Karyotype

A karyotype 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.^p28

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. The preparation and study of karyotypes is part of cytogenetics.

staining.]] The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) 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). The subsequent history of the concept can be followed in the works of Darlington and White.

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

New techniques were needed to definitively solve the problem: # Using cells in culture # Pretreating cells in a hypotonic solution, which swells them and spreads the chromosomes # Arresting mitosis in metaphase by a solution of colchicine # Squashing the preparation on the slide forcing the chromosomes into a single plane # Cutting up a photomicrograph and arranging the result into an indisputable karyogram.

It took until the mid 1950s until it became generally accepted that the karyotype of humans included only 46 chromosomes. Rather interestingly, the great apes have 48 chromosomes. Human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number.

Observations on karyotypes

Staining

The study of karyotypes is made possible by staining. Usually, a suitable dye, such as Giemsa, 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. 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

Six different characteristics of karyotypes are usually observed and compared: # Differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication. # Differences in the position of centromeres. This is brought about by translocations. # Differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths. # Differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes. # Differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread. # Differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.

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: # Between the sexes # Between the germ-line and soma (between gametes and the rest of the body) # Between members of a population (chromosome polymorphism) # Geographical variation between races # Mosaics or otherwise abnormal individuals.

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.

Changes during development

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.

Chromosome elimination. In some species, as in many sciarid flies, entire chromosomes are eliminated during development.

Chromatin diminution (founding father: Theodor Boveri). In this process, found in some copepods and roundworms such as Ascaris suum, portions of the chromosomes are cast away in particular cells. This process is a carefully organised genome rearrangement where new telomeres are constructed and certain heterochromatin regions are lost. In A. suum, all the somatic cell precursors undergo chromatin diminution.

X-inactivation. The inactivation of one X chromosome takes place during the early development of mammals (see Barr body and dosage compensation). In placental mammals, the inactivation is random as between the two Xs; thus the mammalian female is a mosaic in respect of her X chromosomes. In marsupials it is always the paternal X which is inactivated. In human females some 15% of somatic cells escape inactivation.

Number of chromosomes in a set

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.

:"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 Top score for animals might be the shortnose sturgeon Acipenser brevirostrum at a mere 372 chromosomes. 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

The fundamental number, FN, of a karyotype is the number of visible major chromosomal arms per set of chromosomes. Thus, FN ≤ 2n, the difference depending on the number of chromosomes considered single-armed (acrocentric or telocentric) present. Humans have FN = 82, due to the presence of five acrocentric chromosome pairs (13, 14, 15, 21 and 22).

Ploidy

Ploidy is the number of complete sets of chromosomes in a cell. Polyploidy, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants. It has been of major significance in plant evolution according to Stebbins. The proportion of flowering plants which are polyploid was estimated by Stebbins to be 30-35%, but in grasses the average is much higher, about 70%. Polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants.Polyploidy in animals is much less common, but it has been significant in some groups.Polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid.

Haplo-diploidy, where one sex is diploid, and the other haploid. It is a common arrangement in the Hymenoptera, and in some other groups.

Endopolyploidy occurs when in adult differentiated tissues the cells have ceased to divide by mitosis, but the nuclei contain more than the original somatic number of chromosomes. In the endocycle (endomitosis or endoreduplication) chromosomes in a 'resting' nucleus undergo reduplication, the daughter chromosomes separating from each other inside an intact nuclear membrane. In many instances, endopolyploid nuclei contain tens of thousands of chromosomes (which cannot be exactly counted). The cells do not always contain exact multiples (powers of two), which is why the simple definition 'an increase in the number of chromosome sets caused by replication without cell division' is not quite accurate.This process (especially studied in insects and some higher plants such as maize) may be a developmental strategy for increasing the productivity of tissues which are highly active in biosynthesis. The phenomenon occurs sporadically throughout the eukaryote kingdom from protozoa to man; it is diverse and complex, and serves differentiation and morphogenesis in many ways.

See palaeopolyploidy for the investigation of ancient karyotype duplications.

Aneuploidy

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. 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.

Chromosomal polymorphism

Some animal species are polymorphic for chromosome fusions or dissociations. 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.

Species trees

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 , 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.

All of the native Drosophila and Scaptomyza species in Hawaii 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.

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

Depiction of karyotypes

Types of banding

Cytogenetics employs several techniques to visualize different aspects of chromosomes:

Spectral karyotype (SKY technique)

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. 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

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. This method is also known as virtual karyotyping.

Chromosome abnormalities

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

Turner syndrome results from a single X chromosome (45, X or 45, X0).

Klinefelter syndrome, the most common male chromosomal disease, otherwise known as 47, XXY is caused by an extra X chromosome.

Edwards syndrome is caused by trisomy (three copies) of chromosome 18.

Down syndrome, a common chromosomal disease, is caused by trisomy of chromosome 21.

Patau syndrome is caused by trisomy of chromosome 13.

Also documented are trisomy 8, trisomy 9 and trisomy 16, although they generally do not survive to birth.

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

Cri du chat (cry of the cat), from a truncated short arm on chromosome 5. The name comes from the babies' distinctive cry, caused by abnormal formation of the larynx.

1p36 Deletion syndrome, from the loss of part of the short arm of chromosome 1.

Angelman syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the maternal genes, example of imprinting disorder.

Prader-Willi syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the paternal genes, example of imprinting disorder.

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

Genetics

Cytogenetics

Genome screen

Human genome

References

External links

Making a karyotype, an online activity from the University of Utah's Genetic Science Learning Center.

Karyotyping activity with case histories from the University of Arizona's Biology Project.

Printable karyotype project from Biology Corner, a resource site for biology and science teachers.

Chromosome Staining and Banding Techniques

Category:Cell biology Category:Chromosomes Category:Cytogenetics Category:Evolutionary biology