name="Ramsey_1998" /> In this case, allopolyploidy can actually restore normal, [[Bivalent (genetics)|bivalent]] meiotic pairing by providing each homoeologous chromosome with its own homologue. If divergence between homoeologous chromosomes is even across the two subgenomes, this can theoretically result in rapid restoration of bivalent pairing and disomic inheritance following allopolyploidization. However multivalent pairing is common in many recently formed allopolyploids, so it is likely that the majority of meiotic stabilization occurs gradually through selection.<ref name="Justin_2002" /><ref name="Le Comber_2010" />

Because pairing between homoeologous chromosomes is rare in established allopolyploids, they may benefit from fixed [[heterozygosity]] of homoeologous alleles.<ref name="Comai_2005">{{cite journal | vauthors = Comai L | title = The advantages and disadvantages of being polyploid | journal = Nature Reviews. Genetics | volume = 6 | issue = 11 | pages = 836–846 | date = November 2005 | pmid = 16304599 | doi = 10.1038/nrg1711 }}</ref> In certain cases, such heterozygosity can have beneficial [[Heterosis|heterotic]] effects, either in terms of fitness in natural contexts or desirable traits in agricultural contexts. This could partially explain the prevalence of allopolyploidy among crop species. Both bread [[wheat]] and [[triticale]] are examples of an allopolyploids with six chromosome sets. [[Cotton]], [[peanut]], and [[quinoa]] are allotetraploids with multiple origins. In [[Brassicaceae|Brassicaceous]] crops, the [[Triangle of U]] describes the relationships between the three common diploid Brassicas (''[[Brassica oleracea|B. oleracea]], [[Brassica rapa|B. rapa]],'' and ''[[Brassica nigra|B. nigra]]'') and three allotetraploids (''[[Rapeseed|B. napus]], [[Brassica juncea|B. juncea]],'' and ''[[Brassica carinata|B. carinata]]'') derived from hybridization among the diploid species. A similar relationship exists between three diploid species of ''[[Tragopogon]]'' (''[[Tragopogon dubius|T. dubius]], [[Tragopogon pratensis|T. pratensis]],'' and ''[[Tragopogon porrifolius|T. porrifolius]]'') and two allotetraploid species (''[[Tragopogon mirus|T. mirus]]'' and ''[[Tragopogon miscellus|T. miscellus]]'').<ref>{{Cite journal| vauthors = Ownbey M |date=January 1950|title=Natural Hybridization and Amphiploidy in the Genus Tragopogon|journal=American Journal of Botany|volume=37|issue=7|pages=487–499|doi=10.2307/2438023|jstor=2438023}}</ref> Complex patterns of allopolyploid evolution have also been observed in animals, as in the frog genus ''[[Xenopus]].''<ref>{{cite journal | vauthors = Schmid M, Evans BJ, Bogart JP | title = Polyploidy in Amphibia | journal = Cytogenetic and Genome Research | volume = 145 | issue = 3–4 | pages = 315–330 | year = 2015 | pmid = 26112701 | doi = 10.1159/000431388 | doi-access = free }}</ref>

==== Aneuploid ====
{{main|Aneuploidy}}
Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are said to be [[Aneuploidy|aneuploid]] (from the Greek words meaning "not", "good", and "fold"). Aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes.<ref name="isbn0-7167-3520-2">{{cite book| vauthors = Griffiths AJ |title=An Introduction to genetic analysis|publisher=W.H. Freeman|year=1999|isbn=978-0-7167-3520-5|location=San Francisco, CA}}{{page needed|date=September 2013}}</ref>

==== Endopolyploidy ====
Polyploidy occurs in some tissues of animals that are otherwise diploid, such as human [[muscle]] tissues.<ref name="pmid19571289">{{cite journal | vauthors = Parmacek MS, Epstein JA | title = Cardiomyocyte renewal | journal = The New England Journal of Medicine | volume = 361 | issue = 1 | pages = 86–88 | date = July 2009 | pmid = 19571289 | pmc = 4111249 | doi = 10.1056/NEJMcibr0903347 }}</ref> This is known as '''endopolyploidy'''. Species whose cells do not have nuclei, that is, [[prokaryotes]], may be polyploid, as seen in the large [[bacterium]] ''[[Epulopiscium fishelsoni]]''.<ref>{{cite journal | vauthors = Mendell JE, Clements KD, Choat JH, Angert ER | title = Extreme polyploidy in a large bacterium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 105 | issue = 18 | pages = 6730–6734 | date = May 2008 | pmid = 18445653 | pmc = 2373351 | doi = 10.1073/pnas.0707522105 | doi-access = free | bibcode = 2008PNAS..105.6730M }}</ref> Hence [[ploidy]] is defined with respect to a cell.

