Ploidy is the number of the complete set of chromosomes in a cell , and hence the number of possible alleles for autosomal and pseudoautosomal genes . Somatic cells , tissues , and individual organisms can be described according to the number of sets of chromosomes present (“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 general term polyploid is often used to describe cells with three or more chromosome sets.
Virtually all sexually reproducing organisms are composed of somatic cells that are diploid or more, but ploidy levels vary widely between different organisms, between different tissues within the same organism, and at different stages in an organism’s life cycle. It is possible Half of all known plant species are polyploid species, and about two-thirds of all grasses are polyploid. [6] Many animals are equally diploid, although polyploidy is common in invertebrates, reptiles, and amphibians. In some species, ploidy varies between individuals of the same species (as in social insects ), and in others entire tissues and organ systems can be polyploid, despite the whole body being diploid (such as in the mammalian liver)., For many organisms, especially plants and fungi, changes in ploidy levels between generations are major drivers of species . In mammals and birds, ploidy changes are usually fatal. [7] However, there is evidence of polyploidy in organisms now considered diploid, suggesting that polyploidy has contributed to evolutionary diversification in plants and animals through successive periods of polyploidization and rediploidization. [8] [9]
Humans are diploid organisms, normally having two complete sets of chromosomes in their somatic cells: one set of 23 chromosomes from their father and one set of 23 chromosomes from their mother. The two sets combined provide the full complement of 46 chromosomes. This total number of individual chromosomes (counting all complete sets) is called chromosome number or chromosome complement . The number of chromosomes found in a complete set of chromosomes is called monoploid number ( x ). The haploid number ( n ) refers to the total number of chromosomes found in a gamete (a sperm or egg ).Meiosis produced by the cell in preparation for sexual reproduction). Under normal conditions, the haploid number is half of the total number of chromosomes present in the somatic cells of the organism. For diploid organisms, the monoploid number and the haploid number are equal; In humans, both equal 23. When a human germ cell undergoes meiosis, the diploid 46 chromosomes split in complementary half to form haploid gametes. After the fusion of male and female gametes (each having 1 set of 23 chromosomes) during fertilization , the resulting zygote again has a full complement of 46 chromosomes: 2 sets of 23 chromosomes. Euploidy and aneuploidydescribe having multiple chromosomes which is an exact multiple of the number of chromosomes in a normal gamete; and have any other number respectively. For example, a person with Turner syndrome may have one sex chromosome (X or Y) missing, resulting in a (45, X) karyotype instead of the normal (46, XX) or (46, XY). It is a type of aneuploidy and individual cells with a 45K (diploid) chromosome complement can be called aneuploid.
Word – medium
The term ploidy is a stage of haploidy and diploidy from the formation of the back . “Ploidy” is a combination of Ancient Greek -πλόος (-plos, “-fold”) and -ειδής (- idis ) , from ( eidos , “form, likeness”). [a] The main meaning of the Greek word (haplos) is “single”, [10] ] – (ha-, “one, similar”). 11 means “duplex” or “two-fold”. Hence diploid means “duplex-shaped” (compare “humanoid”, “human-sized”).
Polish botanist Eduard Strasberger coined the terms haploid and diploid in 1905 . [b] Some authors suggest that Strasberger based the term on August Weisman ‘s concept of the id (or germ plasm ), [14] [15] [16] hence the haplo- id and diplo – id . The two words were brought from German to the English language through William Henry Lang ‘s 1908 translation of a 1906 textbook by Strasberger and his colleagues .
Types of ploidy
Haploid and monoploid
The term haploid is used with two different but related definitions. In the most general sense, the number of sets of haploid chromosomes normally found in a gamete . [18]Because the two gametes necessarily join together during sexual reproduction to form a single zygote from which somatic cells are produced, healthy gametes always have half the number of sets of chromosomes found in somatic cells, and are therefore “haploid” in this sense. ” means having exactly half the number of sets of chromosomes found in a somatic cell. By this definition, an organism that has one copy of each chromosome (one set of chromosomes) in its gamete cells can be considered haploid, while somatic cells, which have 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 describe in diagrams of genetics concepts. But this definition is more than one of the chromosomes.Also allows set haploid gametes. 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, by meiosis will produce gametes that contain two sets of chromosomes. Despite being numerically diploid, these gametes can still be called haploid.
