Karyotype

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A karyotype (Greek karyon = kernel, seed or nucleus) is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.[1][2][3]

Karyotypes describe the number of chromosomes, and what they look like under a light microscope. Attention is paid to their length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.[4] The preparation and study of karyotypes is part of cytogenetics.

Karyogram of human male using Giemsa staining

The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a photomicrograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.

The basic number of chromosomes in the somatic cells of an individual or a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. In the germ-line (the sex cells) the chromosome number is n (humans: n = 23).[2]p28

So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.

The study of karyotypes is important for cell biology and genetics, and the results may be used in evolutionary biology (karyosystematics)[5] and medicine. Karyotypes can be used for many purposes; such as to study chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.

History of karyotype studies[edit]

Chromosomes were first observed in plant cells by Carl Wilhelm von Nägeli in 1842. Their behavior in animal (salamander) cells was described by Walther Flemming, the discoverer of mitosis, in 1882. The name was coined by another German anatomist, von Waldeyer in 1888.

The next stage took place after the development of genetics in the early 20th century, when it was appreciated that chromosomes (the karyotype) was the carrier of genes. Levitsky seems to have been the first person to define the karyotype as the phenotypic appearance of the somatic chromosomes, in contrast to their genic contents.[6][7] The subsequent history of the concept can be followed in the works of Darlington[8] and White.[2][9]

Investigation into the human karyotype took many years to settle the most basic question: how many chromosomes does a normal diploid human cell contain?[10] In 1912, Hans von Winiwarter reported 47 chromosomes in spermatogonia and 48 in oogonia, concluding an XX/XO sex determination mechanism.[11] Painter in 1922 was not certain whether the diploid of humans was 46 or 48, at first favouring 46,[12] but revised his opinion from 46 to 48, and he correctly insisted on humans having an XX/XY system.[13] Considering the techniques of the time, these results were remarkable.

Fusion of ancestral chromosomes left distinctive remnants of telomeres, and a vestigial centromere

In textbooks, the number of human chromosomes remained at 48 for over thirty years. New techniques were needed to correct this error. Joe Hin Tjio working in Albert Levan's lab[14] was responsible for finding the approach:

  1. Using cells in tissue culture
  2. Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes
  3. Arresting mitosis in metaphase by a solution of colchicine
  4. Squashing the preparation on the slide forcing the chromosomes into a single plane
  5. Cutting up a photomicrograph and arranging the result into an indisputable karyogram.

The work took place in 1955, and was published in 1956. The karyotype of humans includes only 46 chromosomes.[15][16] Rather interestingly, the great apes have 48 chromosomes. Human chromosome 2 is now known to be a result of an end-to-end fusion of two ancestral ape chromosomes.[17][18]

Observations on karyotypes[edit]

Staining[edit]

The study of karyotypes is made possible by staining. Usually, a suitable dye, such as Giemsa,[19] is applied after cells have been arrested during cell division by a solution of colchicine. For humans, white blood cells are used most frequently because they are easily induced to divide and grow in tissue culture.[20] Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn fetus can be determined by observation of interphase cells (see amniotic centesis and Barr body).

Observations[edit]

Six different characteristics of karyotypes are usually observed and compared:[21]

  1. Differences in absolute sizes of chromosomes.  Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family. For example, the legumes Lotus tenuis and Vicia faba each have six pairs of chromosomes, yet V. faba chromosomes are many times larger.  These differences probably reflect different amounts of DNA duplication.
  2. Differences in the position of centromeres.  These differences probably came about through translocations.
  3. Differences in relative size of chromosomes.  These differences probably arose from segmental interchange of unequal lengths.
  4. Differences in basic number of chromosomes. These differences could have resulted from successive unequal translocations which removed all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis) or through fusion.  Humans have one pair fewer chromosomes than the great apes.  Human chromosome 2 appears to have resulted from the fusion of two ancestral chromosomes, and many of the genes of those two original chromosomes have been translocated to other chromosomes.
  5. Differences in number and position of satellites. Satellites are small bodies attached to a chromosome by a thin thread.
  6. Differences in degree and distribution of heterochromatic regions.  Heterochromatin stains darker than euchromatin.  Heterochromatin is packed tighter.  Heterochromatin consists mainly of genetically inactive and repetitive DNA sequences.  Euchromatin is usually under active transcription.

A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.

Variation is often found:

  1. between the sexes,
  2. between the germ-line and soma (between gametes and the rest of the body),
  3. between members of a population (chromosome polymorphism),
  4. in geographic specialization, and
  5. in Mosaics or otherwise abnormal individuals.[9]

Human karyotype[edit]

The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for females contain two X chromosomes and are denoted 46,XX; males have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.

Diversity and evolution of karyotypes[edit]

Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable. There is variation between species in chromosome number, and in detailed organization, despite their construction from the same macromolecules. This variation provides the basis for a range of studies in evolutionary cytology.

In some cases there is even significant variation within species. In a review, Godfrey and Masters conclude:

"In our view, it is unlikely that one process or the other can independently account for the wide range of karyotype structures that are observed... But, used in conjunction with other phylogenetic data, karyotypic fissioning may help to explain dramatic differences in diploid numbers between closely related species, which were previously inexplicable.[22]

Although much is known about karyotypes at the descriptive level, and it is clear that changes in karyotype organization has had effects on the evolutionary course of many species, it is quite unclear what the general significance might be.

"We have a very poor understanding of the causes of karyotype evolution, despite many careful investigations... the general significance of karyotype evolution is obscure." Maynard Smith.[23]

Changes during development[edit]

Instead of the usual gene repression, some organisms go in for large-scale elimination of heterochromatin, or other kinds of visible adjustment to the karyotype.

