Ploidy
Ploidy (
Virtually all
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 the chromosome number or chromosome complement. The number of chromosomes found in a single complete set of chromosomes is called the monoploid number (x). The haploid number (n) refers to the total number of chromosomes found in a
Etymology
The term ploidy is a back-formation from haploidy and diploidy. "Ploid" is a combination of Ancient Greek -πλόος (-plóos, "-fold") and -ειδής (-eidḗs), from εἶδος (eîdos, "form, likeness").[a] The principal meaning of the Greek 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 botanist Eduard Strasburger coined the terms haploid and diploid in 1905.[b] Some authors suggest that Strasburger based the terms on August Weismann's conception of the id (or germ plasm),[14][15][16] hence haplo-id and diplo-id. The two terms were brought into the English language from German through William Henry Lang's 1908 translation of a 1906 textbook by Strasburger and colleagues.[17][citation needed]
Types of ploidy
Haploid and monoploid
The term haploid 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 a gamete.[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 with more 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 a eukaryotic cell is said to be haploid only if it has a single set of chromosomes, 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 term monoploid 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]
All
In some cases there is evidence that the n 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 the monoploid number,[21] also known as basic or cardinal number,[22] or fundamental number.[23][24] As an example, the chromosomes of common 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 general n is a multiple of x. 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 number n is 21 and the monoploid number x 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
Diploid cells have two
Polyploidy
Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, usually 3 or more. Specific terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid
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 unreduced
Polyploidy occurs commonly in plants, but rarely in animals. Even in diploid organisms, many somatic cells are polyploid due to a process called endoreduplication, where duplication of the genome occurs without mitosis (cell division). The extreme in polyploidy occurs in the fern genus Ophioglossum, 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 by haploidisation.[citation needed]
In bacteria and archaea
Variable or indefinite ploidy
Depending on growth conditions, prokaryotes such as bacteria 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 called ampliploid, because only part of the genome is amplified.[44]
Mixoploidy
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.[45] There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69,[46] and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.
Dihaploidy and polyhaploidy
Dihaploid and polyhaploid cells are formed by haploidisation 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[47] to combine in one word the number of genome copies (diploid) and their origin (haploid). The term is well established in this original sense,[48][49] but it has also been used for doubled monoploids or doubled haploids, which are homozygous and used for genetic research.[50]
Euploidy and aneuploidy
Euploidy (Greek eu, "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 the sex-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. Euploid karyotypes would consequentially be a multiple of the haploid 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 in Down syndrome, where affected individuals have three copies of chromosome 21) or missing a chromosome (as in Turner syndrome, where affected individuals have only one sex chromosome). Aneuploid karyotypes are given names with the suffix -somy (rather than -ploidy, used for euploid karyotypes), such as trisomy and monosomy.
Homoploid
Homoploid means "at the same ploidy level", i.e. having the same number of
Zygoidy and azygoidy
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.[51][52] 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.)
Special cases
More than one nucleus per cell
In the strictest sense, ploidy refers to the number of sets of chromosomes in a single
Ancestral ploidy levels
It is possible on rare occasions for ploidy to increase in the
Common wheat (Triticum aestivum) is an organism in which x and n 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 number x = 7 and the haploid number n = 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]
Over evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less apparent by karyotype – for example, humans are generally regarded as diploid, but the 2R hypothesis has confirmed two rounds of whole genome duplication in early vertebrate ancestors.
Haplodiploidy
Ploidy can also vary between individuals of the same species or at different stages of the
In the Australian bulldog ant,
Tissue-specific polyploidy
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 of endoreduplication as an aspect of cellular 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.[59]
Adaptive and ecological significance of variation in ploidy
There is continued study and debate regarding the fitness advantages or disadvantages conferred by different ploidy levels. A study comparing the
When a germ cell with an uneven number of chromosomes undergoes meiosis, the chromosomes cannot be evenly divided between the daughter cells, resulting in
In
Older WGDs have also been investigated. Only as recently as 2015 was the ancient
Glossary of ploidy numbers
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 common potato (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.
However, commercial potato crops (as well as many other crop plants) are commonly propagated vegetatively (by asexual reproduction through mitosis),[68] 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.
Specific examples
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 | 2 |
Chicken | 78 | 2 |
Gold fish | 100 or more | 2 or polyploid |
Notes
- ^ Compare the etymology of tuple, from the Latin for "-fold".
- ^ The original text in German is as follows: "Schließlich wäre es vielleicht erwünscht, wenn den Bezeichnungen Gametophyt und Sporophyt, die sich allein nur auf Pflanzen mit einfacher und mit doppelter Chromosomenzahl anwenden lassen, solche zur Seite gestellt würden, welche auch für das Tierreich passen. Ich erlaube mir zu diesem Zwecke die Worte Haploid und Diploid, bezw. haploidische und diploidische Generation vorzuschlagen."[12][13]
References
- ISBN 978-0-7637-7364-9.
- ^ S2CID 45850598.
- ^ S2CID 38029072.
- S2CID 10163081.
- ^ Darlington, C. D. (Cyril Dean) (1937). Recent advances in cytology. Philadelphia: P. Blakiston's son & co. p. 60.
- ISBN 978-0-470-90359-9.
- S2CID 10054182.
