Index
Cells and polyploid organisms contain more than two pairs of paired (homologous) chromosomes.
Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, one set inherited from each parent.
However, polyploidy is found in some organisms and is especially common in plants. Additionally, polyploidy occurs in some otherwise diploid animal tissues, such as human muscle tissues.
This is known as endopolyploidy. Species whose cells do not have nuclei, prokaryotes, can be polyploid, as seen in the large bacterium Epulopiscium fishelsoni. Therefore, ploidy is defined concerning a cell.
Most eukaryotes have diploid somatic cells but produce haploid gametes (eggs and sperm) through meiosis. A monoploid has only one set of chromosomes, and the term generally only applies to cells or organisms that are typically diploid.
Male bees and other hymenopterans, for example, are monoploid. Unlike animals, plants and multicellular algae have life cycles with two alternating multicellular generations.
The gametophyte generation is haploid and produces gametes by mitosis; the sporophyte generation is diploid and produces spores by meiosis.
Polyploidy refers to a numerical change in a complete set of chromosomes. Organisms in which a particular chromosome or chromosome segment is under-or over-represented are said to be aneuploid (from the Greek words for “no,” “good,” and “fold”).
Aneuploidy refers to a numerical change in one part of the chromosome set, while polyploidy refers to a numerical change in the entire chromosome set.
Polyploidy can occur due to abnormal cell division during mitosis or commonly during metaphase I in meiosis.
Furthermore, it can be induced in plants and cell cultures by some chemicals: the best known is colchicine, which can result in double the chromosome, although its use may have other less obvious consequences. Oryzalin will also duplicate the existing chromosome content.
Polyploidy occurs in highly differentiated human tissues in the liver, heart muscle, and bone marrow.
It occurs in the somatic cells of some animals, such as goldfish, salmon, and salamanders. However, it is widespread among ferns and flowering plants (see Hibiscus rosa-Sinensis), including wild and cultivated species.
Wheat, for example, after millennia of hybridization and modification by humans, has diploid strains (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum wheat or macaroni, and hexaploid (six sets of chromosomes) with the common name of bread wheat.
Many plants of agricultural importance in the genus Brassica are also tetraploid.
Polyploidization is a sympatric speciation mechanism because polyploids generally cannot interbreed with their diploid ancestors. An example is the Erythranthe peregrine plant.
Sequencing confirmed that this species originated from E. x Roberts, a sterile triploid hybrid between E. guttate and E. lutea, both introduced and naturalized in the UK.
New populations of E. peregrina emerged on the Scottish mainland and the Orkney Islands through genome duplication of local populations of E. x Roberts. Due to a rare genetic mutation, E. peregrine is not sterile.
Types
Polyploid types are labeled according to the number of chromosome sets in the nucleus. The letter x represents the number of chromosomes in a single set.
- Triploid (three sets; 3x), e.g., seedless watermelons, standard in the phylum Tardigrada.
- Tetraploid (four sets; 4x), e.g., Salmonidae fish, cotton Gossypium hirsutum.
- Pentaploid (five sets; 5x), for example, a tree species, Kenai birch (Betula papyrifera var. Kenaica)
- Hexaploid (six sets; 6x), wheat, and kiwi.
- Heptaploid or sepaloid (seven series; 7x), for example, Tasmanian Devil.
- Octaploid or octoploid (eight sets; 8x), Acipenser (genus of sturgeon fish), dahlias.
- Decaploid (ten sets, ten times), for example, particular strawberries.
- Dodecaploid (twelve sets; 12x), e.g., Celosia argentea and Spartina anglica plants or Xenopus ruwenzoriensis amphibian.
Animals
Animal examples are more common in non-vertebrates, such as flatworms, leeches, and brine shrimp. Within vertebrates, examples of stable polyploidy include salmonids and many cyprinids (i.e., carp).
Some fish have up to 400 chromosomes. Polyploidy also occurs commonly in amphibians; for example, the biomedically important genus Xenopus contains many different species with up to 12 sets of chromosomes (dodecaphonic).
Polyploid lizards are also quite common but are sterile and must reproduce by parthenogenesis.