An alternative usage defines “haploid” as having one 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 contains a single set of chromosomes, each not being part of a pair. By extension, a cell can be called haploid if it has one set of chromosomes in its nucleus, and an organism can be said to be haploid if the cells of its body (somatic cells) have one set of chromosomes per cell. By this definition, haploid would not be used to refer to the gametes produced by a tetraploid organism in the above example, as these gametes are numerically diploid. monoploidThe term is often used as a less ambiguous way to describe a set of chromosomes; According to this second definition, haploid and monoploid are the same and can be used interchangeably.
Comparison of sexual reproduction in predominantly haploid organisms and predominantly diploid organisms .
1) A haploid organism is on the left and a diploid organism is on the right.
2 and 3) The haploid egg and sperm carry the dominant purple gene and the recessive blue gene, respectively. These gametes are produced by simple mitosis of cells in the germ line.
4 and 5) diploid sperm and egg carry the recessive blue gene and the dominant purple gene, respectively. These gametes are produced by meiosis, which halves the number of chromosomes in diploid germ cells.
6) The short-lived diploid state of haploid organisms, a zygote produced by the union of two haploid gametes during sex.
7)A diploid zygote that has just been fertilized by the union of a haploid egg and a sperm during sex.
8) Cells of the diploid structure soon undergo meiosis, producing spores containing the meiotic number of the meiotic chromosomes, which are haploid. These spores express either the mother’s dominant gene or the father’s recessive gene and proceed by mitotic division to form a new completely haploid organism.
9) The diploid zygote proceeds by mitosis to form a new fully diploid organism. These cells contain both purple and blue genes, but only the purple gene is expressed because it is dominant over the recessive blue gene.
The term haploid is used with two different but related definitions. In the most general sense,
Gametes (sperm and egg) are haploid cells. The haploid gametes produced by most organisms combine with n pairs of chromosomes to form a zygote, i.e. a total of 2 n chromosomes. The chromosomes in each pair, one from the sperm and one from the egg, are called homologous. Cells and organisms having pairs of homologous chromosomes are called diploid. For example, most animals are diploid and produce haploid gametes. During meiosis, sex cell precursors halve their number of chromosomes by randomly “choosing” one member of each pair of chromosomes, resulting in haploid gametes. Since homologous chromosomes are usually genetically different, gametes are usually genetically different from each other.
All plants and many fungi and algae switch between a haploid and a diploid stage, with one stage asserting the other. This is called alternation of generations. Most fungi and algae are haploid during the major phase of their life cycle, as are some primitive plants such as mosses. Recently evolved plants, such as gymnosperms and angiosperms, spend most of their life cycle in the diploid stage. Most animals are diploid, but male bees, wasps, and ants are haploid organisms because they develop from unfertilized, haploid eggs, while females (workers and queens) are diploid, making their systems haploidiploid.
In some cases there is evidence that the n chromosomes in the haploid set originally resulted from duplication of shorter chromosomes. This “base” number – the number of apparently originally unique chromosomes in a haploid set – is called the monoploid number , [20] also known as the basic or cardinal number , [21] or the fundamental number. . [22] [23] As an example, the common wheat chromosome is believed to be derived from three different parental species, each of which has 7 chromosomes in its haploid gametes. Thus the monoploid number is 7 and the haploid number is 3 × 7 = 21. in general nis a multiple of x . Somatic cells in a wheat plant have six sets of 7 chromosomes: three sets from the egg and three sets from the sperm that are fused to form the plant, giving a total of 42 chromosomes. As a formula, 2 n = 6 x = 42 for wheat, so that the haploid number n is 21 and the monoploid number x is 7. Normal wheat gametes are considered haploid, as they contain half the genetic information of the somatic. cells, but they are not monoploid, as they still contain three complete sets of chromosomes ( n = 3 x ).
