Polyploidy

Polyploidy is a condition in which the
Polyploidy is the result of whole-genome duplication during the evolution of species. It may occur due to abnormal cell division, either during mitosis, or more commonly from the failure of chromosomes to separate during meiosis or from the fertilization of an egg by more than one sperm.[1] In addition, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in chromosome doubling, though its use may have other less obvious consequences as well. Oryzalin will also double the existing chromosome content.
Among
Polyploidization can be a mechanism of
On the other hand, polyploidization can also be a mechanism for a kind of 'reverse speciation',[7] whereby gene flow is enabled following the polyploidy event, even between lineages that previously experienced no gene flow as diploids. This has been detailed at the genomic level in Arabidopsis arenosa and Arabidopsis lyrata.[8] Each of these species experienced independent autopolyploidy events (within-species polyploidy, described below), which then enabled subsequent interspecies gene flow of adaptive alleles, in this case stabilising each young polyploid lineage.[9] Such polyploidy-enabled adaptive introgression may allow polyploids at act as 'allelic sponges', whereby they accumulate cryptic genomic variation that may be recruited upon encountering later environmental challenges.[10]
Terminology
Types
Polyploid types are labeled according to the number of chromosome sets in the nucleus
- haploid (one set; 1x), for example male European fire ants
- diploid (two sets; 2x), for example humans
- triploid (three sets; 3x), for example sterile Tardigrada[11]
- tetraploid (four sets; 4x), for example, Plains viscacha rat, Salmonidae fish,[12] the cotton Gossypium hirsutum[13]
- pentaploid (five sets; 5x), for example Kenai Birch (Betula kenaica)
- hexaploid (six sets; 6x), for example some species of wheat,[14] kiwifruit[15]
- heptaploid or septaploid (seven sets; 7x), for example some cultured Siberian sturgeon[16]
- octaploid or octoploid, (eight sets; 8x), for example Acipenser (genus of sturgeon fish), dahlias
- decaploid (ten sets; 10x), for example certain strawberries
- dodecaploid or duodecaploid (twelve sets; 12x), for example the plants Xenopus ruwenzoriensis.
- tetratetracontaploid (forty-four sets; 44x), for example black mulberry[18]
Classification
Autopolyploidy
Autopolyploids are polyploids with multiple chromosome sets derived from a single taxon.
Two examples of natural autopolyploids are the piggyback plant, Tolmiea menzisii[19] and the white sturgeon, Acipenser transmontanum.[20] Most instances of autopolyploidy result from the fusion of unreduced (2n) gametes, which results in either triploid (n + 2n = 3n) or tetraploid (2n + 2n = 4n) offspring.[21] 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.[21] Triploids may also persist through asexual reproduction. In fact, stable autotriploidy in plants is often associated with apomictic mating systems.[22] In agricultural systems, autotriploidy can result in seedlessness, as in watermelons and bananas.[23] Triploidy is also utilized in salmon and trout farming to induce sterility.[24][25]
Rarely, autopolyploids arise from spontaneous, somatic genome doubling, which has been observed in apple (Malus domesticus) bud sports.[26] This is also the most common pathway of artificially induced polyploidy, where methods such as protoplast fusion or treatment with colchicine, oryzalin or mitotic inhibitors are used to disrupt normal 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.[27]
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 aneuploid gametes.[28] 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[29] and Arabidopsis lyrata,[30] with specific adaptive alleles of these species shared between only the evolved polyploids.[31][32]
The high degree of
About half of all polyploids are thought to be the result of autopolyploidy,[37][38] although many factors make this proportion hard to estimate.[39]
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 (2n) gametes, which can take place before or after
Because pairing between homoeologous chromosomes is rare in established allopolyploids, they may benefit from fixed
Aneuploid
Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are said to be 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.[44]
Endopolyploidy
Polyploidy occurs in some tissues of animals that are otherwise diploid, such as human
is defined with respect to a cell.Monoploid
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
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