Polyploid (mostly triploid) molar salamanders are all female and reproduce by leptogenesis, “stealing” spermatophores from diploid males of related species to trigger egg development but not incorporating the DNA of the males into the offspring.
Although mammalian liver cells are polyploid, rare cases of polyploid mammals are known, but more frequently, they cause prenatal death.
An octodontid rodent from the harsh desert regions of Argentina, known as the plains viscacha rat (Tympanoctomys barrier), has been reported to be an exception to this ‘rule.’
However, careful analysis using chromosome paints shows that there are only two copies of each chromosome in T. Barriere, not the four expected if it were a tetraploid. This rodent is not a rat but a relative of guinea pigs and chinchillas.
Its “new” diploid number [2n] is 102, so its cells are approximately twice the standard size. Its closest life relationship is Octomys mimax, the Andean vizcacha rat of the same family, whose 2n = 56.
Therefore, it is assumed that an Octomys-like ancestor produced tetraploid offspring (i.e., 2n = 4x = 112) that were, by their duplicated chromosomes, reproductively isolated from their parents.
Polyploidy was induced in fish by Har Swarup (1956) using cold shock treatment of the eggs near fertilization, producing triploid embryos that matured successfully.
Cold or heat shock has also been shown to produce non-reduced amphibian gametes, although this occurs more frequently in eggs than in sperm.
John Gurdon (1958) transplanted intact somatic cell nuclei to produce diploid eggs in the frog, Xenopus (an extension of Briggs and King’s work in 1952) that we can develop to the tadpole stage.
British scientist JBS Haldane praised the work for its potential medical applications and, in describing the results, became one of the first to use the word “clone” about animals.
Shinya Yamanaka’s later work showed how mature cells could be reprogrammed to become pluripotent, extending the possibilities to non-stem cells. Gurdon and Yamanaka were awarded the Nobel Prize in 2012 for this work.
Humans
True polyploidy rarely occurs in humans, although polyploid cells occur in highly differentiated tissues, such as the liver parenchyma, heart muscle, and bone marrow. Aneuploidy is more common.
Polyploidy occurs in humans in the form of triploidy, with 69 chromosomes (sometimes called 69, XXX) and tetraploidy with 92 chromosomes (sometimes called 92, XXXX).
Triploidy, usually due to polyspermia, occurs in about 2-3% of all human pregnancies and ~ 15% of miscarriages. The vast majority of triploid conceptions end as a miscarriage; those who survive to term generally die shortly after birth.
In some cases, survival after birth may be further extended by myxoploidy with the presence of a population of diploid and triploid cells.
Triploid can result from digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father).
Diandry is mainly caused by reduplicating the paternal haploid set of a single sperm, but it can also be the consequence of dispermic fertilization (two sperm) of the ovum.
Digyny is most commonly caused by the failure of a meiotic division during oogenesis leading to a diploid oocyte, or by the failure to extrude a polar body from the oocyte.
Diandry appears predominant among early abortions, whereas digyny predominates among triploid zygotes that survive into the fetal period.
However, among early abortions, digyny is also more common in those <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in placentas and triploid fetuses that depend on the origin of the extra haploid set.
In digyny, there is typically an underdeveloped asymmetric fetus with marked adrenal hypoplasia and a very small placenta. In diandry, a partial hydatidiform mole develops. These parental-of-origin effects reflect the effects of genomic imprinting.
Complete tetraploidy is rarely diagnosed as triploidy but is seen in 1–2% of early miscarriages. However, some tetraploid cells are commonly found on chromosome analysis in prenatal diagnosis and are generally considered “harmless.”
It is unclear whether these tetraploid cells tend to arise during cell culture in vitro or are also present in placental cells in vivo. There are, in any case, very few clinical reports of fetuses/babies diagnosed with tetraploidy mosaicism.
Myxoploidy is commonly seen in preimplantation human embryos and includes mixed haploid/diploid and diploid/tetraploid cell populations.
It is unknown if these embryos do not implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process that favors diploid cells.
Floors
Polyploidy is dominant in plants, and some estimates suggest that 30-80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleoploploidy) in their genomes.
The multiple explosions in angiosperm species diversity appear to have coincided with the time of duplication of the ancient genome shared by many species.