In the case of wheat, the origin of the haploid number of its 21 chromosomes from three sets of 7 chromosomes can be demonstrated. In many other organisms, although the number of chromosomes may have arisen in this way, it is no longer clear, and the monoploid number is assumed to be the same as the haploid number. Thus, in humans x = n = 23.
Duplicate
Diploid cells have two homologous copies of each chromosome, usually one from the mother and one from the father. All or almost all mammals are diploid organisms. The suspected tetraploid (four-chromosome set) plains Wiscacha rat ( Tympanoctomys barrere ) and the golden Wiscacha rat ( Pipanacotomys aureus ) [25] are considered the only known exceptions (as of 2004). [26] However, some genetic studies have ruled out any polyploidy in mammals as unlikely, and suggest that the amplification and dispersal of repetitive sequences best explain the large genome sizes of these two rodents. [27]All normal diploid individuals have some small fraction of cells that exhibit polyploidy. Human diploid cells have 46 chromosomes (somatic number, 2n ) and human haploid gametes (egg and sperm) have 23 chromosomes ( n ). Retroviruses in which each viral particle contains two copies of their RNA genome are also called diploid. Examples include human foamy virus, human T-lymphotropic virus, and HIV. [28]
Polyploidy
Polyploidy is the state where all cells have multiple sets of chromosomes beyond the original set, usually 3 or more. Typical terms are triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid [2] or septaploid [3] (7 sets), octoploid (8 sets), nonploid (9 sets) ) Huh. sets), decaploid (10 sets), undecaploid (11 sets), dodecaploid (12 sets), tridecaploid (13 sets), tetradecaploid (14 sets), etc. [29] [30] [31] [32] Some higher ploidies include hexadecaploid (16 sets), dotriacontaploid (32 sets), and tetrahexacontaploid (64 sets), [33]However in cases of high ploidy (such as “16-ploidy”) the Greek terminology may be set aside for readability. [31] The polytene chromosomes of plants and fruit flies may be 1024-ploidy. [34] [35] The ploidy of systems such as the salivary gland, elaiosome, endosperm and trophoblast can exceed 10,48576-ploids in the silk glands of the commercial silkworm Bombyx mori . [36]
Chromosome sets may be from the same species or from closely related species. In the latter case, these are referred to as allopolyploids (or amphidiploids, which are allopolyploids that behave as if they were normal diploids). Allopolyploids are formed by the hybridization of two different species. In plants, this probably occurs most often by pairing of gametes in meiosis, and not by diploid–diploid hybridization followed by chromosome doubling. [37] The so-called Brassica triangle is an example of allopolyploidy, where three different parent species hybridize in all possible pair combinations to produce three new species.
Polyploidy usually occurs in plants, but rarely in animals. Even in diploid organisms, many somatic cells are polyploid due to a process called endoreduplication, where the duplication of the genome occurs without mitosis (cell division). Polyploidy occurs in the extreme in the fern genus Ophioglossum , in adder’s-tongue, in which polyploidy results in chromosome counts in the hundreds, or, in at least one case, well over a thousand.
It is possible for polyploid organisms to revert to reduced ploidy by haploidisation.
In bacteria and archaea
Polyploidy is a feature of the bacterium Deinococcus radiodurans [38] and of the archaeon Halobacterium salinarum . [39] These two species are highly resistant to ionizing radiation and drying, conditions that induce DNA double-strand breaks. [40] [41] This resistance appears to be due to efficient home recombination repair.
Variable or indefinite ploidy
Depending on growth conditions, prokaryotes such as bacteria can have a chromosome copy number of 1 to 4, and this number usually varies, with portions of the chromosome partially replicated at a given time. This is because cells in a state of exponential growth are able to replicate their DNA faster than they can divide.
In ciliates , the macronucleus is said to be ampliploid , as only part of the genome is amplified. [42]
Mixoploidy
Mixoploidy is the case where two cell lines, one diploid and one polyploid, coexist within the same organism. Although polyploidy is not viable in humans, mixoploidy has been found in living adults and children. [43] There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, [44] and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. . This 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.
