Ploidy (/ˈplɔɪdi/) is the number of complete sets ofchromosomes in acell, and hence the number of possiblealleles forautosomal andpseudoautosomalgenes. Heresets of chromosomes refers to the number of maternal and paternal chromosome copies, respectively, in each homologous chromosome pair—the form in which chromosomes naturally exist.Somatic cells,tissues, andindividual organisms can be described according to the number of sets of chromosomes present (the "ploidy level"):monoploid (1 set),diploid (2 sets),triploid (3 sets),tetraploid (4 sets),pentaploid (5 sets),hexaploid (6 sets),heptaploid[2] orseptaploid[3] (7 sets), etc. The generic termpolyploid is often used to describe cells with three or more sets of chromosomes.[4][5]
Virtually allsexually reproducing organisms are made up of somatic cells that are diploid or greater, but ploidy level may vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism's life cycle. Half of all known plant genera contain polyploid species, and about two-thirds of all grasses are polyploid.[6] Many animals are uniformly diploid, though polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in thesocial insects), and in others entire tissues and organ systems may be polyploid despite the rest of the body being diploid (as in the mammalianliver[citation needed]). For many organisms, especially plants and fungi, changes in ploidy level between generations are major drivers ofspeciation. In mammals and birds, ploidy changes are typically fatal.[7] There is, however, evidence of polyploidy in organisms now considered to be diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive rounds of polyploidization and rediploidization.[8][9]
Humans are diploid organisms, normally carrying two complete sets of chromosomes in their somatic cells: one copy of paternal and maternal chromosomes, respectively, in each of the 23 homologous pairs of chromosomes that humans normally have. This results in two homologous pairs within each of the 23 homologous pairs, providing a full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called thechromosome number orchromosome complement. The number of chromosomes found in a single complete set of chromosomes is called themonoploid number (x). Thehaploid number (n) refers to the total number of chromosomes found in agamete (asperm oregg cell produced bymeiosis in preparation for sexual reproduction). Under normal conditions, the haploid number is exactly half the total number of chromosomes present in the organism's somatic cells, with one paternal and maternal copy in each chromosome pair. For diploid organisms, the monoploid number and haploid number are equal; in humans, both are equal to 23. When a humangerm cell undergoes meiosis, the diploid 46 chromosome complement is split in half to form haploid gametes. After fusion of a male and a female gamete (each containing 1 set of 23 chromosomes) duringfertilization, the resultingzygote again has the full complement of 46 chromosomes: 2 sets of 23 chromosomes.Euploidy and aneuploidy describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person withTurner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY). This is a type of aneuploidy and cells from the person may be said to be aneuploid with a (diploid) chromosome complement of 45.
The termploidy is aback-formation fromhaploidy anddiploidy. "Ploid" is a combination of Ancient Greek -πλόος (-plóos, "-fold") and -ειδής (-eidḗs), from εἶδος (eîdos, "form, likeness").[a] The principal meaning of theGreek word ᾰ̔πλόος (haplóos) is "single",[10] from ἁ- (ha-, "one, same").[11] διπλόος (diplóos) means "duplex" or "two-fold". Diploid therefore means "duplex-shaped" (compare "humanoid", "human-shaped").