It has been established that an increase in ploidy accompanies 15% of angiosperms and 31% of fern speciation events.
Polyploid plants can arise spontaneously in nature by various mechanisms, including meiotic or mitotic failures and fusion of non-reduced gametes (2n).
Autopolyploids (e.g., potato) and allopolyploids (e.g., canola, wheat, cotton) can be found among wild and domesticated plant species.
Most polyploids show new variations or morphologies about their parent species, which can contribute to speciation and eco-niche exploitation processes.
Mechanisms that lead to a new variation in newly formed allopolyploids may include gene dose effects (resulting from more copies of genome content), the assembling of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect the gene content and expression levels.
Many of these rapid changes can contribute to reproductive isolation and speciation.
However, seeds generated from intrepid crosses, such as between polyploids and their parent species, generally undergo aberrant endosperm development that impairs their viability, contributing to polyploid speciation.
Lomatia tasmanica is an extremely rare Tasmanian shrub that is triploid and sterile; reproduction is completely vegetative, and all plants have the same genetic makeup.
There are few naturally occurring polyploid conifers. An example is the Coast Redwood Sequoia sempervirens, a hexaploid (6x) with 66 chromosomes (2n = 6x = 66), although the origin is unclear.
Aquatic plants, especially monocots, include a large number of polyploids.
Crops
Polyploidy induction is a common technique to overcome the sterility of a hybrid species during plant reproduction. For example, TriticaleTriticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale).
It combines the desired characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can spread further to become TriticaleTriticale.
In some situations, polyploid cultures are preferred because they are sterile. For example, wide varieties of seedless fruits are seedless due to polyploidy. These crops are propagated using asexual techniques, such as grafting.
Polyploidy in crop plants is often induced by treating seeds with the chemical colchicine.
Examples
Triploid crops: some apples (e.g. Belle de Boskoop, Jonagold, Mutsu, Ribston Pippin), banana, citrus, ginger, watermelon.
Tetraploid crops: very few varieties of apples, durum wheat or macaroni, cotton, potato, canola/rape, leeks, tobacco, peanuts, know, Pelargonium.
Hexaploid crops: chrysanthemum, bread wheat, TriticaleTriticale, oats, kiwi.
Octaploid crops: strawberry, dahlia, sugar cane, goose (Oxalis tuberosa).
Dodecaploid crops: some sugarcane hybrids.
Some crops are found in various ploidies: tulips and lilies are commonly found as diploids and triploids; daylilies (Hemerocallis cultivars) are available as diploids or tetraploids; know apples and mandarins can be diploids, triploid, or tetraploid.
Mushrooms
In addition to plants and animals, the evolutionary history of various species of fungi is punctuated by past and recent genome duplication events. Several examples of polyploids are known:
Autopolyploid: the aquatic fungi of the genus Allomyces, some strains of Saccharomyces cerevisiae used in bakery, etc.
Allopolyploid: the widespread cyathus stertorous, the allotetraploid lager yeast saccharomyces pastorianus, the allotriploid yeast from wine decay Dekker bruxellensis, etc.
Paleopolyploid: the human pathogen rhizopus oryzae, the genus saccharomyces, etc.
Furthermore, polyploidy is frequently associated with hybridization and lattice evolution that appear to be highly prevalent in various fungal taxa.
In fact, homoploid speciation (hybrid speciation without a change in chromosome number) has been evidenced for some species of fungi (for example, Basidiomycota microbotryum violaceum).
Regarding plants and animals, fungal and polyploid hybrids show structural and functional modifications compared to their diploid progenitors and counterparts.
In particular, the structural and functional results of Saccharomyces polyploid genomes remarkably reflect the evolutionary fate of plant polyploids.
Large chromosomal rearrangements leading to chimeric chromosomes have been described, as well as more specific genetic modifications, such as the loss of genes.
The homoalles of the allotetraploid yeast S. pastorianus show an uneven contribution to the transcriptome.
After polyploidization and hybridization in fungi, phenotypic diversification is also observed, producing the fuel for natural selection and subsequent adaptation and speciation.