Dihaploids (which are diploid) are important for the selective breeding of tetraploid crop plants (especially potatoes), as selection is faster with diploids than with tetraploids. Tetraploids can be reconstituted from diploids, for example by somatic fusion.
The term “diploid” was coined by Bender [45] to combine the number of genome copies (diploid) and their origin (haploid) in a single term. The term is well established in this sense, [46] [47] but has also been used for doubly monoploids or doubly haploids, which are homozygous and used for genetic research. [48]
Euploidy and Euploidy
Euploidy (Greek eu , “true” or “even”) is the state of a cell or organism having one or more sets of chromosomes, possibly excluding sex-determining chromosomes. For example, most human cells have 2 in each of the 23 homologous monoploid chromosomes, for a total of 46 chromosomes. A human cell with an extra set of 23 normal chromosomes (functionally triploid) would be considered euploid. The euploid karyotype will result in a multiple of the haploid number, which is 23 in humans.
Aneuploidy is the state where one or more individual chromosomes of a normal set are absent or present in more than their normal number of copies (except for the absence or presence of complete sets, which is considered to be euploidy). Unlike euploidy, aneuploid karyotypes will not be multiples of the haploid number. In humans, examples of aneuploidy include having an 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). It happens). Aneuploid karyotypes are named with the suffix -somy (instead of -ploidy , such as, used for euploid karyotypes), trisomy and monosomy.
Homoploid
Homoploid means “at the same ploidy level”, i.e. having the same number of homologous chromosomes. For example, homoploidy hybridization is hybridization where the offspring have the same ploidy level as the two parental species. This is in contrast to a common situation in plants where chromosome doubling occurs with or immediately after hybridization. Similarly, homoploid speciation is the opposite of polyploid speciation. [ citation needed ]
Zygoidy and Azygoidy
Zygody is the stage in which chromosomes pair up and can undergo meiosis. The zygoid stage of a species can be diploid or polyploid. [49] [50] In the azygoid stage, the chromosomes are unpaired. This may be the natural state of some asexual species or may occur after meiosis. In diploid organisms, the azygoid stage is monoploid. (See below for dihaploidy.)
Special cases
More than one nucleus per cell
In the strict sense, ploidy refers to the number of sets of chromosomes in a single nucleus rather than in an entire cell. Because there is only one nucleus per cell in most situations, it is common to talk about ploidy of the cell, but in cases in which there is more than one nucleus per cell, more specific definitions are needed when discussing ploidy. Is performed. Authors can sometimes report the total combined ploidy of all nuclei present within the cell membrane of a syncytium, [36] although usually the ploidy of each nucleus is described individually. For example, a fungal dikaryon with two separate haploid nuclei differs from a diploid cell in that the chromosomes share a nucleus and can be shuffled together. [51]
Paternal ploidy level
It is possible on rare occasions that there may be an increase in germline to ploidy, which may result in polyploid progeny and eventually polyploid species. It is an important evolutionary mechanism in both plants and animals and is known to be the primary driver of species. [8] As a result, it may be desirable to distinguish between a species or variety of ploidy as it reproduces in the present and that of an ancestor. The number of chromosomes in the parental (non-homologous) set is called the monoploid number ( X ), and differs from the haploid number ( N ) in the organism as it is now reproducing.
Common wheat ( Triticum aestivum ) is one such organism in which x and n are different. Each plant has a total of six sets of chromosomes (two sets probably derived from each of three different diploid species that are its distant ancestors). Somatic cells are hexaploid, 2 n = 6 x = 42 (where the monoploid number x = 7 and the haploid number n = 21). Gametes are haploid for their species, but triploid, with three sets of chromosomes, compared to a possible evolutionary ancestor, einkorn wheat. [ citation needed ]
Tetraploidy (four sets of chromosomes, 2 n = 4 x ) is common in many plant species, and also occurs in amphibians, reptiles and insects. For example, species of Xenopus (African toads) characterized a form ploidy series diploid ( X. tropicalis , 2n=20), tetraploid ( X. laevis , 4n=36), octaploid ( X. wittei , 8n=72), and dodecaploid ( X. ruvenzoriensis , 12n=108) species. [52]
On the evolutionary time scales in which chromosomal polymorphisms accumulate, these changes become less pronounced by karyotype—for example, humans are generally thought to be diploid, but the 2R hypothesis suggested two rounds of whole-genome duplication in early vertebrate ancestors. is confirmed.