Polish-German botanistEduard Strasburger coined the termshaploid anddiploid in 1905.[b] Some authors suggest that Strasburger based the terms onAugust Weismann's conception of the id (orgerm plasm),[14][15][16] hence haplo-id and diplo-id. The two terms were brought into the English language from German throughWilliam Henry Lang's 1908 translation of a 1906 textbook by Strasburger and colleagues.[17][citation needed]
The termhaploid is used with two distinct but related definitions. In the most generic sense, haploid refers to having the number of sets of chromosomes normally found in agamete.[18] Because two gametes necessarily combine during sexual reproduction to form a single zygote from which somatic cells are generated, healthy gametes always possess exactly half the number of sets of chromosomes found in the somatic cells, and therefore "haploid" in this sense refers to having exactly half the number of sets of chromosomes found in a somatic cell. By this definition, an organism whose gametic cells contain a single copy of each chromosome (one set of chromosomes) may be considered haploid while the somatic cells, containing two copies of each chromosome (two sets of chromosomes), are diploid. This scheme of diploid somatic cells and haploid gametes is widely used in the animal kingdom and is the simplest to illustrate in diagrams of genetics concepts. But this definition also allows for haploid gametes withmore than one set of chromosomes. As given above, gametes are by definition haploid, regardless of the actual number of sets of chromosomes they contain. An organism whose somatic cells are tetraploid (four sets of chromosomes), for example, will produce gametes by meiosis that contain two sets of chromosomes. These gametes might still be called haploid even though they are numerically diploid.[citation needed]
An alternative usage defines "haploid" as having a single copy of each chromosome – that is, one and only one set of chromosomes.[19] In this case, the nucleus of aeukaryotic cell is said to be haploid only if it has a single set ofchromosomes, each one not being part of a pair. By extension a cell may be called haploid if its nucleus has one set of chromosomes, and an organism may be called haploid if its body cells (somatic cells) have one set of chromosomes per cell. By this definition haploid therefore would not be used to refer to the gametes produced by the tetraploid organism in the example above, since these gametes are numerically diploid. The termmonoploid is often used as a less ambiguous way to describe a single set of chromosomes; by this second definition, haploid and monoploid are identical and can be used interchangeably.[citation needed]
Gametes (sperm andova) are haploid cells. The haploid gametes produced by most organisms combine to form azygote withn pairs of chromosomes, i.e. 2n chromosomes in total. The chromosomes in each pair, one of which comes from the sperm and one from the egg, are said to behomologous. Cells and organisms with pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. Duringmeiosis, sex cell precursors have their number of chromosomes halved by randomly "choosing" one member of each pair of chromosomes, resulting in haploid gametes. Because homologous chromosomes usually differ genetically, gametes usually differ genetically from one another.[20]
Allplants and manyfungi andalgae switch between a haploid and a diploid state, with one of the stages emphasized over the other. This is calledalternation of generations. Most fungi and algae are haploid during the principal stage of their life cycle, as are some primitive plants likemosses. More recently evolved plants, like thegymnosperms andangiosperms, spend the majority of their life cycle in the diploid stage. Most animals are diploid, but malebees,wasps, andants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their systemhaplodiploid.[citation needed]
In some cases there is evidence that then chromosomes in a haploid set have resulted from duplications of an originally smaller set of chromosomes. This "base" number – the number of apparently originally unique chromosomes in a haploid set – is called themonoploid number,[21] also known asbasic orcardinal number,[22] orfundamental number.[23][24] As an example, the chromosomes ofcommon wheat are believed to be derived from three different ancestral species, each of which had 7 chromosomes in its haploid gametes. The monoploid number is thus 7 and the haploid number is 3 × 7 = 21. In generaln is a multiple ofx. The somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm which fused to form the plant, giving a total of 42 chromosomes. As a formula, for wheat 2n = 6x = 42, so that the haploid numbern is 21 and the monoploid numberx is 7. The gametes of common wheat are considered to be haploid, since they contain half the genetic information of somatic cells, but they are not monoploid, as they still contain three complete sets of chromosomes (n = 3x).[25]
In the case of wheat, the origin of its haploid number of 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although the number of chromosomes may have originated in this way, this is no longer clear, and the monoploid number is regarded as the same as the haploid number. Thus in humans,x = n = 23.
