Polyploidy

Source: Wikipedia, the free encyclopedia.
Not to be confused with "polypoid", resembling a polyp.
This image shows haploid (single), diploid (double), triploid (triple), and tetraploid (quadruple) sets of chromosomes. Triploid and tetraploid chromosomes are examples of polyploidy.

Polyploidy is a condition in which the

spores by meiosis
.

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

durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids. Sugarcane can have ploidy levels higher than octaploid.[4]

Polyploidization can be a mechanism of 

Orkney Islands via genome duplication from local populations of E. × robertsii.[5] Because of a rare genetic mutation, E. peregrina is not sterile.[6]

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

"Triploid" redirects here. For the human chromosomal disorder (69 XXX, etc.), see Triploid syndrome.

Organ-specific patterns of endopolyploidy (from 2x to 64x) in the giant ant Dinoponera australis

Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter x is used to represent the number of chromosomes in a single set:

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

polysomic inheritance.[33] 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.[34] 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.[35][36]

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

homoeologous chromosomes or nondisjunction during meiosis.[40] In this case, allopolyploidy can actually restore normal, 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.[28][34]

Because pairing between homoeologous chromosomes is rare in established allopolyploids, they may benefit from fixed

B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploid species. A similar relationship exists between three diploid species of Tragopogon (T. dubius, T. pratensis, and T. porrifolius) and two allotetraploid species (T. mirus and T. miscellus).[42] Complex patterns of allopolyploid evolution have also been observed in animals, as in the frog genus Xenopus.[43]

Aneuploid

Main article: Aneuploidy

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

Epulopiscium fishelsoni.[46] Hence ploidy
is defined with respect to a cell.

Monoploid

Main article:
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

This phylogenetic tree shows the relationship between the best-documented instances of paleopolyploidy in eukaryotes.
Main article: 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

Angiosperms (flowering plants) have paleopolyploidy in their ancestry. All eukaryotes
probably have experienced a polyploidy event at some point in their evolutionary history.

Other similar terms

Karyotype

Main article: 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

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 article:
Homoeology

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 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.

Examples

Animals

Examples in animals are more common in non-vertebrates

kleptogenesis,[54] "stealing" spermatophores
from diploid males of related species to trigger egg development but not incorporating the males' DNA into the offspring.

While some tissues of mammals, such as

Octomys mimax, the Andean
Viscacha-Rat of the same family, whose 2n = 56. It was therefore surmised that an Octomys-like ancestor produced tetraploid (i.e., 2n = 4x = 112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents.

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

diploid (that is, non-polyploid) karyotype. It shows 22 homologous chromosomes, both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left).
Further information: Karyotype
Further information: Triploid syndrome

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

Pneumocystis carinii infection, which indicates a weak immune system.[65]

Triploidy may be the result of either

genomic imprinting.[citation needed
]

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

prenatal diagnosis
and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism.

Mixoploidy
is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells.

Fish

A polyploidy event occurred within the stem lineage of the teleost fish.[48]

Plants

diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote
.

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 (

angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species.[73] It has been established that 15% of angiosperm and 31% of fern speciation events are accompanied by ploidy increase.[74]

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

polyploid speciation. Polyploids may also interbreed with diploids and produce polyploid seeds, as observed in the agamic complexes of Crepis.[82]

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

conifers.[83] One example is the Coast Redwood Sequoia sempervirens, which is a hexaploid (6x) with 66 chromosomes (2n = 6x = 66), although the origin is unclear.[84]

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

Some crops are found in a variety of ploidies:

daylilies (Hemerocallis cultivars) are available as either diploid or tetraploid; apples and kinnow mandarins
can be diploid, triploid, or tetraploid.

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:

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]).

Schematic phylogeny of the Chromalveolata. Red circles indicate polyploidy, blue squares indicate hybridization. From Albertin and Marullo, 2012[89]

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

Phenotypic diversification is also observed following polyploidization and/or hybridization in fungi,[102] producing the fuel for natural selection and subsequent adaptation
and speciation.

Chromalveolata

Other eukaryotic

Alveolata group, the remarkable species Paramecium tetraurelia underwent three successive rounds of whole-genome duplication[106]
and established itself as a major model for paleopolyploid studies.

Bacteria

Each

synthesis-dependent strand annealing (SDSA).[108]

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

DNA binding protein and is likely homologous recombinational repair.[113]

See also

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Further reading

Look up mesopolyploid in Wiktionary, the free dictionary.
Look up neopolyploid in Wiktionary, the free dictionary.
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