Haplodiploidy
Ploidy may also differ between individuals of the same species or at different stages of the life cycle. [53] [54] In some insects it varies depending on the species. In humans, only gametes are haploid, but in many social insects including ants, bees and termites, some individuals develop from fertilized eggs, making them haploid for their entire lives even as adults. In the Australian bulldog ant, Myrmesia pilosula , a haplodiploid species, haploid individuals of this species have a single chromosome and diploid individuals have two chromosomes. [55] In Entamoeba , the ploidy level varies from 4n to 40n in a single population .[56] The alternation of generations occurs in most plants, in which individuals “alternate” the ploidy level between different stages of their sexual life cycle.
Tissue-specific polyploidy
In large multicellular organisms, variation in the level of ploidy between different tissues, organs, or cell lineages is common. Since the chromosome number is normally 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 through endoreduplication as an aspect of cellular differentiation. For example, the hearts of two-year-old human children have 85% diploid and 15% tetraploid nuclei, but by the age of 12 the ratio becomes roughly equal, and adults tested have 27% diploid, 71% tetraploid and 2%. There are octaploids. Nucleus [57]
Adaptive and ecological significance of variation in ploidy
There is ongoing study and debate about the advantages or disadvantages of fitness conferred by different ploidy levels. A study comparing the karyotypes of endangered or invasive plants with those of their relatives found that being polyploid as opposed to diploid has a 14% lower risk of becoming endangered and a 20% higher chance of being invasive. [58] Polyploidy may be associated with increased vigor and adaptability. [59] Some studies suggest that the host species are more likely to be diploid by selection and haploid in the parasitic species. [60]
When a germ cell with an unequal number of chromosomes undergoes meiosis, the chromosomes cannot be divided equally among daughter cells, resulting in aneuploid gametes. For example, triploid organisms are generally sterile. Because of this, triploids are commonly used in agriculture to produce seedless fruits such as bananas and watermelons. If the fertilization of human gametes results in three sets of chromosomes, the condition is called triploid syndrome.
Glossary of Ploidy Numbers
| period | description |
|---|---|
| ploidy number | number of chromosome sets |
| monoploid number ( x ) | number of chromosomes found in a single complete set |
| chromosome number | The total number of chromosomes in all sets combined |
| zygotic number | number of chromosomes in zygote cells |
| haploid or gamete 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, having 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 the 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: a total of 48 chromosomes equals a monoploid number of 12 divided by a ploidy level of 4. Therefore, in this example the monoploid number (12) and the haploid number (24) are different.
However, commercial potato crops (as well as many other crop plants) are usually propagated vegetatively (by asexual reproduction through mitosis), [61] in this case without the involvement of gametes and fertilization, with a single mother. New individuals arise from the father, and all offspring are genetically identical to each other and to the parent, including chromosome number. The parents of these vegetative clones may still be able to produce haploid gametes in preparation for sexual reproduction, but these gametes are not used to produce vegetative offspring by this route.
Specific example
| Caste | ploidy | number of chromosomes |
|---|---|---|
| Eucalyptus spp. | Duplicate | 2 x = 22 |
| Banana ( Musa spp.) | triploid | 3 x = 33 |
| coffee arabica | tetraploid | 4 x = 44 |
| sequoia sempervirens | hexaploid | 6 x = 66 |
| Opuntia ficus-indica | octoploid | 8 x = 88 |
| Caste | number of chromosomes | ploidy number |
|---|---|---|
| vinegar/fruit fly | 8 | 2 |
| wheat | 14, 28 or 42 | 2, 4 or 6 |
| Crocodile | 32, 34, or 42 | 2 |
| Apple | 34, 51, or 68 | 2, 3 or 4 |
| man | 46 | 2 |
| Horse | 64 | 2 |
| chicken | ७८ | 2 |
| Golden fish | 100 or more | 2 or polyploid |