Chromalveolata
Other eukaryotic taxa have experienced one or more polyploidization events during their evolutionary history. Oomycetes, members of non-true fungi, contain several examples of polyploid and paleopolypoid species, as in the genus Phytophthora.
Some species of brown algae (focal, laminar, and diatom) contain apparent polyploid genomes. In the Alveolata group, the remarkable species Paramecium tetraurelia underwent three successive rounds of whole-genome duplication and established itself as a primary model for allopolyploid studies.
Terminology
Autopoliploidy
Autopolyploids are polyploids with multiple chromosome sets derived from a single taxon. Two examples of natural autopolyploids are the piggyback plant, Tolmiea menisci, and the white sturgeon, Acipenser transmontane.
Most autopolyploid cases result from the fusion of non-reduced gametes (2n), resulting in triploid (n + 2n = 3n) or tetraploid (2n + 2n = 4n) offspring.
Triploid offspring are typically sterile (as in the “triploid block” phenomenon), but in some cases, they can produce high proportions of non-reduced gametes and thus aid tetraploid formation.
This pathway to tetraploidy is known as the “triploid bridge.” Triploids can also persist through asexual reproduction. Stable autotriploidy in plants is often associated with apomictic mating systems.
In agricultural systems, autotriploidy can lead to a lack of seeds, as in watermelons and bananas. Triploid is also used in salmon and trout farming to induce sterility.
On rare occasions, autopolyploids arise from spontaneous duplication of the somatic genome, which has been observed in sports apple buds (Malus domesticus).
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, resulting in the production of polyploid cells.
This process can be helpful in plant improvement, especially when it comes to introgressing germplasm through poloidal levels.
Autopolyploids possess at least three sets of homologous chromosomes, which can lead to high rates of multivalent pairing during meiosis (particularly in newly formed autopolyploids, also known as neoproploids) and an associated decrease in fertility due to the production of aneuploid gametes.
Natural or artificial selection for fertility can rapidly stabilize meiosis in autopolyploids by restoring bivalent pairing during meiosis. However, the high homology between duplicated chromosomes causes autopolyploids to show polysomal inheritance.
This trait is often used as a diagnostic criterion to distinguish autopolyploids from allopolyploids, which commonly show disomic inheritance after progressing beyond the neopolyploid stage.
While most polyploid species are unambiguously characterized as autopolyploid or allopolyploid, these categories represent the extremes of a spectrum between divergence between parental subgenera.
Polyploids between these two extremes, often referred to as segmental allopolyploids can exhibit intermediate levels of polysomal inheritance that vary by locus.
About half of all polyploids are believed to be the result of autopolyploids, although many factors make this ratio challenging to estimate.
Alopoliploide
Allopolyploids, amphipoliploids, or heteropolyploids are polyploids with chromosomes derived from two or more divergent taxa. As in the autopolyploid, this occurs mainly through the fusion of non-reduced gametes (2n), which can occur before or after hybridization.
In the former case, the unreduced gametes of each diploid taxon (or the reduced gametes of two autotetraploid taxa) combine to form allopolyploid offspring.
In the latter case, one or more F1 diploid hybrids produce non-reduced gametes that fuse to form an allopolyploid progeny.
Hybridization followed by genome duplication may be a more common pathway for allopolyploids because F1 hybrids between taxa often have relatively high rates of gamete formation without reducing:
The divergence between the genomes of the two taxa results in abnormal pairing between homologous chromosomes or nondisjunction during meiosis.
Allopolyploidy can restore regular bivalent meiotic pairing in this case by providing each homologous chromosome with its homolog.
If the divergence between homologous chromosomes is even across the two subgenera, this can theoretically result in rapid restoration of bivalent pairing and disomic inheritance after allopolyploidization.
However, multivalent pairing is common in many newly formed allopolyploids, so, likely, most of the meiotic stabilization occurs gradually through selection.
Because pairing between homologous chromosomes is rare in established allopolyploids, they can benefit from the fixed heterozygosity of homologous alleles.
In some instances, such heterozygosity can have beneficial heterotic effects, either in terms of fitness in natural contexts or desirable traits in agricultural contexts.
