Editing Polyploidy (section)

==== 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>