==== Monoploid ====
{{main|Monoploidy}}
A monoploid has only one set of chromosomes and the term is usually only applied to cells or organisms that are normally diploid. The more general term for such organisms is [[haploid]].

=== Temporal terms ===

==== Neopolyploidy ====
A polyploid that is newly formed.

==== Mesopolyploidy ====
That has become polyploid in more recent history; it is not as new as a neopolyploid and not as old as a paleopolyploid. It is a middle aged polyploid. Often this refers to whole genome duplication followed by intermediate levels of diploidization.

==== Paleopolyploidy ====
[[File:PaleopolyploidyTree.jpg|upright=1.5|thumb|This [[phylogenetic tree]] shows the relationship between the best-documented instances of [[paleopolyploidy]] in eukaryotes.]]

{{Main|Paleopolyploidy}}
Ancient genome duplications probably occurred in the evolutionary history of all life. Duplication events that occurred long ago in the history of various [[Lineage (evolution)|evolutionary lineages]] can be difficult to detect because of subsequent [[diploidization]] (such that a polyploid starts to behave cytogenetically as a diploid over time) as [[mutation]]s and gene translations gradually make one copy of each chromosome unlike the other copy. Over time, it is also common for duplicated copies of genes to accumulate mutations and become inactive pseudogenes.<ref>{{cite journal | vauthors = Edger PP, Pires JC | title = Gene and genome duplications: the impact of dosage-sensitivity on the fate of nuclear genes | journal = Chromosome Research | volume = 17 | issue = 5 | pages = 699–717 | year = 2009 | pmid = 19802709 | doi = 10.1007/s10577-009-9055-9 | doi-access = free }}</ref>

In many cases, these events can be inferred only through comparing [[DNA sequencing|sequenced genomes]]. Examples of unexpected but recently confirmed ancient genome duplications include [[baker's yeast]] (''[[Saccharomyces cerevisiae]]''), mustard weed/thale cress (''[[Arabidopsis thaliana]]''), [[rice]] (''[[Oryza sativa]]''), and two rounds of whole genome duplication (the [[2R hypothesis]]) in an early [[evolution]]ary [[ancestor]] of the [[vertebrates]] (which includes the [[human]] lineage) and another near the origin of the [[teleost]] [[fishes]].<ref name="Clarke_2016" /> [[Angiosperm]]s ([[flowering plant]]s) have paleopolyploidy in their ancestry. All [[eukaryote]]s probably have experienced a polyploidy event at some point in their evolutionary history.

=== Other similar terms ===

==== Karyotype ====
{{Main|Karyotype}}
A karyotype is the characteristic chromosome complement of a [[eukaryote]] [[species]].<ref>{{cite book| vauthors = White MJ |url= https://archive.org/details/chromosomes01whit |title=The Chromosomes|date=1973|publisher=Chapman & Hall|edition=6th|location=London|page=28|url-access=registration}}</ref><ref>{{cite book| vauthors = Stebbins GL |title=Variation and Evolution in Plants|date=1950|publisher=Columbia University Press|location=New York, NY|chapter=Chapter XII: The Karyotype}}{{page needed|date=September 2013}}</ref> The preparation and study of karyotypes is part of [[cytopathology|cytology]] and, more specifically, [[cytogenetics]].

Although the replication and transcription of DNA is highly standardized in [[eukaryotes]], the same cannot be said for their karyotypes, which are highly variable between species in chromosome number and in detailed organization despite being constructed out of the same macromolecules. In some cases, there is even significant variation within species. This variation provides the basis for a range of studies in what might be called evolutionary cytology.

==== Homoeologous chromosomes ====
{{Main|Homoeology}}
[[Homoeologous chromosome]]s are those brought together following [[Hybrid (biology)|inter-species hybridization]] and [[Allopolyploidy|allopolyploidization]], and whose relationship was completely homologous in an ancestral species. For example, [[Durum#Genealogy|durum wheat]] is the result of the inter-species hybridization of two diploid grass species ''Triticum urartu'' and ''Aegilops speltoides''. Both diploid ancestors had two sets of 7 chromosomes, which were similar in terms of size and genes contained on them. Durum wheat contains a [[Eukaryote hybrid genome|hybrid genome]] with two sets of chromosomes derived from ''Triticum urartu'' and two sets of chromosomes derived from ''Aegilops speltoides''. Each chromosome pair derived from the ''Triticum urartu'' parent is '''homoeologous''' to the opposite chromosome pair derived from the ''Aegilops speltoides'' parent, though each chromosome pair unto itself is '''homologous'''.