Diploid describes a cell or nucleus which contains two copies of genetic material, or a complete set of chromosomes, paired with their homologs (chromosome carrying the same information from the other parent).[26]Diploid cells have twohomologous copies of eachchromosome, usually one from themother and one from thefather. All or nearly all mammals are diploid organisms. The suspected tetraploid (possessing four-chromosome sets) plains viscacha rat (Tympanoctomys barrerae) and golden viscacha rat (Pipanacoctomys aureus)[27] have been regarded as the only known exceptions (as of 2004).[28] However, some genetic studies have rejected anypolyploidism in mammals as unlikely, and suggest that amplification and dispersion of repetitive sequences best explain the large genome size of these two rodents.[29] All normal diploid individuals have some small fraction of cells that displaypolyploidy.Human diploid cells have 46 chromosomes (thesomatic number,2n) and human haploidgametes (egg and sperm) have 23 chromosomes (n).Retroviruses that contain two copies of their RNA genome in each viral particle are also said to be diploid. Examples includehuman foamy virus,human T-lymphotropic virus, andHIV.[30]
Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms aretriploid (3 sets),tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid[2] or septaploid[3] (7 sets), octoploid (8 sets), nonaploid (9 sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc.[31][32][33][34] Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets),[35] though Greek terminology may be set aside for readability in cases of higher ploidy (such as "16-ploid").[33]Polytene chromosomes of plants and fruit flies can be 1024-ploid.[36][37] Ploidy of systems such as thesalivary gland,elaiosome,endosperm, andtrophoblast can exceed this, up to 1048576-ploid in the silk glands of the commercial silkwormBombyx mori.[38]
The chromosome sets may be from the same species or from closely related species. In the latter case, these are known as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed from the hybridization of two separate species. In plants, this probably most often occurs from the pairing of meiotically unreducedgametes, and not by diploid–diploid hybridization followed by chromosome doubling.[39] The so-calledBrassica triangle is an example of allopolyploidy, where three different parent species have hybridized in all possible pair combinations to produce three new species.[40]
Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, manysomatic cells are polyploid due to a process calledendoreduplication, where duplication of thegenome occurs withoutmitosis (cell division). The extreme in polyploidy occurs in thefern genusOphioglossum, the adder's-tongues, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over one thousand.[citation needed]
It is possible for polyploid organisms to revert to lower ploidy byhaploidisation.[citation needed]
Polyploidy is a characteristic of the bacteriumDeinococcus radiodurans[41] and of thearchaeonHalobacterium salinarum.[42] These two species are highly resistant toionizing radiation anddesiccation, conditions that induceDNA double-strand breaks.[43][44] This resistance appears to be due to efficienthomologous recombinational repair.
Depending on growth conditions,prokaryotes such asbacteria may have a chromosome copy number of 1 to 4, and that number is commonly fractional, counting portions of the chromosome partly replicated at a given time. This is because under exponential growth conditions the cells are able to replicate their DNA faster than they can divide.[citation needed]
In ciliates, the macronucleus is calledampliploid, because only part of the genome is amplified.[45]
Mixoploidy is the case where two cell lines, one diploid and one polyploid,coexist within the same organism. Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children.[46] There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69,[47] and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.
Dihaploid and polyhaploid cells are formed byhaploidisation of polyploids, i.e., by halving the chromosome constitution.[citation needed]
Dihaploids (which are diploid) are important for selective breeding of tetraploid crop plants (notably potatoes), because selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from the diploids, for example by somatic fusion.[citation needed]
The term "dihaploid" was coined by Bender[48] to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense,[49][50] but it has also been used for doubled monoploids ordoubled haploids, which are homozygous and used for genetic research.[51]
Euploidy (Greekeu, "true" or "even") is the state of a cell or organism having one or more than one set of the same set of chromosomes, possibly excluding thesex-determining chromosomes. For example, most human cells have 2 of each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with one extra set of the 23 normal chromosomes (functionally triploid) would be considered euploid. Euploidkaryotypes would consequentially be a multiple of thehaploid number, which in humans is 23.[citation needed]
Aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their usual number of copies (excluding the absence or presence of complete sets, which is considered euploidy). Unlike euploidy, aneuploid karyotypes will not be a multiple of the haploid number. In humans, examples of aneuploidy include having a single extra chromosome (as inDown syndrome, where affected individuals have three copies of chromosome 21) or missing a chromosome (as inTurner syndrome, where affected individuals have only one sex chromosome). Aneuploidkaryotypes are given names with the suffix-somy (rather than-ploidy, used for euploid karyotypes), such astrisomy andmonosomy.[citation needed]
Homoploid means "at the same ploidy level", i.e. having the same number ofhomologous chromosomes. For example, homoploidhybridization is hybridization where the offspring have the same ploidy level as the two parental species. This contrasts with a common situation in plants where chromosome doubling accompanies or occurs soon after hybridization. Similarly, homoploid speciation contrasts withpolyploid speciation.[citation needed]
Zygoidy is the state in which the chromosomes are paired and can undergo meiosis. The zygoid state of a species may be diploid or polyploid.[52][53] In the azygoid state the chromosomes are unpaired. It may be the natural state of some asexual species or may occur after meiosis. In diploid organisms the azygoid state is monoploid. (See below for dihaploidy.)