This could partially explain the prevalence of allopolyploids among crop species. Bread wheat and TriticaleTriticale are examples of an allopolyploid with six chromosome systems.
Cotton is an allotetraploid with multiple origins. In Brassicaceous cultures, the U Triangle describes the relationships between the three common diploid Brassicas (B. oleracea, B. Rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived of hybridization between diploid species.
There is a similar relationship between three diploid species of Tragopogon (T. dubius, T. pratensis, and T. porrifolius) and two allotetraploid species (T. mirus and T. micelles).
Complex patterns of allopolyploid evolution have also been observed in animals, such as the frog genus Xenopus.
Paleopoliploide
Duplications of the ancient genome 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 due to subsequent diploidization (such that a polyploid begins to behave cytogenetically diploid over time) as mutations and Gene translations make one copy of each chromosome the other copy.
Over time, it is also common for duplicate copies of genes to accumulate mutations and become inactive pseudogenes.
In many cases, these events can be inferred only by comparing sequenced genomes.
Examples of unexpected but recently confirmed duplications of the ancient genome include baker’s yeast (Saccharomyces cerevisiae), mustard grass/watercress (Arabidopsis thaliana), rice (Oryza sativa), and an early evolutionary ancestor of vertebrates (which includes the human lineage) and another near the origin of teleost fish.
Angiosperms (flowering plants) have paleopolyploidy in their ancestors. All eukaryotes have likely experienced a polyploidy event at some point in their evolutionary history.
Karyotype
A karyotype is the characteristic chromosomal complement of a eukaryotic species. The preparation and study of karyotypes are part of cytology and, more specifically, cytogenetics.
Although DNA replication and transcription are highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable between species in terms of the number of chromosomes and a detailed organization despite being built from the identical macromolecules.
In some cases, there is even significant variation within species. This variation provides the basis for various studies in what could be called evolutionary cytology.
Paralogous
The term describes the relationship between duplicate genes or portions of chromosomes that derive from a common ancestral DNA and reside in the same genome (as opposed to orthologs that are present in different species and diverge after speciation events).
Paralogue DNA segments can arise spontaneously due to errors during DNA replication, copying, and pasting of transposons or duplications of the entire genome.
Counterpart
The term describes the relationship of similar chromosomes containing nearly identical sets of genes and pairs during the prophase of meiosis. In a diploid organism, one homolog in one pair is derived from the male parent (sperm) and the other from the female parent (egg).
During meiosis and gametogenesis, homologous chromosomes pair up and exchange genetic material by recombination, producing sperm or eggs with chromosomal haplotypes that contain new allelic combinations.
Homoegogo
The term homoeologous, also spelled homeologous, is used to describe the relationship of similar chromosomes or parts of chromosomes reunited after interspecies hybridization and allopolyploidization and whose relationship was utterly homologous in an ancestral species.
In allopolyploids, the homologous chromosomes within each parental subgenome must match faithfully during meiosis, leading to disomic inheritance.
However, in some allopolyploids, the homologous chromosomes of the parental genomes can be almost as similar to each other as the homologous chromosomes, leading to tetrasomy inheritance (pairing of four chromosomes in meiosis), intergenomic recombination, and reduced fertility.
Example of homoeologous chromosomes
Durum wheat is the result of cross-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 in them.
Durum wheat contains 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 homologous to the opposite chromosome pair derived from the Aegilops speltoides parent, although each chromosome pair itself is homologous.
Bacteria
Each Deinococcus radiodurans bacterium contains 4 to 8 copies of its chromosome. Exposure of D. radiodurans to X-ray irradiation or desiccation can destroy its genomes into hundreds of short random fragments.
However, D. radiodurans is highly resistant to such exposures. The mechanism by which the genome is precisely restored involves RecA-mediated homologous recombination and a process called synthesis-dependent strand annealing.
Azotobacter vinelandii can contain up to 80 copies of chromosomes per cell. However, this is only seen in fast-growing cultures, whereas cultures grown on minimal synthetic media are not polyploid.
Arches
The archaeon Halobacterium salination is polyploid and, like D. radiodurans, is highly resistant to X-ray irradiation and desiccation, conditions that induce DNA double-strand breaks.