Number of chromosomes in a set[edit]

A spectacular example of variability between closely related species is the muntjac, which was investigated by Kurt Benirschke and his colleague Doris Wurster. The diploid number of the Chinese muntjac, Muntiacus reevesi, was found to be 46, all telocentric. When they looked at the karyotype of the closely related Indian muntjac, Muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes.[29]

"They simply could not believe what they saw... They kept quiet for two or three years because they thought something was wrong with their tissue culture... But when they obtained a couple more specimens they confirmed [their findings]" Hsu p73-4[16]

The number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. The low record is held by the nematode Parascaris univalens, where the haploid n = 1; and an ant: Myrmecia pilosula.[30] The high record would be somewhere amongst the ferns, with the Adder's Tongue Fern Ophioglossum ahead with an average of 1262 chromosomes.[31] Top score for animals might be the shortnose sturgeon Acipenser brevirostrum at 372 chromosomes.[32] The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.

Fundamental number[edit]

The fundamental number, FN, of a karyotype is the number of visible major chromosomal arms per set of chromosomes.[33][34] Thus, FN ≤ 2 x 2n, the difference depending on the number of chromosomes considered single-armed (acrocentric or telocentric) present. Humans have FN = 82,[35] due to the presence of five acrocentric chromosome pairs: 13, 14, 15, 21 and 22. The fundamental autosomal number or autosomal fundamental number, FNa[36] or AN,[37] of a karyotype is the number of visible major chromosomal arms per set of autosomes (non-sex-linked chromosomes).

Ploidy[edit]

Ploidy is the number of complete sets of chromosomes in a cell.

Aneuploidy[edit]

Aneuploidy is the condition in which the chromosome number in the cells is not the typical number for the species. This would give rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost. Abnormalities in chromosome number usually cause a defect in development. Down syndrome and Turner syndrome are examples of this.

Aneuploidy may also occur within a group of closely related species. Classic examples in plants are the genus Crepis, where the gametic (= haploid) numbers form the series x = 3, 4, 5, 6, and 7; and Crocus, where every number from x = 3 to x = 15 is represented by at least one species. Evidence of various kinds shows that that trends of evolution have gone in different directions in different groups.[48] Closer to home, the great apes have 24x2 chromosomes whereas humans have 23x2. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.[49]

Chromosomal polymorphism[edit]

Some animal species are polymorphic for chromosome fusions or dissociations.[50] When this happens, the chromosome number is variable from one individual to another. Well-researched examples are the ladybird beetle Chilocorus stigma, some mantids of the genus Ameles, the European shrew Sorex araneus. There is some evidence from the case of the mollusc Thais lapillus (the dog whelk) on the Brittany coast, that the two chromosome morphs are adapted to different habitats.[51]

Species trees[edit]

The detailed study of chromosome banding in insects with polytene chromosomes can reveal relationships between closely related species: the classic example is the study of chromosome banding in Hawaiian drosophilids by Hampton Carson.

In about 6,500 sq mi (17,000 km2), the Hawaiian Islands have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows. These roughly 800 Hawaiian drosophilid species are usually assigned to two genera, Drosophila and Scaptomyza, in the family Drosophilidae.

The polytene banding of the 'picture wing' group, the best-studied group of Hawaiian drosophilids, enabled Carson to work out the evolutionary tree long before genome analysis was practicable. In a sense, gene arrangements are visible in the banding patterns of each chromosome. Chromosome rearrangements, especially inversions, make it possible to see which species are closely related.

The results are clear. The inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. There are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. Using K-Ar dating, the present islands date from 0.4 million years ago (mya) (Mauna Kea) to 10mya (Necker). The oldest member of the Hawaiian archipelago still above the sea is Kure Atoll, which can be dated to 30 mya. The archipelago itself (produced by the Pacific plate moving over a hot spot) has existed for far longer, at least into the Cretaceous. Previous islands now beneath the sea (guyots) form the Emperor Seamount Chain.[52]

All of the native Drosophila and Scaptomyza species in Hawaiʻi have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. The subsequent adaptive radiation was spurred by a lack of competition and a wide variety of niches. Although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species.[53][54][55][56]

There are other animals and plants on the Hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations.[57][58]

Depiction of karyotypes[edit]

Types of banding[edit]

Cytogenetics employs several techniques to visualize different aspects of chromosomes:[20]

Classic karyotype cytogenetics[edit]

Karyogram from a human female lymphocyte probed for the Alu sequence using FISH.

In the "classic" (depicted) karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern.

Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of p15.2 (the locus on the chromosome), which is written as 46,XX,del(5)(p15.2).[59]

Spectral karyotype (SKY technique)[edit]

Spectral karyogram of a human female

Spectral karyotyping is a molecular cytogenetic technique used to simultaneously visualize all the pairs of chromosomes in an organism in different colors. Fluorescently labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Because there are a limited number of spectrally distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.[60]

Spectral human karyotype

This technique is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.

Digital karyotyping[edit]

Digital karyotyping is a technique used to quantify the DNA copy number on a genomic scale. Short sequences of DNA from specific loci all over the genome are isolated and enumerated.[61] This method is also known as virtual karyotyping.

Chromosome abnormalities[edit]

Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in derivative chromosome, translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells.

Chromosomal abnormalities that lead to disease in humans include

Some disorders arise from loss of just a piece of one chromosome, including

Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well-documented example is the Philadelphia chromosome, a translocation mutation commonly associated with chronic myelogenous leukemia and less often with acute lymphoblastic leukemia.

See also[edit]

References[edit]

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