In the strictest sense, ploidy refers to the number of sets of chromosomes in a singlenucleus rather than in the cell as a whole. Because in most situations there is only one nucleus per cell, it is commonplace to speak of the ploidy of a cell, but in cases in which there is more than one nucleus per cell, more specific definitions are required when ploidy is discussed. Authors may at times report the total combined ploidy of all nuclei present within the cell membrane of asyncytium,[38] though usually the ploidy of each nucleus is described individually. For example, a fungaldikaryon with two separate haploid nuclei is distinguished from a diploid cell in which the chromosomes share a nucleus and can be shuffled together.[54]
It is possible on rare occasions for ploidy to increase in thegermline, which can result inpolyploid offspring and ultimately polyploid species. This is an important evolutionary mechanism in both plants and animals and is known as a primary driver ofspeciation.[8] As a result, it may become desirable to distinguish between the ploidy of a species or variety as it presently breeds and that of an ancestor. The number of chromosomes in the ancestral (non-homologous) set is called themonoploid number (x), and is distinct from the haploid number (n) in the organism as it now reproduces.[citation needed]
Common wheat (Triticum aestivum) is an organism in whichx andn differ. Each plant has a total of six sets of chromosomes (with two sets likely having been obtained from each of three different diploid species that are its distant ancestors). The somatic cells are hexaploid, 2n = 6x = 42 (where the monoploid numberx = 7 and the haploid numbern = 21). The gametes are haploid for their own species, but triploid, with three sets of chromosomes, by comparison to a probable evolutionary ancestor,einkorn wheat.[citation needed]
Tetraploidy (four sets of chromosomes, 2n = 4x) is common in manyplant species, and also occurs inamphibians,reptiles, andinsects. For example, species ofXenopus (African toads) form aploidy series, featuring diploid (X. tropicalis, 2n=20), tetraploid (X. laevis, 4n=36), octaploid (X. wittei, 8n=72), and dodecaploid (X. ruwenzoriensis, 12n=108) species.[55]
Over evolutionary time scales in whichchromosomal polymorphisms accumulate, these changes become less apparent bykaryotype – for example, humans are generally regarded as diploid, but the2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors.[citation needed]
Ploidy can also vary between individuals of the same species or at different stages of thelife cycle.[56][57] In some insects it differs bycaste. In humans, only the gametes are haploid, but in many of thesocial insects, includingants,bees, andtermites, males develop from unfertilized eggs, making them haploid for their entire lives, even as adults.[citation needed]
In the Australian bulldog ant,Myrmecia pilosula, a haplodiploid species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes.[58] InEntamoeba, the ploidy level varies from 4n to 40n in a single population.[59]Alternation of generations occurs in most plants, with individuals "alternating" ploidy level between different stages of their sexual life cycle.[citation needed]
In large multicellular organisms, variations in ploidy level between different tissues, organs, or cell lineages are common. Because the chromosome number is generally reduced only by the specialized process of meiosis, the somatic cells of the body inherit and maintain the chromosome number of the zygote by mitosis. However, in many situations somatic cells double their copy number by means ofendoreduplication as an aspect ofcellular differentiation. For example, the hearts of two-year-old human children contain 85% diploid and 15% tetraploid nuclei, but by 12 years of age the proportions become approximately equal, and adults examined contained 27% diploid, 71% tetraploid and 2% octaploid nuclei.[60]
There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. A study comparing thekaryotypes of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid is associated with a 14% lower risk of being endangered, and a 20% greater chance of being invasive.[61] Polyploidy may be associated with increased vigor and adaptability.[62] Some studies suggest that selection is more likely to favor diploidy in host species and haploidy in parasite species.[63] However, polyploidization is associated with an increase intransposable element content[64][65] and relaxedpurifying selection onrecessive deleterious alleles.[66][67]