Ancient genome duplications probably occurred in the evolutionary history of all life. Duplication events that occurred long ago in the history of various 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 mutations 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.[47]
In many cases, these events can be inferred only through comparing
Other similar terms
Karyotype
A karyotype is the characteristic chromosome complement of a eukaryote species.[49][50] The preparation and study of karyotypes is part of cytology and, more specifically, cytogenetics.
Although the replication and transcription of DNA is highly standardized in
Homoeologous chromosomes
Examples
Animals
Examples in animals are more common in non-vertebrates
While some tissues of mammals, such as
Polyploidy was induced in fish by Har Swarup (1956) using a cold-shock treatment of the eggs close to the time of fertilization, which produced triploid embryos that successfully matured.[59][60] Cold or heat shock has also been shown to result in unreduced amphibian gametes, though this occurs more commonly in eggs than in sperm.[61] John Gurdon (1958) transplanted intact nuclei from somatic cells to produce diploid eggs in the frog, Xenopus (an extension of the work of Briggs and King in 1952) that were able to develop to the tadpole stage.[62] The British scientist J. B. S. Haldane hailed the work for its potential medical applications and, in describing the results, became one of the first to use the word "clone" in reference to animals. Later work by Shinya Yamanaka showed how mature cells can be reprogrammed to become pluripotent, extending the possibilities to non-stem cells. Gurdon and Yamanaka were jointly awarded the Nobel Prize in 2012 for this work.[62]
Humans
True polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissue, such as liver parenchyma, heart muscle, placenta and in bone marrow.[63][64] Aneuploidy is more common.
Polyploidy occurs in humans in the form of
Triploidy may be the result of either
Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1–2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at
Fish
A polyploidy event occurred within the stem lineage of the teleost fish.[48]
Plants
Polyploidy is frequent in plants, some estimates suggesting that 30–80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (
Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes.[41] Both autopolyploids (e.g. potato[75]) and allopolyploids (such as canola, wheat and cotton) can be found among both wild and domesticated plant species.
Most polyploids display novel variation or morphologies relative to their parental species, that may contribute to the processes of
Some plants are triploid. As meiosis is disturbed, these plants are sterile, with all plants having the same genetic constitution: Among them, the exclusively vegetatively propagated saffron crocus (Crocus sativus). Also, the extremely rare Tasmanian shrub Lomatia tasmanica is a triploid sterile species.
There are few naturally occurring polyploid
Aquatic plants, especially the Monocotyledons, include a large number of polyploids.[85]
Crops
The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale.
In some situations, polyploid crops are preferred because they are sterile. For example, many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques, such as grafting.
Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine.
Examples
- Triploid crops: some apple varieties (such as Belle de Boskoop, Jonagold, Mutsu, Ribston Pippin), banana, citrus, ginger, watermelon,[86] saffron crocus, white pulp of coconut
- Tetraploid crops: very few
- Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit[15]
- Octaploid crops: sugar cane, oca (Oxalis tuberosa)[87]
- Dodecaploid crops: some sugar cane hybrids[88]
Some crops are found in a variety of ploidies:
Fungi
Besides plants and animals, the evolutionary history of various fungal species is dotted by past and recent whole-genome duplication events (see Albertin and Marullo 2012[89] for review). Several examples of polyploids are known:
- autopolyploid: the aquatic fungi of genus Allomyces,[90] some Saccharomyces cerevisiae strains used in bakery,[91] etc.
- allopolyploid: the widespread Dekkera bruxellensis,[94]etc.
- paleopolyploid: the human pathogen Rhizopus oryzae,[95] the genus Saccharomyces,[96] etc.
In addition, polyploidy is frequently associated with hybridization and reticulate evolution that appear to be highly prevalent in several fungal taxa. Indeed, homoploid speciation (hybrid speciation without a change in chromosome number) has been evidenced for some fungal species (such as the basidiomycota Microbotryum violaceum[97]).

As for plants and animals, fungal hybrids and polyploids display structural and functional modifications compared to their progenitors and diploid counterparts. In particular, the structural and functional outcomes of polyploid Saccharomyces genomes strikingly reflect the evolutionary fate of plant polyploid ones. Large chromosomal rearrangements
Chromalveolata
Other eukaryotic
Bacteria
Each
Azotobacter vinelandii can contain up to 80 chromosome copies per cell.[109] However this is only observed in fast growing cultures, whereas cultures grown in synthetic minimal media are not polyploid.[110]
Archaea
The
See also
- Diploidization
- Eukaryote hybrid genome
- Ploidy
- Polyploid complex
- Polysomy
- Reciprocal silencing
- Sympatry
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- Eakin GS, Behringer RR (December 2003). "Tetraploid development in the mouse". Developmental Dynamics. 228 (4): 751–766. PMID 14648853.
- Gaeta RT, Pires JC, Iniguez-Luy F, Leon E, Osborn TC (November 2007). "Genomic changes in resynthesized Brassica napus and their effect on gene expression and phenotype". The Plant Cell. 19 (11): 3403–3417. PMID 18024568.
- Gregory, T. Ryan; Mable, Barbara K. (2005). "Polyploidy in Animals". The Evolution of the Genome. pp. 427–517. ISBN 978-0-12-301463-4.
- Jaillon O, Aury JM, Brunet F, Petit JL, Stange-Thomann N, Mauceli E, et al. (October 2004). "Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype". Nature. 431 (7011): 946–957. PMID 15496914.
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- Simillion C, Vandepoele K, Van Montagu MC, Zabeau M, Van de Peer Y (October 2002). "The hidden duplication past of Arabidopsis thaliana". Proceedings of the National Academy of Sciences of the United States of America. 99 (21): 13627–13632. PMID 12374856.
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External links
- Polyploidy on Kimball's Biology Pages
- The polyploidy portal a community-editable project with information, research, education, and a bibliography about polyploidy.