- ^ ISSN 0024-4066.
- PMID 23149459.
- ^ "Greek Word Study Tool". www.perseus.tufts.edu.
- ^ "Greek Word Study Tool". www.perseus.tufts.edu.
- ^ Strasburger, Eduard; Allen, Charles E.; Miyake, Kilchi; Overten, James B. (1905). "Histologische Beiträge zur Vererbungsfrage". Jahrbücher für Wissenschaftliche Botanik. 42: 62. Retrieved 2017-03-11.
- ISBN 978-3-476-02317-9.
- ^ Battaglia E (2009). "Caryoneme alternative to chromosome and a new caryological nomenclature" (PDF). Caryologia. 62 (4): 48.
- S2CID 207403936.
- .
- ^ Strasburger, E.; Noll, F.; Schenck, H.; Karsten, G. 1908. A Textbook of botany, 3rd English ed. (1908) [1], rev. with the 8th German ed. (1906) [2], translation by W. H. Lang of Lehrbuch der Botanik für Hochschulen. Macmillan, London.
- ^ "MGI Glossary". Mouse Genome Informatics. Bar Harbor, Maine: The Jackson Laboratory. Retrieved 6 July 2019.
- ^ "Talking Glossary of Genetic Terms". National Human Genome Research Institute. Retrieved 6 July 2019.
- ^ "Homologous chromosomes". Genomics Education Programme. 23 September 2021. Retrieved 10 March 2023.
- ^ Langlet, 1927.
- ^ Winge, 1917.
- ^ Manton, 1932.
- ^ Fabbri F (1963). "Primo supplemento alle tavole cromosomiche delle Pteridophyta di Alberto Chiarugi". Caryologia. 16: 237–335.
- ^ "LECTURE 10: CHANGES IN CHROMOSOME NUMBER" (PDF). Mcb.berkeley.edu. Retrieved 2022-03-10.
- PMID 16580173.
- hdl:11336/102012.
- PMID 15780745.
- ^ "Human Retroviruses". Archived from the original on 2003-03-30. Retrieved 2008-05-14.
- PMID 19589857.
- PMID 20150197.
- ^ PMID 21628226.
- ^ Talyshinskiĭ, G. M. (1990). "Study of the fractional composition of the proteins in the compound fruit of polyploid mulberry". Shelk (5): 8–10.
- PMID 11831358.
- ISBN 9780849389191.
- ^ "Genes involved in tissue and organ development: Polytene chromosomes, endoreduplication and puffing". The Interactive Fly. Archived from the original on 2005-05-04. Retrieved 2012-12-16.
- ^ a b Encyclopedia of the Life Sciences (2002) "Polyploidy" Francesco D'Amato and Mauro Durante
- .
- PMID 33599732.
- PMID 649572.
- S2CID 31385928.
- S2CID 4412830.
- S2CID 8391234.
- ^ Schaechter, M. Eukaryotic microbes. Amsterdam, Academic Press, 2012, p. 217.
- PMID 7810564.
- PMID 8301657.
- ^ Bender K (1963). "Über die Erzeugung und Entstehung dihaploider Pflanzen bei Solanum tuberosum"". Zeitschrift für Pflanzenzüchtung. 50: 141–166.
- ^ Nogler, G.A. 1984. Gametophytic apomixis. In Embryology of angiosperms. Edited by B.M. Johri. Springer, Berlin, Germany. pp. 475–518.
- S2CID 32122774.
- PMID 17247970.
- ISBN 978-0-12-017603-8.
- ISBN 978-3-642-14635-0.
- ^ James B. Anderson; Linda M Kohn. "Dikaryons, diploids, and evolution" (PDF). University of Toronto. Archived from the original (PDF) on 2013-05-27. Retrieved 2012-12-16.
- PMID 26112701.
- PMID 18258610.
- S2CID 40564254.)
{{cite journal}}
: CS1 maint: multiple names: authors list (link - S2CID 25465053.
- ^ "Archived copy" (PDF). Archived from the original (PDF) on 2014-02-23. Retrieved 2014-02-18.
{{cite web}}
: CS1 maint: archived copy as title (link) - ISBN 9783718605187.
- S2CID 38197332.
- . Retrieved 2011-04-07.
- PMID 15252199.
- PMID 15739260.
- PMID 21238281.
- ISSN 1469-5073.
- S2CID 209420359.
- ^ S2CID 92205236.
- ^ "The Biology of Solanum tuberosum (L.) (Potatoes)". Canadian Food Inspection Agency. 2012-03-05.
Sources
- Griffiths, A. J. et al. 2000. An introduction to genetic analysis, 7th ed. W. H. Freeman, New York ISBN 0-7167-3520-2
External links
Some eukaryotic genome-scale or genome size databases and other sources which may list the ploidy levels of many organisms:
- Animal genome size database
- Plant genome size database
- Fungal genome size database
- Protist genome-scale database of Ensembl Genomes
- Nuismer S.; Otto S.P. (2004). "Host-parasite interactions and the evolution of ploidy". Proc. Natl. Acad. Sci. USA. 101 (30): 11036–11039. PMID 15252199. (Supporting Data Set, with information on ploidy level and number of chromosomes of several protists)
- Chromosome number and ploidy mutations YouTube tutorial video