When a germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting inaneuploid gametes. Triploid organisms, for instance, are usually sterile. Because of this, triploidy is commonly exploited in agriculture to produce seedless fruit such as bananas and watermelons. If the fertilization of human gametes results in three sets of chromosomes, the condition is calledtriploid syndrome.[citation needed]
Inunicellular organisms theploidy nutrient limitation hypothesis suggests thatnutrient limitation should encourage haploidy in preference to higher ploidies. This hypothesis is due to the highersurface-to-volume ratio of haploids, which eases nutrient uptake, thereby increasing the internal nutrient-to-demand ratio. Mable 2001 findsSaccharomyces cerevisiae to be somewhat inconsistent with this hypothesis however, as haploid growth is faster than diploid under high nutrient conditions. The NLH is also tested in haploid, diploid, and polyploid fungi by Gerstein et al. 2017. This result is also more complex: On the one hand, underphosphorus and other nutrient limitation, lower ploidy is selected as expected. However under normal nutrient levels or under limitation of onlynitrogen, higher ploidy was selected. Thus the NLH – and more generally, the idea that haploidy is selected by harsher conditions – is cast into doubt by these results.[68]
Older WGDs have also been investigated. Only as recently as 2015 was the ancientwhole genome duplication inBaker's yeast proven to beallopolyploid, by Marcet-Houben and Gabaldón 2015. It still remains to be explained why there are not more polyploid events in fungi, and the place ofneopolyploidy andmesopolyploidy infungal history.[68]
The concept that thosegenes of an organism that are expressed exclusively in the diploid stage are under less efficientnatural selection than those genes expressed in thehaploid stage is referred to as the “masking theory”.[69] Evidence in support of this masking theory has been reported in studies of the single-celled yeastSaccharomyces cerevisiae.[70] In further support of the masking theory, evidence of strong purifying selection in haploid tissue-specific genes has been reported for the plantScots Pine.[69]
| Term | Description |
|---|---|
| Ploidy number | Number of chromosome sets |
| Monoploid number (x) | Number of chromosomes found in a single complete set |
| Chromosome number | Total number of chromosomes in all sets combined |
| Zygotic number | Number of chromosomes in zygotic cells |
| Haploid or gametic number (n) | Number of chromosomes found in gametes |
| Diploid number | Chromosome number of a diploid organism |
| Tetraploid number | Chromosome number of a tetraploid organism |
The commonpotato (Solanum tuberosum) is an example of a tetraploid organism, carrying four sets of chromosomes. During sexual reproduction, each potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a full complement of 48 chromosomes. The haploid number (half of 48) is 24. The monoploid number equals the total chromosome number divided by the ploidy level of the somatic cells: 48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12. Hence, the monoploid number (12) and haploid number (24) are distinct in this example.[citation needed]
However, commercial potato crops (as well as many other crop plants) are commonly propagatedvegetatively (by asexual reproduction through mitosis),[71] in which case new individuals are produced from a single parent, without the involvement of gametes and fertilization, and all the offspring are genetically identical to each other and to the parent, including in chromosome number. The parents of these vegetative clones may still be capable of producing haploid gametes in preparation for sexual reproduction, but these gametes are not used to create the vegetative offspring by this route.
| Species | Ploidy | Number of chromosomes |
|---|---|---|
| Eucalyptus spp. | Diploid | 2x = 22 |
| Banana (Musa spp.) | Triploid | 3x = 33 |
| Coffea arabica | Tetraploid | 4x = 44 |
| Sequoia sempervirens | Hexaploid | 6x = 66 |
| Opuntia ficus-indica | Octoploid | 8x = 88 |
| Species | Number of chromosomes | Ploidy number |
|---|---|---|
| Vinegar/fruit fly | 8 | 2 |
| Wheat | 14, 28 or 42 | 2, 4 or 6 |
| Crocodilian | 32, 34, or 42 | 2 |
| Apple | 34, 51, or 68 | 2, 3 or 4 |
| Human | 46 | 2 |
| Horse | 64, 65, or 66 | 2 |
| Chicken | 78 | 2 |
| Gold fish | 100 or more | 2 or polyploid |
{{cite journal}}: CS1 maint: multiple names: authors list (link){{cite web}}: CS1 maint: archived copy as title (link)Some eukaryotic genome-scale orgenome size databases and other sources which may list the ploidy levels of many organisms: