Polyploidy is a condition in which the cells of an organism have more than two paired sets of (homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one from each of two parents; each set contains the same number of chromosomes, and the chromosomes are joined in pairs of homologous chromosomes. However, some organisms are polyploid. Polyploidy is especially common in plants. Most eukaryotes have diploid somatic cells, but produce haploid gametes (eggs and sperm) by meiosis. A monoploid has only one set of chromosomes, and the term is usually only applied to cells or organisms that are normally diploid. Males of bees and other Hymenoptera, 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 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. 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.
Whole-organism polyploidy is rare among mammals; however, endopolyploidy (polyploid cells in certain tissues of an otherwise-diploid organism) occurs in mammals at a high frequency in organs such as the brain, liver, heart, and bone marrow. Endopolyploidy also occurs in the somatic cells of other animals, such as goldfish, salmon, and salamanders; and in other Kingdoms.
Polyploidy is common among ferns and flowering plants (see Hibiscus rosa-sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of 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.This review cites this study:
Polyploidization can be a mechanism of sympatric speciation because polyploids are usually unable to interbreed with their diploid ancestors. An example is the plant Erythranthe peregrina. Sequencing confirmed that this species originated from E. × robertsii, a sterile triploid hybrid between E. guttata and E. lutea, both of which have been introduced and naturalised in the United Kingdom. New populations of E. peregrina arose on the Scottish mainland and the Orkney Islands via genome duplication from local populations of E. × robertsii. Because of a rare genetic mutation, E. peregrina is not sterile.
On the other hand, polyploidization can also be a mechanism for a kind of 'reverse speciation', 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. 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. 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.
Terminology
Types
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:
monoploid (one set; 1x), for example male European fire ants diploid (two sets; 2x), for example humans triploid (three sets; 3x), for example sterile saffron crocus, or seedless watermelons, also common in the phylum Tardigrada tetraploid (four sets; 4x), for example, Plains viscacha rat, Salmonidae fish, the cotton Gossypium hirsutum pentaploid (five sets; 5x), for example Kenai Birch (Betula kenaica) hexaploid (six sets; 6x), for example some species of wheat, kiwifruit heptaploid or septaploid (seven sets; 7x), for example some cultured Siberian sturgeon octaploid or octoploid, (eight sets; 8x), for example Acipenser (genus of sturgeon fish), dahlias decaploid (ten sets; 10x), for example certain strawberries hendecaploid or undecaploid (eleven sets; 11x), for example some Lepidium species and rose cultivars dodecaploid or duodecaploid (twelve sets; 12x), for example the plants Celosia argentea and Spartina anglica or the amphibian Xenopus ruwenzoriensis. tetratetracontaploid (forty-four sets; 44x), for example black mulberryThis review and book cite this research.
The monoploid number x should be distinguished from the haploid number n, which instead describes the number of chromosome sets in a gamete – if one can ever be formed. In a fertile tetraploid (4x) species, there usually is : each gamete would be diploid with respect to the monoploid set x, but "haploid" (n) if treating the overall 4x genome as "diploid" (2n). The distinction of monoploid vs. haploid and x vs. n is often overlooked in informal usage.
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 and the white sturgeon, Acipenser transmontanum. 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. 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. Triploids may also persist through asexual reproduction. In fact, stable autotriploidy in plants is often associated with apomictic mating systems. In agricultural systems, autotriploidy can result in seedlessness, as in watermelons and bananas. Triploidy is also utilized in salmon and trout farming to induce sterility.
Rarely, autopolyploids arise from spontaneous, somatic genome doubling, which has been observed in apple (Malus domesticus) bud sports. 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.
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. 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 and Arabidopsis lyrata, with specific adaptive alleles of these species shared between only the evolved polyploids.
The high degree of homology among duplicated chromosomes causes autopolyploids to display polysomic inheritance. 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. 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.
About half of all polyploids are thought to be the result of autopolyploidy, although many factors make this proportion hard to estimate.
Allopolyploidy Allopolyploids or amphipolyploids or heteropolyploids are polyploids with chromosomes derived from two or more diverged taxa. They most commonly exhibit disomic inheritance, having functionally diploidized (diploidization).
As in autopolyploidy, this primarily occurs through the fusion of unreduced (2n) gametes, which can take place before or after 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 hybrids produce unreduced gametes that fuse to form allopolyploid progeny. Hybridization followed by genome duplication may be a more common path to allopolyploidy because F1 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. 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 (diploidization) 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.
Because pairing between homoeologous chromosomes is rare in established allopolyploids, there often ends up being fixed heterozygosity of homoeologous alleles. In certain cases, 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 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 Brassicaceous crops, the Triangle of U 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 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). Complex patterns of allopolyploid evolution have also been observed in animals, as in the frog genus Xenopus.
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.
Endopolyploidy